Technical Field

The invention relates to a method and device for generating proximity warnings, in particular for use in a mining environment.

Background Art

Surface mines and similar sites or areas are generally operated by means of a large number of vehicles and staff. Some of the vehicles may be exceedingly large, heavy, and difficult to control.

It has been proposed to use GNSS-devices (GNSS = global navigation satellite system, such as GPS) on board of vehicles and other objects, such as cranes, to generate proximity warnings in order to reduce the risk of collisions between vehicles. Such a system is e.g. described in WO 2004/047047 and it is based on devices mounted to the objects. Each device comprises a GNSS receiver, a control unit deriving positional data using the signal of the GNSS receiver, a radio circuit for wireless exchange of the positional data with the other devices, and an output device for outputting proximity warnings .

Such systems allow the driver of a vehicle to obtain information on some of the obstacles nearby.

Disclosure of the Invention

The problem to be solved by the invention is to provide a method and a monitoring apparatus of a similar type that enables improved safety and/or positioning accuracy, even in situations with poor or absent GNSS re ^¬ ception .

This problem is solved by the method and devices of the independent claims.

Accordingly, a method for generating a proximity warning on an area (e.g., in a surface mine) is carried out by a plurality of monitoring devices mounted on movable objects like vehicles, persons, etc. on said area. At least a first monitoring device at a first position and a second monitoring device at a second position each comprises a radio transceiver for transmitting and receiving radio pulses and position information. It should be noted here that the radio receiver is not necessarily a single device but can comprise a plurality of physical devices. The first and the second monitoring de vices additionally comprise a receiver for a radio based positioning system such as a GNSS (e.g., GPS, Galileo, ... each. The method comprises the following steps:

- A determination of a first position of the first monitoring device and a determination of a second position of the second monitoring device using the respective receivers for the radio based positioning system. The determined second position is then transmitted (advantageously together with a unique identifier of sec ond monitoring device) by means of the radio transceiver of the second monitoring device such that other monitoring devices in range are aware of the second position. Specifically, the transmitted second position of the sec ond monitoring device is received by means of the radio transceiver of the first monitoring device such that the first monitoring device knows about the second position.

The method comprises further steps of a) A transmission of a first radio pulse by said radio transceiver of said first monitoring device. The first radio pulse advantageously comprises a unique identifier of the first monitoring device. Furthermore, this first radio pulse is advantageously a frequency chirped pulse (i.e., a radio pulse with a frequency modulation over time) . Preferred frequencies of the first radio pulse are in the range between 0.3 and 6 GHz, particularly in the range between 868 and 928 MHz, in the range between 2 and 3 GHz, in the range between 2.3 and 2.5 GHz, or in the range between 5 and 6 GHz. Thus, the signal-to-noise ratio and radio range can be improved, e.g., also by means of suitable post-processing steps (e.g., correlation algorithms, filtering, etc.).

b) Then, the first radio pulse from the first monitoring device is received by means of said radio transceiver of said second monitoring device. Advantageously, suitable post-processing steps such as, e.g., filtering, are applied to the received radio pulse as it is obvious to the person skilled in the art.

c) Then, after processing, the second moni ^¬ toring device transmits a second radio pulse (reply pulse) by means of its radio transceiver. As discussed above, the second radio pulse advantageously also comprises a unique identifier of the second monitoring device such that the first monitoring device is aware that the reply pulse originates from the second monitoring device. Furthermore, the second radio pulse is advanta ^¬ geously also a frequency chirped pulse. Preferred frequencies of the second radio pulse are in the range be ^¬ tween 0.3 and 6 GHz, particularly in the range between 868 and 928 MHz, in the range between 2 and 3 GHz, in the range between 2.3 and 2.5 GHz, or in the range between 5 and 6 GHz.

d) Then, the second radio pulse is received by means of the radio transceiver of the first monitoring device, and - again - suitable post-processing is advantageously applied to the received radio pulse. As part of this, coarse (i.e., ± 20 degrees) relative bearing information can be derived, e.g., using at least one directional antenna. As discussed below, ambiguities in posi- tion determinations using, e.g., triangulation methods, can be resolved.

e) Then, the first monitoring device measures a first flight time t__TOF of the first and second radio pulses between the transmission of the first radio pulse (from its own radio transceiver) and the receiving of said second radio pulse which originates from the second monitoring device.

As an alternative to the bidirectional exchange of the first and second radio pulses as discussed above under the steps a) - d) , an unidirectional transmission and receiving of a single first radio pulse is also possible:

a) The second monitoring device transmits a first radio pulse by means of its radio transceiver. This first radio pulse advantageously comprises a unique identifier of the second monitoring device such that the first monitoring device is aware that the first radio pulse originates from the second monitoring device. Furthermore, the first radio pulse is advantageously a frequency chirped pulse. Preferred frequencies of the first radio pulse are in the range between 0.3 and 6 GHz, particularly in the range between 868 and 928 MHz, in the range between 2 and 3 GHz, in the range between 2.3 and 2.5 GHz, or in the range between 5 and 6 Ghz.

b) Then, the first radio pulse is received by means of the radio transceiver of the first monitoring device, and - advantageously - suitable post-processing is applied to the received first radio pulse. As part of this, coarse (i.e., ± 20 degrees) relative bearing information can be derived, e.g., using at least one directional antenna. As discussed below, ambiguities in position determinations using, e.g., triangulation methods, can be resolved.

c) Then, the first monitoring device measures a first flight time t_TOF of the first radio pulse between the transmission of the first radio pulse (from the radio transceiver of the second monitoring device) and the receiving of said first radio pulse by means of the first monitoring device's radio transceiver. For this, the first radio pulse comprises a synchronized timestamp or equivalent time information. The term "synchronized timestamp" herein relates to a transmission timestamp attached to the radio pulse, wherein clocks of the first and the second monitoring device are synchronized. Such synchronized time information/clocks is/are advantageously derived by means of the receiver for the radio based positioning system, e.g., from information transmitted by GPS satellites. As an alternative to transmission timestamps, the first radio pulse can also be transmitted at a predefined synchronized time between the first and the second monitoring device. The term "predefined synchronized time" herein relates to a predefined transmission time of the radio pulse, wherein clocks of the first and the second monitoring device are synchronized .

In both scenarios, i.e., the bidirectional radio pulses scenario (ping-pong-scenario) and the unidirectional radio pulse scenario (pong-scenario) , the method comprises the following further steps:

- The first monitoring device derives a first distance value d_12 or a value indicative of this dis ^¬ tance value between said first and said second monitoring devices, i.e., between the first and the second posi ^¬ tions. This is achieved using

* said measured first flight time t_TOF and advantageously a known response delay of the second moni ^¬ toring device in the ping-pong-scenario as described above. This known response delay is due to necessary processing steps in the second monitoring device between receiving the first radio pulse and transmitting the second radio pulse. Thus, the precision of the derivation of the first distance value d_12 can be improved. In addition to solely using radio pulse time- of-flight measurements as discussed above for deriving the first distance value d_12, also

* the determined first position of the first monitoring device and the received second position of the second monitoring device are used.

Both distance derivation schemes, i.e., the position based and the time-of-flight based scheme, are weighted and combined to improve the precision and/or reliability of the derivation of the first distance value d_12. One possibility of deriving the first distance value is based on the equation: d_12 = w_TOF * d_T0F + w_POS * d_POS with d_TOF and d_POS being the time-of-flight based and the position based distance values, respectively, and with w_TOF and w_POS being associated weighting factors with w_TOF + w_POS = 1. The weighting factors can be fixed or changed on the fly, e.g., based on the reliability and/or accuracies of the respective position determinations (see below) .

- Then, an, e.g., acoustic, tactile (vibrational), electric shock, or optical proximity warning or approach warning is issued to, e.g., a driver of a vehi ^¬ cle or an operator of an object as a function of said first distance value d_12. As an example, an acoustic beeping can be triggered as soon as the first distance value decreases below a threshold, e.g., 10 or 50 m. Another warning can be triggered as soon as, e.g., a cur ^¬ rent approach speed of the first monitoring device and the second monitoring device would result in an impact in less than a threshold time, e.g., 4 s.

In an advantageous embodiment, a first positioning accuracy of the first position of the first monitoring device is determined. The positioning accuracy of the first position as determined by the receiver for the radio based positioning system can vary, e.g., due to changing satellite reception levels and/or signal reflec ^¬ tions. Thus, a reliability level or precision of the de ^¬ termined first position can be assessed.

In another advantageous embodiment, a second positioning accuracy of the second position of the second monitoring device is determined. As discussed above, the positioning accuracy of the second position as determined by the receiver for the radio based positioning system can also vary, e.g., due to changing satellite reception levels and/or signal reflections. Thus, a reliability level of precision of the determined second position can also be assessed. This second positioning accuracy is then transmitted by means of the radio transceiver of the second monitoring device. Then, the transmitted second positioning accuracy is received by the radio transceiver of the first monitoring device. Thus, the first monitor ^¬ ing device is aware of the accuracy/precision of the second position of the second monitoring device.

Then, preferably, the first positioning accuracy of the first position and/or the second positioning accuracy of the second position are used in the step of deriving the first distance value d_12 between the first and the second monitoring device. Thus, e.g., the above described radio-pulse time-of-flight contribution can be uprated (i.e., its weighting factor is increased) in the derivation of the first distance value d_12 in a case when one or both determined positions have a decreased reliability. In a border case, when the first and/or the second position cannot be determined at all, e.g., due to a lack of satellite reception, a fallback to solely using the time-of-flight based distance value derivation is also possible (i.e., with w_TOF = 1 in the above example) . This improves the reliability of the proximity warning generation method.

In another advantageous embodiment, the proximity warning issued by, e.g., the first monitoring de- vice, comprises the second position of the second monitoring device. Then, an operator of a machine who receives the proximity warning is aware of the position of the second monitoring device and can take suitable action. Thus, the overall safety is increased.

In an advantageous embodiment, the derived first distance value d_12 between the first and the second monitoring devices is transmitted to all or selected other monitoring devices in range, such that these monitoring devices are aware of the distance value d_12 be ^¬ tween the first and the second monitoring devices. Thus, a more complete "picture" of the spatial distribution of the monitoring devices on the area is derivable. Advantageously, using additional positional data from the re ^¬ ceiver (s) for said radio based positioning system and, e.g., triangulation methods (see below), also position information of monitoring devices without a receiver for the radio based positioning system are derivable. Thus, the overall safety is increased.

As another aspect of the invention, a monitoring device comprises a radio transceiver for transmitting and for receiving a radio pulse. Furthermore, the monitoring device comprises an interface (e.g., an acoustic, tactile, e.g., vibrating, and/or optical user interface or - in another embodiment - a computer interface) for issuing a proximity warning as a function of the derived distance value (s). Furthermore, the monitoring device comprises an analysis and control unit which is adapted and structured to carry out the steps of a method as described. Specifically, the analysis and control unit can comprise a computer program element comprising computer program code means for, when executed by the analy ^¬ sis and control unit, implementing a method for generating a proximity warning as described.

Note: The described embodiments and/or features similarly pertain to both the apparatuses and the methods. Synergetic effects may arise from different combina ^¬ tions of these embodiments and/or features although they might not be described in detail.

Brief Description of the Drawings

The invention and its embodiments will be more fully appreciated by reference to the following detailed description of presently preferred but nonetheless illustrative embodiments in accordance with the present invention when taken in conjunction with the accompanying drawings .

Fig. 1 schematically shows a surface mine 100 with a monitoring apparatus comprising three monitoring devices 1,2,3,

fig. 2 shows a monitoring apparatus 1,2,3 comprising a radio transceiver 10, an interface 5, an optional receiver for a radio based positioning system 20, and an analysis and control unit 6.

Modes for Carrying Out the Invention Definitions :

A "movable object" is any object that can change and is expected to change its position and/or orientation or configuration in space. It may e.g. be a truck or any other vehicle that moves from place to place and changes its orientation in respect to the general north-south direction, e.g., by steering, or it may be an object positioned at a fixed location but able to rotate about its axis or to change its physical configuration, e.g. by extending an gripper or shovel, in such a manner that the volume of safety space attributed to it varies in significant manner.

The term GNSS stands for "Global Navigation Satellite System" and encompasses all satellite 16 based navigation systems, including GPS and Galileo.

The term "radio based positioning system" stands for a GNSS or for any other type of positioning system using radio signals, such as a pseudolite system.

The term "monitoring apparatus" as used here ^¬ in designates an assembly of monitoring devices distrib ^¬ uted over several locations, with the single monitoring devices communicating with each other as described. Some of the monitoring devices are installed on movable objects while others may be installed at fixed locations, e.g., fixed obstacles.

The term "mounting a device to a person" can be understood as affixing the monitoring device to the person in such a manner that the person will carry it without requiring the use of his/her hands. For example, the term expresses that the monitoring device is affixed to a piece of clothing or equipment, such as a belt or a helmet, that the person is wearing.

The area:

Fig. 1 schematically depicts an area, such as a surface mine 100, to be monitored by the present sys ^¬ tem. Typically, such a site covers a large area, in the case of a surface mine, e.g., in the range of several square kilometers, with a network of roads (bold lines) and other traffic ways, such as rails. A plurality of ob ^¬ jects is present in the mine, such as:

- Large vehicles, such as haul trucks 40, cranes, or diggers. Vehicles of this type may easily weigh several 100 tons, and they are generally difficult to control, have very large braking distances, and a large number of blind spots that the vehicle operator is unable to visually monitor without monitoring cameras. - Medium sized vehicles, such as regular trucks 50. These vehicles are easier to control, but they still have several blind spots and require a skilled driver .

- Small vehicles. Typically, vehicles of this type weigh 3 tons or less. They comprise passenger vehi ^¬ cles and small lorries.

- Trains.

- Individual persons 8, in particular pedestrians .

Ά further type of object within the mine is comprised of stationary obstacles, such as temporary or permanent buildings, open pits, boulders, non-movable excavators, stationary cranes, deposits, etc.

The risk of accidents in such an environment is high, specifically under adverse conditions as bad weather, during night shifts, etc. In particular, the large sized vehicles can easily collide with other vehi ^¬ cles, or obstacles.

For this reason, the mine is equipped with a monitoring apparatus comprising a plurality of monitoring devices 1,2,3 that allows to generate proximity warnings for the personnel of the site, thereby reducing the risk of collisions and accidents.

The monitoring apparatus:

Basically, the monitoring apparatus comprises a plurality of monitoring devices 1,2,3 that are affixed to fixed objects and/or movable objects and/or persons. Specifically, monitoring device 1 is affixed to haul truck 40 which is at position P_l, monitoring device 2 is affixed to truck 50 which is at position P_2, and monitoring device 3 is affixed to person 8 who is at position P_3. These components communicate in a wireless manner, in particular by radio signals by means of integrated radio transceivers 10. They are described in more detail in the following sections. In addition, the monitoring apparatus can comprise a central server (not shown) , whose role is also explained below.

The monitoring devices:

As stated above, the monitoring devices 1,2,3 are affixed to different objects in the area.

In general, the larger the number of installed monitoring devices 1,2,3, the higher the safety level .

The monitoring devices 1,2,3 comprise an analysis and control unit 6, such as a microprocessor system, which controls the operations of the monitoring device and the communication with the other monitoring devices .

The monitoring devices 1,2,3 further comprise a radio transceiver 10 comprising a first and a second transceiver unit. The second transceiver unit comprises a digital transceiver for exchanging digital data such as position information, positioning accuracies, distance values, etc. with other monitoring devices 1,2,3. Due to the digitally exchanged signals, error detection/ correction algorithms can be used which increase the reliability of the data exchange. The radio transceiver 10, specifically its first transceiver unit, is also adapted to transmit and receive time-of-flight radio pulses and optionally timestamp information. By measuring a flight time of these radio pulses (e.g., first pulse P_TOF from first monitoring device 1 to second monitoring device 2, reply pulse R__TOF from second monitoring device 2 to first monitoring device 1) between two monitoring devices 1,2, a first distance value d_12 of a distance between the two monitoring devices, i.e., between the first position P_l of the first monitoring device 1 and the second position P_2 of the second monitoring device 2 can be derived (dotted line and dotted circle segment) . The monitoring devices 1,2 further comprises a GNSS receiver 20. Although it is called a GNSS receiver in the following, it can also be a receiver interoper- ating with any other radio based positioning system for determining its position. The present invention can be used on various types of radio based positioning systems.

Control unit 6 accesses a memory (e.g., RAM, ROM) that comprises programs as well as various parame ^¬ ters, such as a unique identifiers of the monitoring de ^¬ vices which is transmitted with each message over the radio transceiver. Thus, the identity or origin of the transmitted signals can be determined.

An interface 5, here, an acoustic and optical user interface 5 advantageously comprises an optical dis ^¬ play 22 as well as an acoustic signal source 23, such as a loudspeaker. Furthermore, a tactile interface such as a vibrating unit (not shown) for alerting a user can be comprised in the interface 5.

The primary purpose of monitoring device 1,2,3 is to generate proximity warnings or approach warn ^¬ ings in case that there is a danger of collision between two or more objects. This is achieved by a two-step ap ^¬ proach :

a) By receiving position signals through GNSS receiver 20, a position of the respective monitoring de ^¬ vice (s) is determined. This positio (s) is or are advantageously transmitted by means of the radio transceiver 10 to all the other monitoring devices 1,2,3 that are in range such that all monitoring devices 1,2,3 are aware of the respective positions, velocities, and probabilities for collisions.

b) By deriving distance values (e.g., d_12) between the monitoring devices 1,2,3 using position based and time-of-flight based measurements of radio pulses that are exchanged between the two monitoring devices. Again, advantageously, the distance values are exchanged with other monitoring devices 1,2,3 in order to calculate relative positions, velocities, and probabilities for collisions. The method for calculating relative positions is described in the next section, while further informa ^¬ tion about various aspects of the monitoring device fol ^¬ lows later.

The advantage of using such a two-step ap ^¬ proach is that proximity warnings or approach warnings can also be issued in a case where one or more of the monitoring devices 1,2,3 do not or only have poor GNSS reception and thus positioning accuracy and/or where one or more of the monitoring devices 1,2,3 are not equipped with a GNSS receiver 20.

In an advantageous embodiment, monitoring device 1,2,3 can also comprises an acceleration detector (not shown here) . Acceleration values can also be transmitted by means of the radio transceiver 10. This acceleration detector can be used to reduce the energy con ^¬ sumption of the monitoring device. Since GNSS receiver 20 is one of the major power drains, GNSS receiver 20 can have a "disabled mode" where it is not operating and an "enabled mode" where it is operating. When analysis and control unit 6 detects an acceleration by means of the acceleration detector, it puts GNSS receiver 20 into its enabled state to obtain the current position of the monitoring device. Otherwise, it puts GNSS receiver 20, e.g., after a predetermined amount of time, into its disabled state. In addition to this, to account for the unlikely event that no acceleration is measured even though the monitoring device 1,2,3 is moving, control unit 6 can be adapted to put GNSS receiver 20 into its enabled state at regular intervals in order to perform sporadic position measurements .

In addition or alternatively to switching GNSS receiver 20 between a disabled an enabled state, other parts of monitoring device 1,2,3 can be switched between an idle and an active state in response to signals from the acceleration detector. In general terms, monitoring device 1,2,3 can have an "idle state" and an "active state", wherein, in said idle state, monitoring device 1,2,3 has a smaller power consumption than in said active state. Control unit 6 is adapted to put monitoring device 1,2,3 into its active state upon detection of an acceleration by the acceleration detector, while the monitoring device is, e.g., brought back to its inactive state if no acceleration has been detected for a certain period of time.

Monitoring device 1,2,3 advantageously com ^¬ prises a rechargeable battery (not shown) for feeding power to its components. A battery charger comprises circuitry for charging the battery. The battery charger can draw power from at least one power source. Such power sources can, e.g., be

- a power plug for directly connecting moni ^¬ toring device 1,2,3 to an external power supply;

- an inductive coupler comprising a coil adapted to generate electrical current from an alternating magnetic field generated by an external primary coil; such inductive power couplers are known to the skilled person; and/or

- a solar power supply mounted at the outer surface of the monitoring device 1,2,3 or in a separate unit electrically connected to the monitoring device

Relative position determination:

If all monitoring devices 1,2,3 would have a GNSS receiver 20, the operation of the monitoring devices could be basically as in conventional systems of this type, such as, e.g., described in WO 2004/047047 and need not be described in detail herein.

In such a simple approach, each monitoring device 1,2, and 3 would obtain information about its respective position P_l , P_2 , and P_3 derived from a signal from GNSS receiver 20. This data is stored in a "device status dataset". The device status dataset also contains a unique identifier (i.e. an identifier unique to each of the monitoring devices 1,2,3 used on the same area 100) .

The device status dataset is then transmitted as a digital radio signal through radio transceiver 10. At the same time, the monitoring device receives the cor ^¬ responding signals from neighboring monitoring devices and, for each such neighboring monitoring device, it calculates the relative distance d_xy (where x and y denote the index of the involved monitoring devices, e.g., d_12 relates to a distance between monitoring device 1 and monitoring device 2) by subtracting its own coordinates from those of the neighboring monitoring device.

In the present invention, however, the derivation of the distance values d_xy does not simply rely on the described position based procedure but also on a radio pulse flight time approach. Both distance deriva ^¬ tion schemes are weighted and combined as described to improve the reliability and safety of the system, in par ^¬ ticular in situations with poor GNSS reception.

Proximity warnings:

Proximity warnings can be generated by means of various algorithms. Examples of such algorithms are described in the following.

In a very simple approach, it can be tested if the absolute value of the relative distance d_xy is below a given threshold. If yes, a proximity warning can be issued. This corresponds to the assumption that a circular volume in space is reserved for each object. The radius of the circular volume attributed to an object can, e.g., be encoded in its device status dataset.

A more accurate algorithm can, e.g., take into account not only the relative position, but also the driving velocities and directions of the vehicles.

An improvement of the prediction of collisions can be achieved by storing data indicative of the size and/or shape of the object that a monitoring device 1,2,3 is mounted to. This is especially true for large vehicles, e.g., haul truck 40, which may have non- negligible dimensions. In a most simple embodiment, a vehicle can be modeled to have the same size in all directions, thereby defining a circle/sphere "covered" by the vehicle. If these circles or spheres of two vehicles are predicted to intersect in the near future, a proximity warning can be issued. It should be noted here that it is also possible to affix more than one monitoring device to an object which, e.g., enables the monitoring of orientations. These multiple monitoring devices on the same object would then, e.g., be configured not to exchange time-of-flight radio pulses with each other.

Instead of modeling an object or vehicle by a simple circle or sphere, a more refined modeling and therefore proximity prediction can be achieved by storing the shape (i.e. the bounds) of the vehicle in the data- set. In addition, not only the shape of the vehicle, but also the position of the GNSS-receiver 20 (or its antenna) in respect to this shape or bounds can be stored in memory.

In some cases, the signal strength of a received radio signal can be used to determine a range of distance where the monitoring device may be, thus improving warning accuracy in such a case. Hence, a first monitoring device receiving a signal from a second monitoring device assesses the signal strength of said signal and generates a proximity warning based on the assessed signal strength, in particular by comparing it to a maximum value .

Other functions:

In addition to issuing proximity warnings as described above, monitoring devices 1,2,3 can provide other uses and functions. In one embodiment, which is particularly use ^¬ ful if monitoring device 1,2,3 is worn on a person, the monitoring device 1,2,3 can issue a warning when it leaves the site or enters a "restricted area" of the site. This can, e.g., happen when a user of the monitor ^¬ ing device forgets to return the apparatus when leaving the site or tries to steal it, or when a user enters an area, such as a blast area, that is not safe for him.

This type of warning can be generated by executing the following steps:

1) In a first step, analysis and control unit 6 obtains the position of the monitoring device by means of GNSS receiver 20 or via time-of-flight triangulation (see below) .

2) In a second step, analysis and control unit 6 compares this position to a predefined geographi ^¬ cal area. This geographical area can, e.g., be stored in memory and describes the area where the monitoring device is allowed to be operated. If it is found that the posi ^¬ tion is not within the geographical area, the following step 3 is executed:

3a) ^' A warning which can comprise one or more positions of monitoring devices is issued. This warning can, e.g., be displayed on a display or issued as a sound by acoustic signal source 23.

3b) Alternatively, or in addition to step 3a, the warning can be sent, e.g., by means of a cellular phone transceiver (not shown) integrated into the monitoring device 1,2,3 or by means of the radio transceiver 10 to a central monitoring system (i.e. a central server) , together with the current position and identity of the respective monitoring device 1,2,3. Then, the warning can be displayed by the central server and brought to the attention of personnel that can then take further necessary steps. 3c) Alternatively or in addition to steps 3a and/or 3b, the apparatus can be made unusable by blocking and/or destroying at least part of its functionality.

In general, a cellular phone network (or any other wireless network) can be used to transmit information from the monitoring devices 1,2,3 to the central server. As mentioned, this information can, e.g., comprise any warnings issued by the monitoring devices

1,2,3, and/or it may comprise the position of the moni ^¬ toring device.

Another application of a cellular phone transceiver integrated into monitoring device 1,2,3 is to send messages from the central server to any monitoring device 1,2,3. Such messages are received the respective monitoring device 1,2,3 and displayed or brought to the operator's attention acoustically. This, e.g., allows to issue warnings, alerts or information to the person using the monitoring device 1,2,3.

The monitoring devices 1,2,3 can also be used for generating automatic response to the presence of a vehicle or person at a certain location. For example, when a pedestrian with a monitoring device approaches a gate, such as door of a building, that door can open automatically. Similarly, an entry light can switch to red or to green, depending on the type of object that a monitoring device is attached to, or a boom can open or close. This can be achieved by mounting a receiver device to a selected object (such as a door, a gate, boom or an entry light) . The receiver device is equipped with a radio receiver adapted to detect the proximity of monitoring devices 1,2,3. When the receiver device detects the proximity of a monitoring device 1,2,3, it actuates an actuator (such as the door, gate or entry light) after testing access rights of the object attributed to the monitoring device 1,2,3. For example, the actuator may be actuated depending on the type of the object that the monitoring device is attached to and/or to its distance from the receiver. The type and/or distance information is transmitted as part of the device status dataset of the monitoring device 1,2,3.

Furthermore, the control unit of the monitor ^¬ ing device 1,2,3 can have an "alert mode", which can be activated by a user, e.g., by pressing an alert button on a keyboard of the monitoring device 1,2,3 and/or by voice control. It can, e.g., be used to indicate that the person using the monitoring device is in need of urgent help or needs all activity around it to be stopped immedi ^¬ ately. The device status dataset comprises a flag indica ^¬ tive of whether the monitoring device is in alert mode. Another monitoring device receiving a device status data- set that indicates that the sender is in alert mode may take appropriate action. For example, the central control room operator can be informed, closeby machinery can be shut down, etc.

Persons on the site:

As mentioned above, the monitoring devices 1, 2, 3 can not only be mounted to vehicles in the area, but also to individual persons on the site. By using the same type of device for persons as well as vehicles, costs can be reduced. In such a situation, the monitoring device 1,2 mounted to vehicles can cooperate with the monitoring device 3 mounted to a person in such a manner that

a) the drivers of the vehicles are alerted of the presence of pedestrians, and/or

b) the pedestrians are alerted of the pres ^¬ ence of the vehicles 40,50.

While option a) is the primary purpose of the present invention, option b) may also have its advantages, too.

In order to alert the drivers of the presence of a pedestrian, each pedestrian-mounted monitoring device can transmit a flag indicative of the fact that it is mounted to a pedestrian, e.g., as part of its device status dataset. Thus, all other monitoring devices in range are aware of this fact and can adapt their warning strategy accordingly.

Time of flight measurement/triangulation:

Under adverse conditions, e.g. when one or more satellite signals are partially or fully blocked by obstacles, GNSS receiver 20 of a given monitoring device 1,2 may not be able to reliably derive its position

P_1,P_2, or the determined position P_l , P_2 will be inaccurate. In another situation, one monitoring device 3 may not equipped with a GNSS receiver 20 at all (therefore, GNSS receiver 20 is dotted in fig. 2) .

Therefore, in order to improve the reliabil ^¬ ity and versatility of the proximity warning method, the monitoring devices 1,2,3 are adapted and structured for mutual time-of-flight distance derivation. For this, the monitoring devices 1,2,3 are equipped with radio transceivers 10 to perform a "time-of-flight (TOF) measurement and/or triangulation" . This TOF measurement/triangulation allows to at least approximately determine the mutual distances and/or positions of several monitoring devices 1,2,3, even if the monitoring device 3 is unable to determine its position P_3 because of the lack of a GNSS receiver 20.

In the given example situation, a first position P__l of the first monitoring device 1 and a second position P_2 of the second monitoring device 2 are deter ^¬ mined using the GNSS receivers 20 of the respective moni ^¬ toring devices 1,2. The first and second positions P_l and P_2 are then transmitted by the monitoring devices 1,2 such that all monitoring devices 1,2,3 are aware of these positions P_l and P_2.

Then, using a bidirectional exchange of TOF- pulses P_TOF and R__TOF (see above) between the first and the second devices 1,2 and using the first and second positions P_l and P_2, the first monitoring device 1 de- rives a first distance value d_12 (dotted lines and cir ^¬ cle segments in figure 1) between the first and the second monitoring devices 1,2. Then, this distance value d_12 is transmitted in a way that all monitoring devices 1,2,3 in range are aware of it.

As another step, by means of a bidirectional exchange of TOF-pulses P_T0F (i.e., a third radio pulse) and R_TOF (see above) between the second and the third monitoring devices 2,3, the second monitoring device 2 derives a second distance value d_23 (dotted lines and circle segments in figure 1) between the second and the third monitoring devices 2,3. It should be noted here that this distance value d_23 is not derivable because position P_3 is not (yet) known. Then, the distance value d_23 is also transmitted in a way that all monitoring devices 1,2,3 in range are aware of it.

As another step, by means of a bidirectional exchange of TOF-pulses P_T0F and R_T0F (see above) be ^¬ tween the first and the third monitoring devices 1,3, the first monitoring device 1 derives a third distance value d_13 (dotted lines and circle segments in figure 1) be ^¬ tween the fist and the third of the monitoring devices 1,3. The P_TOF pulse can be the first radio pulse which is also used for deriving d_12 as discussed above or it can be a fourth radio pulse. It should be noted here that this distance value d_13 is also not derivable from the respective positions because the third position P_3 is not yet known. Then, the distance value d_13 is also transmitted in a way that all monitoring devices 1,2,3 in range are aware of it .

Now, the monitoring apparatus (specifically all the monitoring devices 1,2,3 in range) are aware of P_l, P_2, d__12, d_13, and d_23. From this information, the position P__3 of the third of the monitoring device 3 is derived and optionally transmitted by means of the radio transceive ( s ) . An ambiguity between P_3 and an also possible "mirrored position P_3'" is resolved by means of a radio transceiver antenna with directional resolution, i.e., a direction sensitive antenna, in any of the moni ^¬ toring devices 1,2,3.

Then, a proximity warning can be issued as a function of the first, the second, and/or the third dis ^¬ tance values d__12, d_23, d_13 and/or optionally of the first, second, and/or third position P__l , P__2 , P_3.

Thus, even without a GNSS receiver 20 in the third of the monitoring devices 3, the position P_3 is derivable which leads to higher reliability and increased safety of the monitoring apparatus.

Note :

The above mentioned steps of transmitting a determined position P_l , P_2 , and/or P__3 of a monitoring device 1,2,3 by means of the radio transceiver 10 of the respective monitoring device 1,2,3 can comprise absolute position information, e.g., altitude, longitude, and latitude and/or relative position information with regard to a predefined position or regular grid points on the surface mine 100. Furthermore, an incremental position information transmission is possible as well in which only position changes are transmitted.

While there are shown and described presently preferred embodiments of the invention, it is to be dis ^¬ tinctly understood that the invention is not limited the ^¬ reto but may be otherwise variously embodied and practiced within the scope of the following claims.