Patent GB-1296601-A (GB)
IPC: G01S13/78
Filed: 1970-11-13
Abstract: 1296601 Road vehicle radar STANDARD TELEPHONES & CABLES Ltd 13 Nov 1970 54098/70 Heading H4D A road vehicle radar comprises an interrogation unit including means transmitting a sequence of at least two pulses of predetermined amplitude and spacing on a first carrier frequency and a further pulse of predetermined amplitude on a second carrier frequency at a fixed time from the first of said first frequency pulses and a response unit including means for receiving signals of said first frequency; logic responsive to pulses therefrom when the time interval pattern corresponds to a predetermined pattern; means for receiving signals on said second frequency; means for comparing the amplitude of the first and second frequency signals; means for initiating the transmission of a response on the second frequency only in response to a signal from the logic in conjunction with a signal from said comparing means, wherein the response signal is transmitted a fixed time after the receipt of the first pulse of the first carrier frequency. The system, Fig. 4, includes a controller 101 which initially holds switch 102 as shown, generator 104 produces a pulse pair whose spacing, e.g. T o + + 2t a  indicates the lane in which the vehicle is travelling. These signals pass via switch 103 to the primary radar transmitter 106 and aerial 107 which is bi-directional. After a time T c , switch 103 changes over and a third, cancel, pulse passes to the secondary radar transmitter 109 and transmitting aerial which points backwards only. In a receiving vehicle received signals at the primary frequency are detected and passed to a process unit 115. The secondary frequency signals are received by a forward pointing aerial 121 and are passed to amplifier 117 whose gain varies in a sawtooth of period equal to the secondary transmitter period, 800 ns. The gain is also controlled by the output of a detector 116 which provides a signal representing the intensity of the primary frequency signals. If this is small compared to the secondary frequency signals the cancel pulse is passed to switch 124, normally in the position shown. In the process unit, Fig. 5, the primary frequency pulse pair is applied to AND gates 201-206 directly and through delay lines each representing the code of a particular lane. An output from any one of these gates indicates interrogation and bistable 218 is set inhibiting generator 104 for 10 ms. Switch SW applies an enabling signal to that one of AND gates 207-212 connected to the output of the AND gate representing the lane in which the receiving vehicle is travelling. If the interrogating vehicle is in the same lane bi-stable 219 is set. The received pulse pair is also passed through a delay T c  and the first pulse compared with the secondary receiver output. If a cancel pulse has been received which is large compared to the primary signals indicating the receiving vehicle is behind the interrogating vehicle and 11 delayed by Tc from the first thereof gate 217 is inhibited and no response is initiated. Otherwise bi-stable 220 is set and controller 101 in response thereto changes over switch 102 and generator 125 generates a response signal which is transmitted by the secondary radar. The response comprises a number of pulses, e.g. 15, the first always being present to enable range measurement and spaced from the first received primary signal by a fixed time. Subsequent pulses are at 800 ns. intervals and are present or not according to the response code. At the interrogating vehicle switch 124 is changed after the transmission of the interrogating signal and the received response pulses are gated by AND gate 126 to a store 127.