Patent GB-1249630-A (GB)
Title: Method and apparatus for digitally measuring distance
Inventor: AKER JOHN LEWIS
IPC: G01S13/78
Filed: 1968-08-27
Abstract: 1,249,630. Pulse radar. KING RADIO CORP. 27 Aug., 1968, No. 40893/68. Heading H4D. ' In pulse-echo secondary radar two digital counting registers are provided for changing the delay between each interrogation transmission and a " marker " signal to be made coincident with the received reply, and the registers determine the instant at which the marker signal is produced without transfer of numerical information therebetween. The embodiment is an airborne D.M.E. for use in a TACAN system, in which each transmission from the aircraft is a pair of pulses separated by 12 microseconds and the ground beacon responds in a different channel; to avoid interference each aircraft pulse repetition frequency is varied randomly. A strobe scans in range until a valid response is received (i.e. a minimum proportion of responses are received at that range), and transfer is effected from searching to tracking. The strobe position is numerically indicated. Ground speed is measured by rate of change of range and is also indicated. Counters are employed and the number of pulses to clear a counter correspond to its previous state. The embodiment is composed mainly of solid-state circuitry. General, Fig. 6. The leading edge of a single D.C. pulse (corresponding to receipt of a pair of echo pulses with the 12 microseconds spacing) and herein termed a return pulse causes a monostable circuit Q601, Q602 to produce a fifteen microsecond pulse and the leading edge of an inverted marker (i.e. strobe) pulse causes a monostable circuit Q603, Q604 to generate a pulse also of fifteen microseconds duration: together with diode gates CR609, CR615 a phase detection circuit is thereby formed. If the leading edge of the marker pulse follows the leading edge of the return pulse by up to 15 microseconds there is a " subtract error ", indicating that a stored distance number should be reduced ; if the marker pulse leading edge precedes leading edge of the return pulse by up to 1-4 microseconds (1À4 microseconds is the duration of the marker pulse) there is a zero error condition; and if the marker pulse trailing edge precedes the return pulse by up to 15 microseconds there is, an "add error" and the stored distance number should be increased. Corresponding waveforms are given Figs. 24, 25, 26 (none shown). (If a return pulse does not fall within 15 microseconds of the marker pulse it is ignored). Valid returns are separated from random ones by integrators Q609, Q610 which control " To- From " binary Q615, Q616 which in turn controls binaries Q613, Q614 and Q617, Q618; and the circuit is so arranged that integration is longer on reversal of aircraft direction relative to the ground transponder and that tracking can continue in the same direction and at the same rate during momentary loss of signal. Stand by circuitry in Fig. 6 senses the average decoded " squitter " rate and if this drops below e.g. 300 pulses/second the reception path is regarded as unsuitable and the D.M.E. transmitter is shut off. Timing and counting, Figs. 4, 7. Accuracy of all timing functions is determined by a crystalcontrolled oscillator Q507 Fig. 4, the frequency of which is divided to give the equivalent of a radar range of <SP>1</SP>/ 10  nautical mile. After interrogation transmission (Fig. 5 not shown)-initiated by driver Q512-and allowing for the ground responder and internal system delays, derived clock pulses are fed to digital counting and storage circuits in ten mile range, one mile range and 1/ 10  mile range circuit boards Fig. 7, from which the marker pulse is generated by a monostable Q816, Q817. Pulse repetition frequency generation, Fig. 4. The pulse repetition frequency of the D.M.E. is derived from a unijunction relaxation oscillator Q502 which is arranged to be at 30 c./s. in the track mode and 150 c/s during the search mode, Fig. 21 (not shown); the interrogation rate during both modes is jittered utilizing a noise voltage from a Zener diode CR501 and is effectively synchronized to the clock pulses by an AND gate CR507. Response validity. Fig. 4. When a return pulse falls within 15 microseconds of a marker pulse the search process is stopped for 4 interrogation cycles; the average D.C. voltage at the collector of transistor Q517 (see also Fig. 21, not shown) is an inverse function of the ratio of the number of such returns to the number of interrogations. When there are four consecutive returns a generator incorporating a unijunction Q524 and a binary Q525, Q526 advances the marker during the search mode in 0.2 n. mile steps and allows interpolation in the track mode. Means by which the track mode is initiated following the search is described. Counter details, Fig. 7. Each range decade incorporates a binary counter e.g. Q921, Q922 and a " quinary " counter e.g. Q929-Q933: the quinary counter has five transistors giving five stable states depending on which transistor is off, Fig. 11 (not shown). Identification signalling, Fig. 7. A ground responder transmits its call sign in Morse code at intervals e.g. 45 seconds as a 135 c./s. modulation which is filtered, Q811 and Q812, in a Darlington circuit incorporating a Twin-T network and applied after phase inversion to a push-pull transistor amplifier Q814, Q815, Fig. 18 (not shown).