Patent GB-1126301-A (GB)
Title: Navigational range computer system
IPC: G01S13/78
Filed: 1966-06-29
Abstract: 1,126,301. Electronic counters. GENERAL DYNAMICS CORP. 29 June, 1966 [29 June, 1965], No. 29297/66. Heading G4A. [Also in Division H4] General.-The Specification describes a navigational range computer system in particular the range computing portion of a TACAN receiver. An interrogation pulse (a), Fig. 3, from the TACAN transmitter (not shown) operates input control unit 12, Fig. 1, to cause the transmission of clock pulses (d) to the range calculation unit 10. The clock pulses (d) are stopped by a signal (preferably a reply signal) which is range gated by control unit 14 to control unit 12. For the first interrogation period (dwell) the range gate is started by the interrogation signal and the clock pulses are stopped by the first received signal even if this signal is interference. On subsequent dwells, unit 10 controls the time of the range gate in dependance on the time of occurrence of a signal in the preceding dwell. A " filler " pulse is generated by unit 18 at a time synchronous with the occurrence of signal in the preceding dwell. Such filler pulses are fed to the units 12 and 14, and serve a memory function. An aquisition control unit 16, on counting six filler pulses gives an indication that the TACAN reply has been lost, and the system is changed from a track to a search mode. Conversely unit returns the system to a track mode, on counting six actual reply signals. Unit 16 controls unit 14 such that in the search mode a filler pulse based on the time of a previous dwell interference pulse, will not prematurely terminate the range gate. During the track mode, unit 16 causes unit 10 to read out its clock count into indicator 20, to give a range indication. Detailed embodiment.-Pulses synchronous with the leading and trailing edges of the interrogation pulse (a) are formed by shaping circuit 36 and are used respectively to reset clock pulse counter 22 in unit 10 to zero and to start a clock gating pulse (c) from bi-stable circuit 38. A reply signal (b) via range gate 58 and OR circuit 60 stops the clock gating pulse. Clock pulses (d) are thus gated through gate 40 to counter 22. Reply (b) may be interference. The range gate (h 1 ), Fig. 3, for the first dwell is initiated from generator 44 by the interrogation pulse (a 1 ) via gate 48 and OR circuit 48 and is terminated by the first received signal via gate 58, OR circuit 60 and gate 62. Unit 16 is initially in a search mode, such that circuit 80 in a 0 state, which is changed to a 1 state by invertor 64, such that track gate 82 is disabled and first signal reset gate 62 is enabled. The received signal, after delay at 42 activates transfer gates 24, whereby the range count in counter 22 is transfered to down-counter 26. The second interrogation pulse starts the clock pulse feed to up-counter 22 and to downcounter 26, wherein said clock pulses are subtracted from the count previously transferred thereinto. When the count in counter 26 is zero, a filler pulse is produced by the counter and fed to gate 65 and via 3Ás delay 68 to gate 66. A bi-stable circuit 69 is in the reset state or " miss " state such that an enabling signal is fed to gate 65 and the undelayed filler pulse passes to counter 56, to a minus input of counter 74, to reset input of circuit 69 and to inhibited gate 86. The delayed filler pulse (e) by-passes gate 86 and resets range gate generator. Generator 44 is set by an " early " pulse from counter 26 occurring a predetermined short " gate width " time before the filler pulse. The range gate (h 2 ) thus embraces the position of the first received signal in the previous dwell. The second interrogation pulse does not start the range gate as gate 46 at that time is inhibited. The filler pulse stops the clock gating pulse and transfers the count in counter 22, the same as the previous count, into counter 26-now holding zero count. The third interrogation pulse (a 3 ) starts the clock pulse count. The range gate pulse h 3  is started by the early pulse but is now terminated by the reply pulse RP via gates 58, 60, 62. The thus range gated reply pulse terminates the clock pulse count such that counter 22 contains a count representing the true range. This count is transferred to counter 26 by the delayed reply pulse. The gated reply pulse is also fed to actuate circuit 69 to a " hit " state and to the plus input of counter 74 to register a count of one therein. The fourth interrogation pulse (a 4 ) starts the clock pulse count, and the early pulse now starts the range gate, such that the reply pulse RP is again gated through to terminate the range gate to and increase the count in 74 to two. When the total count in 74 reaches six, gate 76 gives an output whereby circuit 80 gives a 1 or " track " output, which operates, via gate 82, transfer gates 28, whereby the range count in counter 22 is fed via a storage register 30 and decoding matrix 32 to range indicator means. A count in counter 22, indicating a range of 300 miles causes a " loss of signal " signal to reset register 30 to zero and initiates a new search mode when the count in 74 falls below six. A velocity detector may be used providing an error signal indicating in sense and magnitude the difference between the actual reply pulse time and the expected (filler pulse) time. This error signal is averaged over a number of dwells and fed to the range.