GB2155720A

GB2155720A - Vehicle location system

Info

Publication number: GB2155720A
Application number: GB08406075A
Authority: GB
Inventors: Peter Southcott
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1984-03-08
Filing date: 1984-03-08
Publication date: 1985-09-25
Also published as: AU3911685A; GB2155720B

Abstract

A transponder in response to an interrogation from a control station, gives not only its usual identification response, but also the position of the vehicle carrying the transponder. The position is derived from NAVSTAR- GPS navigational satellites. <IMAGE>

Description

SPECIFICATION Vehicle location system This invention relates to a system for locating, from a central position, suitably-equipped mobile vehicles.

A vehicle, such as an aircraft or ship, can obtain high accuracy information about its position by using an external source, such as a satellite navigational system, such as NAVSTAR-GPS. This is a useful aid to the vehicle's navigator, but as it is derived on board the vehicle the positional information is only useful for surveillance and control if it is sent from the vehicle to a control centre. This is conceptually simple as the information can be sent over a radio channel, but in proctice it is often difficult to obtain sufficient radio channels to cope with a large number of continuously and independently transmitting vehicles.

As object of the invention is to provide systems whereby mobile vehicles can be located from a central station and which exploits information derived from an external source such as a navigational satellite.

According to the invention there is provided an arrangement for determining the position of mobile vehicles, in which interrogation signals sent from a secondary radar central station to a mobile vehicle cause the emission from that vehicle of signals which include the identity of that vehicle positioned information derived from a source of navigational information outside the vehicle, so that the vehicle can be located by the central station.

In such an arrangement, the responses may resemble those from a conventional secondary radar transponder, and can thus be used, if needed, in the normal way.

Embodiments of the invention will now be described with reference to the two highly schematic diagrams attached hereto.

In many surveillance/control problems it is not necessary to obtain continuous indication of a vehicle's position and frequent sampling at discrete intervals may be adequate, the sampling rate depending on the vehicle's speed, and the purpose of the surveillance. In effect, communication with the vehicle is on a TDM basis, each of a number of targets being interrogated in sequence and then replying with its satellite-derived positional information on a radio channel common to all cooperating targets.

In the arrangement of Fig. 1, conventional interrogation signals and transponders are used with an extra transmitter. However, the preferred arrangement shown in Fig. 2 uses secondary radar transmitter additionally for sending positional information. In conventional secondary systems, the radar pulses emitted from the fixed station impinge on a co-operating vehicle and cause it to emit a group of pulses coded with the vehicle's identity, possibly plus other information. The fixed station then derives the vehicle's position in the conventional way, the advantage over primary radar being that: (a) the signals received at the fixed station are much stronger, because they originate from a transmitter and not via skin reflection; (b) An identity label is attached to the responses; (c) a data link facility can be provided.

In some versions, selective addressability on an identity, rather than positional, basis is provided.

However, accuracy is not very high.

In the arrangements to be described herein, the secondary radar interrogating signal from the control centre is used to initiate the transmission to the control centre of high-precision, locally determined, positional information, e.g. derived from a satellite navigation system. This transmission takes place either over a data link facility built in to the secondary radar signal format (Fig. 2) or, as mentioned, over a conventional oneway communications channel (Fig. 1) (e.g. VHFI UHF) which could be shared between many vehicles. It is desirable to retransmit this information in as "raw" a form as possible to eliminate any effects of corruption by malfunctioning on-board processors.

The use of one navigation system as a bearer for another, with each providing the same kind of positional information, is justified by the accuracy claimed for systems such as NAVSTAR-GPS which is much higher than that obtained with conventional secondary radar. This is of particular relevance to operations such as ground movement control at airfields, where conventional secondary radar would not be precise enough. Thus in such a system the secondary radar gives 'coarse' positional information, with the satellite derived information acting as a 'vernier' to give greater precision. This minimises the amount of information which would have to be transmitted over the data link, and this approach will be referred to in more detail later.

We now refer to Fig. 1, in which the block 1 represents the receiving equipment for the signals from the satellite system, e.g. NAVSTAR-GPS, which supplies information to the aircraft's display, auto-pilot, etc,. It also supplies this information to a coder 2 which converts the information into a coded form for application with the aircraft's identity to the aircraft's communications transmitter 3.

The interrogation request from the control centre is received at the aircraft by a receiver 4, which triggers the secondary radar transmitter 5. This emits the aircraft identity in the usual way. In addition, the receiver 4 triggers the transmitter 3, which now sends out the positional information from a memory associated with the transmitter 3.

This memory is continuously updated by the information received from the navigational satellite, so that the latest version of this information is sent out.

The system shown in Fig. 2 is simpler in that the coded positional information obtained from the coder is applied via a data link to the secondary radar transmitter 5 so that the latter emits the secondary radar response, which includes the aircraft identity with the positional information.

Here also the transmitter 5 has a memory in which the coded output from the coder is stored and which is continuously updated.

The arrangements described have dual applicability. If the target vehicle carries a high precision navigational aid, then the secondary radar interrogates it in response to the signal from the central station, obtains a high accuracy fix and ignores the secondary radar response, except, perhaps, for ambiguity resoiution -- see later. If however, the craft is not so equipped, the ground station has to be content with the secondary radar derived fix. Thus the ground secondary radar equipment remains standard and automatically works with either type of craft.

We referred above to the satellite's use as a 'vernier'. This means that whereas secondary radar gives positional information (originally in polar coordinates) to a maximum range of about 200 miles and an accuracy of (say) half a mile, NAVSTAR-GPS covers a much greater range, with accuracies, it is claimed, down to a few metres, or even better. The transmission of the complete NAVSTAR-GPS position would thus involve a very long string of figures. So if one can obtain the approximate position from the secondary radar with an accuracy, say, of one mile, one can ignore those digits of the much more precise NAVSTAR-GPSfix which refer to distances greater than a few miles.

Thus, with a "granularity" of say, about one meter, and if the secondary radar can be trusted to be correct within about two nautical miles, we only have to transit number up to about 4,000 for each co-ordinate. This number increases to 400,000 if a 200 nautical miles area had to be covered without secondary radar. The positional coder can easily be programmed as to which figures to ignore.

Claims

1. An arrangement for determining the position of mobile vehicles, in which interrogation signals sent from a secondary radar central station to a mobile vehicle cause the emission from that vehicle of signals which include the identity of that vehicle positioned information derived from a source of navigational information outside the vehicle, so that the vehicle can be located by the central station.

2. An arrangement as claimed in claim 1, and in which the control station successively polls all vehicles in the area of interest in a time divided manner, so as to receive therefrom updated information as to the positions of those vehicles.

3. An arrangement for determining the positions of mobile vehicles, substantially as described with reference to Fig. 1 or Fig. 2 of the accompanying drawing.

4. A secondary radar installation for use in a mobile vehicle, which includes first receiving means responsive to radio signals from an external source of navigational information, such as a navigational satellite, indicating means for displaying positional information derived from said radio signals to the crew of the vehicle, a secondary radar transponder responsive to an interrogation signal from a land based installation to transmit to said land based installation radio signals which identify that vehicle, and means also responsive to said interrogation signal to transmit to the land based installation part at least of the positional information derived from the first-mentioned radio signals, so that a highly accurate indication of the vehicle's position is transmitted to the land based installation.

5. An installation as claimed in claim 4, and in which the positional information is transmitted by a radio transmitter additional to that of the secondary radar transponder.

6. An installation as claimed in claim 4, and in which the positional information is transmitted by the transmitter of the secondary radar transponder.

7. A secondary radar transponder for use in a mobile vehicle, substantiaily as described with reference to Fig. 1 or Fig. 2 of the accompanying drawing.