Patent GB-1429350-A (GB)
Title: Range finding systems and components thereof
IPC: G01S13/87
Filed: 1973-04-16
Abstract: 1429350 Acoustic position finding; radio position finding FRANCE ARMED FORCES MINISTER OF 16 April 1973 18303/73 Heading H4D Relates to a secondary radar or sonar system for determining at an interrogator-equipped first vehicle (e.g. a vessel): (i) the distances d1 and d2 of said vehicle from at least two fixed spaced responders R1, R2 of unknown separation d1,2 which is also determined at the vehicle; and (ii) the distances d1,3 and d2,3 from said responders R1 and R2 of a second vehicle carrying a responder R3. The system also allows measurement at the second vehicle of: (i) its distances d1,3 and d2,3 from the responders R1, R2; and (ii) the position of the first vehicle relative to the responders R1, R2 and the second vehicle expressed as two path differences (d3 - d1) and (d3 - d2) where d3 is the distance of R3 from the first vehicle. The resultant distance information can be converted into corresponding position co-ordinates by a known form of coordinate computer. As described each responder Ri is characterized by a binary frequency address code F(Ri) such that 2<SP>n</SP> responders can be uniquely characterized by n different carrier frequencies f1...fn, the responders being such that when the interrogator transmits an interrogation signal having frequency components Fg, Fo and F(Ri) then the corresponding responder Ri transmits a reply signal of frequency Fg and all the other responders transmit their characteristic frequency codes; in addition any responder Ri transmits its characteristic frequency code F(Ri) in response to the reception of a signal from any source which comprises the component Fg without the component Fo. In operation the interrogator first transmits a frequency code [Fg, Fo, F(Ri)] so that the responder R1 transmits a reply signal Fg which (i) is received back at the interrogator to give d1 and (ii) initiates transmission from R2 of a signal F(R2) which is received at the interrogator to give (d1+d1,2+d2). The interrogatothen transmits a frequency code [Fg, Fo, F(R2)] which provides measurement of d2 and (d2+d1,2+d1) so that d1, d2 and d1,2 can be determined from these two interrogations. In order to locate the second vehicle carrying the responder R3, the interrogator transmits a frequency code [Fg, Fo, F(R3)] and the resultant reply signal Fg from R3 (i) is received back at the interrogator to give d3 and (ii) initiates transmissions from R1 and R2 of respective reply signals F(R1) and F(R2) which are received at the interrogator to give (d3+d1,3+ d1) and (d3+d2,3+d2) from which d1,3 and d2,3 can be derived since d1 and d2 have already been determined. The reply signals F(R1) and F(R2) from R1 and R2 are also received at R3 thereby giving d1,3 and d2,3 at the second vehicle; the interrogation signal [Fg, Fo, F(R3)] also initiates a different set of reply signals F(R1) and F(R2) from R1 and R2 so that the time intervals between reception at R3 of said different reply signals F(R1), F(R2) and the interrogation signal [Fg, Fo, F(R3)] gives the path differences (d1+d1,3-d3) and (d2+d2,3-d3) from which the required path differences (d3-d1) and (d3-d2) can be derived. In an acoustic system using submerged responders, the interrogator may be additionally frequency coded to (i) cause release or launch of a selected responder from a carrier vessel and (ii) cause a selected responder to transmit a reply code indicating its depth of immersion, said depth signals being used to convert the measured slant distances into corresponding horizontal (plan) distances. In the Specification the various code combinations are given in tabular form. An acoustic transmitter/receiver which can be used as the interrogator/receiver or (with some modification) as a fixed or mobile responder is illustrated in the single Figure accompanying the Specification.