GB2256106A - Vehicle position monitoring - Google Patents

Abstract

In a method for determining the location of a vehicle (10), a transmitter on the vehicle emits an identifying signal pulse at a random trigger rate. The signal pulse has a fast leading edge to reduce multi-path signal effects and the random trigger rate permits a large number of vehicles to be identified whilst avoiding repeated synchronous emission of identifying signal pulses. Receivers (16, 18) and associated circuitry are provided to monitor identifying signal pulses and to determine vehicle location by monitoring signal pulses received at two receivers (16,18) relative to a known path (12) of the vehicle (10) or at three receivers and by using trilateration. A method for synchronising receiver clock pulses is disclosed. <IMAGE>

Description

VEII POSITION MONITORING The present invention relates to a method and apparatus for determining the location of vehicles. In particular, the present invention relates to a method and apparatus for monitoring the position of a large number of vehicles within a predetermined area, such as an airport.

The term "vehicle" used herein will be understood as referring to not only powered and unpowered vehicles (such as cars and trailers) but also to other movable bodies which it is desired to monitor the position of.

As described in British Patent Specification GB-A-1262223 (dullard), vehicle location systems generally fall into one of two categories, that is to say either on-board navigational systems or remote vehicle location systems. In known on-board navigational systems used for vehicle fleet management, signals are transmitted to the vehicle fram each of at least three transmission sites. Equipment on-board the vehicle calculates its position relative to the transmission sites by trilateration, based on the relative delays of received signals, and then reports the vehicle position to a remote control centre via a dedicated cammunications channel.Such systems are known to have problems with accuracy, particularly in built-up areas where loss of contact occurs due to fades in the navigational signals and corrmohication channel.

With remote vehicle location systems, the vehicle emits a signal which is detected at each of at least three receiver sites. By measurement of the relative arrival times of the signal at the receiver sites the vehicle position may again be determined by trilateration.

One area in which there is a particular need for vehicle location is at airports. Up to thirty vehicles service an aircraft before its departure: failure of a vehicle to arrive when required results in a delayed departure and the loss of an airspace slot. Such delays are very costly to the airlines and, with air travel increasing and airspace slots becoming more scarce, the cost of such delays is rising.

Known on-board navigational and vehicle location systems are unsuitable for use at airports due to the large numbers of vehicles which must be monitored in a relatively small, built-up area. At london Heathrow Airport there is a need to monitor over 6000 vehicles in an area of approximately 8 square miles (20 square kilometres). With on-board navigational systems, the problems with location signal fade are exacerbated by the need to monitor vehicles within buildings such as baggage halls and hangars. Furthermore, the dedicated communication channel on which each vehicle reports its position would be swamped by so many vehicles. The-cost of equipping each vehicle with receiver, transmitter and positional determination apparatus would also be prohibitive.

In known vehicle location systems, the problems of continuously monitoring signals from each of the large number of vehicles at each of at least three receiver sites would be extensive. Also, the poor accuracy of such known systems does not make them suitable for use in areas of such high vehicle density.

In accordance with the present invention there is provided a method of determining the location of a vehicle along a known path within a predetermined area, in which the vehicle transmits an identifying signal pulse at a random trigger rate, the signal is received directly by two detectors at known locations within the said area, the time differential between receipt of the pulse at the two receivers defines a hyperbolic line relative to the two receivers, and the location of the vehicle is determined by the intersection of the hyperbolic line and the known path.

Also in accordance with the present invention there is provided a method of determining the location of a vehicle within a predetermined area, in which the vehicle transmits an identifying signal pulse at a random trigger rate, the signal is received directly by at least three detectors at known locations within the said area, and the location of the vehicle is determined by trilateration.

The identifying signal pulse, which is preferably internally modulated to carry information, may be transmitted at a trigger rate which is random within a predetermined range, that is to say a pulse may be triggered once during each of successive intervals, each pulse at a random point during the respective interval. A first random trigger rate, such as one pulse per minute, may be used when the vehicle is moving or (in the case of motor driven vehicles) when the motor is running. A second, slower, random trigger rate, such as one pulse per ten minutes, may be used when the vehicle is stationary or the motor is not running. The identifying signal pulse preferably has a fast leading edge and may be modulated to carry additional information relating to the vehicle.

A random trigger rate of one pulse per minute may be provided by successive pulses being spaced by one minute subject to a random variation of plus or minus five to ten percent. Thus, with a random variation of plus or minus ten percent, successive signals may be spaced by as little as 48 seconds or as much as 72 seconds. Over a long period however, the pulse spacing will average out at one minute. Alternatively, a random trigger rate of one pulse per minute may result from pulses being triggered during successive one minute intervals, each at a random point during the interval. A random trigger rate of one pulse per ten minutes may be provided in a similar manner.

Also in accordance with the present invention there is provided a transmitter for use in a method of determining the location of a vehicle, comprising circuitry operable to generate a signal pulse at a random trigger rate, to modulate the signal pulse and to transmit it, and means for attachment of the said circuitry to a vehicle.

In accordance with the present invention there is provided receiver apparatus for use in a method of vehicle location, comprising two spaced apart detectors, each detector being operable to receive a transmitted identifying signal pulse, storage means storing coordinates of a known path, and calculation circuitry to which each detector is connected, the calculation circuitry being operable to determine the point along the known path of transmission of the received transmitted identifying signal pulse from the time differential between receipt of the said pulse at the two detectors.

Further in accordance with the present invention there is provided receiver apparatus for use in a method of determining the location of a vehicle, comprising three or more spaced apart detectors, each detector being operable to receive a transmitted identifying signal pulse, and calculation circuitry to which each detector is connected, the calculation circuitry being operable to determine the point of transmission of the received transmitted identifying signal pulse by trilateration.

The applicants have appreciated that a number of benefits may be obtained by the use of a random self-triggered identifying pulse, particularly one having a fast leading edge. Firstly, there is no requirement for each vehicle to carry on-board apparatus for determining its own position and for reporting this back to a central station, and no requirement for receiver circuitry on board the vehicle or transmitters at fixed sites. Accordingly, the cost of the circuitry required per vehicle is kept low - an important consideration where it is desired to so equip several thousand vehicles.

Secondly, the use of random self-triggering avoids repeated synchronous emissions from several vehicles which would result in mutual interference at the receivers.

Thirdly, the use of pulses with fast leading edges minimizes the effects of multipath on the measured arrival time of a pulse at a detector. In a crowded environment, such as at an airport terminal, the problems of multiple reflected signals arriving at the receiver or detector would be substantial if signals having long rise times were used. The use of internally phase modulated signals also helps to overcome the problems of multipath.

One particular preferred embodiment of the present invention will now be described, by way of example only, with reference to a vehicle location system for use in monitoring the movement of vehicles at airports, and with reference to the accompanying drawings in which: Figure 1 schematically illustrates a first embodiment of the method of vehicle location where the vehicle is travelling along a known path; and Figure 2 schematically illustrates a second embodiment of the method of vehicle location, using trilateration.

At an airport, many types of vehicles are required, including baggage tugs, motorised and unmotorised aircraft steps, mobile conveyors, aircraft tugs, catering vehicles, passenger and crew buses and cargo vans. In order to ensure that aircraft are not delayed, it is important to know the location of each of these vehicles at all times.

Each vehicle to be tracked carries a transmitter which emits a pulse at a random trigger rate of once per minute, if the vehicle is moving, or once per ten minutes, if the vehicle is parked. Unlike known SSR (secondary surveilance radar) systems, there is no interrogation system to activate a transponder and hence the size and complexity of apparatus carried by each vehicle is minimised.

The transmitted pulse contains internal modulation to convey the individual indentity of the vehicle, together with other information relating to the status of the vehicle.

Differential phase shift keying is used to modulate the pulse to reduce the effects of multipath on the data carried.

Vehicle location is determined by multiple receivers provided around the airport, and by using one of two methods, schematically represented in Figures 1 and 2 repectively. In Figure 1, a vehicle 10 equipped with a transmitter travels along a known path 12 which may, for example, be an airport perimeter road. The transmitted identifying signal pulse (shown schematically by lines 14 radiating from the vehicle 10) is received at each of two receivers 16, 18 at known positions relative to the known path 12 and the area as a whole. The time differential between arrival of the pulse at each receiver allows a hyperbolic line 20 to be plotted, the hyperbolic line 20 defining a path on which the vehicle 10 lies. The point of intersection of the known path 12 and the hyperbolic line 20 locates the vehicle.

The second method, shown in Figure 2 uses trilateration to determine the position of a vehicle relative to the (known) positions of at least three receivers 16, 18, 22. As with the first method, a hyperbolic line 20 is plotted relative to the first and second receivers 16,18. A second hyperbolic line 24 is then plotted relative to the first and third receivers 16, 22 or (not shown) relative to the second and third receivers 18, 22. The point of intersection of the two hyperbolic lines 20, 24 locates the vehicle 10.

When using the method of Figure 2, it may occur that the transmitted pulse is only received at two 16, 18 of the three transmitters 16, 18, 22 possibly due to a second vehicle or other obstruction blocking the path of the signal. When this occurs, the previously determined location for the vehicle and the direction in which it is travelling are used to plot a path along which the vehicle lies. The plotted path corresponds to the known path 12 of the first method and hence the vehicle location may be determined using the first method.

In an extreme case, where the path of the signal to all but one receiver is blocked, the location of the vehicle cannot be determined more accurately than the area covered by that one receiver. When such a condition occurs, the strength of the received signal may be used to determine the distance of the vehicle from the receiver.

The location of an identified vehicle 10 is calculated in a first computer which then passes the information to a central computer which may add further data relating to the identified vehicle, either by having the additional data keyed in or by calling it up from a back-up store. A local area network, such as Ethernet, then makes the data available wherever required around the airport.

A suitable specification for the transmitter on each vehicle would be as shown in Table 1 below.

Table 1 Frequency: 2450 MHz crystal controlled.

Transmitter: 0.1 W peak Transmitter pulse width: 10uS.

Transmission rate: Moving - 1 pulse per minute.

Parked - 1 pulse per 10 mins.

Mdulation: Binary phase shift keying.

32 bits spaced 0.25uS.

MdLlation source: 16 bits internal switch for identity.

1 bit internal indicating battery state.

1 bit external indicating vehicle ignition on.

9 bits external unassigned.

5 bits internal parity.

Antenna: Integral.

mounting: On vehicle roof using screws or clamps.

Some could have magnetic clamps for temporary mounting.

Construction: Rugged, likely to be knocked.

Power Source: External - vehicle 12v battery.

Internal re-chargeable.

The 16 bit identity number allows individual identification of up to 65536 vehicles. The 9 bits unassigned may be used to carry further information relating to the vehicle, to avoid the need for the central computer to call this information from a back-up store. The 5 bits of internal parity allow use of a Hamming code to provide single bit error correction.

Phase modulation, at a bit rate of 4MHz, of an RF pulse is used to reduce corruption of signals due to the effects of multipath. The 4MHz bit rate is the same as that used by the Mode S SSR systems currently used to locate aircraft on the ground and commonality of components may permit a reduction in system installation costs.

Where the vehicle is not in use or powered, an emission rate of one pulse in ten minutes is sufficient to confirm its location. The mean transmitter power in this instance is 0.002uW so that battery drain (whether an internal battery or the vehicle battery) is low.

For a powered vehicle in service, which may be indicated by the vehicle ignition being switched on, an emission rate of once per minute is used. In this case the mean transmitter power is 0.02uW.

A pulse risetime of 100ns permits the pulse position to be measured to an accuracy of +/- iOns which corresponds to a positional accuracy of about 3 metres in distance. The arrival time of the pulse is measured using a clock frequency of about 100MEz with the clocks of all receivers synchronised, as will be described in greater detail below. The position of the pulse leading edge is detected by differentiation techniques to identify the time of steepest rise.

As will be appreciated, a wicker pulse risetime and a higher clock frequency will each improve the positional accuracy, although this may be at the expense of increased costs for transmitter and receiver equipment.

The receiving antennae are positioned at multiple sites around the airport. To enable accurate positional determination by trilateration, sufficient receivers are provided that any location where a vehicle may be found is in direct line of sight with at least two antennae for the first method or at least three antennae for the second. As will be appreciated, open areas can be sufficiently monitored by relatively few receiving antennae, whilst additional receivers will be required in built-up areas such as around terminal buildings, baggage halls and hangars.

The system may be extended to cover indoor portions of the airport such as baggage halls, with one or more receivers being located within the hall. It is generally the case that, where the building is relatively small, only a single receiver is required to detect the presence and identity of vehicles within the building. The signals received by such a single receiver are combined with information identifying the location of the building, thereby providing a location for those vehicles identified within the building, which location and vehicle identities are then passed to the central computer in the same way as the location and vehicle identities determined by the first and second methods.

To calibrate (synchronise) the receiver clocks, a number of fixed "vehicles" - stationary transmitters at known locations - are used. These fixed transmitters emit pulses at a fixed rate which may suitably be once per second. Since the position of the fixed transmitters is known, the differential reception time of the transmitted pulse at the receivers will also be known and hence the clocks may be synchronised. With a clock frequency of 100MHz, recalibration once per second will permit a frequency accuracy of 0.01 ppm for the clock frequency to be in error by no more than one cycle.

The fixed clock frequency of 100MHz is generated by a temperature stabilized crystal oscillator. In extremes of ambient temperature, the frequency accuracy of the crystal oscillator may worsen, with a corresponding drop in the accuracy of vehicle position determination, although the size of the drop is likely to be small.

The present invention provides a low cost system of monitoring the position of a large number of vehicles, due to the simple randomly triggered transmitters used on each vehicle and by avoiding the need for on-board receivers and position determining equipment. Good coverage is achieved by virtue of the vehicles continuing to transmit, even when in a totally closed room. No dedicated communication channel is required with each vehicle and the system as a whole is resistant to multipath errors due to the use of pulses with fast leading edges which should be detected by the receivers ahead of later arriving reflected signals.

Claims (21)

1. A method of determining the location of a vehicle along a known path within a predetermined area, in which the vehicle transmits an identifying signal pulse at a random trigger rate, the signal is received directly by two detectors at known locations within the said area, the time differential between receipt of the pulse at the two receivers defines a hyperbolic line relative to the two receivers, and the location of the vehicle is determined by the intersection of the hyperbolic line and the known path.

2. A method according to claim 1, in which the known path is determined by the direction in which the vehicle is travelling away from a known location.

3. A method of determining the location of a vehicle within a predetermined area, in which the vehicle transmits an identifying signal pulse at a random trigger rate, the signal is received directly by at least three detectors at known locations within the said area, and the location of the vehicle is determined by trilateration.

4. A method according to claim 1, 2 or 3, in which the identifying signal pulse is transmitted at a first random trigger rate when the vehicle is moving and at a second random trigger rate when the vehicle is stationary.

5. A method according to claim 1, 2 or 3, in which, where the vehicle is a motor driven vehicle, the identifying signal pulse is transmitted at a first random trigger rate when the vehicle motor is running and at a second random trigger rate when the motor vehicle motor is not running.

6. A method according to claim 4 or claim 5 in which the first random trigger rate is one pulse per minute.

7. A method according to claim 4, 5 or 6 in which the second random trigger rate is one pulse per ten minutes.

8. A method according to any preceding claim in which the identifying signal pulse is modulated to carry information relating to the vehicle.

9. A method according to claim 8 in which the identifying signal pulse is modulated by differential phase shift keying.

10. A method according to any preceding claim in which the identifying pulse has a fast leading edge with a risetime of about 100ns or less.

11. A method according to any preceding claim in which each detector measures the time of arrival of received identifying signal pulses using a clock frequency of about 100MHz or greater.

12. A method according to any preceding claim in which each detector measures the time of arrival of a received identifying signal using substantially the same clock frequency, the clock frequency being synchronised between all detectors.

13. A method according to claim 12, in which the clock frequency is synchronised by use of a transmitter at a known position transmitting a calibrating signal pulse at a predetermined rate.

14. A method according to claim 13, in which the calibrating signal pulse is transmitted once per second.

15. A transmitter for use in a method according to claim 1 or claim 3, comprising circuitry operable to generate a signal pulse at a random trigger rate, to modulate the signal pulse and to transmit it, and means for attachment of the said circuitry to a vehicle.

16. apparatus according to claim 15, further comprising monitoring circuitry operable to obtain information relating to a vehicle to which the apparatus is attached, and to modulate the signal pulse to carry the obtained information.

17. Receiver apparatus for use in a method according to claim 1, comprising two spaced apart detectors, each detector being operable to receive a transmitted identifying signal pulse, storage means storing coordinates of a known path, and calculation circuitry to which each detector is connected, the calculation circuitry being operable to determine the point along the known path of transmission of the received transmitted identifying signal pulse from the time differential between receipt of the said pulse at the two detectors.

18. Receiver apparatus for use in a method according to claim 3 comprising three or more spaced apart detectors, each detector being operable to receive a transmitted identifying signal pulse, and calculation circuitry to which each detector is connected, the calculation circuitry being operable to determine the point of transmission of the received transmitted identifying signal pulse by trilateration.

19. A method of determining the location of a vehicle substantially as hereinbefore described.

20. A transmitter for use in a method of determining the location of a vehicle substantially as hereinbefore described.

21. Receiver apparatus for use in a method of determining the location of a vehicle substantially as hereinbefore described.