Patent GB-663709-A (GB)
Title: Wave-signal communication system
IPC: G01S13/933
Filed: 1946-08-24
Abstract: 663,709. Electric control systems. HAZELTINE CORPORATION. Aug. 24, 1946. [Sept. 18, 1945] No. 25365/46. Class 40 (i). [Also in Group XL (c)] In a wave signal communication system comprising a transmitting station spaced from a receiving station in which one of said stations is carried by a mobile object, the transmitter is time-modulated by pulse signals the modulation, e.g. pulse duration or the spacing between the two component pulses of a pulse pair, being characteristic of a particular traffic zone, e.g. altitude zone, which may be occupied by the mobile object and the receiver is adapted to select signals having a modulation characteristic of a single traffic zone. The invention is described as applied to an aircraft radio navigation system in which each aircraft is equipped with a pulse modulated interrogator and a responder. The modulation of the interrogating pulses is automatically made proportional to the altitude of the interrogating aircraft and each responder responds only to interrogating signals having a modulation corresponding to the altitude of the responder so that the interrogating aircraft only receives reply pulses from responding aircraft at the same altitude. The modulation of the interrogating signals may also be controlled manually to effect the interrogation of adjacent altitudes. When the aircraft are to fly along a particular course at some predetermined altitude, responders responsive to signals characteristic of that altitude are spaced along the course so that interrogating aircraft will only receive reply pulses when they are at the correct altitude and responders responsive to all signals characteristic of altitudes less than some particular value may be mounted on the top of dangerous obstacles. Interrogators with manually adjustable modulation may also be located on the ground along the courses for determining the location of aircraft flying at any particular altitude and these stations may be connected by line or radio link with a traffic control centre. As described each interrogator includes, a switched-beam aerial and a type A cathoderay tube display for indicating the range and bearing of the responders and the interrogators and responders preferably transmit on different carrier frequencies. In the airborne interrogator 10, Fig. 1, the output pulses A, Fig. 2, from the generator 11 are converted at 12 into pulse pairs B, the spacing a being controlled by an aneroid altimeter 13. The pulse pairs radiated by the transmitter 15 are received at the responder 19 and the first pulse of each pair of received pulses C generates a delayed gate pulse D having a delay time t2 controlled by an aneroid altimeter 21 which is applied to gate the pulses C so that a pulse E is produced at the decoder 20 when the delay time t2 is equal to the spacing a of the interrogating pulse pairs. The pulse E is applied to the reply signal generator 22 where it is coded in width or grouping characteristic of the identity of the responding aircraft and/or its heading and the output from 22 is applied to block the receiver 17 and to modulate the transmitter 24. The radiated reply signals are received at the receiver portion 26 of the interrogator by the two aerials 27, 28 which have overlapping directional characteristics and which are alternately rendered operative by the switch 29 controlled by the generator 34, the output from the receiver 30 being alternately applied to the two X plates of the cathode-ray tube 32 in synchronism with the aerial switching, and a time-base synchronised by the pulse generator 11 being applied to the Y plates to give a backto-back type A display, Fig. 3 (not shown), indicating the range and bearing of the responder. The receiver 30 is blocked by the output from 12 to prevent, reception of the directly radiated interrogating pulses and the output from receiver 30 is also applied to headphones P to provide aural identification of the responder. Pulse modulating circuit, Figs. 4 and 5. Each pulse G, Fig. 5, from the generator 11, Fig. 4, is applied to trigger a cathode-coupled flip-flop 39, 40 which produces a negative square wave H whose duration is determined by the variable resistor 41 controlled by the aneroid altimeter 13 or by the manually variable resistor 47. The wave H is differentiated at 51, 49 and the positive peak of the resultant waveform I renders valve 50 conductive thereby triggering the blocking oscillator 54 since the anodes and cathodes of valves 50 and 54 are connected in parallel. When valve 54 is operative grid current flow produces a negative pulse in resistor 55 connected across the input terminals of an opencircuited delay line 57 so that the reflected negative pulse cuts off valve 54 and a current pulse K is produced in the output winding 59 of transformer 53 of duration determined by the constants of line 57 and which is fed through the cathode-follower 58 (output pulse L) and mixed in amplifier 35 with an inverted G pulse from generator 11 to give a pulse pair. If the pilot of an aircraft wishes to interrogate responders operating at altitudes different from his own, he can adjust the pulse spacing a by the variable resistor 47 and when a grounded interrogator wishes to interrogate all responders which differ in altitude from the interrogator by a constant amount, the difference may be set in by the control 48. The circuit arrangement of Fig. 4 is also used at the responders, Fig. 6 (not shown), to generate the gating pulse, the flip-flop being triggered by the first pulse of each received pulse pair and the output pulse from the blocking oscillator being used as the gating pulse. A responder mounted on top of an obstacle should be triggered by all interrogating aircraft of lower altitude and accordingly the square wave output H from the flip-flop is inverted and used as the gate pulse, Fig. 11 (not shown). In a modification, Fig. 9 (not shown), the means for producing the interrogating pulse pairs described above is replaced by a pulse radar altimeter operating in synchronism with the interrogator, the altimeter echo pulse being used as the second pulse of each interrogating pulse pair and at the responder, Fig. 10 (not shown), a similar pulse altimeter is triggered by the first pulse of each interrogating pulse pair the altimeter echo pulse being used as the gating pulse. In a further modification, Fig. 7 (not shown), each interrogating pulse pair is replaced by a single pulse with a duration proportional to altitude and at the responder Fig. 8 (not shown) the received pulse is differentiated to give a pulse pair which is handled as above.