FIELD OF THE INVENTION

The invention relates to tracking systems for tracking the position of objects moving around a tracking area and methods of their operation. In particular, the invention is concerned with sports tracking systems for tracking the position of objects, such as players or balls, moving around an area of play, in particular sports fields.

BACKGROUND

In many conventional tracking systems, a network of fixed or mobile tracking devices, commonly referred to as “anchors”, are used to locate a number of mobile devices, commonly referred to as “tags”. An example of such a tracking system would be an ultra wide-band (UWB) tracking system, which uses an array of UWB anchors to track the position of one or more UWB tags using the distance measurements that are possible using radio-frequency messages. However, in order to accurately track the position of tags using the anchor devices, the relative position of the anchor devices, i.e. a device topology, must be known and their absolute position must be known relative to a coordinate system within which the tags are to be tracked.

In many applications a coordinate system for a tracking area is established using well-known surveying techniques and equipment. One such standard piece of surveying hardware is the total station, which can locate objects to millimetre or even sub-millimetre precision. Upon installation of stationary UWB anchors around the tracking area, a coordinate system is established using a total station and the location of the fixed anchors is then determined relative to that coordinate system.

Wth the coordinate system established to high accuracy, and the locations of the anchors known, the location of the tags relative to the established coordinate system can be calculated using a number of techniques well known to those skilled in the art, for example multilateration. As an example, in sports applications, anchors are often installed in stadiums in the grandstand, particularly under the I-beams that support the seating tiers. For these fixed installations, using a total station to determine the anchor positions very accurately upon installation is efficient, since the procedure only needs to be performed once and the devices will not move for many years. However, there are contexts where using a total station (or other equivalent surveying technique) is too expensive, cumbersome and restrictive. For example, for systems that need to be transportable and able to be rapidly deployed, such a technique is not practical. Alternatively, anchors that are installed around a training field where the field lines are periodically re-painted would also need to be periodically recalibrated, which is costly and operationally burdensome. Further still, if an anchor’s position changes, e.g. because it was moved by a maintenance worker, or because the fixture to which it was attached was moved, then the whole system would need to be recalibrated before it could be used again, otherwise the data would be corrupted by anchor location inaccuracy.

While UWB is referred to specifically above, the same set of problems are applicable to camera tracking systems which use a combination of video and computer vision algorithms to track objects of interest. If the cameras are moved for any reason, then the whole system needs to be recalibrated.

It is therefore desirable to provide a faster, more convenient and more cost- effective solution for operating a tracking system.

SUMMARY OF INVENTION

In accordance with a first aspect of the invention, there is provided a method of operating a tracking system, in particular a sports tracking system, fortracking the position of objects moving around a tracking area, in particular an area of play, the method comprising: arranging at least three tracking devices around the tracking area, wherein each tracking device comprises a linear distance measuring unit; positioning a reference device at least at a first position, wherein the reference device comprises a linear distance measuring unit, and wherein the at least first position has a known relationship with a first reference point in the area of play; wherein the linear distance measuring units are configured to allow a linear distance to be measured between each tracking device or reference device and at least three other tracking devices and/or reference devices; for each tracking device and reference device, calculating a linear distance between said tracking device or reference device and at least three other tracking devices and/or reference devices using the linear distance measuring units to create a set of linear distance measurements; creating a device topology of the at least three tracking devices based on the set of linear distance measurements; and determining an arrangement of the device topology relative to the area of play based on linear distance measurements to the at least first position and the known relationship of the at least first position with the first reference point..

This method uses linear distance measuring units located on the tracking devices themselves to establish the relative position of the tracking devices and create a device topology. As will be discussed in more detail below, each linear distance measurement unit is typically a transmitter and/or receiver, such that a pair of said linear distance measurement units allow the linear distance between that pair to be measured. The device topology established using the set of linear distance measurements is fixed to a coordinate system by positioning a reference device at a reference position within the tracking area, and using linear distance measuring units, e.g. of the tracking devices, to measure the distance to the reference position. In particularly preferable embodiments, the device topology is created and then fixed relative to the area of play; however, it is also possible for the device topology to be created in a reference frame that is already fixed to the area of play, i.e. by using the reference device(s) in the starting set of devices when creating the topology. The present technique prioritises convenience over accuracy by having the tracking devices themselves contribute to the measurements used to establish the device topology and its position relative to the coordinate system within which objects are to be tracked. The result is a tracking system that is simple, cost-effective and fast to set up and operate, and so allows for rapid deployment and re-calibration of tracking systems at scale. The tracking devices are typically arranged with unknown positions, e.g. at any convenient positions around an area of play (although this is not essential). In contrast, the reference devices(s) are positioned in at least partially prescribed positions such that there is a predetermined known relationship between the position of the reference device and one or more reference points and/or reference directions in the area of play, as will be described in more detail below. In other words, the known relationship should be predetermined, i.e. it should be known without the linear distance measurements.

The present technique is preferably a method of operating a sports tracking system for tracking the position of objects moving around an area of play, such as a sports field. However, this technique could also be used for other types of tracking system, such as in warehouses, at music concerts, or in television studios etc. In a sports tracking system, preferably a plurality of the tracking devices (most preferably each tracking device) will be arranged outside an area of play, e.g. outside a sports field. However, alternatively a tracking device could be positioned above the sports field, for example.

The tracking devices here each include a linear distance measuring unit that allow a linear distance to be measured between each tracking device or reference device and at least three other tracking devices and/or reference devices. It will be appreciated that the invention may be used with more than three tracking devices (and indeed this will be preferred); however, three tracking devices is sufficient to establish a device topology and then track objects around the tracking area when the devices can be assumed to be arranged in a single plane, as is the case for many set-ups, particularly around sports fields. Where more than three tracking devices are used, typically each tracking device will have a linear distance measuring unit to allow a linear distance to be measured to each other tracking device and reference device; however, a topology can still be established without measurements between each of the tracking devices and every other device. Where the tracking devices are not arranged in the same plane, i.e. as the objects tracked around the area of play, at least four tracking devices will be preferred, with each tracking device comprising a linear distance measuring unit configured to allow a linear distance to be measured to at least three, preferably at least four, other tracking devices and/or reference devices, although again typically they will measure the linear distance to each other tracking device and reference device.

This technique also involves positioning a reference device at least at a first position and using the linear distance measuring units to measure a linear distance to the first position. As mentioned above, the first position should be at least partially prescribed. For example, a user of the system may be instructed to place the reference device on a particular field marking or somewhere along one of the field lines of a sports field. This is because this step is used to gain information about the device topology relative to a first reference point in the tracking area, in order to establish the position of the tracking devices relative to a coordinate system within which objects are to be tracked. It should be noted that the at least first position has a predetermined “known relationship” with the first reference point, which means that the position may not correspond to the first reference point, but will provide information about the location of the first reference point. For example, the first position may be on the first reference point or offset from the first reference point by a known distance, and this can be factored in when determining an arrangement of the device topology relative to the tracking area. Alternatively, if multiple reference devices are used, the two reference devices could be positioned so that the first reference point is at the centre point between the devices. This is why it is merely required that the at least first position measured with the reference devices has a known relationship with the first reference point, i.e. if there is more than just a first position then it may be these positions collectively that have the known relationship with the first reference point.

By providing the reference device at the first position, which has a (predetermined) known relationship with at least a first reference point in the tracking area, this can allow the device topology to be accurately mapped to a coordinate system of the tracking area.

In some particularly preferred embodiments, setup is split into two distinct phases in which the topology of the tracking devices is established in an arbitrary reference frame without the reference devices and then the arrangement of the device topology is determined relative to the tracking area using linear distance measurements calculated to the reference device(s). For example, this method may comprise arranging at least three tracking devices around the tracking area, wherein each tracking device comprises a linear distance measuring unit configured to measure a linear distance to at least two other tracking devices of the at least three tracking devices; for each tracking device, calculating a linear distance between said tracking device and at least two other tracking devices using the linear distance measuring units to create a set of linear distance measurements; creating a device topology of the at least three tracking devices based on the set of linear distance measurements; positioning a reference device at least at a first position, wherein the at least first position has a known relationship with a first reference point in the tracking area; calculating a linear distance between each of at least three of said tracking devices and the reference device at the first position to create a set of reference measurements; and determining an arrangement of the device topology relative to the tracking area based on the set of reference measurements.

It will be appreciated that the above generally describes a method of setting up or calibrating the sports tracking system. The method of operating the sports tracking system will typically further comprise, after the steps defined above, tracking one or more objects (preferably mobile electronic devices, i.e. tags) around the area of play using the at least three tracking devices, preferably using the linear distance measuring units of the at least three tracking devices.

In some embodiments, one reference device may be all that is required to determine the arrangement of the device topology relative to the tracking area. For example, if the tracking area is circular, with rotational symmetry about a centre point, then a single reference device may be placed on the centre point to determine the arrangement of the device topology relative to the tracking area. This may require one or more assumptions, such as assuming that the tracking devices are arrange in a plane parallel to the ground of the tracking area. In other embodiments, the reference device could include means for identifying its orientation. For example, embodiments will be described below which use UWB transceivers with multiple antennae to allow phase difference of arrival (PDOA) or Angle of Arrival (AOA) to be used to determine orientation. The reference device may then be configured to be placed at a position with its orientation aligned with one or more reference directions of the tracking area, and measurements as to the linear distance to that reference device and its orientation relative to the device topology may then be used to determine the arrangement of the device topology relative to the tracking area. For example, if the tracking area is an athletics track, the reference device may be positioned at the centre of the athletics track and aligned with the longer dimension of the athletics track to allow the arrangement of the device topology relative to the athletics track to be determined.

While it may be possible in some scenarios to determine the arrangement of the device topology relative to the tracking area using only one reference device, the majority of embodiments preferably comprise positioning a reference device at each of at least the first position and a second position, wherein the at least first position and second position together have a known relationship with the first reference point in the tracking area and a second reference point or a first reference direction in the tracking area, and wherein determining the arrangement of the device topology relative to the area of play is based on linear distance measurements to the first position and the second position and the known relationships of the first position and the second position with the first reference point and the second reference point or first reference direction. For example, this may comprise calculating a linear distance between each of at least three of said tracking devices and the reference device at the second position. This provides more flexibility and greater accuracy for determining the arrangement of the device topology relative to the tracking area. As will be discussed below, where a reference device is required at a first position and a second position, this could involve one reference device positioned sequentially at the first and second position, or two reference devices positioned respectively at the first and second position.

Again, the at least first position and second position should be each at least partially prescribed and together may have a (predetermined) “known relationship” with a first reference point and a second reference point. The first position and second position will preferably coincide with the first and second reference points, but again there could be a known offset. In one example, for an association football pitch of a known size, it may be prescribed that the reference devices should be placed on the two penalty spots so that position of the tracking devices relative to the field can be determined using the measurements between the tracking devices and the first and second reference points. Additionally, the at least first position and second position together may have a “known relationship” with a first reference direction. The devices could be spaced along the direction of the reference direction, for example being spaced along one of the field lines, but could also be at some known angle to the reference direction, e.g. spaced perpendicular to the field line, if desired. In a second example, for a rugby field of a known size, one reference device could be positioned at one corner of the try line and the touch line and another reference device could be spaced from the first reference device along the touch line. This may provide information about a reference point, namely the corner of the try line and the touch line, and information about a reference direction, i.e. the direction of the touchline, being the direction along which the two reference devices are spaced. It will be appreciated that these are merely two examples given in particular sports contexts, and that any system of reference points for positioning the device topology relative to references in a coordinate scheme for tracking may be used.

While the system may be set up with two reference devices in some contexts, in many cases it is preferred to extract information about the tracking area through the use of still further reference devices. Therefore, preferably, the method comprises positioning a reference device at an at least partially prescribed third position such that the first, second and third positions do not lie along a straight line, wherein preferably the third position together with the first and/or second position have a known relationship with a second reference direction in the tracking area, and wherein determining the arrangement of the device topology relative to the area of play is based on linear distance measurements to the third position and the known relationships of the third position together with the first and/or second position with the second reference direction, and wherein preferably determining the arrangement of the device topology relative to the tracking area is further based on the order of the first, second and third positions. In many applications, measurements to a reference device at a third position are required in order to properly orient the device topology to an area of interest, such as a sports field. For example, three positions not on a straight line can allow the system to identify a plane, such as the plane of a sports field, regardless of the arrangement of the device topology to this plane. Measurements to a reference device at a third position can also allow the system to identify a second reference direction in the tracking area. To continue the earlier rugby field example, the third position could be positioned somewhere along the try line, so that the first position, at the corner of the try line and the touch line, together with the third position enable the system to identify the direction of both the try line and touch line. A series of three positions can also convey information through the order of the positions, for example in a manner analogous to the “right-hand rule”. This can be used so that the three positions are used to measure a plane, e.g. ground level, and the order of the positions is used to identify which way is up in the direction perpendicular to the plane. It should be noted that, alternatively, the order of the tracking devices themselves could be used to determine the arrangement of the device topology relative to the tracking area. For example, each tracking device could be numbered, and set up may require positioning the tracking devices clockwise around a sports field, with the up direction then being determined in a manner analogous to the “right-hand rule”. Again, in these examples, there may be a separate reference device positioned at each position, or one or more reference devices could be sequentially moved between the positions in order to make measurements to the positions with fewer reference devices.

As mentioned above, the first, second and third positions preferably define a plane substantially parallel with ground level, e.g. a sports field, in the tracking area. This allows the system to identify the plane of a sports field. The or each reference device may be positioned substantially at ground level, for example. Alternatively, one or more reference device could be located at a known height above ground level. For example, one or more reference devices could be provided on supports, such as tripods, having a known height. In order to identify ground level, the system may then factor in the known height of the reference device when using the measurements to the reference devices.

In many embodiments, the first reference point is a notable point within a coordinate system in which objects are going to be tracked. For example, the first reference point could be the intersection of the halfway line and the touch line on a rugby pitch. A preferred way of obtaining information about this reference point would be to provide that the first position has a known positional relationship with the first reference point, wherein most preferably the first position substantially coincides with the first reference point. The reference device would not need to be placed physically on the first reference point, although this would be preferred. For example, if play is currently ongoing, it may be necessary to position the reference device 5 metres from the reference point. However, if this offset is known, then the reference point may still be identified with the measurements to the reference device at the first position. It should be noted that it is not essential to position one of the reference devices having a known positional relationship with the first reference point. For example, two reference devices could be spaced arbitrarily along the halfway line of a rugby field and two reference devices spaced arbitrarily along the touch line. In this case, the reference position, i.e. the intersection of these two lines, may be determined from the combination of the positions, since the direction of the two lines can be determined from the reference measurements.

Regardless of how the reference device(s) are positioned, preferably linear distance measurements to the reference positions are further used to measure at least one dimension of the tracking area. It should be noted here that this may include linear distance measurements made between the reference devices at the at least first position and second position. That is, the dimension may be measured either by linear distance measurements from the tracking devices to the reference devices, or based on linear distance measurements directly between the reference devices, or some combination thereof. The dimension may be measured by ensuring that the at least first position and second position together have a (predetermined) “known relationship” with respect to a dimension to be measured. The dimension may then be determined by the distance between the positions used for the reference device(s).

Further to the above, preferably, the second position has a (predetermined) known positional relationship with the second reference point in the tracking area, wherein preferably the second position substantially coincides with the second reference point. To continue the above rugby field example, a second reference point may be the intersection of the same touch line and the try line. Again, the reference device may be placed to coincide with this reference point, or could be spaced a known distance from the reference point. An advantage of a system that requires the user to place reference devices with known positional relationships with respect to two or more reference points is that at least one dimension of the tracking area may be conveniently determined based on the set of linear distance measurements. Continuing the rugby field example, the system may then be able to measure the distance between the halfway line and the try line, and so more accurately understand the tracking area. This is preferred over, for example, assuming standard field dimensions or requiring the user to separately enter certain field measurements. In a further preferred embodiment, the third position has a known positional relationship with a third reference point in the tracking area, wherein preferably the third position substantially coincides with the third reference point. Again continuing the rugby field example, this third reference point could be the intersection of the opposite touch line with the 22-metre line. This builds upon the advantages noted above, and allows further measurements about the field to be taken during set up. As indicated above, another alternative to positioning the reference devices with known positions relative to reference points is to rely on a combination of reference devices to provide information about one or more reference points, along with reference directions. For example, if a pair of reference devices are arbitrarily spaced along each field line, then this would allow identification of reference points, as well as a measurement of the distance between these reference points or between the field lines in order to measure the field. Therefore, in the more general case the at least first position and second position together may have a (predetermined) “known relationship” with respect to a dimension to be measured, as stated above. However, this may require the use of reference devices at more positions in order to acquire the same information about the field.

As indicated above, in addition to obtaining information about reference points in the tracking area, it may be desirable to identify reference directions in the tracking area. One way this may be achieved is to provide that the first and second positions have a (predetermined) known angular relationship with the first reference direction, wherein preferably the first and second positions define a line substantially parallel with the first reference direction. It should be noted here that a line parallel with the first reference direction is considered to include a line that coincides with the reference direction. For example, if the reference direction is considered to be defined by one of the touch lines, then the first and second positions may be arranged along that touch line. Generally, it will be easiest for the user to position reference devices along a reference direction, e.g. along a field line of a sports field. However, provided an angle to the reference direction is known, this may provide enough information to identify the reference direction in the tracking area.

As will already be clear from the above discussion, in the most preferred examples, the reference points and reference directions referred to correspond to the field markings of a sports field in the tracking area. More generally, the first and/or second reference point (and/or any third or further reference point) may correspond to a field marking on a sports field in the tracking area. Preferably, the first and/or second reference point (and/or any third or further reference point) corresponds to the intersection of at least two field lines on the sports field and/or wherein the first reference direction corresponds to a direction of a field line on the sports field. Example reference points already given have included the penalty spots of an association football field, or the intersection of the halfway line and touch line of a rugby field; however, it will be clear that any field marking may be taken as a reference point in establishing the coordinate system within which objects are to be tracked, and further the direction of any field lines could correspond to a reference direction in the coordinate system. Preferably, the set of linear distance measurements are further used to measure one or more dimensions of the sports field, e.g. using linear distance measurements to the first and second positions.

In particularly preferred examples, the first position is at the intersection of at least two field lines on a sports field in the tracking area, and the second position is located along one of said two field lines from the first position, wherein preferably the second position is at the intersection of a third field line with one of said at least two field lines, such that at least one dimension of the sports field may be determined based on linear distance measurements to the first and second positions. As has already been noted, the reference device may be placed directly on the field, or could be supported above the field, e.g. on a tripod. This represents a particularly convenient way of determining the arrangement of the device topology relative to the tracking area and also taking measurements of one or more dimensions of the field, since a user simply has to position the reference device(s) at certain positions on the field lines, which is simple for a user to understand and execute.

As has already been briefly noted above, this technique may involve the use of any number of reference devices for the array of positions at which reference measurements are to be obtained. For example, a first reference device may be positioned at the first position and a second reference device positioned at the second position, or a first reference device may positioned at the first position, and then, after the linear distance between each of at least three of said tracking devices and the first reference device at the first position is calculated, the first reference device may be positioned at the second position. Then, the linear distance between each of at least three of said tracking devices and the first reference device at the second position may be calculated and the linear distance measurement set updated. This principle may be expanded when further positions are used, with anywhere between one reference device and the same number of reference devices as the number of reference positions (i.e. one reference device for each position) being used.

In particularly preferred embodiments, one or more of the or each reference device is a further tracking device, and further comprising, after obtaining linear distance measurements to the corresponding at least first position, arranging the or each reference device around the tracking area, calculating a linear distance between the or each further tracking device and at least three tracking devices of the at least three tracking devices using one or more linear distance measuring units, and updating the device topology to include the or each further tracking device. In these embodiments, the reference devices may each be used as a tracking device once they have been used to determine the arrangement of the device topology relative to the tracking area. For example, the reference devices could be essentially the same hardware as the tracking devices. In order to do this, the device topology has to be updated to include the reference devices once linear distance measurements to the reference device positions have been obtained; however, at this stage the arrangement of the device topology to the tracking area is already known, and so the reference devices are no longer needed for their original function. It will be appreciated that tracking capabilities will be enhanced with additional tracking devices, and the present embodiment makes the most efficient use of the hardware used to set up the tracking system. While the reference devices could be further tracking devices, they could alternatively be “tags”, i.e. the mobile devices normally tracked by the tracking devices.

In some embodiments, wherein one or more of the tracking devices and/or reference devices comprises a GNSS receiver configured to make a GNSS position measurement of said device, and wherein determining the arrangement of the device topology relative to the tracking area is further based on the GNSS position measurement. There are various advantages to also using GNSS position measurements. For example, the GNSS position measurement data may be used for an initial coarse alignment of the device topology with the tracking area, or the data may be fused with the linear distance measurement data to provide a more accurate coordinate system.

The present method may be performed by a controller, and the measurements from each tracking device and/or reference device may be communicated to the controller, which may then calculate the linear distances and create the device topology and determine the arrangement of the device topology to the tracking area based on the measurements it receives from the tracking devices (and reference devices).

One problem with conventional tracking systems is that when one of the tracking devices is moved, the whole system would need to be recalibrated before it could be used again, otherwise the data would be corrupted by tracking device location inaccuracy. Therefore, preferably, the method comprises, after creating the device topology, detecting a movement of one of the at least three tracking devices (preferably at least four tracking devices) relative to the other tracking devices of the at least three (preferably at least four) tracking devices, re-calculating a linear distance between the moved tracking device and each of at least two other tracking devices (preferably at least three other tracking devices) using the linear distance measuring units, and updating the device topology to account for the movement of said moved tracking device based on the re-calculated linear distance between the moved tracking device and each of at least two (preferably at least three) other tracking devices of the at least three (preferably at least four) tracking devices. That is, because in the present system the tracking devices are able to measure the distance between each other, they are able to update the device topology automatically, without the need to use surveying equipment again, as would be the case for a conventional system. This part of the method will typically take place after initial calibration, e.g. after the reference devices are removed, therefore in order to most accurately re-locate the moved tracking device, preferably the method involves at least four tracking devices.

This method may involve, for each tracking device, periodically re-calculating a linear distance between said tracking device and at least two other (preferably at least three other) tracking devices using the linear distance measuring units, and detecting movement of one of the at least three tracking devices is based on a detected change in linear distance between said moved tracking device and said other at least two other tracking devices after said re-calculation. In other words, the tracking devices constantly check the linear distance to other tracking devices in order to identify when one device has moved. This will be visible in the data, since one device will detect movement of all of the other devices it is measuring, while the other devices will detect only the movement of that one tracking device. In addition, or alternatively, each tracking device may further comprise one or more of an accelerometer, gyroscope and magnetometer and detecting a movement of one of the at least three tracking devices relative to the other tracking devices is based on readings from said accelerometer, gyroscope and/or magnetometer. It should be noted that there are also other advantages to including these sensors in the tracking device. For example, in some applications, it may be desirable to identify a north-south direction as part of the coordinate system and a magnetometer would allow this. These sensors would also allow for a form of dead reckoning, in order to update the position of the device within the topology even if the measurement to other tracking devices is blocked, such as if the line of sight is blocked.

Linear distance measuring units can produce particularly accurate device topology when the position of the tracking devices is varied in three-dimensional space, i.e. not all within substantially the same plane, so that a number of measurements can be made from significantly different directions for each device. Similarly, when all tracking devices are positioned in a single plane, e.g. all at the same height above a sports field, then the linear distance measuring units can produce an accurate device topology. However, it is very common for there to be small height differences between the tracking devices due to infrastructure around the tracking area, which can introduce significant inaccuracies into the device topology. For example, if tracking devices are all positioned in roughly the same plane, but have height differences of up to, for example, 1 metre, then when linear distances are measured at, for example, 200 metres, this difference in height is very difficult to accurately discern and incorporate into a device topology. Therefore, preferably, each tracking device further comprises a pressure sensor configured to measure air pressure at the position of the tracking device, and the method further comprises calculating a relative altitude of each tracking device based on the measured air pressure at the position of each tracking device, and wherein creating the device topology is further based on the calculated relative altitude of each tracking device. The or each reference device may also further comprise a pressure sensor configured to measure air pressure at the position of the reference device. In this embodiment, the relative heights can be reasonably accurately deduced from air pressure measurements at the tracking devices.

The method may further include determining the absolute heights of each of the tracking devices. In practice, this may involve providing one or more tracking devices (or reference devices) with a known altitude, and then comparing the pressure reading of each tracking device with the pressure reading of the known device to determine the absolute altitude of each device.

Preferably, one or more reference devices further comprise a pressure sensor configured to measure air pressure at the position of the reference device, and determining an arrangement of the device topology relative to the tracking area is further based on the pressure measurements of the tracking devices and the pressure measurements of the one or more reference devices. Alternatively, the altitudes may only be determined relative to one another among the tracking devices so that the device topology reflects the variation in height, and then the device topology determined relative to the reference devices using linear distance measurements to the reference positions.

It should be noted for the above embodiments that pressure naturally varies over time, and this will affect the pressure measurements made by the devices. Therefore, it may be preferred to take each pressure reading over the same time interval, preferably a known time interval, e.g. two minutes. For example, the reading for one device (or an average of several devices), preferably having a known altitude, over the time interval may then be subtracted from the pressure reading of each tracking device in order to remove the natural variability of the atmospheric pressure, leaving only the difference that is caused by altitude difference of the tracking devices.

The use of pressure sensors is preferably provided in a method that involves arranging at least four tracking devices around the tracking area, wherein each tracking device is arranged substantially within the same plane within a tolerance of 10 metres, preferably within a tolerance of 5 metres, more preferably within a tolerance of 2 metres, most preferably within a tolerance of 1 metre, and wherein preferably the tracking devices are arranged at least 50 metres apart, preferably at least 70 metres apart, more preferably at least 100 metres apart. That is, the use of pressure sensors becomes more valuable as the height different between the tracking devices becomes smaller, i.e. and harder to measure from the linear distance measurements, and as the distance between the tracking devices becomes larger, i.e. and so the effect on distance caused by the height variation has a smaller proportional impact on the total measured distance.

In many embodiments, the linear distance measuring unit of each tracking device is further configured allow a linear distance to be measured between each tracking device and one or more mobile electronic devices moving around the tracking area. Such systems are particularly advantageous, as they make dual use of the linear distance measuring unit.

The linear distance measuring unit of each tracking device preferably comprises an ultra-wideband (UWB) transmitter and/or receiver, which would include the use of an UWB transceiver. Preferably each tracking device comprises an UWB transmitter and receiver so each tracking device can measure the linear distance to each other tracking device. Other examples of suitable linear distance measurement devices include, for example, lidar, ultrasound, and laser range finder units. An UWB transmitter and/or receiver is an example of a linear distance measuring unit that can also measure the distance to corresponding electronic devices that may move through the tracking area.

Preferably, the linear distance measuring unit of at least one tracking device comprises at least a first antenna and a second antenna used in making the linear distance measurements, and further comprising calculating a relative orientation of said at least one tracking device using the linear distance measurements obtained using the first and second antennae. It will be preferred that a plurality of tracking devices, preferably each tracking device, each comprise at least a first antenna and a second antenna, so that angular orientation can be determined for each of those tracking devices. As mentioned above, the reference devices may be structured identically to the tracking devices, and so one or more, preferably each reference device, may have at least a first antenna and a second antenna, and the method may include calculating an angular orientation of the or each reference device relative to one or more tracking devices. This orientation of the reference device(s) may be used in determining the arrangement of the device topology relative to the tracking area. The use of at least two antennae may allow orientation to be determined, for example, using phase difference of arrival (PDOA) or angle of arrival (AOA) of the measurements made using said linear distance measuring unit. Two antennae may allow angular orientation to be determined in two-dimensional space, e.g. if all devices are known to be positioned in the same plane. Three antennae may be preferred to allow orientation to be determined in three-dimensional space.

While antennae may be preferred for determining orientation, in other embodiments, at least one tracking device comprises a plurality of linear distance measuring units, and the method further comprises calculating a linear distance between each of the plurality of linear distance measuring units on said at least one tracking device and at least one other tracking device, and calculating a relative orientation of said at least one tracking device based on the differences in the calculated linear distances. Again, preferably a plurality of tracking devices, preferably each tracking device, comprises a respective plurality of linear distance measuring units, so that orientation may be determined for each of those tracking devices. Again, the reference devices may be structured in the same way to allow their orientation to be determined. In this case, orientation is determined based on the differences in the linear distance measurements. For example, if a first linear distance measuring unit on one of said tracking devices is measured to be closer to another tracking device than the second linear distance measuring unit on that tracking device, then it can be determined that the tracking device is oriented so that the first linear distance measuring unit is on the side closer to the other tracking device. Again, two linear distance measuring units on each tracking device may allow orientation to be determined in two-dimensional space, e.g. if all devices are known to be positioned with their linear distance measuring units in the same plane. However, it may be preferred to provide three linear distance measuring units on each tracking device in order to allow orientation to be determined in three-dimensional space. In other examples, one or more tracking devices comprise a camera for optically tracking the position of objects moving around the tracking area. The use of several cameras to track objects in three-dimensional space is known in various sports contexts in particular. In these embodiments, the distance between the tracking devices must still be measured using a linear distance measuring unit, such as an UWB transmitter and/or receiver. It is particularly preferred where the tracking device comprises a camera to allow the orientation of the tracking device to be determined relative to other tracking devices, e.g. using the techniques described above. This is because object tracking with cameras typically requires the orientation of each camera to be known. Therefore, where one or more tracking devices comprises a camera, preferably each of those tracking devices with a camera comprises a linear distance measuring unit with at least a first antenna and a second antenna or comprises at least two linear distance measuring units.

Preferably, the arranging at least three tracking devices around the tracking area comprises fixedly positioning each tracking device around the tracking area. This is in contrast to systems which use, for example, UWB transmitters and/or receivers attached to objects, such as players or a sports ball, moving around a tracking area. For example, the tracking devices may be attached to infrastructure around the tracking area, or provided on a fixed support structure, such as a tripod, that remains in position throughout the duration of the tracking process. Preferably, each of the at least three tracking devices remains stationary during one or more subsequent steps of tracking the position of objects moving around the area of play.

As mentioned above, preferably more than three tracking devices will be used, in order to improve the accuracy of the tracking process and introduce redundancy in terms of the number and positioning of the tracking devices. Preferably, the method comprises arranging at least four tracking devices, preferably at least six tracking devices, more preferably at least ten tracking devices, around the tracking area, wherein preferably each tracking device comprises a linear distance measuring unit configured to allow a linear distance to be measured to at least three other tracking devices. A system with four tracking devices that measures a distance to three other tracking devices is particularly useful when the objects to be tracked are not essentially confined to a single plane, as are players on a sports field. With more tracking devices, it would still only be necessary to measure the distance to three other tracking devices, however, typically each tracking device will comprises a linear distance measuring unit configured to allow a linear distance to be measured to each other tracking device.

When using four or more tracking devices, preferably creating a device topology of the at least four tracking devices based on the set of linear distance measurements comprises identifying a starting set of three tracking devices and/or reference devices based on the magnitude of the linear distance measurements in the set of linear distance measurements and the differences between the linear distance measurements, defining a coordinate system using the linear distance measurements between each of the starting set of tracking devices and/or reference devices, and then locating a fourth tracking device or reference device in the coordinate system defined for the starting set of tracking devices and/or reference devices based on the linear distance measurements between the fourth tracking device or reference device and each tracking device and/or reference device in the starting set of tracking devices and/or reference devices. It will typically be desirable to identify a starting set of three tracking devices and/or reference devices that defines a relatively large and approximately equilateral triangle. This is discernible from the magnitude of the linear distance measurements and the differences between the linear distance measurements. From this starting set, additional tracking devices and/or reference devices can be incorporated into the topology relatively precisely from a robust starting set of tracking devices and/or reference devices. When operating in three-dimensional space, as opposed to tracking devices and/or reference devices all approximately in the same plane, a starting set should be selected based on the magnitude of the linear distance measurements and the differences between the linear distance measurements that defines a tetrahedron, with further tracking devices and/or reference devices then being measured relative to the starting set and incorporated into the device topology. Preferably, creating a device topology of the at least four tracking devices comprises creating a preliminary device topology using each of a plurality of different starting sets of at least three tracking devices and/or reference devices, preferably each possible starting set of at least three tracking devices and/or reference devices, and creating the device topology using the plurality of preliminary device topologies, preferably by taking an average of the preliminary device topologies. Calibration in the manner described above would be vulnerable to error propagation if the one and only starting set of devices contains a measurement error. In order to prevent this, the device topology may be created several times from several different starting sets and then averaged, as described. This reduces the impact of measurement errors in the starting set. It should be noted that the average may be a mean position or a median position of each device, with a median being more resilient to measurement errors.

In particularly preferable embodiments, the method comprises comparing the preliminary device topologies created from each starting set, disregarding one or more preliminary device topologies, the position of one or more devices within one or more preliminary device topologies, or one or more linear distance measurements based on the results of the comparison, and creating the device topology using a plurality of remaining preliminary device topologies. In this embodiment, the different preliminary device topologies are compared and recognisable measurement errors are accounted for. In particular, either the whole topology containing the measurement error may be disregarded, individual device positions within a device topology may be disregarded, or it may be determined which linear distance measurement(s) led to the incorrect device position and said measurement(s) disregarded and the preliminary device topology recalculated. For example, most preliminary device topologies may agree on relative device position to within a relatively small tolerance and so any preliminary device topology that includes one or more device positions that differ, for example, from an average device position or from device positions of a plurality of other preliminary device topologies by more than a threshold amount, e.g. 30 cm, preferably 20 cm, more preferably 10 cm, even more preferably 5 cm, may be disregarded, or that device position within that preliminary device topology disregarded, or the problematic linear distance measurement(s) disregarded, to remove the measurement error from the calibration process.

In accordance with a second aspect of the invention, there is provided a method of operating a sports tracking system for tracking the position of objects moving around an area of play, the method comprising: fixedly arranging at least three tracking devices around the area of play, wherein each tracking device comprises a linear distance measuring unit configured to allow a linear distance to be measured between each tracking device and at least two other tracking devices of the least three tracking devices, wherein the at least three tracking devices have a known device topology; detecting a movement of one of the at least three tracking devices relative to the other tracking devices of the at least three tracking devices; calculating a linear distance between the moved tracking device and each of at least two other tracking devices using the linear distance measuring units; updating the device topology to account for the movement of said moved tracking device based on the calculated linear distance between the moved tracking device and each of at least two other tracking devices of the at least three tracking devices.

Compared with the first aspect of the invention, this may involve calibration of the system using conventional techniques, such as the use of a total station. However, in this case, in the event that one of the tracking devices is moved, instead of having to recalibrate the system, the topology of the devices is automatically updated. The tracking accuracy will decrease overtime in this case, and recalibration may still be required periodically, but this technique will prevent wildly inaccurate tracking results due to tracking device location inaccuracy.

The preferred features described above with respect to the first aspect apply equally to this aspect of the invention. For example, this is also preferably a method of operating a sports tracking system for tracking the position of objects moving around an area of play, such as a sports field, preferably uses more than three tracking devices configured to measure a linear distance to each other tracking device, and preferably uses linear distance measuring units in the form of an ultra-wideband (UWB) transmitter and/or receiver. In this aspect, each tracking device is fixedly arranged around a tracking area. For example, the tracking device may be affixed to infrastructure around the tracking area, or provided on fixed support structures, such as a tripod. This is in contrast with the tags which may be attached to objects moving around the tracking area, such as players moving around a sports field. Indeed, in a sports tracking system, preferably each tracking device will be arranged outside an area of play, e.g. outside a sports field. In conventional tracking systems, fixed tracking devices are not expected to move, and so any change in the position of one of these fixed anchors will corrupt the tracking data. This is addressed in the present aspect by detecting a movement of one of the at least three tracking devices and updating the device topology accordingly. This method may occur cotemporally with one or more steps of tracking the position of objects moving around the area of play.

The technique of updating the device topology to account for movement of one of the tracking devices is the same as described above. Preferably, detecting the movement of one of the at least three tracking devices comprises, for each tracking device, periodically calculating a linear distance between said tracking device and at least two (preferably each of the) other tracking devices using the linear distance measuring units, and detecting movement of one of the at least three tracking devices based on a detected change in linear distance between said moved tracking device and said other at least two other tracking devices after said calculation. In other words, the tracking devices constantly check the linear distance to other tracking devices in order to identify when one device has moved. This will be visible in the data, since one device will detect movement of all of the other devices it is measuring, while the other devices will detect only the movement of that one tracking device. In addition, or alternatively, each tracking device may further comprise one or more of an accelerometer, gyroscope and magnetometer and detecting a movement of one of the at least three tracking devices relative to the other tracking devices is based on readings from said accelerometer, gyroscope and/or magnetometer. As also mentioned above, each tracking device preferably further comprises a pressure sensor configured to measure air pressure at the position of the tracking device, and the method further comprises calculating a relative altitude of each tracking device based on the measured air pressure at the position of each tracking device, and wherein updating the device topology is further based on the calculated relative altitude of each tracking device. This may be particularly useful if the movement of the device includes raising or lowering the tracking device compared with the others, particularly where the tracking devices are in roughly the same plane.

In accordance with a third aspect of the present invention, there is provided a method of operating a sports tracking system for tracking the position of objects moving around an area of play, the method comprising arranging at least three tracking devices around the tracking area, wherein each tracking device comprises a linear distance measuring unit configured to allow a linear distance to be measured between each tracking device and at least two other tracking devices of the least three tracking devices, and a pressure sensor configured to measure air pressure at the position of the tracking device; for each tracking device, calculating a linear distance between said tracking device and at least two other tracking devices using the linear distance measuring units to create a set of linear distance measurements; calculating a relative altitude of each tracking device based on the measured air pressure at the position of each tracking device; and creating a device topology of the at least three tracking devices based on the set of linear distance measurements and the calculated relative altitude of each tracking device.

Again, the preferred features described above with respect to the first aspect apply equally to this aspect of the invention. For example, this is also preferably a method of operating a sports tracking system for tracking the position of objects moving around an area of play, such as a sports field, preferably uses more than three tracking devices configured to measure a linear distance to each other tracking device, and preferably uses linear distance measuring units in the form of an ultra-wideband (UWB) transmitter and/or receiver. Compared with the first aspect of the invention, this technique establishes a device topology using both of linear distance measurements and pressure measurement, but the position of the device topology relative to the tracking area, e.g. a sports field, may be determined in any known way, including through the use of a calibration device, such as a total station.

As with the first aspect, the method may further include determining the absolute heights of each of the tracking devices, e.g. by providing one or more tracking devices (or reference devices) with a known altitude, and then comparing the pressure reading of each tracking device with the pressure reading of the known device to determine the absolute altitude of each device.

Again, it may be preferred to take each pressure reading over the same time interval, preferably a known time interval, e.g. two minutes, and then preferably subtract one the reading for one device (or an average of several devices), preferably a known device, over the time interval may then be subtracted from the pressure reading of each tracking device in order to remove the natural variability of the atmospheric pressure, leaving only the difference that is caused by altitude difference of the tracking devices.

As mentioned above, preferably this method involves arranging at least four tracking devices around the tracking area, wherein each tracking device is arranged substantially within the same plane within a tolerance of 10 metres, preferably within a tolerance of 5 metres, more preferably within a tolerance of 2 metres, most preferably within a tolerance of 1 metre, and wherein preferably the tracking devices are arranged at least 50 metres apart, preferably at least 70 metres apart, more preferably at least 100 metres apart.

In accordance with a fourth aspect of the invention, there is provided a sports tracking system for tracking the position of objects moving around an area of play, the system comprising: at least three tracking devices configured to be arranged around an area of play, wherein each tracking device comprises a linear distance measuring unit; a reference device, wherein the reference device comprises a linear distance measuring unit; wherein the linear distance measuring units are configured to allow a linear distance to be measured between each tracking device or reference device and at least three other tracking devices and/or reference devices; and a data processing apparatus configured to create a device topology of the at least three tracking devices based on a set of linear distance measurements comprising linear distance measurements calculated for each tracking device and reference device between said tracking device or reference device and at least three other tracking devices and/or reference devices using the linear distance measuring units; wherein the data processing apparatus is further configured to receive linear distance measurements to the reference device positioned at least at a first position, wherein the at least first position has a known relationship with a first reference point in the area of play, and further configured to determine an arrangement of the device topology relative to the area of play based on the linear distance measurements to the at least first position and the known relationship of the at least first position with the first reference point.

This corresponds to a system suitable for use in accordance with the method of the first aspect of the invention. Accordingly, all the preferred features described above with regard to the first aspect apply equally to the system according to this aspect.

In accordance with a fifth aspect of the invention, there is provided a sports tracking system for tracking the position of objects moving around an area of play, the system comprising: at least three tracking devices configured to be arranged around an area of play with a known device topology, wherein each tracking device comprises a linear distance measuring unit configured to allow a linear distance to be measured between each tracking device and at least two other tracking devices of the at least three tracking devices to create a set of linear distance measurements; and a data processing apparatus configured to detect a movement of one of the at least three tracking devices relative to the other tracking devices of the at least three tracking devices, and to update the device topology to account for the movement of said moved tracking device based on the set of linear distance measurements. This corresponds to a system suitable for use in accordance with the method of the second aspect of the invention. Again, all of the preferred features described above apply equally to this system.

In accordance with a sixth aspect of the invention, there is provided a sports tracking system for tracking the position of objects moving around an area of play, the system comprising: at least three tracking devices configured to be arranged around a tracking area, wherein each tracking device comprises a linear distance measuring unit configured to allow a linear distance to be measured between each tracking device and at least two other tracking devices of the at least three tracking devices to create a set of linear distance measurements, and a pressure sensor configured to measure air pressure at the position of the tracking device.

This corresponds to a system suitable for use in accordance with the method of the third aspect of the invention. Again, all of the preferred features described above apply equally to this system.

Preferably, this system further comprises a data processing apparatus configured to calculate a relative altitude of each tracking device based on the measured air pressure at the position of each tracking device, and to create a device topology of the at least three tracking devices based on the set of linear distance measurements and the estimated relative altitude of each tracking device. This may operate as described above with regard to the third aspect of the invention.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described with reference to the accompanying drawings, of which:

Figure 1 shows, schematically, the positioning of elements of a sports tracking system around a sports field during set up;

Figure 2 show, schematically, the structure of a tracking device of a sports tracking system; Figure 3 shows, schematically, the structure of certain elements of a sports tracking system;

Figure 4 is a flow diagram illustrating the set up of a sports tracking system;

Figure 5 is a flow diagram illustrating the operation a sports tracking system;

Figures 6 and 7 illustrate, schematically, the creation of the tracking device topology of a sports tracking system; and

Figures 8 to 10 illustrate, schematically, the determination of the arrangement of the tracking device topology to a sports field.

DETAILED DESCRIPTION

Figure 1 illustrates the elements of a sports tracking system during set up. This sports tracking system is being set up on a rugby field 10. As mentioned previously, the present technique is applicable to other sports fields or play areas, and is applicable in contexts outside of sports, such as the tracking of objects in a warehouse, at a music concert, or in a television studio. The tracking system comprises an array of tracking devices 100. The tracking devices are arranged around the sports field 10, outside the field markings. For example, the tracking devices 100 may be fixed to convenient infrastructure around the sports field or supported on fixed supports, such as tripods, positioned outside of the field markings. In this embodiment, 12 tracking devices are used; however, any number of tracking devices of 3 or more would also be possible. Generally, a larger number of tracking devices improves the tracking ability of the system, and provides for redundancy of the tracking devices, should any fail or be obstructed. The tracking devices 100 are relatively evenly distributed around the sports field, which also improves the tracking ability of the system.

Figure 2 shows the structure of a tracking device 100. The tracking device 100 comprises an ultra wide-band (UWB) transceiver 110, which acts as a linear distance measuring unit. This UWB transceiver is preferably provided with three spaced apart antennae used for transmitting and receiving UWB signals, the antennae being arranged in a triangle; however, alternatively, the tracking device could be provided with three separate UWB transceivers, again not arranged in a straight line. The tracking device also comprises a communications module for communication with a central computer 300, shown in Figure 3. The communications module may be any means of transferring data to the central computer 300, including Wi-Fi, a wired ethernet port or Bluetooth etc. In some embodiments, the UWB transceiver may used to transfer data to the central computer 300, in which case a dedicated communication module 120 would not be required. The tracking device 100 also includes a GNSS receiver, such as a GPS receiver, configured to obtain a position measurement using a GNSS. The tracking device may include a camera 140 in some embodiments in which the tracking is intended to be an optical tracking of objects on the sports field 10. Any camera may be used in this context, although tracking systems often make use of high speed cameras. Finally, the tracking device comprises a series of microelectromechanical systems (MEMS) sensors 150, including a pressure sensor 151 , an accelerometer 152, a gyroscope 153 and a magnetometer 154. Each tracking device may be battery powered (not shown in Figure 2), preferably using a rechargeable battery, so as to be conveniently portable. Alternatively, the tracking devices may include a connector cable for powering by an external power source, such as a mains power source.

As also shown in Figure 1 , the sports tracking system includes a number of mobile electronic devices, i.e. tags, positioned on the field 10. Where the tracking devices are intended to track objects using the UWB transceiver 110, the tags 50 may include corresponding UWB transceivers which communicate with the tracking devices. While Figure 2 shows only two tags 50 on the field, it will be appreciated that more may be used (e.g. if each player wears a tag 50) or only one could be used (e.g. if the system is only intended to track the ball in play). It should also be noted that where tracking is optical, e.g. with a camera 140, there may be no tags 50 required for the tracking system to operate.

Figure 1 also shows three reference devices 200 that have been positioned on the sports field. In particular, a first reference device 200a has been positioned at the intersection of the halfway line and one of the touch lines, a second reference device 200b has been positioned at the intersection of the same touch line and one of the try lines, and a third reference device 200c has been positioned at the intersection of the opposite touch line and the 22-metre line between the halfway line and the same try line. In preferable embodiments, the reference devices are constructed identically to the tracking devices 100, shown and described with respect to Figure 2.

Figure 3 shows part of a sports tracking system similar to that shown in Figure 1 . In particular, this Figure shows the way in which a central computer 300 (not shown in Figure 1) and two tracking devices 100 communicate with each other. In this embodiment, each tracking device comprises only a UWB transceiver 110, a communication module 120, and the MEMS sensors 150. It will be noted that, while Figure 3 shows the interaction of two tracking devices 100, since a reference device 200 is preferably constructed identically to a tracking device, the illustrated communication will apply analogously for the communication of a tracking device 100 with a reference device 200.

As shown in Figure 3, the UWB transceiver 110 of each tracking device 100 is configured to communicate with the UWB transceiver of each other tracking device 100, although only two are shown in Figure 3. This communication between the UWB transceivers 110 of each tracking device 100 allows for the distance between the two tracking devices to be established, e.g. using time of flight (TOF) or time difference of arrival (TDOA). The relative orientation of the tracking devices may also be determined using phase difference of arrival (PDOA) or angle of arrival (AOA), as measured using the three separate antennae on each UWB transceiver. The tracking devices also collect data using their respective MEMS sensors 150. The data from the UWB transceiver and the MEMS sensors of each tracking device 100 is communicated to a central computer 300 via the respective communication module 120. The central computer 300 comprises a controller 310 and a communication module 320. The communication module 320 receives the UWB and MEMS sensor data from the tracking devices and passes this to the controller 310. The controller is responsible for interpreting the data received from all of the tracking devices during the calibration and operation of the system, which will now be described.

Figure 4 illustrates a method of setting up and calibrating a tracking system. Firstly, in step S100, the tracking devices 100 are arranged around a tracking area. In this step, the tracking devices 100 should be placed in the position they are intended occupy during operation of the tracking system. However, as will be described below, the system may be configured to account for movement of the tracking devices 100 after calibration. In the example of Figure 1 , the tracking devices are arranged around a sports field, each being outside of the sports field. The tracking devices should be arranged so that each has a clear line of sight with at least two other tracking devices, preferably at least three other tracking devices, more preferably at least four tracking devices. In the most preferred case, each tracking device 100 will have a clear line of sight with each other tracking device. Step S100 will be dependent on the particular area to be tracked. For example, on a sports field with stands in close proximity, one or more tracking devices may need to be positioned in or above the stands. Other tracking devices 100 may be positioned on support structures, such as a tripod, intended to be positioned around the tracking area. As will be explained further below, it is preferable that three of the tracking devices 100 are arranged to define a large substantially equilateral triangle, and this may be factored into the positioning of the tracking devices. As was mentioned above, it is also preferred that the tracking devices are relatively evenly positioned around the sports field, e.g. at least two on each side of the sports field, in order to ensure good tracking capabilities.

In step S110, a linear distance is calculated between each tracking device 100 using their UWB transceivers 110. For example, each tracking device 100 may use their UWB transceivers 110 to send a TOF signal to each other tracking device. From this TOF data, which is sent from the tracking devices 100 to the computer 300 using the communication modules, the controller 310 may compute the linear distance between each tracking device.

In step S120, the air pressure is measured at each tracking device over an interval of two minutes using their respective pressure sensors 151. It should be noted that while step S120 is illustrated as taking place after step S110, these steps could take place at least partly simultaneously, or could be reversed. The pressure measurements of each tracking device are then passed to the computer 300 using the communication modules 120, 320.

The illustrated method uses only the linear distance measurements and pressure measurements; however, the method may also involve here collecting GNSS position data using the GNSS receiver 130 of each tracking device and sending this to the controller 310.

In step S130, the controller 310 uses the received data to create a device topology reflective of the position of the tracking devices 100 around the sports field 10. In this embodiment, the device topology is based on both the linear distance measurements calculated from the UWB TOF data and the pressure data. Figures 6 and 7 illustrate this process of creating the device topology.

The first part of this step may involve determining the relative height differences of the tracking devices 100 based on the measured pressures. This may involve firstly comparing the pressure reading of each tracking device 100 with a first tracking device - or number of tracking devices - which is/have been positioned at a known height. This may require the height of the device to be measured, or the tracking device may be positioned on a tripod of a known height, for example. The pressure reading at this first tracking device over the measured two minutes may then be subtracted from the pressure readings of the other tracking devices over the same two minutes in order to remove the natural variability of the atmospheric pressure. The resulting difference in the measured pressure of each tracking device can then be converted into a height difference of the tracking devices.

Next, the controller 310 may start to build the device topology, and this may start by selecting a starting set of three of the tracking devices 100 for this substantially two-dimensional arrangement of tracking devices. The ideal starting set of tracking devices 100 is a large substantially equilateral triangle. This may be identified from the linear distance measurements. In particular, a set of three tracking devices with large linear distance measurements among themselves will be preferred, as well as linear distances among themselves that have a low variance. Figure 6 illustrates a suitable starting set of tracking devices in this particular example, with the dashed lines illustrating the distance measurements for this starting set. Here, two tracking devices are selected adjacent to one touch line of the rugby field 10, located at opposite ends of the touch line, with a third tracking device is selected adjacent to the opposite touch line, level with the halfway line.

With these three tracking devices 100 selected, one of the tracking devices may be designated as the origin P0, and the position of any of the remaining devices P1 may be chosen to define the x-axis. The position of the third device may be asserted to be in the y-direction from the tracking device designated as the origin. Then, a unit vector for x may be found as:

P1 - P0 x ~ |Pl - P0|

A unit vector that is indicative of the y direction, but which is not the y direction, may then be found as:

P2 - P0 y ~ |P2 - P0|

The cross product of x and y will provide the correct unit vector for the z-direction by the right hand rule, therefore: z = x x y

Thus, the true unit vector for the y direction can be found using: y = z x x

Therefore, three tracking devices are now located within a coordinate system

With the first three tracking devices now located within a coordinate system, additional tracking devices may now be added into the coordinate system. As shown in Figure 7, any one of the remaining tracking devices 100 may be selected to be incorporated within the device topology next. Figure 7 shows the linear distance measurements between the starting set of tracking devices 100 in solid lines and shows the linear distance measurements between each of these tracking devices in the starting set and a next tracking device to be added in dashed lines. With the coordinate system established and three linear distance measurements from the starting set of tracking devices 100 already located in that coordinate system, the position of the next tracking device can be determined in the same way as finding the intersection of three circles centered on each of the starting set of tracking devices 100 and whose radiuses are defined as the linear distance measured to the next tracking device

This step may be repeated for subsequent tracking devices, until all tracking devices have been positioned within the defined coordinate system. It will be appreciated that each further tracking device 100 could be incorporated on the basis of only linear distance measurements to the starting set, or to any three tracking devices already located within the coordinate system, or to more than three tracking devices for further certainty of position and to help prevent error propagation.

Optionally, the method may repeat the above process for producing a device topology for each possible starting set of tracking devices. This would produce as many device topologies (referred to as preliminary device topologies in this context) as there are unique starting sets of tracking devices. In order to account for measurement errors in the starting sets, each of these preliminary device topologies may now be compared. The position of each device within each preliminary device topology may be compared with an average position for that device determined from all of the other device topologies. If the position of any device within any preliminary device topology differs from the average position by more than, for example, 5 cm, then that preliminary device topology may be disregarded, or the device position within that preliminary device topology disregarded. Alternatively, the comparison may be used to identify which one or more linear distance measurements led to the incorrect calculated device position and those measurements disregarded. In this case, the preliminary device topology may need to be updated. Once each preliminary device topology is assessed in this way, the device topology that will be used in the rest of the method may be established, for example, by taking an average of the device positions within the remaining preliminary device topologies. This process may also be performed as a random sample consensus in which the process iteratively repeats the steps of estimating the position of the devices based on the preliminary device topology, e.g. by taking an average (such as a median), and disregarding information associated with any outliers that are outside of the estimated device position by a threshold amount. This iterative process will gradually increase the accuracy of the estimate until all remaining data is in agreement to within a threshold amount.

Once the position of each of the tracking devices is established within the device topology, the orientation of each of those tracking devices may be set relative to each other. As described above, the relative orientation of the tracking devices may be determined using PDOA or AOA, as determined by the measurements made with the three antennae of each UWB transceiver.

With the positions and orientations of the devices calculated using only the linear distance measurements, their heights can be optimized by incorporating the pressure data. The calculated positions define a new topology which can be compared to the set of UWB measurements which were originally taken. These topologies can be expressed as two matrices of inter-device measurements. If the UWB measurements were exact, then the calculated positions of the devices would be exact, and the two matrices would be identical. However, because of measurement noise and normal variability, the calculated positions will not be exact. At this stage a stochastic optimization algorithm can be used to incorporate the pressure sensor data to ensure the correct relative height differences between the anchors, while optimizing the positions such that the calculated topology agrees with both the original UWB measurements and the pressure sensor data. This can be achieved by minimizing an objective function. It will be appreciated that the above example is considered to be essentially two- dimensional. However, this technique would apply analogously for a three- dimensional arrangement of tracking devices. Instead of identifying a starting set of three tracking devices defining an equilateral triangle, a starting set of four tracking devices defining a tetrahedron with edges of approximately the same length would be required. The coordinate system is defined in essentially the same manner described above for a starting set of three, with the fourth device being added as the solution of the intersection of three spheres centered on the first three tracking devices and having radii determined by the measured distances to the fourth tracking device. Thereafter, additional tracking devices can be incorporated again by the intersection of four or more spheres. This may be repeated for each starting set in the same way described above.

Once all tracking devices have been located within the coordinate system defined initially relative to the starting set of tracking devices, a device topology is known, which indicates the relative position in space of each of the tracking devices 100. However, at this stage, the position of these devices relative to the area to be tracked, i.e. the sports field 10, is not known.

With the device topology created, in step S140, three reference devices 200 are positioned within the tracking area. While step S140 is described as occurring after steps S100 to S130, this step could also be performed before or during any of the preceding steps. As has been mentioned above, the reference devices may be constructed identically to the tracking devices 100.

As mentioned above, in this embodiment, a first reference device 200a is positioned at the intersection of the halfway line and one of the touch lines of the rugby field 10, a second reference device 200b is positioned at the intersection of the same touch line and one of the try lines, and a third reference device 200c is positioned at the intersection of the opposite touch line and the 22-metre line between the halfway line and the same try line that the second reference device is positioned on. These reference devices could be placed directly on the field, or could be on tripods, for example, a known height above the field. It will be noted that this positioning of the reference devices will allow for dimensions of the field to be measured. In particular, the distance between the first reference device and the second reference device will indicate the distance between the try line and the halfway line. The perpendicular distance between the third reference device and the line joining the first and second reference devices will indicate the distance between the opposing touch lines. These distanced could be determined using measurements directly between the reference devices, since these have UWB transceivers themselves, or could be determined using distance measurements between the tracking devices 100 and the reference devices, or some combination. It will also be appreciated that many more dimensions of the pitch could be determined using further combinations of reference devices, as desired. In this embodiment, opposing halves of the pitch are assumed to be identically sized, but this could also be confirmed with further reference devices 200.

In step S150, a linear distance is calculated between each tracking device 100 and each reference device 200 using their UWB transceivers 110. For example, each tracking device 100 may use their UWB transceivers 110 to send a TOF signal to each reference device. From this TOF data, which is sent from the tracking devices 100 to the computer 300 using the communication modules, the controller 310 may compute the linear distance between each tracking device 100 and each reference device 200.

It should also be noted that this method may also involve pressure data being obtained at the reference devices 200 and the tracking devices 100. The pressure data of the reference devices may be taken at the same time as the tracking devices, i.e. step S120, if all devices are positioned at that stage, or the pressure data may be collected for all devices again after step S140. This pressure data may again be used in the subsequent step, as described above, to determine the height difference between the reference devices and the tracking devices.

Finally, in step S160, the method involves determining the arrangement of the created device topology relative to the sports field based on the linear distance measurements, and based on any pressure measurements taken by the reference devices. Figures 8 to 10 illustrate this process.

Figure 8 shows the first reference device 200a being positioned in the coordinate system relative to the tracking devices 100. Figure 8 shows the linear distance measurements between four of the tracking devices 100 and the reference device 200a in dashed lines; however, it will be appreciated that linear distance measurements from all of the tracking devices 100 may be used, if they were able to make such a measurement. Based on these linear distance measurements, the controller is able to establish the position of each of the tracking devices relative to the intersection of the halfway line and the touch line of the rugby field. This reference point may be designated the origin of the coordinate system; however, alternatively, the origin may be arbitrarily set relative to this reference point.

Knowing the relative position of the tracking device topology to one point on the field is not enough to fix the sports field in the coordinate system. For example, the sports field could rotate within the coordinate system about this fixed reference point and still satisfy the measured distances between the tracking devices 100 and the first reference device 200a. Therefore, at least a second reference device is required in order to fix the sports field in the coordinate system.

Figure 9 shows the second reference device 200b being positioned in the coordinate system relative to the tracking devices 100. This Figure now shows the measurements to the first reference device 200a in solid lines, representing that this point is now fixed in the coordinate system, and shows the linear distance measurements between four of the tracking devices 100 and the second reference device 200b in dashed lines. Again, linear distance measurements from all of the tracking devices 100 will preferably be used, but these are not shown for clarity in the Figures. Based on these linear distance measurements, the controller 310 is able to establish the position of each of the tracking devices 100 relative to the intersection of the try line and the touch line of the rugby field 10. It will also be noted that the controller is able to determine a reference direction within the coordinate system based on these measurements, namely the direction of the touch line of the rugby field.

In some embodiments, the fixing of two such reference points within the coordinate system may be sufficient to fix the rugby field 10 within the coordinate system. For example, assumptions may be made, e.g. such that the plane of the tracking devices is the same as the plane of the rugby field. However, in this embodiment, measurements to a third reference device are used to fix the rugby field 10 within the coordinate system.

Figure 10 shows the third reference device 200c being positioned in the coordinate system relative to the tracking devices 100. This Figure now shows the measurements to the first and second reference devices 200a, 200b in solid lines, representing that these points are now fixed in the coordinate system, and shows the linear distance measurements between four of the tracking devices 100 and the third reference device 200c in dashed lines. Again, linear distance measurements from all of the tracking devices 100 will preferably be used, but these are not shown for clarity in the Figures. Based on these linear distance measurements, the controller 310 is able to establish the position of each of the tracking devices 100 relative to the intersection of the 22-metre line and the touch line of the rugby field 10. It will also be noted that the controller is able to determine a second reference direction within the coordinate system based on these measurements, namely the direction of the halfway line, try line and 22-metre line of the rugby field.

It will be noted that even with these three reference points fixed within the coordinate system of the tracking devices, the vertical direction may still not be known, i.e. it may not be known on which side of this planar sports field play is to take place. Again, this could be resolved with assumptions, e.g. about the height of the tracking devices relative to the height of the reference devices. However, the reference devices could be numbered sequentially, and the order of the reference devices on the field could be used to confirm the vertical direction. For example, the reference devices may be required to be arranged sequentially counter-clockwise as viewed from above the sports field in order to convey this information.

With the tracking device topology established relative to a coordinate system, and the sports field fixed within this coordinate system, the tracking system is now calibrated, and able to track the movement of objects, such as tags 50, within its coordinate system.

It should also be noted that the reference devices 200 can now be moved to outside of the sports field 10 and used to act as additional tracking devices after calibration has been performed. In this case, the position of the reference device 200 (in its capacity as a tracking device) will need incorporating within the tracking device topology. This may be achieved by arranging the reference devices 200 in their desired position around the sports field 10, and then adding the reference devices to the device topology in the same manner described above with respect to Figure 7. In particular, the linear distance measurements between the any of the existing tracking devices 100 and each reference device may be used to position the reference device within the coordinate system in the same way as finding the intersection of circles centered on the existing tracking devices 100 and whose radiuses are defined as the linear distance measured to the reference device that is to be used as a tracking device.

While the above method is described as producing a device topology using only the tracking devices, and then using measurements to reference devices to fix the topology to the coordinate system of the sports field, the method could also build a combined device topology for all tracking devices and reference devices in one single process. That is, steps 110 to 130 may be performed while treating the reference devices identically to tracking devices, measuring linear distances between the reference devices and other tracking devices and/or reference devices to determine a topology including both sets of devices. In this case, the arrangement of the device topology relative to the area of apply may be determined as soon as the reference devices are included in the device topology, since the position of those reference devices brings with it the knowledge of the reference points in the area of play. If the reference devices make up the starting set, for example, then, the arrangement of the tracking devices relative to the area of play may be known as the tracking devices are included in the device topology. The reference devices may then be removed from the device topology at the end of the process to leave only the tracking devices with a known arrangement relative to the area of play.

As referred to above, a problem with tracking systems of this sort is that movement of any of the tracking devices can corrupt the tracking data. Figure 5 illustrates a method for addressing this problem.

The method of Figure 5 begins with a set up and calibrated sports tracking system. This method may follow the calibration steps just described, or could be employed after any other calibration technique, including those described in the background section above.

In step S200, the tracking system monitors for and detects the movement of one of the tracking devices. Movement of the tracking devices may be detected in a number of ways.

A first way in which movement may be detected is based on distance measurements taken between the tracking devices of the tracking system. That is, the system may be configured to periodically use the UWB transceiver 110 of each tracking device 100 to send a TOF signal to each other tracking device 100. From this TOF data, which is sent from the tracking devices 100 to the computer 300 using the communication modules, the controller 310 may check the linear distances between each tracking device. Movement of one of the tracking devices may be identified from this data, i.e. based on a change in the measured linear distance. The movement of one of the tracking devices relative to the others will typically be identifiable as the moved tracking device will see linear distance measurements to substantially all of the other tracking devices change, while substantially all other tracking devices will see linear distance measurements to the moved tracking device change, with the others remaining substantially the same. An alternative way that movement may be detected is using the measurements of one or more of the MEMS sensors, i.e. the pressure sensor 151 (for vertical movements), the accelerometer 152, the gyroscope 153 and the magnetometer 154. Again, the controller may monitor the data from these sensors to identify when a tracking device has moved.

With movement detected, in step S210, a linear distance between the moved tracking device and each other tracking device is calculated. This is performed in the same way described above for step S110.

In optional step S220, the air pressure of each tracking device is measured, again in a manner analogous to step S120. This step will allow the system to update to reflect any change in height of the moved tracking device 100.

Finally, in step S230, the position of the moved tracking device within the device topology is updated based on the measured linear distances and any pressure measurements. This process is analogous to the process described above, particularly with respect to Figure 7. That is, the new position of the moved tracking device 100 can be determined in the same way as finding the intersection of circles centered on each of the other tracking devices and whose radius is defined as the linear distance measured to the moved tracking device.

The invention may be further understood with reference to the following numbered clauses.

Clause 1. A method of operating a tracking system for tracking the position of objects moving around a tracking area, the method comprising: arranging at least three tracking devices around the tracking area, wherein each tracking device comprises a linear distance measuring unit configured to measure a linear distance to at least two other tracking devices of the at least three tracking devices; for each tracking device, calculating a linear distance between said tracking device and at least two other tracking devices using the linear distance measuring units to create a set of linear distance measurements; creating a device topology of the at least three tracking devices based on the set of linear distance measurements; positioning a reference device at least at a first position, wherein the at least first position has a known relationship with a first reference point in the tracking area; calculating a linear distance between each of at least three of said tracking devices and the reference device at the first position to create a set of reference measurements; and determining an arrangement of the device topology relative to the tracking area based on the set of reference measurements.

Clause 2. A method according to clause 1 , comprising positioning a reference device at each of at least the first position and a second position, wherein the at least first position and second position together have a known relationship with a first reference point in the tracking area and a second reference point or a first reference direction in the tracking area, and further comprising calculating a linear distance between each of at least three of said tracking devices and the reference device at the second position to create the set of reference measurements.

Clause 3. A method according to clause 2, comprising positioning a reference device at a third position such that the first, second and third positions do not lie along a straight line, the third position together with the first and/or second position having a known relationship with a second reference direction in the tracking area, and calculating a linear distance between each of at least three of said tracking devices and the reference device at the third position using the linear distance measuring units to create the set of reference measurements, and wherein preferably determining the arrangement of the device topology relative to the tracking area is further based on the order of the first, second and third positions.

Clause 4. A method according to clause 3, wherein the first, second and third positions define a plane substantially parallel with a sports field in the tracking area.

Clause 5. A method according to any of the preceding clauses, wherein the first position has a known positional relationship with the first reference point in the tracking area, wherein preferably the first position substantially coincides with the first reference point. Clause 6. A method according to clause 5 when dependent on at least clause 2, wherein the second position has a known positional relationship with the second reference point in the tracking area, wherein preferably the second position substantially coincides with the second reference point.

Clause 7. A method according to clause 6 when dependent on at least clause 3, wherein the third position has a known positional relationship with a third reference point in the tracking area, wherein preferably the third position substantially coincides with the third reference point.

Clause 8. A method according to at least clause 2, wherein the first and second positions have a known angular relationship with the first reference direction, wherein preferably the first and second positions define a line substantially parallel with the first reference direction.

Clause 9. A method according to any of the preceding clauses, wherein the first reference point corresponds to a field marking on a sports field in the tracking area, wherein preferably the first reference point corresponds to the intersection of at least two field lines on the sports field.

Clause 10. A method according to at least clause 2, wherein the first reference direction corresponds to a direction of a field line on a sports field in the tracking area.

Clause 11 . A method according to at least clause 2, wherein the first position is at the intersection of at least two field lines on a sports field in the tracking area, and wherein the second position is located along one of said two field lines from the first position, wherein preferably the second position is at the intersection of a third field line with one of said at least two field lines, such that at least one dimension of the sports field may be determined based on the set of reference measurements.

Clause 12. A method according to at least clause 2, wherein either: a first reference device is positioned at the first position and a second reference device is positioned at the second position; or a first reference device is positioned at the first position, and then, after the linear distance between each of at least three of said tracking devices and the first reference device at the first position is calculated, the first reference device is positioned at the second position.

Clause 13. A method according to any of the preceding clauses, wherein the or each reference device is a further tracking device comprising a linear distance measuring unit configured to measure a linear distance to at least two tracking devices of the at least three tracking devices, and further comprising, after creating the corresponding set(s) of reference measurements, arranging the or each reference device around the tracking area, calculating a linear distance between the or each further tracking device and at least two tracking devices of the at least three tracking devices using one or more linear distance measuring units, and updating the device topology to include the or each reference device.

Clause 14. A method according to any of the preceding clauses, wherein one or more of the tracking devices and/or reference devices comprises a GNSS receiver configured to make a GNSS position measurement of said device, and wherein determining the arrangement of the device topology relative to the tracking area is further based on the GNSS position measurement.

Clause 15. A method according to any of the preceding clauses, further comprising, after creating the device topology, detecting a movement of one of the at least three tracking devices relative to the other tracking devices of the at least three tracking devices, re-calculating a linear distance between the moved tracking device and each of at least two other tracking devices using the linear distance measuring units, and updating the device topology to account for the movement of said moved tracking device based on the re-calculated linear distance between the moved tracking device and each of at least two other tracking devices of the at least three tracking devices.

Clause 16. A method according to clause 15, comprising, for each tracking device, periodically re-calculating a linear distance between said tracking device and at least two other tracking devices using the linear distance measuring units, and wherein detecting movement of one of the at least three tracking devices is based on a detected change in linear distance between said moved tracking device and said other at least two other tracking devices after said re-calculation.

Clause 17. A method according to clause 15 or clause 16, wherein each tracking device further comprises one or more of an accelerometer, gyroscope and magnetometer and wherein detecting a movement of one of the at least three tracking devices relative to the other tracking devices is based on readings from said accelerometer, gyroscope and/or magnetometer.

Clause 18. A method according to any of the preceding clauses, wherein each tracking device further comprises a pressure sensor configured to measure air pressure at the position of the tracking device, and further comprising calculating a relative altitude of each tracking device based on the measured air pressure at the position of each tracking device, and wherein creating the device topology is further based on the calculated relative altitude of each tracking device.

Clause 19. A method according to clause 18, comprising arranging at least four tracking devices around the tracking area, wherein each tracking device is arranged substantially within the same plane within a tolerance of 10 metres, preferably within a tolerance of 5 metres, more preferably within a tolerance of 2 metres, most preferably within a tolerance of 1 metre, and wherein preferably one or more of the tracking devices are arranged at least 50 metres apart, preferably at least 70 metres apart, more preferably at least 100 metres apart.

Clause 20. A method according to any of the preceding clauses, wherein the linear distance measuring unit of each tracking device is further configured to measure a linear distance to one or more mobile electronic devices moving around the tracking area.

Clause 21 . A method according to any of the preceding clauses, wherein the linear distance measuring unit of each tracking device comprises an ultra-wideband (UWB) transmitter and/or receiver.

Clause 22. A method according to any of the preceding clauses, wherein the linear distance measuring unit of at least one tracking device comprises at least a first antenna and a second antenna used in making the linear distance measurements, and further comprising calculating a relative orientation of said at least one tracking device using the linear distance measurements obtained using the first and second antennae.

Clause 23. A method according to any of the preceding clauses, wherein at least one tracking device comprises a plurality of linear distance measuring units, and further comprising calculating a linear distance between each of the plurality of linear distance measuring units on said at least one tracking device and at least one other tracking device, and calculating a relative orientation of said at least one tracking device based on the differences in the calculated linear distances.

Clause 24. A method according to any of the preceding clauses, wherein one or more tracking devices comprise a camera for optically tracking the position of objects moving around the tracking area.

Clause 25. A method according to any of the preceding clauses, wherein arranging at least three tracking devices around the tracking area comprises fixedly positioning each tracking device around the tracking area.

Clause 26. A method according to any of the preceding clauses, comprising arranging at least four tracking devices, preferably at least six tracking devices, more preferably at least ten tracking devices, around the tracking area, wherein preferably each tracking device comprises a linear distance measuring unit configured to measure a linear distance to at least three other tracking devices.

Clause 27. A method according to clause 26, wherein creating a device topology of the at least four tracking devices based on the set of linear distance measurements comprises identifying a starting set of three tracking devices based on the magnitude of the linear distance measurements in the set of linear distance measurements and the differences between the linear distance measurements, defining a coordinate system using the linear distance measurements between each of the starting set of tracking devices, and then locating a fourth tracking device in the coordinate system defined for the starting set of tracking devices based on the linear distance measurements between the fourth tracking device and each tracking device in the starting set of tracking devices.

Clause 28. A method of operating a tracking system for tracking the position of objects moving around a tracking area, the method comprising: fixedly arranging at least three tracking devices around the tracking area, wherein each tracking device comprises a linear distance measuring unit configured to measure a linear distance to at least two other tracking devices of the least three tracking devices, wherein the at least three tracking devices have a known device topology; detecting a movement of one of the at least three tracking devices relative to the other tracking devices of the at least three tracking devices; calculating a linear distance between the moved tracking device and each of at least two other tracking devices using the linear distance measuring units; updating the device topology to account for the movement of said moved tracking device based on the calculated linear distance between the moved tracking device and each of at least two other tracking devices of the at least three tracking devices.

Clause 29. A method according to clause 28, wherein detecting the movement of one of the at least three tracking devices comprises, for each tracking device, periodically calculating a linear distance between said tracking device and at least two other tracking devices using the linear distance measuring units, and detecting movement of one of the at least three tracking devices based on a detected change in linear distance between said moved tracking device and said other at least two other tracking devices after said calculation.

Clause 30. A method according to clause 28 or clause 29, wherein each tracking device further comprises one or more of an accelerometer, gyroscope and magnetometer and wherein detecting a movement of one of the at least three tracking devices relative to the other tracking devices is based on readings from said accelerometer, gyroscope and/or magnetometer.

Clause 31 . A method according to any of clauses 28 to 30, wherein each tracking device further comprises a pressure sensor configured to measure air pressure at the position of the tracking device, and further comprising calculating a relative altitude of each tracking device based on the measured air pressure at the position of each tracking device, and wherein updating the device topology is further based on the calculated relative altitude of each tracking device.

Clause 32. A method of operating a tracking system for tracking the position of objects moving around a tracking area, the method comprising: arranging at least three tracking devices around the tracking area, wherein each tracking device comprises a linear distance measuring unit configured to measure a linear distance to at least two other tracking devices of the least three tracking devices, and a pressure sensor configured to measure air pressure at the position of the tracking device; for each tracking device, calculating a linear distance between said tracking device and at least two other tracking devices using the linear distance measuring units to create a set of linear distance measurements; calculating a relative altitude of each tracking device based on the measured air pressure at the position of each tracking device; and creating a device topology of the at least three tracking devices based on the set of linear distance measurements and the calculated relative altitude of each tracking device.

Clause 33. A tracking system for tracking the position of objects moving around a tracking area, the system comprising: at least three tracking devices configured to be arranged around an tracking area, wherein each tracking device comprises a linear distance measuring unit configured to measure a linear distance to at least two other tracking devices of the at least three tracking devices to create a set of linear distance measurements; and a data processing apparatus configured to create a device topology of the at least three tracking devices based on the set of linear distance measurements; at least one reference device, the at least one reference device being configured to be positioned at least at a first position, wherein the at least first position has a known relationship with a first reference point in the tracking area; wherein the data processing apparatus is further configured to receive a set of reference measurements, the set of reference measurements including linear distance measurements between each of at least three of said tracking devices and the reference device at the first position, and further configured to determine an arrangement of the device topology relative to the tracking area based on the set of reference measurements.

Clause 34. A tracking system for tracking the position of objects moving around an tracking area, the system comprising: at least three tracking devices configured to be arranged around an tracking area with a known device topology, wherein each tracking device comprises a linear distance measuring unit configured to measure a linear distance to at least two other tracking devices of the at least three tracking devices to create a set of linear distance measurements; and a data processing apparatus configured to detect a movement of one of the at least three tracking devices relative to the other tracking devices of the at least three tracking devices, and to update the device topology to account for the movement of said moved tracking device based on the set of linear distance measurements.

Clause 35. A tracking system for tracking the position of objects moving around an tracking area, the system comprising: at least three tracking devices configured to be arranged around an tracking area, wherein each tracking device comprises a linear distance measuring unit configured to measure a linear distance to at least two other tracking devices of the at least three tracking devices to create a set of linear distance measurements, and a pressure sensor configured to measure air pressure at the position of the tracking device.

Clause 36. A system according to clause 35, further comprising a data processing apparatus configured to calculate a relative altitude of each tracking device based on the measured air pressure at the position of each tracking device, and to create a device topology of the at least three tracking devices based on the set of linear distance measurements and the estimated relative altitude of each tracking device.