(54) IMPROVEMENTS IN OR RELATING TO SYSTEMS TO ASSIST IN THELOCATION OF REMOTE RADIO TRANSMITTERS(71) We, THE MARCONI COMPANYLIMITED, of Marconi House, New Street,Chelmsford, CMl 1PL, Essex, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:This invention relates to systems to assist in the location of remote radio transmitters. Such a system may determine the range, or some other location parameter, of a remote radio transmitter. This determined information may be used with other information, for example, the bearing of the remote transmitter, to enable the position of the transmitter to be located.
The invention is based on the realisation that if the aerial of a radio transmitter operating at a frequency in, say, the H.F., V.H.F., or U.H.F.
band lies in a relatively strong radiation field from a high power microwave transmitter, cross-modulation may occur with the result that the radio signal actually transmitted has superimposed on it any modulation of the microwave radiation. Such cross-modulation may occur if both the radio signal required to be transmitted and the microwave signal picked up by the aerial are applied to a nonlinear impedance, for example the output stage of the transmitter. Alternatively cross-modulation may be produced in the transmitter aerial structure or neighbouring metalwork due to nonQinearity of the electric and/or magnetic characteristics of the metal used. The level of such crosslnodulation is, of course, usually very low.
According to one aspect of the present invention, a system to assist in the location of a remote radio transmitter comprises a radio transmitting system which is adapted to transmit modulated radio signals, a radio receiving system that is arranged to receive radio signals transmitted by the remote transmitter, and means to determine a measure of the sum of the distances from the radio transmitting system to the remote transmitter and from the remote transmitter to the radio receiving system by time correlating the modulation trans mitted by the radio transmitting system with the same modulation superimposed on the radio signals received by said receiving system from the remote transmitter.
If the radio transmitting system and the radio receiving system are at the same location, the time correlation gives a measure of twice the range of the remote transmitter from that location.
According to another aspect of the present invention, a system to assist in the location of a remote radio transmitter comprises a first radio receiving system for receiving modulated radio signals transmitted by another remote radio transmitter, a second radio receiving system for receiving radio signals from the remote radio transmitter that is to be located, and means to determine a location parameter of the latter transmitter by tirne correlating the modulation received by the first radio receiving system with the same modulation superimposed on the radio signals received by the second radio receiving system.
Examples of systems in accordance with the present invention will now be described with reference to the two figures of the accompanying drawing which show two systems respectively.
Refering now to Figure 1 of the accompanying drawing, the first system to be described comprises a microwave radio transmitting system 1, for example having an operating frequency of l000MHz, which has a narrow beam steerable aerial 2. The transmitted carrier is amplitude (or otherwise) modulated by a pulse or other signal. During use the radiated beam is periodically scanned over the area in which lies a remote transmitter 3 that is to be located.
The system also includes a radio receiving system 4 which is located at a distance from the transmitting system 1 and which is tuned to the frequency of the transmitter 3. A correlator 5, which may conveniently be formed by a suitably programmed general purpose digital computer,is arranged to effect time correlation of themodulation transmitted by the system 1 andthe cross-modulation superimposed on to theradio signals transmitted by the system 3.
When the correlator 5 establishes time correlation, the information supplied thereby is ameasure of the time taken for radio waves totravel from the transmitting system 1 to the ) remote transmitter 3 and from the transmitter3 to the receiving system 4 i.e. the sum of thepath lengths L1 and L2. It follows thereforethat the remote transmitter 3 lies on an ellipse6, its major axis passing through the locationsof the systems 1 and 4. Furthermore, whencorrelation is established, the direction inwhich the aerial 2 is then pointing gives a bearing of the remote transmitter 3.
The aerial 7 of the radio receiving system 4 may also be directional, but less so than theaerial 2 of the transmitting system 1 ,and maybe operated so that, at any time, it is pointingin the same general direction as the aerial 2.
From the information obtained as aforesaid, the position of the transmitter 3 may readily bedetermined, for example by a computer.
It is, of course, desirable for the fieldstrength of the radiation from the radio transmitting system 1 to be as high as possible in theI region of the remote transmitter 3. To this endit is convenient for the radio transmitting system 1 to be the transmitting end of a pulsedradar. Since however the operation of time correlation takes a relatively long time to carryout, the rate of scan is preferably rather lowerthan that utilised in its alternative radar modeof operation. Furthermore the pulse recurrencefrequency is preferably not greater than 7,500 per second so that measurements of the rangeof a remote transmitter can be made up to 20kilometres without ambiguity.
Instead of utilising the modulation of a cooperating radio transmitter system (from whichthe modulation signal is directly available) tocross-modulate the radiated signal of the remote transmitter, some other radio transmitting system to which there is no direct accessmay be used. Such an arrangement will now bedescribed with reference to Figure 2 of theaccompanying drawing (which utilises the samereference numbers as Figure 1 for corresponding items.)Referring now to Figure 2, the system forlocating the remote transmitter 3 again includesa radio transmitting system 1 which may, forexample, be provided by a pulsed radar that isperiodically scanning the area in which thetransmitter 3 is located.In this case, in additionto the receiving system 4 that responds to thesignal radiated by the transmitter 3 and carrying the cross-modulation, a second radio receiving system 8 is provided at the same location topick up directly the signal transmitted by theradar or other radio transmission system 1. Thereceiving system 8 thus derives a modulation signal in which the cross-modulation signal supplied by the receiving system 4 is time correlated by means of the correlator 5.
In the arrangement described in the last paragraph, the result of effecting the time correlation is to determine a measure of the distance from the radar 1, say, to the transmitter 3 plus the distance from that transmitter to the location of the two receiving systems 4 and 8 less the distance from the radar 1 to said location. This identifies a particular ellipse on which the transmitter 3 lies, this ellipse having a major axis passing through the location of the systems 4 and 8 and the location of the radar 1.
If now the two radio receiving systems are duplicated at another location, a second ellipse on which the transmitter 3 lies can be determined in similar manner and the intersection of the two ellipses gives the position of the transmitter 3. More generally, if the location of the radar 1 is not otherwise known, the information available by time correlating the crossmodulation received at each location with the corresponding modulation directly received from the radar 1, comparison of the time receipt of the radar transmission at the two locations and, possibly, the bearing of the radar l from one of the locations, the positions of both the radar 1 and the transmitter 3 may be determined. A digital computer is preferably utilised for carrying out the necessary calculations.
WHAT WE CLAIM IS:1. A system to assist in the location of a remote radio transmitter comprising a radio transmitting system which is adapted to transmit modulated radio signals, a radio receiving system that is arranged to receive radio signals transmitted by the remote transmitter, and means to determine the measure of the sum of the distances from the radio transmitting system to the remote transmitter and from the remote transmitter to the radio receiving system by time correlating the modulation transmitted by the radio transmitting system with the same modulation superimposed on the radio signals received by said receiving system from the remote transmitter.
2. A location system according to Claim 1 wherein the radio transmitting system and the radio receiving system are at the same location.
3. A location system according to Claim 1 or Claim 2 wherein the radio transmission system has a narrow beam steerable aerial which is arranged periodically to scan the area in which the remote transmiiter lies.
4. A location system according to Claim 3 wherein a computer is arranged to determine the position of the remote transmitter utilising the time correlation information obtained as aforesaid and the bearing of the steerable aerial of the transmission system at the time of correlation.
5. A location system according to any preceeding claim wherein said radio transmitting
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