Patent GB-887864-A (GB)
Title: Improvements in proximity warning radio devices
Inventor: FREEDMAN HAROLD PHILIP
IPC: G01S13/933
Filed: 1958-02-12
Abstract: 887,864. Radio navigation; radio signalling. AVEL CORPORATION GENEVA. Feb. 2, 1959 [Feb. 12, 1958], No. 4615/58. Classes 40(5) and 40(7). In an aircraft range indicator apparatus of the transponder type, two or more spaced pulses transmitted by the interrogator aircraft are received by the responder aircraft which transmits only if the spacing of the received pulses corresponds to its own height, receiving means in the interrogating aircraft being operative only during the time within which a retransmission would be received from the second aircraft if it were within a predetermined range of the first aircraft. The time between the transmission and reception of pulses at the interrogator gives an indication of the range of the responder. If the interrogation and response are on different frequencies each aircraft either carries two transmitters and two -receivers or, by using frequency shift technique, carries only a single transmitter and receiver. If, however, only one frequency is used a time sharing technique is employed. Operation of the interrogator transmitter and receiver. The interrogator transmitter Txl, Figs. 1A and 1B, is keyed to give two 20 Á-second pulses on a frequency X spaced by a time t1 corresponding to the height of the interrogator above a datum level. An oscillator V1, square wave generator V2, and differentiating circuits produce single pulses at a rate of 100 per second. These are fed to a delay, which is made proportional to the interrogator height and is controlled by a height sensitive device, and are also fed direct to V7b. As shown the delay is a phantastron V4. By combining the delayed and undelayed pulses in the valve V7a, V7b pulse pairs having the required spacing are produced. The pulses are also fed to the responder section of the apparatus to mute the receiver Rxl. On reception of a pulse on a frequency Y from a responder aircraft carrying apparatus similar to that of Figs. 1A and 1B, the 20 Á-second pulse output of Rx2 is passed to a gate valve V20. This gate is opened by a long pulse from V18, the pulse being initiated by a pulse from V4 and its length being adjustable by means of VR2. The valve V20 therefore acts as an adjustable gate and if the responder transmitter is positioned within a range from the interrogator aircraft which corresponds to the long pulse fed to V18 a pulse will pass to V22. Reception of a number of pulses causes V22 to actuate an anticollision equipment. Operation of the responder transmitter and receiver. On reception of two interrogator pulses spaced by a time t1 corresponding to the height of the interrogator transmitter of another aircraft carrying similar apparatus, the pulses are differentiated and fed to V11 which gives an output pulse of length t2 dependent on the height of the responder receiver. The pulse length is controlled by a height sensitive device. This pulse is differentiated and the negative pulse obtained fed to V13. This valve feeds a 20 Á-second pulse, delayed by t2 relative to the first of the two pulses received on Rxl, to a coincidence valve V15 which is also fed with the undelayed pulses from Rxl. If there is any output from V15, i.e. if the heights of the interrogator and responder are substantially the same, so that t1 is approximately equal to t2 or V16 gives a 20 Á-second output pulse which is used to mute Rx2 and to key the responder transmitter Tx2 to give a pulse on frequency Y. The responder thus transmits only if its own height corresponds to the time spacing of the interrogator pulses. To ensure that the response received by an interrogator is in reply to an interrogation that originated from that interrogator and from no other, three pulses may be transmitted by the interrogator and the spacing of the received responder pulses, a pair of pulses in this case, compared with the spacing of the pulses originally transmitted, Figs. 3 and- 4 (not shown). Reception of signals from the responder station may cause data to be exchanged between the stations. It is possible that one aircraft may be unaware of the presence of the other due to their transmissions synchronizing. To avoid this danger the pulse repetition frequency may be varied according to the course followed by each craft. By manual adjustment of the pulse spacing at the interrogator, aircraft at heights other than that of the interrogator may be detected. Specifications 874,261, 874,262 and 874,263 are referred to.