Field of the Invention

The present invention relates generally to a system and method for determining the position of an object, and more particularly, but not exclusively, to determining the position of an object provided with a GNSS (Global Navigation Satellite Systems) receiver and a radio positioning system.

Background of the Invention

It is known for mobile units to incorporate both a GNSS receiver and other positioning systems, such as an ultra-wideband (UWB) positioning system. Further examples of other positioning systems include both non-radio (such as laser, video or inertial) and radio systems (such as signal strength measurement). However, a position fix for the mobile unit is usually only possible if either one or both of the GNSS and the other positioning system can work on its own. Thus by introducing the UWB system to GNSS one only adds positional coverage in those places where UWB positioning works and the GNSS does not (and vice versa if one adds GNSS to UWB). This leaves no positioning coverage at locations where neither GNSS nor the UWB positioning systems works to provide an accurate location of the mobile unit. This is a significant disadvantage of the prior art.

Referring to figure 1 there is shown a schematic representation of a prior art mobile unit 2 wherein location and time data are used to determine its position using a GNSS and UWB positioning system. It will be appreciated that not all of the components making up the GNSS and UWB positioning systems are illustrated in figure 1. The components of the position determining systems that are shown include a GNSS receiver 4 and a UWB transceiver 6, each connected to a respective antenna 12 and 14. Both the GNSS receiver 4 and UWB transceiver 6 are coupled to a processor 8 which in turn is coupled to a memory 10. The processor 8 transmits an output signal at an output 16. The outputs of the GNSS receiver 4, the UWB transceiver 6, and the processor 8 provide position estimates and other information including the time of validity for each of these estimates.

The operation of the unit 2 shown in figure 1 can be understood more clearly with reference to figure 2, which is a flowchart of a software routine for choosing which receiver element has yielded data of use in determining the location of the unit 2. It would be appreciated by the skilled person in the art that this operation is also applicable to alternative UWB systems in which the mobile unit contains a UWB receiver instead of a transceiver.

The GNSS receiver 4 is monitored to determine whether or not a GNSS position fix is available at step 20. If the GNSS position fix is not available, the UWB receiver is monitored to determine whether or not a UWB position fix is available at step 22. If the UWB position fix is also not available, an output will be provided to indicate that no position is found at step 24, and transmitted to a host computer at step 30. If the UWB position fix is available, the position of the unit 2 is determined using UWB signals as shown in step 26. The positional data for the unit 2 can be displayed as indicated by step 28 on a suitable display device and also passed on to a host computer if desired (step 30).

If the GNSS position fix is available, the UWB receiver is also monitored to determine whether or not a UWB position fix is available at step 32. If the UWB position fix is not available, the position of the unit 2 is determined using measurements of GNSS signals alone as shown in step 34. The location data is then provided at step 28 and, as described before, the data can, if desired, be transmitted to a host computer at step 30. However, if the UWB position fix is available, the position of the unit 2 can be determined using both GNSS and UWB signals as shown in step 36.

As mentioned above, one of the disadvantages of such a system arises when the unit 2 is located in a place where neither the GNSS nor UWB can accurately measure the location of the unit, although both may have some measurements available relating to the location of the unit.

As is known, one limitation of GNSS systems is that several satellites are required to be in line of sight (LOS) of the receiver in order to provide an accurate location of the unit. Problems can therefore occur in receiving GNSS in areas where there is no or few line of sight opportunities, for example, in a mountainous area, within a building or adjacent to the walls of a building, or in a forested area. In such locations the unit 2 and its GNSS receiver may not be able to obtain repeatable readings of desired quality, or indeed any signal, from enough satellites. Usually the unit 2 is required to be in line of sight (LOS) with at least four satellites to obtain any location fix. If less than four satellites are in LOS with the unit, the GNSS receiver will not be able to determine a position. When insufficient GNSS signals of adequate quality are present the units switch to UWB positioning. However, the signals from the UWB system may themselves be insufficient to give an accurate location for the unit 2. Furthermore, this approach requires a considerable number of UWB terminals to be deployed in order to ensure that locations that do not receive good-quality GNSS solution to have sufficient UWB ranges for a complete UWB solution.

An object of the present invention is to strive to overcome the above-mentioned disadvantages of the prior art systems. According to a first aspect of the invention there is provided a method of determining the position of a unit having a GNSS receiver as part of a GNSS positioning system and a radio transceiver associated with another radio positioning system, the method comprising:

receiving signals by the GNSS receiver from one or more earth orbiting satellites to provide one or more pseudoranges for the position of the unit;

transmitting and/or receiving signals by the radio transceiver in communication with one or more radio devices to determine a time offset between a clock associated with said radio transceiver and a clock associated with said one or more radio devices; and

processing the data received from said one or more earth orbiting satellites and from said one or more radio devices, whereby said determined time offset is compared with a clock associated with the GNSS receiver in order to resolve the pseudoranges for the position of the unit so as to determine the position of the unit. In one embodiment of the invention there is provided a method of determining the position of a first unit, the first unit provided with a GNSS receiver and a radio transceiver, the GNSS receiver and the radio transceiver each controlled by a clock signal generated from a respective or common clock, the method comprising determining the time offset between the clock associated with the radio transceiver in said first unit and a clock associated with a radio transceiver in a second unit spaced a distance from said first unit, said second unit having a GNSS receiver controlled by a respective or common clock, comparing said determined time offset to the clocks associated with the GNSS receivers whereby one or more pseudoranges measured from data received at the GNSS receiver of the first unit is resolved into a range measurement.

In one embodiment of the method the radio transceiver is a UWB transceiver. However, other types of radio transceivers in other radio positioning systems can be employed.

In one embodiment the first unit is a mobile unit, and the second unit is either a mobile or fixed ground based unit.

In one embodiment the distance between the first and second units is determined by the radio transceivers in those units engaging in a signal exchange whereby the time for the signals to pass between the units is first determined, the distance being of product of time and speed of signal.

According to a second aspect of the invention there is provided a system comprising a first unit and a second unit, the first unit provided with a GNSS receiver and a radio transceiver, the GNSS receiver and the radio transceiver each controlled by a clock signal generated from a respective or common clock, the system further comprising a processor including determining means for determining the time offset between the clock associated with the radio transceiver in said first unit and a clock associated with a radio transceiver in the second unit spaced a distance from said first unit, said second unit having a GNSS receiver controlled by a respective or common clock, and means for comparing said determined time offset to the clocks associated with the GNSS receivers whereby one or more pseudoranges position measured from data received at the GNSS receiver of the first unit can be resolved into a range measurement.

In one embodiment of the system the radio transceiver is a UWB transceiver

According to a third aspect of the invention there is provided an apparatus comprising a GNSS receiver and a radio transceiver, the GNSS receiver and the radio transceiver each controlled by a clock signal generated from a respective or common clock, the apparatus having determining means operable for determining the time offset between the clock associated with the radio transceiver and a clock associated with a radio transceiver in a second apparatus spaced a distance from said apparatus, said second apparatus having a GNSS receiver controlled by a respective or common clock, said apparatus further comprising application means configured to compare the time offset to the clocks associated with the GNSS receivers whereby one or more pseudoranges from data received at the GNSS receiver of the said apparatus can be resolved into a range measurement.

According to a further aspect of the invention there is provided a computer program product storing computer executable instructions operable to cause a general purpose computer apparatus to become configured to perform a method in accordance with the method of the invention defined above.

According to a further aspect of the invention there is provided a signal carrying computer receivable information, the information defining computer executable instructions operable to cause a general purpose computer apparatus to become configured to perform a method in accordance with the invention defined above.

Brief Description of the Drawings

Further preferred features of these aspects will now be set forth by way of the following description of specific embodiments of the invention, provided by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of a prior art unit wherein radio signal and time data are measured by one of two receivers to determine its position;

Figure 2 is a flowchart of a software routine according to the prior art to operate the unit of Figure 1 to obtain location data;

Figure 3 is a schematic representation of a system according to an embodiment of the present invention wherein the location and the time data associated with two transceivers/receivers in a unit is processed to determine the position of the unit; and Figure 4 is a flow diagram of the steps for determining an accurate location of the unit of Figure 3 in accordance with an embodiment of the present invention.

Detailed Description of the Invention

Specific embodiments of the present invention will be described in further detail on the basis of the attached drawings. It will be appreciated that this is by way of example only, and should not be viewed as presenting any limitation on the scope of protection sought. The embodiments to be described herein relate to determining the location of mobile units, in real time. Referring to Figure 3, there is shown schematically a system for determining the position of one or both of mobile units designated 26 and 28. The mobile unit 26 has a GNSS receiver 30 and a UWB transceiver 32, the outputs from which are coupled to a processor 34. The inputs to the GNSS receiver 30 and UWB transceiver 32 are connected to receive signals from respective antennas 46, 48. In a similar manner the mobile unit 28 comprises a GNSS receiver 36 and a UWB transceiver 38 both coupled to receive signals by means of antennas 47 and 49 respectively. The GNSS receiver 36 and the UWB transceiver 38 are both coupled to a processor 40. In the embodiment illustrated in Figure 3, the outputs from the processor 34 and the processor 40 are both coupled to a host computer 42.

A plurality of satellites, for the sake of this example four satellites S1, S2, S3 and S4, transmit signals coded with location and time data for use by the GNSS receivers 30 and 36.

The GNSS signals from each of the satellites S1, S2, S3 and S4 may or may not arrive at the GNSS receiver 30 via antenna 46 along a line of sight (LOS). Essentially, the GNSS receiver 30 tracks as many signals as it can and measures the time of arrival of each signal against its internal clock using timing information encoded in the signal. The internal clocks of all the satellites are accurately synchronised to a GNSS system reference time, so that all the satellites provide this timing information in a common time. Each signal also contains information about the precise position of the satellite when the signal was transmitted or the same information is provided in the receiver by other means. The receiver can use this information to calculate a pseudorange from each time of arrival. A skilled reader would understand that a pseudorange is derived from the time delay of a radio message from a transmitter to a receiver, wherein the respective clocks have an unknown time offset. Thus, the each pseudorange of a set measured simultaneously is essentially the sum of the true range to one satellite and a common unknown clock offset distance.

In this example, the term "clock" refers to a combination of one or more elements, generally comprising (1) a stable clock oscillator that generates timing signals at uniform intervals that are used for the hardware components to perform actions in sequence or simultaneously as required, (2) a counter that increments regularly so as to provide a number representing the current time, and (3) processes usually implemented as software which use this current time to provide an accurate estimate of the time of a number of defined events, using parameters that are kept up to date by processing the times produced by this clock for events for which the true time is known. Two such clocks in the same unit can be compared if there is a suitable link between them so that they can synchronise themselves to the same event which may be generated by one of them or of external origin.

The pseudorange becomes an estimate of the range to the satellite at a precisely defined time, once the receiver's internal clock is exactly synchronised with the GNSS system clock. This can be achieved by determining the offset of the receiver's clock from the GNSS system time, and correcting the pseudoranges arithmetically. If a set of pseudoranges, which all contain the same clock offset, is measured at the same time at a GNSS receiver, the GNSS receiver will be able to determine its position and its clock offset by simply solving geometrical equations.

Therefore, in this example, if the number of pseudoranges is four or more, the GNSS receiver 30 will be able to determine the position of the unit 26 and the actual time difference of its internal clocks from the GNSS system time, subject to a number of small errors (such as noise, atmospheric propagation variations, antenna phase variations, non-linearity in electronic circuits and so on).

It is noted that additional information about the position of the antenna 46 can be used to allow the position to be computed with a smaller minimum number of pseudoranges. For example, in a situation where the position of the unit can be constrained, the minimum number of satellites needed could be less than four. If the height of the unit is known, its position can be determined in two dimensions (as opposed to three). In this case, positioning is possible using three satellites. If the receiver is known to be at some point along an approximately straight line, such as for example, a railway line, its position has only to be determined in one dimension rather than two or three dimensions. In this case, positioning is possible using two satellites.

However, if the number of pseudoranges is smaller than the minimum, then the GNSS receiver will not be able to determine its position or the clock time offset. Furthermore, it is noted that the computed position and the clock offset will be liable to error if the number of pseudoranges is equal to or even slightly above the minimum, and that errors can be reduced as the number of pseudoranges increases. For example, if any of the GNSS signals from the satellites S1 , S2, S3, and S4 is not in LOS with the GNSS receiver 30 but there is sufficient power received at antenna 46 for its pseudorange to be measured then a substantial error may occur in the measured pseudorange. The presence of more pseudoranges than the minimum (generally referred to as redundancy) allows erroneous measurements to be discarded so as to provide an accurate location for unit 26.

In principle, if the GNSS receiver has an estimate of its clock offset from the GNSS system clock, derived from any source, its pseudoranges can be converted into ranges. This can be achieved by using a UWB ranging transceiver to determine the local clock offset by exchanging signals with a unit in which the clock offset is known, which has the effect of transferring the clock offset estimate around the system. Furthermore, any unit with a good GNSS solution that has knowledge of its local coordinates can estimate its local clock offset from the GNSS system clock with greater accuracy than if it has to determine its position and time. This information can be sent to and applied in other units.

In one example, the unit 28 may be established in a known location and therefore can be employed as an accurate position "reference" point. In another example, units 26 and 28 may be spaced apart and unit 28 may have LOS with sufficient satellites to determine its own position reasonably accurately, and therefore can also be used as a position "reference" point. If it is possible for the units 26 and 28 and their radio transceivers to measure the distance ^u d" between the units using signals passing in both directions, and for digital radios this is particularly easy, then a distance between the two UWB transceivers for unit 26 and unit 28 can be determined.

For practical reasons this usually has to be done successively, hence it is called "round trip" or "ping-pong" ranging as illustrated by line 66 in Figure 3. For example, using four clock times recorded, the path delay can be found as a simple exercise in algebra, and inevitably at the same time the offset between the clocks must become known as well. It is self-evident that all digital radios (and GNSS receivers) must have a clock to control their operation.

If we take the distance "d" between units 26 and 28 to be an unknown, then the distance "d" can be calculated by measuring the time "t" for a UWB signal to travel between the two units, the time Ύ being d/c, where "c" is the known speed of travel of radio waves. In practice a signal from unit 26, once received by unit 28, triggers transmission of a signal back to unit 26. The time T for the return trip of the signal is 2t and the total distance covered is 2d. It follows

T

d =—x c

2 where c is the speed of light.

Each transceiver is capable of recording the time at which a signal is transmitted or received on its own master clock. The recorded time is relative to the master clock. By way of an example, assume that the clocks in units 1 and 2 are offset from a common reference time by E1 and E2 respectively. A first signal is recorded as being transmitted at time T2 from unit 2 and recorded as being received at time R1 in unit 1 , and a replying signal is recorded as being transmitted at time T1 in unit 1 and recorded as being received at time R2 in unit 2. The time between transmission and reception is the same for each signal in common time, and can be expressed as follows:

(R1 +E1) - (T2+E2)

(R2+E2) - (T1 +E1) The two equations can be added and expressed as: 2d/c = R1-T2+R2-T1

The equations can also be subtracted and expressed as: 2(E1-E2) = R2-T1-R1+T2. Thus, if the measurements at the two units are combined, the distance between the two units and the offset between the clocks in each of these units can be determined. The skilled person would appreciate that, in practice, other variables may also be present in the above equations. These variables may include time delays within the units, relative motion between the units, drift in the clocks of these units, and other effects. It would also be appreciated that conventional techniques can be applied to resolve any errors caused by the introduction of these terms.

As can be seen in Figure 3, the units 26 and 28 are UWB ranging radios, and hence the time (and range) accuracy will be better than 1 ns (i.e. range errors of less than 30 cm). If these radio system clocks can be used as clocks for associated GNSS receivers in the units or can be accurately compared with such clocks, then the clocks in two GNSS receivers can be compared to a commensurate accuracy.

If one clock is known (e.g. its receiver has a good GNSS fix, so it has found its own clock offset from GNSS system time), the error will be known too. Thus, each pseudorange as measured in a second receiver can be resolved into a true range, even if there are too few satellites for it to fix its own offset.

The ranges computed from the GNSS pseudoranges can be combined with the UWB ranges and with the positions of the satellites and the other UWB transceivers to compute the position of unit 26. The use of more range measurements in this computation increases its accuracy. This increase is of particular advantage where the greater redundancy allows grossly erroneous ranges or pseudoranges to be discarded, but this cannot be achieved by using either the GNSS or the UWB measurement alone. When UWB ranging is performed between two units where neither clock is accurately known, this method will still establish the time offset between them. This permits pseudoranges to the same satellite to be used differentially between the two receivers, which is otherwise not possible, and this is similar in effect to an extra UWB range measurement for one of the UWB units.

Referring to Figure 4 there is illustrated a flow diagram of the method steps according to a further embodiment of the invention. In the embodiment of Figure 4 it is assumed that the location of mobile unit 1 is to be determined, whereas it will be self-evident to those skilled in the art that similar steps can be taken to determine the location of the mobile unit 2.

In steps 80 and 82, the mobile unit 1 determines whether or not any signals are being received and measured by the GNSS receiver of unit 1. In other words, the mobile unit checks whether there are sufficient pseudoranges for it to determine its position. Essentially, the mobile unit checks whether or not there are sufficient signals of appropriate quality from sufficient satellites, usually four, in LOS with unit 1 in order to determine an accurate fix of the position of unit 1. If there are sufficient signals to provide such a fix in position, the position of unit 1 is calculated in step 84. Otherwise, the mobile unit 1 will examine the results of communication with other radio positioning units in step 86.

As part of their routine operations the units exchange information about the data being present in each of the units such as location, accuracy of the location and time estimates. This also includes information about other units or sources that were used to obtain these estimates. It is noted that this information is important to prevent closed loops in the sequence of time and position estimates. Essentially each unit decides which of the other units are to be used for ranging (step 88). For example unit 2 may have been set in a known location or its GNSS positioning system may have already provided unit 2 with its exact location which is stored in the memory of unit 2. If it is determined at step 88 that the location of unit 2 is accurately known then the time for the clock operating the GNSS positioning system in unit 2 can be considered as operating at a true time for that known position. Units 1 and 2 then exchange signals (step 92), the average time of transmission being measured between unit 1 and unit 2 and used to determine the distance of unit 1 from unit 2, and determines the time offset (step 92) between the clocks controlling the GNSS systems of unit 1 and unit 2. This knowledge enables the pseudoranges already determined for unit 1 to be resolved into ranges for the location of unit 1 at step 98. All the available pseudorange, range, and projected range data are then used to produce the best estimate of the position of unit 1 in step 100.

While the foregoing specific description of an embodiment of the invention has been provided for the benefit of the skilled reader, it will be understood that it should not be read as mandating any restriction on the scope of the invention. The invention should be considered as characterised by the claims appended hereto, as interpreted with reference to, but not bound by, the supporting description.