Patent GB-1511267-A (GB)
Title: Distance measuring equipment
IPC: G01S13/28
Filed: 1975-06-10
Abstract: 1511267 Distance measuring equipment TEXAS INSTRUMENTS Inc 10 June 1975 [12 June 1974] 24848/75 Heading H4D Distance Measuring Equipment comprises a frequency modulated pulse (chirp) interrogator transmitter in combination with a weighted matched filter/pulse compression responder which re-transmits a conventional short pulse response signal. In Figure 1 the interrogator transmitter which may be mounted in an aircraft is triggered by the unique jittered output from synchronizer circuit 2 which causes encoder 4 to produce two bursts of FM encoded IF signal, and in addition initiates time counting in range tracker 16. Each (ground based) transponder operates with a unique fixed pulse spacing, RF carrier frequency and sense of frequency modulation of the successive interrogating pulses, i.e. a 'chirp code'. Channel selector 8 is used to select the transponder to interrogate by selecting a code corresponding to a given combination of these factors. The chirp code may be one of two viz., both pulses having the same FM sense, or, each of opposing sense. The signal chirp coding is effected in encoder 4 which comprises a surface wave dispersive pulse expanding circuit with digitally selectable sense of frequency modulation (see Figs. 6, 7, and 8, not shown where details of the circuit are given in detail). The R.F. signal is transmitted from transmitter 10 via circulator 18 and antenna 20. The signal is received by transponder antenna 22 and coupled by circulator 24 to the transponder receiver 26, which produces an IF signal. Limiter/pulse compressor 27 processes the I.F. signal and comprises a hard limiter 52, Fig. 5, a pulse compression filter 54 (Fig. 9, not shown) which in turn comprises surface wave pulse compressor (129) from which at output (130) 'down chirp' pulses appear, and at output (132) 'up chirp' pulses appear. These outputs are fed to S.W. delay lines (131, 133) which have each two respective outputs having time delays whereby received pulse pairs of the given time spacing appear at respective known instants. Each delay line output is fed via one of four detector 56, peak detector 58, comparator 62 chains and fed separately to digital decoder 28 which produces an output on detection of the correct pulse time spacing, and triggers, synchronously in response to the leading edge of the 1st received compressed pulse, a systematic delay 30 which after a fixed time triggers encoder 32. A squitter 34 also triggers encoder in random fashion. Transmitter 36 re-transmits conventional fast risetime narrow pulse signals via circulator 24 and antenna 22 back to the interrogator. These signals have a broad frequency spectrum and a spectrum validation circuit 40 is included to ensure that they have the correct centre frequency. Decoder circuit 42 tests these received retransmitted signals for the predetermined time spacing, whereupon range tracker 16 on reception of validation signals from circuits 40 and 42 stops counting time. The counted value is therefore proportional to the propagation time less the known system delays and the range may then be calculated and displayed.