GB2432079A

GB2432079A - A position tracking system

Info

Publication number: GB2432079A
Application number: GB0522810A
Authority: GB
Inventors: Alan Weight
Original assignee: Datong Electronics Ltd
Current assignee: Datong Electronics Ltd
Priority date: 2005-11-08
Filing date: 2005-11-08
Publication date: 2007-05-09
Anticipated expiration: 2025-11-08
Also published as: GB0522810D0; GB2432079B

Abstract

A position tracking system comprises a primary unit that transmits a position signal derived from e.g. GPS, and two or more monitoring units that receive and retransmit the position signal. The primary unit 10 is attached to an object to be tracked and comprises a GPS receiver and a transmitter to broadcast its position to an in-range monitoring unit 20A. The monitoring unit 20A receives and retransmits the position signal to other monitoring units 20B which may not be in range of the primary unit. The monitoring units 20 and primary unit 10 are each assigned a time slot within a TDMA transmission cycle to avoid clashes between transmissions. Within the TDMA cycle each monitoring unit 20 is arranged to transmit in its assigned time slot a position signal of the primary unit 10 in the event that the monitoring unit has received such a signal from the primary unit but not from any other monitoring unit.

Description

POSiTION TRACKING SYSTEM

Field of the Invention

The present invention relates to a position tracking system. In particular, but not exclusively, the present invention relates to a position tracking system for providing information about the position of a primary unit, associated with a target, to two or more monitoring units, and a method of tracking the position of a primary unit.

Background to the Invention

In a position tracking system it may be desirable to pass a position signal indicative of the position of a primary unit to a plurality of monitoring units. The primary unit may comprise means for determining it own position, such as a GPS receiver. The primary unit typically may then comprise signal transmission means arranged to transmit its position signal. The primary unit may, for instance, transmit the position signal periodically. The monitoring units may comprise signal receiving means arranged to receive the position signal from the primary unit.

Such a position tracking system may, for instance, be used in a situational awareness system. For instance, one or more of the monitoring units may further comprise a display screen arranged to graphically display the position of the primary unit. This may be used in a number of situations in which it is desirable to be able to monitor the position of an object to which the primary unit is attached, or with which the primary unit is associated in some other way. The object may be referred to as a target, or the target may be a person associated with the object, for example. Such an object could be, for instance, a vehicle, a container in a dockyard or any other moveable or mobile object. The monitoring units may themselves be moveable or mobile. For example, they may be attached to, or carried by, moveable objects such as vehicles. They may be portable devices to be carried by a person. Alternatively they may be at known fixed locations.

Furthermore, in an alternative type of position tracking system, each monitoring unit may also further comprise means for determining the position of itself, such as a GPS receiver and signal transmission means arranged to transmit a position signal indicative of the position of the monitoring unit. Each monitoring unit

I

may then also be arranged to receive position signals from other monitoring units.

Such a position tracking system may thus allow each monitoring unit to provide information (for example by means of a display to a user) relating to the relative position of the primary unit and other monitoring units to itself, as well as the absolute position of each primary unit and monitoring unit.

However, in a position tracking system it may be the case that the position signal indicative of the position of the primary unit is of greater importance than the position signals indicative of the positions of the monitoring units. Furthermore, in some scenarios, the primary unit may be transmitting at a lower power than the monitoring units. This may be because the primary unit is operating from battery power, whereas the monitoring units are powered from a mains electricity supply or some other independent power source, such as a vehicle battery. The primary unit may also be located in a non-ideal location on the tracked object. Thus, the effective range of the primary unit signal transmission means may be less than the effective range of the monitoring unit's signal transmission means. Consequently, one or more monitoring units may be outside of the range of the primary unit, while still within range of other monitoring units. One possible solution for ensuring that all monitoring units receive the primary unit position is for one of the monitoring units to retransmit the primary unit position signal, or to transmit some other related position signal containing that information (i.e. the location of the primary unit).

One option for retransmitting the primary unit position signal is for a single monitoring unit to be assigned the retransmission role. However, this is not desirable as the assigned monitoring unit may itself be one of the monitoring units out of range of the primary unit. Another option is for any of the monitoring units to be able to retransmit the position signal from the primary unit. However, this second option runs the risk of collision between retransmissions from multiple monitoring units that have received the primary unit position signal. Consequently, other monitoring units would not receive the retransmitted primary unit position signal correctly.

Alternatively, after each transmission of the primary unit position signal, some form of communication may be entered into between the monitoring units in order to determine which monitoring unit should retransmit the primary unit position signal.

However, this additional communication is a significant overhead, and reduces the amount of position information that can be transmitted in a particular time interval, or alternatively requires a higher bandwidth communications link for the position tracking system.

It is an object of the present invention to obviate, or mitigate, one or more of the problems associated with the prior art, whether identified herein or elsewhere. It is an object of specific embodiments of the present invention to provide a position tracking system allowing for efficient retransmission of a position signal indicative of the position of a primary unit.

Summary of the Invention

According to a first aspect of the present invention there is provided a position tracking system comprising: a primary unit comprising signal transmission means and assigned a respective time slot within a TDMA transmission cycle comprising a plurality of time slots, the primary unit being adapted to transmit a position signal indicative of its position during its assigned time slot; and at least two monitoring units, each monitoring unit comprising respective signal transmission means and respective signal receiving means and being assigned a further respective time slot in the transmission cycle, each monitoring unit being adapted to receive position signals from the primary unit and from the other monitoring unit or units; wherein each monitoring unit is further arranged to monitor the received position signals and, within a particular transmission cycle, to: transmit in its assigned time slot a position signal indicative of the position of the primary unit in the event that the monitoring unit has received a position signal from the primary unit in the primary unit's assigned time slot but has not received a position signal indicative of the position of the primary unit from another monitoring unit in any other preceding time slot in that same transmission cycle.

An advantage of the present invention is that the identity of the monitoring unit that is to retransmit the primary unit position signal is determined dynamically, with no additional communication required between the monitoring units.

Additionally, this avoids the problem of transmission collision between the multiple retransmitted position signals. In other words, rather than a particular monitoring unit or units being assigned a fixed retransmission role, whether or not a monitoring unit retransmits primary unit position information is determined dynamically, i.e. each monitoring unit during each transmission cycle determines (decides) whether or not to retransmit primary unit location information according to what has occurred in that cycle, in particular, according to what signals it has already received before its allotted time slot.

Preferably, each monitoring unit is further arranged, within said particular transmission cycle, to transmit in its assigned slot a position signal indicative of its own position in the event that it has not received a position signal from the primary unit in the primary unit's assigned time slot and in the event that it has received a position signal from the primary unit in the primary unit's assigned time slot and has received a position signal indicative of the position of the primary unit from another monitoring unit in any other preceding time slot in that same transmission cycle.

More preferably, the primary unit and each monitoring unit are further arranged to transmit an identifier signal along with each position signal, and each monitoring unit is further arranged to receive identifier signals from the primary unit and the other monitoring unit or units.

In certain embodiments of the present invention there may be at least two primary units, each monitoring unit being arranged to receive position signals from each primary unit and from the other monitoring unit or units.

The transmission cycle may further comprise at least one dynamically assigned time slot and each monitoring unit is further arranged to transmit in the or one of the dynamically assigned time slots a position signal indicative of its own position in the event that it transmits in its assigned time slot a position signal indicative of the position of a primary unit.

Preferably, each time slot within a particular transmission cycle assigned to a primary unit is earlier than the time slots assigned to the monitoring units in that same transmission cycle. Preferably, each dynamically assigned time slot within a particular transmission cycle is later than the time slots assigned to the monitoring units in that same transmission cycle.

The primary and monitoring units may be arranged to operate according to a TDMA system comprising at least two said transmission cycles arranged to repeat sequentially in time, each primary unit and each monitoring unit being assigned a respective time slot in at least one of the transmission cycles.

In certain preferred embodiments, each transmission cycle is 1 second long and comprises ten time slots, each time slot being 100 milliseconds long.

Preferably, each of the primary and monitoring units is further adapted to receive a synchronisation signal and to utilise the synchronisation signal to synchronise its operation with the TDMA transmission cycle. The synchronisation signal may be a GPS signal. Alternatively, the system may further comprise a beacon adapted to transmit the synchronisation signal. The beacon may comprise a primary unit. At least one monitoring unit may be adapted to retransmit the received synchronisation signal.

The or each primary unit may further comprise a global positioning system receiver arranged to provide the position signal indicative of the position of that primary unit. Each monitoring unit may further comprise a global positioning system receiver arranged to provide a position signal indicative of the position of that monitoring unit. Each primary unit or monitoring unit comprising a GPS receiver may be adapted to use the signal received by the global positioning receiver to synchronise to the or each transmission cycle.

The or each primary unit may be arranged to transmit position signals at a lower power than the power at which each monitoring unit transmits position signals.

At least one monitoring unit may further comprise a display screen arranged to graphically display the position of that monitoring unit relative to the or each primary unit. The display screen may be further arranged to graphically display the position of that monitoring unit relative to at least one other monitoring unit.

According to a second aspect of the present invention there is provided a method of tracking the position of a primary unit comprising: assigning a primary unit a respective time slot within a TDMA transmission cycle comprising a plurality of time slots, the primary unit comprising signal transmission means; using the primary unit transmission means to transmit a position signal indicative of the primary unit's position during its assigned time slot; assigning at least two monitoring units further respective time slots in the transmission cycle, each monitoring unit comprising respective signal transmission means for transmitting position signals to the other monitoring unit or units and respective signal receiving means for receiving position signals from the primary unit and from the other monitoring unit or units; monitoring in each monitoring unit the position signals received by that unit; and transmitting from each monitoring unit in its assigned time slot within a particular transmission cycle, a position signal indicative of the position of the primary unit in the event that the monitoring unit has received a position signal from the primary unit in the primary unit's assigned time slot but has not received a position signal indicative of the position of the primary unit from another monitoring unit in any other preceding time slot in that same transmission cycle.

Preferably the method further comprises transmitting from each monitoring unit, in its assigned time slot within said particular transmission cycle, a position signal indicative of its own position in the event that it has not received a position signal from the primary unit in the primary unit's assigned time slot and in the event that it has received a position signal from the primary unit in the primary unit's assigned time slot and has received a position signal indicative of the position of the primary unit from another monitoring unit in any other preceding time slot in that same transmission cycle.

Preferably the method further comprises the primary unit and each monitoring unit transmitting an identifier signal along with each transmitted position signal; each monitoring unit receiving the identifier signals; and each monitoring unit identifying which primary unit or monitoring unit the position signal relates to using the identifier signal.

There may be at least two primary units, the method further comprising each monitoring unit receiving position signals from each primary unit and from the other monitoring unit or units.

The transmission cycle may further comprise at least one dynamically assigned time slot, the method further comprising each monitoring unit transmitting in the or one of the dynamically assigned time slots a position signal indicative of its own position in the event that it transmits in its assigned time slot a position signal indicative of the position of a primary unit.

Assigning a primary unit a respective time slot may comprise assigning a primary unit a respective time slot earlier than the time slots assigned to the monitoring units in that same transmission cycle. The or each dynamically assigned time slot within a particular transmission cycle may be later than the time slots assigned to the monitoring units in that same transmission cycle.

The method further comprise operating the primary and monitoring units according to a TDMA system comprising a plurality of said transmission cycles; repeating the plurality of transmission cycles sequentially in time; and assigning each primary unit and each monitoring unit a respective time slot in at least one of the transmission cycle.

In certain preferred embodiments of the method each transmission cycle is 1 second long and comprises ten time slots, each time slot being 100 milliseconds long.

The method may further comprise receiving at each of the primary and monitoring units a synchronisation signal and utilising the synchronisation signal to synchronise the primary or monitoring unit to the TDMA transmission cycle. The synchronisation signal may be a GPS signal. Alternatively, the method may further comprise transmitting the synchronisation signal from a beacon. The beacon may comprise a primary unit. The method may further comprise retransmitting the synchronisation signal from at least one monitoring unit.

The method may further comprise providing the or each primary unit with a global positioning system receiver, the global positioning system receiver providing the position signal indicative of a position of that primary unit.

The method may further comprise providing each monitoring unit with a global positioning system receiver, the global positioning system receiver providing the position signal indicative of a position of that monitoring unit.

The method may further comprise each primary unit or monitoring unit synchronising to the or each transmission cycle using the signal received by the global positioning receiver.

The method may further comprise the or each primary unit transmitting position signals at a lower power than the power at which each monitoring unit transmits position signals.

At least one monitoring unit may further comprise a display screen, the method further comprising the display screen graphically displaying the position of that monitoring unit relative the or each primary unit. The method may further comprise the display screen graphically displaying the position of that monitoring unit relative to at least one other monitoring unit.

According to a third aspect of the present invention there is provided a primary unit comprising signal transmission means and adapted to be assigned a respective time slot within a TDMA transmission cycle comprising a plurality of time slots, the primary unit being further adapted to transmit a position signal indicative of its position during its assigned time slot.

The primary unit may be adapted for use in a system or method as described above.

According to a fourth aspect of the present invention there is provided a monitoring unit comprising signal transmission means and signal receiving means, and adapted to be assigned a respective time slot within a TDMA transmission cycle comprising a plurality of time slots, the monitoring unit being further adapted to receive position signals from a primary unit assigned a further respective time slot and from other monitoring units; wherein the monitoring unit is further arranged to monitor the received position signals and, within a particular transmission cycle, to: transmit in its assigned time slot a position signal indicative of the position of the primary unit in the event that the monitoring unit has received a position signal from the primary unit in the primary unit's assigned time slot but has not received a position signal indicative of the position of the primary unit from another monitoring unit in any other preceding time slot in that same transmission cycle.

The monitoring unit may be adapted for use in a system or method as described above.

Brief DescriDtion of the Drawings The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a position tracking system in accordance with an embodiment of the present invention comprising one primary unit and two monitoring units; Figure 2 schematically illustrates the primary unit of Figure 1; Figure 3 schematically illustrates a secondary unit of Figure 1; Figure 4 is a flow chart showing the decision making process which determines whether a monitoring unit should retransmit the position signal indicative of the position of the primary unit for the position tracking system of Figure 1; and Figure 5 is a flow chart showing the decision making process which determines whether a monitoring unit should retransmit the position signal indicative of the position of either of two primary units in a position tracking system having two primary units in accordance with an embodiment of the present invention.

Description of Embodiments of the Invention

Referring now to Figure 1, this illustrates a position tracking system in accordance with an embodiment of the present invention comprising a single primary unit 10 and two monitoring units 20A, 20B (which may also be described as secondary units). The range of the position signal transmitted by the primary unit 10 is indicated by the dashed circle 1. The ranges of the position signals transmitted by the monitoring units 20A, 20B are indicated by the solid circles 2, 3. It can readily be seen that the range 1 of the primary unit 10 is less than the ranges 2, 3 of the monitoring units 20A, 20B. This may be because the primary unit 10 is operating at a lower power than the monitoring units 20A, 20B, or is in a non-ideal RF transmission position on the tracked object.

Monitoring unit 20A is within range 1 of primary unit 10. However, monitoring unit 20B is located further from primary unit 10 and as such cannot receive position signals from primary unit 10. The monitoring units 20A, 20B are within range of each other and consequently are able to transmit and receive position signals between themselves.

In order for monitoring unit 20B to receive the primary unit 10 position signal it is necessary for monitoring unit 20A to retransmit that position signal. It will be appreciated that such "retransmission" of the primary unit position signal is not necessarily a retransmission of exactly the same physical signal as was received from the primary unit; by "retransmission" it is meant that the monitoring unit transmits a signal that contains the same positional information as that received from the primary unit. The monitoring unit may also transmit further information within the retransmission. In other words, the monitoring unit retransmits the received primary unit position. It will also be appreciated that the position tracking system of Figure 1 may be readily scaled up such that there are more primary units 10 and monitoring units 20, such that each monitoring unit is able to receive position signals from none, one, or more of the primary units 10. As the range of the monitoring units 20 is longer it may bc assumed that each monitoring unit 20 is able to receive position signals from every other monitoring unit 20. However, the present invention does not rely on this. If a position signal from a first monitoring unit 20 is not received by a second monitoring unit 20 this omitted position signal does not affect the ongoing performance of the system or the individual monitoring unit 20.

Figure 2 illustrates the component parts of a primary unit 10 in accordance with an embodiment of the present invention. Transmitter 11 is connected to antenna 12, under the control of transmitter controller 13. Transmitter 11 transmits the primary unit 10 position signal, along with any additional data, which may be provided by optional sensors 14. Sensors 14 could include, for example, a movement sensor, battery charge state sensors, a temperature sensor or a sensor detecting a line-in condition.

The position signal is provided to the transmitter 11 by OPS receiver 15, which is connected to GPS antenna 16. Memory 17 is arranged to store the program code controlling the operation of the primary unit 10, along with other data, for instance an identifier signal to be transmitted with the position signal (described in more detail below). Memory 17, in certain embodiments of the invention, also stores a backlog of the transmitted position data.

Figure 3 illustrates the component parts of a monitoring (secondary) unit 20 in accordance with an embodiment of the present invention. Transceiver 21 is connected to antenna 22, under the control of transceiver controller 23. Transceiver 21 transmits the primary unit 10 position signal or the monitoring unit 20 position signal and receives position signals from the primary unit 10 or other monitoring units 20.

The monitoring unit 20 position signal is provided to the transceiver 21 by GPS receiver 24, which is connected to GPS antenna 25. Memory 26 is arranged to store the program code controlling the operation of the monitoring unit 20, along with other data, for instance an identifier signal to be transmitted with the position signal (described in more detail below). Additionally, the memory 26 is arranged to store position signals received from the primary unit 10 or other monitoring units 20.

For efficient use of the portion of the frequency spectrum allocated to the position tracking means it is desirable that the primary unit and the monitoring units all transmit at the same frequency. Furthermore, in order to maximise the number of primary units 10 and monitoring units 20 that can operate within the position tracking system within a single frequency allocation a Time Division Multiple Access (TDMA) system is employed. The TDMA system defines a TDMA transmission cycle comprising a plurality of time slots. After the transmission cycle has finished it immediately starts again from the beginning. In further embodiments of the present invention, described below, there may be two or more transmission cycles arranged to repeat sequentially.

Within each time slot only a single primary unit 10 or a single monitoring unit is allowed to transmit. When not transmitting, each monitoring unit 20 is arranged to receive position signals from the primary unit 10 or other monitoring units 20.

Each monitoring unit 20 monitors the received position signals in order to determine whether it is required to retransmit the position signal indicative of the position of the primary unit.

Each primary unit 10 and each monitoring unit 20 is allocated a respective unique time slot. This ensures that no two primary 10 or monitoring 20 units attempt to transmit at the same time. This would otherwise lead to a transmission collision and data loss. Each unit is synchronised to the TDMA transmission cycle, in this example using the timing component of the received GPS signal (alternative embodiments use alternative synchronisation techniques). If the GPS signal is interrupted then each unit is arranged to maintain synchronisation using an internally generated clock signal until the GPS signal is received again.

In an alternative embodiment of the present invention, instead of a GPS signal being used as a synchronisation signal, a beacon containing a local clock transmits the synchronisation signal. This synchronisation signal is received by each primary unit or monitoring unit and used to synchronise that unit to the TDMA transmission cycle.

The beacon may be one of the primary units or one of the monitoring units.

Monitoring units that receive the synchronisation signal may retransmit the synchronisation signal, either separately or as part of the retransmission of position data from the or each primary unit. Each unit receiving the synchronisation signal may monitor the signal for information about the length and the start time of the transmission cycle.

In order to enable the position signal from the primary unit 10 to be received by all monitoring units 20, including those outside of the direct range of the primary unit 10, one of the monitoring units 20 that has received this position signal retransmits the signal. The retransmitted primary unit position signal can then be received by all other monitoringunits 20 within range of the first monitoring unit 20.

In accordance with an embodiment of the present invention each transmission cycle is I S long and is divided into 10 time slots, each of I OOmS. For a position tracking system having a single primary unit 10 the first time slot in the transmission cycle is allocated to the primary unit 10. The next 8 time slots are allocated to monitoring units 20, allowing up to 8 monitoring units 20 to form part of the position tracking system. The tenth and last time slot is a dynamically allocated time slot, the purpose of which is explained below. Table I illustrates the allocation of time slots for a position tracking system having one primary unit 10 and eight monitoring units 20. PU indicates that a primary unit 10 transmits a position signal indicative of its position in that time slot. MU indicates that a monitoring unit transmits a position signal in that time slot, the transmitted position signal containing positional information of either the monitoring unit itself, or of the primary unit, as describe below.

Time slot Time Transmitting unit 0 0-lOOmS PUI 100-200mS MU1 2 200-300mS MU2 3 300-400mS MU3 4 400-500mS MU4 500-600mS MU5 6 600-700mS MU6 7 700-800mS MU7 8 800-900mS MU8 9 900-l000mS Dynamic slot I Table 1: Time slot allocation for a position tracking signal having a single primary unit.

Each time slot is assigned to a respective primary 10 or monitoring 20 unit before the position tracking system is operational, such that only a single unit will transmit in each time slot. The numbers following MU table 1 indicate to which of the eight monitoring units each time slot is allocated. The units are synchronised to the transmission cycle using the timing information within the GPS signal as described above.

Normally, each monitoring unit 20 transmits a position signal indicative of its own position in its assigned time slot. However, the monitoring unit 20 that is arranged to retransmit the position signal indicative of the position of the primary unit will transmit that position signal in its assigned time slot in place of its own position signal. The dynamic time slot (time slot 9) is used by the retransmitting monitoring unit 20 to transmit a position signal indicative of its own position. Thus, the dynamic time slot does not have a fixed assignment to any one particular monitoring unit 20. At the beginning of the transmission cycle it can be regarded as unassigned. The identity of the monitoring unit that then uses the dynamic time slot in that transmission cycle is determined dynamically, i.e. according to the signals transmitted and received in the cycle. The dynamic time slot (or slots -there may be more than one) may also be described as an unassigned time slot, or an assignable time slot.

Within the data packet containing the position signal transmitted by each unit there is also an identifier signal. Before the system is operational a unique identifier signal is allocated to each unit and stored in the memory 17, 26. Consequently, when each monitoring unit 20 receives a position signal it is able to determine the primary unit 10 or monitoring unit 20 to which the position signal relates. If a monitoring unit has retransmitted a position signal indicative of the position of the primary unit in place of a position signal indicative of its own position in its assigned time slot a recipient monitoring unit 20 will be able to correctly assign this position data to update its stored memory of the position of that unit. Thus, each position signal may be arranged such that a receiving monitoring unit can determine the identity of the unit from which the signal has been transmitted and the identity of the unit to which the contained positional information relates.

The decision making process by which each monitoring unit 20 independently determines whether it is to be the retransmitting monitoring unit 20 is illustrated within the flow chart in Figure 4.

The flowchart starts at step 30 when the transmission cycle reaches the start of the assigned time slot for that monitoring unit 20. In decision step 31 the monitoring unit 20 decides whether it has received the position signal for the primary unit PU1 directly from the primary unit PU1, i.e. whether it received the signal in time slot 0.

If the answer is no then the monitoring unit 20 transmits its own position signal in its assigned time slot at step 32. It is unable to retransmit the position signal from the primary unit PU1. If the answer is yes, then operation passes to decision step 33.

In step 33 the monitoring unit 20 determines whether it has received the primary unit PU 1 position signal in the time slot allocated to another monitoring unit 20, previously in that same transmission cycle, i.e. whether another monitoring unit has already retransmitted the primary unit PUI position signal. If the answer is yes then there is no need to retransmit the primary unit PUI position signal and the monitoring unit 20 transmits its own position signal in step 32. If the answer is no then the monitoring unit 20 retransmits the primary unit PU1 position signal in the monitoring unit's assigned time slot in step 34. In step 35 the monitoring unit 20, having sacrificed its own time slot, will transmit its own position signal in the dynamic time slot (time slot 9).

In this manner it can readily be seen that it will always be the first monitoring unit 20 in the transmission cycle having received the primary unit position signal directly from primary unit PU1 that will retransmit the primary unit position signal.

This decision making process is autonomous to the individual monitoring unit 20, and does not require additional communication between the monitoring units.

Table 2 illustrates the results of this decision making process within a single transmission cycle for a position tracking system having I primary unit and 8 monitoring units.

Time Time Transmitting unit slot 0 0-1 OOmS PU 1 transmits its data. MU3 and MU4 are the only monitoring units to receive the direct data.

1 l00-200mS MUI hasn't received PU1 data in slot 0 and so it transmits its own data.

2 200-300mS MU2 hasn't received PU1 data in slot 0 and so it transmits its own data.

3 300-400mS MU3 received PU1 data in slot 0 and has not received it in any other time slot. It transmits the data it received from pul.

4 400-500mS MU4 received PU! data in slot 0 but also received it in slot 3. It transmits its own data.

500-600mS MU5 hasn't received PU 1 data in slot 0 but has received it in slot 3. It transmits its own data.

6 600-700mS MU6 hasn't received PU I data in slot 0 but has received it in slot 3. It transmits its own data.

7 700-800mS MU7 hasn't received PU 1 data in slot 0 but has received it in slot 3. It transmits its own data.

8 800-900mS MU8 hasn't received PUI data in slot 0 but has received it in slot 3. It transmits its own data.

9 900-1 000mS MU3 sacrificed its own data slot to broadcast the data it received from PU1. It uses this dynamic slot to transmit its own data.

Table 2: Time line of a single transmission cycle indicating what position signal each monitoring unit transmits for a position tracking system having a single primary unit.

It can be seen from table I that for a position tracking system having a single primary unit 10 and a single iS transmission cycle of 10 1 OOmS time slots the system is limited to eight monitoring units 20. The number of monitoring units could be increased by increasing the length of the transmission cycle and/or reducing the length of each time slot. However, the length of transmission cycle determines the frequency at which the stored primary unit 10 position signal (i.e. position data) at each monitoring unit 20 is updated. It can be desirable that this position signal is updated frequently. The minimum length of each time slot is determined by the physical parameters of the communications link and the amount of data to be transmitted within and alongside each position signal.

An alternative embodiment of the present invention provides for having two transmission cycles that run alternately. The primary unit PU I occupies the first time slot of each transmission cycle. Up to the next eight time slots in each transmission cycle are occupied by a subset of the monitoring units 20. Table 3 illustrates the allocation of time slots for a position tracking system having two transmission cycles, one primary unit 10 and twelve monitoring units 20. Time slots 5-8 in transmission cycle 2 are unallocated, and are therefore available for further expansion of the system.

Transmission Time slot Time Transmitting unit cycle 1 0 0-lOOmS PU1 1 00-200mS MU I 1 2 200-300mS MU2 1 3 300-400mS MU3 1 4 400-500m5 MU4 1 5 500-600mS MU5 1 6 600-700mS MU6 7 700-800mS MU7 8 800-900mS MU8 1 9 900-l000mS Dynamic slot 1 2 0 0-lOOmS PU! 2 1 1 00-200mS MU9 2 2 200300mS MU1O 2 3 300-400mS MUll 2 4 400-500mS MU12 2 5 500-600mS Unallocated 2 6 600-700mS Unallocated 2 7 700-800mS Unallocated 2 8 800-900mS Unallocated 2 9 900-l000mS Dynamic slot I Table 3: Time slot allocation for a position tracking system having two transmission cyclcs and a single primary unit.

It will be readily appreciated that this system may be easily expanded to any number of transmission cycles that run sequentially before repeating. Additionally it will be readily appreciated that the repetition frequency of the primary units and the monitoring units may vary, for instance one monitoring unit could have an assigned time slot within each transmission cycle. Typically, the primary units will have a higher repetition frequency than the monitoring units. For instance in table 3 the primary unit PU I has a repetition frequency of once per second and the monitoring units 20 all have a repetition frequency of once every two seconds.

It will be appreciated that the embodiments described above operating according to a repeating plurality of transmission cycles may be described in alternative terms. The sequence of transmission cycles may itself be described as a single transmission super-cycle, with the individual transmission cycles being respective sub-cycles (i.e. portions) of that super-cycle. A primary unit may thus be assigned a plurality of respective time slots in the super cycle (e.g. one in each sub-cycle). Each monitoring unit may be assigned one time slot in each super cycle (i.e. a slot in just one of the sub-cycles). However, each monitoring unit may alternatively be assigned a time slot in more than one sub-cycle.

In an alternative embodiment of the present invention the position tracking system has two primary units 10. For simplicity, this embodiment is described having a single transmission cycle, and six monitoring units 20. However, it will be appreciated that there may be any number of primary units 10, monitoring units 20 and transmission cycles. Table 4 illustrates the allocation of time slots for each primary unit 10 or monitoring unit 20.

Time slot Time Transmitting unit 0 0-lOOmS PUI 1 100-200mS PU2 2 200-300mS MUI 3 300-400mS MU2 4 400-500mS MU3 500-600mS MU4 6 600-700rnS MU5 7 700-800mS MU6 8 800-900mS Dynamic slot I 9 900-l000mS Dynamic slot 2 Table 4: Time slot allocation for a position tracking system having two primary units.

Primary units PU I and PU2 are assigned the first time slots (time slots 0 and 1). The next six time slots are assigned to the monitoring units 20. The last two time slots are dynamic time slots available for monitoring units 20 to transmit their position signals if they sacrifice their own assigned time slot in order to retransmit the position signal for one or the other primary unit. The first dynamic time slot (time slot 8) is used by the monitoring unit 20 that retransmits the position signal for primary unit PU 1. The second dynamic time slot (time slot 9) is used by the monitoring unit 20 that retransmits the position signal for primary unit PU2.

The decision making process by which each monitoring unit 20 independently determines whether it is to retransmit the position signal for primary unit PU 1 or PU2 is illustrated within the flow chart in Figure 5.

The flowchart starts at step 40 when the transmission cycle reaches the start of the assigned time slot for that monitoring unit 20. In decision step 41 the monitoring unit 20 decides whether it has received the primary unit PUI position signal directly from the primary unit PUI, i.e. whether it received the signal in time slot 0. If the answer is no then operation passes to decision step 42 as the monitoring unit 20 is unable to retransmit the position signal from the primary unit PU 1. If the answer is yes, then operation passes to decision step 43.

In step 43 the monitoring unit 20 determines whether it has received the primary unit PU 1 position signal in the time slot allocated to another monitoring unit 20, previously in that same transmission cycle, i.e. whether another monitoring unit has already retransmitted the primary unit PU1 position signal. If the answer is yes then there is no need to retransmit the primary unit PU 1 position signal and operation passes to decision step 42. If the answer is no then the monitoring unit 20 retransmits the position signal for the primary unit PU I in the monitoring unit 20 assigned time slot at step 44.

Similarly in decision step 42 the monitoring unit 20 decides whether it has received the primary unit PU2 position signal directly from the primary unit PU2, i.e. whether it received the signal in time slot 1. If the answer is no then the monitoring unit 20 transmits its own position signal in its assigned time slot at step 45. It is unable to retransmit the position signal from the primary unit PU2. If the answer is yes, then operation passes to decision step 46.

In step 46 the monitoring unit 20 determines whether it has received the primary unit PU2 position signal in the time slot allocated to another monitoring unit 20, previously in that same transmission cycle, i.e. whether another monitoring unit has already retransmitted the primary unit PU2 position signal. If the answer is yes then there is no need to retransmit the primary unit PU2 position signal and the monitoring unit 20 transmits its own position signal in step 45. If the answer is no then the monitoring unit 20 retransmits the position signal for the primary unit PU2 in the monitoring unit 20 assigned time slot in step 47. In dynamic time slot 2 (time slot 9) the monitoring unit 20 will transmit its own position signal at step 48.

If the monitoring unit 20 retransmits the primary unit PU 1 position signal in its own assigned time slot at step 44 then operation then passes to decision step 49. In step 49 the monitoring unit determines whether it has received the primary unit PU2 position signal directly from the primary unit PU2, i.e. whether it received the signal in time slot 1. If the answer is no then the monitoring unit 20 transmits its own position signal in dynamic time slot 1 at step 50 and the process ends for that transmission cycle. It is unable to retransmit the position signal from the primary unit PU2. If the answer is yes, then operation passes to step 51. In step 51 the operation is halted until the start of dynamic time slot 1 before operation passes to decision step 52.

In step 52 the monitoring unit 20 determines whether it has received the primary unit PU2 position signal in the time slot allocated to another monitoring unit 20, previously in that same transmission cycle, i.e. whether another monitoring unit has already retransmitted the primary unit PU2 position signal. If the answer is yes then there is no need to retransmit the primary unit PU2 position signal and the monitoring unit 20 transmits its own position signal in dynamic time slot 1 at step 50.

If the answer is no then the monitoring unit 20 retransmits the primary unit PU2 position signal in dynamic time slot 1 at step 53. In dynamic time slot 2 (time slot 9) the monitoring unit 20 will transmit its own position signal at step 54.

It will be readily appreciated from Figure 4 that a single monitoring unit 20 may retransmit the position signal relating to more than one primary unit 10 and use more than one dynamic time slot.

Table 5 further illustrates this decision making process for a position tracking system having two primary units 10. In the first line of the table the X symbols indicate that if a monitoring unit 20 has not received either primary unit 10 position signal in the primary unit 1 0 time slots then regardless of what it receives in the preceding monitoring unit 20 time slots it will not retransmit any position signal and will transmit its own position signal in its assigned time slot. As described above, if more monitoring units 20 are required than there are available time slots for in a single transmission cycle at least one further transmission cycle may be added. It may be that both primary units 10 will be assigned a time slot at the start of each transmission cycle. Alternatively, each primary unit 10 may be assigned a single time slot at the start of only one of the two transmission cycles.

PU position signal PU position signal Position Position Position heard in PU slots (0 heard in preceding signal sent signal sent signal sent and 1)? MU time slots (2 to 7 in own time Dynamic Dynamic ___________ ___________ inclusive) slot time slot 1 time slot 2 --X X Owndata -- PUI ---PUI Own Data - PU1 -PU1 -Own data -- PU1 --PU2 PU1 Own data - PU1 -PU1 PU2 Own Data -- -PU2 --PU2 -Own data -PU2 PUI -PU2 -Own data -PU2 -PU2 Own data -- -PU2 PU! PU2 Own data --PU 1 PU2 --PU I PU2 Own data PU1 PU2 PU1 -PU2 -Own data PU1 PU2 -PU2 PU1 Own data - PUI PU2 PUI PU2 Own Data j --Table 5: Decision table for a monitoring unit to determine whether to retransmit a position signal indicative of the position of a primary unit for a position tracking system having two primary units.

Table 6 illustrates the results of this decision making process within a single transmission cycle for a position tracking system having two primary units and six monitoring units. In this example the position signals transmitted by the two primary units PU1, PU2 is only received by a small subset of the monitoring units MU 1-6.

The example demonstrates how the monitoring units retransmit on the position signals and how they make the decisions as to which position signals to retransmit.

Time Time Transmitting unit slot 0 0-lOOmS PU1 transmits its data. MU3 is the only MU to receive the data 1 1 00-200mS PU2 transmits its data. MU5 and MU6 receive the data 2 200-300mS MU1 hasn't received PU data in slots 0 or 1 and has not heard any PU in an MU time slot. It transmits its own data.

3 300-400mS MU2 hasn't received data in slots 1 or 2 and hasn't heard any PU in an MU time slot. It transmits its own Data.

4 400-500mS MU3 received PU1 data in slot 0 and hasn't received a PU data in an MU time slot. It transmits the data received from PUI.

500-600mS MU4 has not received data in slots 0 or 1 but has received PU1 data in an MU time slot. It transmits its own data.

6 600-700mS MU5 has received PU2 data in time slot 1 and PUI data in an MU time slot (4). It transmits the data received from PU2.

7 700-800mS MU6 has received PU2 data in time slot 1, PU1 data in slot 4 and PU2 data in time slot 6. It transmits its own data.

8 800-900mS Dynamic time slot. MU3 transmits its own ID and position because it sacrificed its own data time slot to retransmit PU 1 data.

9 900-1 000mS Second dynamic time slot. MU5 transmits its own ID and position because it sacrificed its own data time slot to retransmit PU2 data.

Table 6: Time line of a single transmission cycle indicating what position signal each monitoring unit transmits for a position tracking system having two primary units.

For certain embodiments of the invention, in the event that the primary unit, or one of the primary units, loses the GPS signal, such that the primary unit can no longer transmit its position, the system reverts to a fall back mode of operation. In the fall back mode, the primary unit continues to transmit a signal within its assigned time slot. Each monitoring unit that is synchronised to the transmission cycle can determine the time that it takes for the signal to travel from the primary unit to the monitoring unit. This provides an indication of the distance from the primary unit to that monitoring unit. This distance information can be retransmitted to the other monitoring units either separately or along with the position information for that monitoring unit, allowing each monitoring unit to estimate the position of the primary unit. Even if the primary unit loses synchronisation with the transmission cycle, such that the time for the signal to travel from the primary unit to each monitoring unit caimot be established, the relative times of arrival of the signal from the primary unit at each monitoring unit allows an estimate of the position of the primary unit to be made.

in one embodiment of the present invention, at least one monitoring unit 20 further comprises a user interface of the position tracking system. For instance, this may comprise a hand held controller e.g. a laptop PC or PDA running a mapping package designed to graphically illustrate the relative positions of the primary units 10 and the monitoring units 20. The controller may also display the absolute position of each primary unit or monitoring unit 20.

The primary unit transmitter ii and the monitoring unit transceiver 21 may be any form of communications link (e.g. RF) known in the art. They may, for instance, comprise VHF/UHF transmitters and transceivers respectively. The operation of the position tracking system is designed to be independent of the precise communications link chosen. Furthermore, at least one monitoring unit 20 may comprise a fixed transceiver site, which may in turn be arranged to forward received position signals to other monitoring units over the same communications link or over a separate communications link.

In alternative embodiments of the present invention each primary unit 10 may further comprise a receiver arranged to receive signals from one or more monitoring units 20. The primary units 10 may be arranged to store received signals in memory 17 or use the signal to actuate connected devices.

The memory 17, 26 provided within each primary unit 10 or monitoring unit may be used to store a log of position data relating to that unit, and in the case of monitoring units 20 received position data from other units.

Further objects and advantages of the present invention will be readily apparent to the appropriately skilled person without departing from the scope of the appended claims.

Claims

CLAIMS

I. A position tracking system comprising: a primary unit comprising signal transmission means and assigned a respective time slot within a TDMA transmission cycle comprising a plurality of time slots, the primary unit being adapted to transmit a position signal indicative of its position during its assigned time slot; and at least two monitoring units, each monitoring unit comprising respective signal transmission means and respective signal receiving means and being assigned a further respective time slot in the transmission cycle, each monitoring unit being adapted to receive position signals from the primary unit and from the other monitoring unit or units; wherein each monitoring unit is further arranged to monitor the received position signals and, within a particular transmission cycle, to: transmit in its assigned time slot a position signal indicative of the position of the primary unit in the event that the monitoring unit has received a position signal from the primary unit in the primary unit's assigned time slot but has not received a position signal indicative of the position of the primary unit from another monitoring unit in any other preceding time slot in that same transmission cycle.

2. A position tracking system in accordance with claim 1, wherein each monitoring unit is further arranged, within said particular transmission cycle, to transmit in its assigned slot a position signal indicative of its own position in the event that it has not received a position signal from the primary unit in the primary unit's assigned time slot and in the event that it has received a position signal from the primary unit in the primary unit's assigned time slot and has received a position signal indicative of the position of the primary unit from another monitoring unit in any other preceding time slot in that same transmission cycle.

3. A position tracking system in accordance with claim 1 or claim 2, wherein the primary unit and each monitoring unit are further arranged to transmit an identifier signal along with each position signal, and each monitoring unit is further arranged to receive identifier signals from the primary unit and the other monitoring unit or units.

4. A position tracking system in accordance with any one of the preceding claims, comprising at least two primary units, each monitoring unit being arranged to receive position signals from each primary unit and from the other monitoring unit or units.

5. A position tracking system in accordance with any one of the preceding claims, wherein the transmission cycle further comprises at least one dynamically assigned time slot and each monitoring unit is further arranged to transmit in the or one of the dynamically assigned time slots a position signal indicative of its own position in the event that it transmits in its assigned time slot a position signal indicative of the position of a primary unit.

6. A position tracking system in accordance with any one of the preceding claims, wherein each time slot within a particular transmission cycle assigned to a primary unit is earlier than the time slots assigned to the monitoring units in that same transmission cycle.

7. A position tracking system in accordance with any one of the preceding claims, wherein each dynamically assigned time slot within a particular transmission cycle is later than the time slots assigned to the monitoring units in that same transmission cycle.

8. A position tracking system in accordance with any one of the preceding claims, wherein the primary and monitoring units are arranged to operate according to a TDMA system comprising at least two said transmission cycles arranged to repeat sequentially in time, each primary unit and each monitoring unit being assigned a respective time slot in at least one of the transmission cycles.

9. A position tracking system in accordance with any one of the preceding claims, wherein each transmission cycle is I second long and comprises ten time slots, each time slot being 100 milliseconds long.

10. A position tracking system in accordance with any one of the preceding claims, wherein each of the primary and monitoring units is further adapted to receive a synchronisation signal and to utilise the synchronisation signal to synchronise its operation with the TDMA transmission cycle.

11. A position tracking system in accordance with claim 10, wherein the synchronisation signal is a GPS signal.

12. A position tracking system in accordance with claim 10, wherein the system further comprises a beacon adapted to transmit the synchronisation signal.

13. A position tracking system in accordance with claim 12, wherein the beacon comprises a primary unit.

14. A position tracking system in accordance with any one of claims 10 to 13, wherein at least one monitoring unit is adapted to retransmit the received synchronisation signal.

15. A position tracking system in accordance with any one of the preceding claims, wherein the or each primary unit further comprises a global positioning system receiver arranged to provide the position signal indicative of the position of that primary unit.

16. A position tracking system in accordance with any one of the preceding claims, wherein each monitoring unit further comprises a global positioning system receiver arranged to provide a position signal indicative of the position of that monitoring unit.

17. A position tracking system in accordance with claim 15 or claim 16, wherein each primary unit or monitoring unit comprising a GPS receiver is adapted to use the signal received by the global positioning receiver to synchronise to the or each transmission cycle.

18. A position tracking system in accordance with any one of the preceding claims, wherein the or each primary unit is arranged to transmit position signals at a lower power than the power at which each monitoring unit transmits position signals.

19. A position tracking system in accordance with any one of the preceding claims, wherein at least one monitoring unit further comprises a display screen arranged to graphically display the position of that monitoring unit relative to the or each primary unit.

20. A position tracking system in accordance with claim 19, wherein the display screen is further arranged to graphically display the position of that monitoring unit relative to at least one other monitoring unit.

21. A method of tracking the position of a primary unit comprising: assigning a primary unit a respective time slot within a TDMA transmission cycle comprising a plurality of time slots, the primary unit comprising signal transmission means; using the primary unit transmission means to transmit a position signal indicative of the primary unit's position during its assigned time slot; assigning at least two monitoring units further respective time slots in the transmission cycle, each monitoring unit comprising respective signal transmission means for transmitting position signals to the other monitoring unit or units and respective signal receiving means for receiving position signals from the primary unit and from the other monitoring unit or units; monitoring in each monitoring unit the position signals received by that unit; and transmitting from each monitoring unit in its assigned time slot within a particular transmission cycle, a position signal indicative of the position of the primary unit in the event that the monitoring unit has received a position signal from the primary unit in the primary unit's assigned time slot but has not received a position signal indicative of the position of the primary unit from another monitoring unit in any other preceding time slot in that same transmission cycle.

22. A method in accordance with claim 21, further comprising transmitting from each monitoring unit, in its assigned time slot within said particular transmission cycle, a position signal indicative of its own position in the event that it has not received a position signal from the primary unit in the primary unit's assigned time slot and in the event that it has received a position signal from the primary unit in the primary unit's assigned time slot and has received a position signal indicative of the position of the primary unit from another monitoring unit in any other preceding time slot in that same transmission cycle.

23. A method in accordance with any claim 21 or claim 22, further comprising: the primary unit and each monitoring unit transmitting an identifier signal along with each transmitted position signal; each monitoring unit receiving the identifier signals; and each monitoring unit identifying which primary unit or monitoring unit the position signal relates to using the identifier signal.

24. A method in accordance with any one of claims 21 to 23, wherein there are at least two primary units, further comprising each monitoring unit receiving position signals from each primary unit and from the other monitoring unit or units.

25. A method in accordance with any one of claims 21 to 24, wherein the transmission cycle further comprises at least one dynamically assigned time slot, the method further comprising each monitoring unit transmitting in the or one of the dynamically assigned time slots a position signal indicative of its own position in the event that it transmits in its assigned time slot a position signal indicative of the position of a primary unit.

26. A method in accordance with any one of claims 21 to 25, wherein assigning a primary unit a respective time slot comprises assigning a primary unit a respective time slot earlier than the time slots assigned to the monitoring units in that same transmission cycle.

27. A method in accordance with any one of claims 21 to 26, wherein the or each dynamically assigned time slot within a particular transmission cycle is later than the time slots assigned to the monitoring units in that same transmission cycle.

28. A method in accordance with any one of claims 21 to 27, further comprising operating the primary and monitoring units according to a TDMA system comprising a plurality of said transmission cycles; repeating the plurality of transmission cycles sequentially in time; and assigning each primary unit and each monitoring unit a respective time slot in at least one of the transmission cycle.

29. A method in accordance with any one of claims 21 to 28, wherein each transmission cycle is I second long and comprises ten time slots, each time slot being milliseconds long.

30. A method in accordance with any one of claims 21 to 29, further comprising: receiving at each of the primary and monitoring units a synchronisation signal; and utilising the synchronisation signal to synchronise the primary or monitoring unit to the TDMA transmission cycle.

31. A method in accordance with claim 30, wherein the synchronisation signal is a GPS signal.

32. A method in accordance with claim 30, wherein the method further comprises transmitting the synchronisation signal from a beacon.

33. A method in accordance with claim 32, wherein the beacon comprises a primary unit.

34. A method in accordance with any one of claims 30 to 33, further comprising retransmitting the synchronisation signal from at least one monitoring unit.

35. A method in accordance with any one of claims 21 to 34, further comprising providing the or each primary unit with a global positioning system receiver, the global positioning system receiver providing the position signal indicative of a position of that primary unit.

36. A method in accordance with any one of claims 21 to 35, further comprising providing each monitoring unit with a global positioning system receiver, the global positioning system receiver providing the position signal indicative of a position of that monitoring unit.

37. A method in accordance with any one of claims 21 to 36, further comprising each primary unit or monitoring unit synchronising to the or each transmission cycle using the signal received by the global positioning receiver.

38. A method in accordance with any one of claims 21 to 37, further comprising the or each primary unit transmitting position signals at a lower power than the power at which each monitoring unit transmits position signals.

39. A method in accordance with any one of claims 21 to 38, wherein at least one monitoring unit further comprises a display screen, further comprising the display screen graphically displaying the position of that monitoring unit relative the or each primary unit.

40. A method in accordance with claim 39, further comprising the display screen graphically displaying the position of that monitoring unit relative to at least one other monitoring unit.

41. A primary unit comprising signal transmission means and adapted to be assigned a respective time slot within a TDMA transmission cycle comprising a plurality of time slots, the primary unit being further adapted to transmit a position signal indicative of its position during its assigned time slot.

42. A primary unit in accordance with claim 41 adapted for use in a system or method in accordance with any one of claims 1 to 40.

43. A monitoring unit comprising signal transmission means and signal receiving means, and adapted to be assigned a respective time slot within a TDMA transmission cycle comprising a plurality of time slots, the monitoring unit being further adapted to receive position signals from a primary unit assigned a further respective time slot and from other monitoring units; wherein the monitoring unit is further arranged to monitor the received position signals and, within a particular transmission cycle, to: transmit in its assigned time slot a position signal indicative of the position of the primary unit in the event that the monitoring unit has received a position signal from the primary unit in the primary unit's assigned time slot but has not received a position signal indicative of the position of the primary unit from another monitoring unit in any other preceding time slot in that same transmission cycle.

44. A monitoring unit in accordance with claim 43 adapted for use in a system or method in accordance with any one of claims 1 to 40.

45. A position tracking system, substantially as hereinbefore described, with reference to the accompanying drawings.

46. A method of tracking the position of a primary unit, substantially as hereinbefore described, with reference to the accompanying drawings.

47. A primary unit, substantially as hereinbefore described, with reference to the accompanying drawings.

48. A monitoring unit, substantially as hereinbefore described, with reference to the accompanying drawings.