Patent GB-655391-A (GB)
Title: Traffic signalling system
IPC: G01S13/933
Filed: 1947-07-23
Abstract: 655,391. Electric control systems. HAZELTINE CORPORATION. July 23, 1947, No. 19786. Convention date, July 31, 1946. [Class 40 (i)] [Also in Group XL (c)] The invention relates to an aircraft navigation system employing radiant wave energy, e.g. radio waves, in which each aircraft is equipped with a pulse modulated interrogator and a responder, the interrogator transmitting recurrent pulse pairs and the spacing between the two component pulses of each pair being variable in steps corresponding to discrete altitude zones, e.g. 0-1000 feet, 1000-2000 feet, &c. Each responder includes a pulse-spacing discriminator which is variable in a similar manner so that when the settings of the interrogator modulator and the responder discriminator are controlled automatically in accordance with the altitude of their respective aircraft, an interrogating aircraft in a particular altitude zone receives reply pulses only from other aircraft in the same zone. The spacing of the interrogating pulse pairs may also be controlled manually in order to interrogate any altitude zone. According to the present invention the spacing of the interrogating pulse pairs is controlled so that when an interrogating aircraft approaches the boundary of two contiguous altitude zones, responders on aircraft in both zones are actuated and similarly the pulse-spacing discriminator in each responder is controlled so that when the aircraft carrying the responder approaches the boundary of two contiguous altitude zones it is actuated by interrogating aircraft in either of the zones. The receiver portion of each interrogator comprises a beam-switched aerial system and a back-to-back type A cathode-ray tube display indicating the range and bearing of the reply pulses which may be coded for communication or for indication of the identity, speed and heading of the responding aircraft; headphones may also be provided for aerial identification of the code. The interrogators and responders preferably transmit at different carrier frequencies. Interrogator pulse-modulating circuit, Figs. 3 and 5. The train of pulse pairs is produced by applying the output from pulse generator 11 directly and through a delay means to a combining circuit 65. As shown, negative pulses from 11 are applied to excite a ringing circuit comprising inductor 37 in parallel with one of the capacitors 44-48 which is selected by relays 40-43 controlled either by a manually-operated switch 54<SP>1</SP> or by a switch 54 controlled by an aneroid altimeter 13 and the sinusoidal output is applied to the grid of the negatively-biassed thyratron 39. The first negative half-cycle of the sinusoidal output has no effect on the thyratron but the succeeding positive half-cycle renders it conductive and this discharges the artificial line 61 to produce a pulse across resistor 64 which is delayed with respect to the triggering pulse by half the periodic time of the ringing circuit. The circuit constants are such that the delays corresponding to two contiguous zones differ by an amount equal to the duration of the delayed pulse, Figs. 5a and 5c, and the movable member 55 of the altimeter switch 54 is so dimensioned that when the interrogating aircraft is adjacent the boundary of two contiguous zones the member. makes contact with two adjacent studs, e.g. 50 and 51, thereby energizing two adjacent relays 40 and 41 which select a capacitor 45 giving a delay intermediate between the delays corresponding to the two zones, Fig. 5b, so that the resulting pulse pairs will trigger responders on aircraft in either zone. Responder pulse-spacing discriminator, Fig. 4. The pulse pairs having a spacing corresponding to the altitude zone of the responder are selected by applying the detected output from the receiver directly to the signal grid 71 and via a tapped delay line 75, 76, 77 to the suppressor grid 89 of a coincidence valve 70 which produces a pulse output only when the delay produced by the selected tapping is equal to the spacing of the received pulse pairs. Each tapping is connected to the suppressor grid 89 through a relay contact, the relays 81-86 being selectively energized by switch 87 whose movable member 88 is actuated by an aneroid altimeter 21 and the movable member being so shaped that when the aircraft carrying the responder approaches the boundary of two contiguous altitude zones the member energizes two adjacent relays so that two pulses are produced having different delays corresponding to the two altitude zones and the responder is thus actuated by interrogating pulses from aircraft in either zone.