Patent GB-1042353-A (GB)
Title: Frequency-modulation doppler radar navigation apparatus
IPC: G01S13/60
Filed: 1963-04-18
Abstract: 1,042,353. Radar; automatic frequency control systems. SPERRY RAND CORPORATION. April 18, 1963 [April 19, 1962], No. 15280/33. Headings H3A and H4D. [Also in Division G1] In a Doppler radar a continuous carrier wave frequency modulated at two frequencies simultaneously is generated and transmitted, and the receiver heterodynes echo signals with the generated wave to produce a beat note having a frequency-modulation (F.M.) component whose F.M. index is indicative of the range of the target and whose mean frequency is indicative of the relative velocity of the target. The F.M. index and mean frequency are separately determined, preferably by closed-loop frequency trackers coupled to the heterodyning means. In the preferred embodiment, applied to an aircraft which may include several directional aerials in a "Janus" configuration, one directional aerial and its associated transmitter and receiving apparatus are described. The F.M. frequencies are at 450 kc/s., from frequency sources 8, and 4 kc/s. originating at an oscillator 30; these are summed and F.M. a klystron 3. The 450 kc/s. signal is varied at 0 5 c/s. to eliminate " altitude holes." The echo signals from the ground are directed via a cross-polarization duplexer 9 to a mixer 10 also fed from a single side band modulator 12, the inputs to the latter being the generated wave f T  and an output at 35À1 Mc/s. from frequency sources 8 " wobbulated " similarly to the 450 kc/s. signal on line 6. The output signal spectrum of mixer 10 includes a component at a centre frequency (35À1 Mc/s. - f D ), where f D  is Doppler frequency shift, and sideband components at where each value of n represents a pair of sidebands. To suppress such leakage from the transmitter into mixer 10 as has been passed by the duplexer, the sideband frequency corresponding to n = 3, i.e. 33À75 Mc/s. -f D , is selected and is mixed with a signal of frequency 33À75 Mc/s. from frequency sources 8 to afford a signal A of mean frequency f D  which is frequency-modulated at 4 kc/s. in accordance with the altitude of the aircraft. Signal A is heterodyned with a 150 kc/s. signal frequencymodulated at 4 kc/s. in a modulator 31 (signal B), to give a signal C on line 17 of frequency 150 kc/s. -f D  having substantially no F.M. at 4 kc/s. Doppler frequency tracking.-A variable-frequency oscillator 24 operates nominally at 400 kc/s. and is constrained to afford a.signal of frequency 250 kc/s. on line 25 irrespective of the value of f D  at the input of a mixer 19; a frequency discriminator not shown, centred on 400 kc/s. and fed from oscillator 24, gives the speed signal output, or the oscillator output may be combined with other similar oscillator outputs. Altitude determination.-The extent of frequency deviation at 4 kc/s. at the input to mixers 14 is proportional to aircraft altitude: the signal on line 25 is applied to a frequency discriminator 27 the peak amplitude of the output of which is extracted by a coherent detector 28 which is fed also with a 4 kc/s. signal from an oscillator 30. The D.C. output of detector 28 controls a servo-motor 35 which drives (i) the wiper of a potentiometer 44 thereby affording an A.C. signal of amplitude dependent on aircraft altitude, (ii) the amplitude of a 4 kc/s. signal from oscillator 30, such signal being utilized via a phase-shifter 46 as the 4 kc/s. transmitter F.M. input and via a phasecontrol device 39 and amplitude control 40 as the modulation input to modulator 31, (iii) device 39, and (iv) amplitude control 40. Thus any 4 kc/s. F.M. on the signal at the output of mixers 14 is annulled; any residual 4 kc/s. F.M. is rejected in the Doppler tracker by a filter 21. A block diagram. Fig. 2, shows the frequency sources 8 and includes a crystal-controlled oscillator (C.C.O.) of frequency 11À25 Mc/s. and a frequency multiplier producing a 33.75 Mc/s. signal; a variable-frequency oscillator (11À7 Mc/s.) frequency-modulated at 0À5 c/s. feeding a frequency multiplier to afford a " wobbulated " 35À1 Mc/s. signal for the modulator 12 and feeding a mixer also receiving the 11À25 Mc/s. C.C.O. signal to produce a " wobbulated " 450 kc/s. signal for the second F.M. input to the transmitter; and a second C.C.O. giving a 150 kc/s. signal for modulator 31.