Patent GB-655396-A (GB)
Title: Improvements in or relating to radio distance-measuring apparatus utilizing the doppler effect
IPC: G01S13/34
Filed: 1947-08-07
Abstract: 655,396. Radiolocation. SPERRY GYROSCOPE CO., Inc. Aug. 7, 1947, No. 21716. Convention date, Oct. 29, 1943. [Class 40 (vii)] Relates to radar systems utilizing the Doppler effect for determining the relative motion of a reflecting object in which frequencies related to the frequencies of the transmitted and reflected signals are heterodyned to give a beat note having a frequency dependent on the relative motion. According to the present invention, the range of the reflecting object is determined by rendering the transmitter and receiver alternately operative and periodically varying the frequency of alternation to amplitudemodulate the beat note at a frequency proportional to the range. As shown in Fig. 1, the output from an R.F. carrier source 11 is amplitude-modulated at 12 by a square wave 33, Fig. 2, from a generator 13 and the resultant wave pulses are coupled to the aerial 15 through switch 14 also controlled by generator 13, the mixer stage 22 of the receiver being coupled to the aerial 15 for alternate periods by means of a switch 19 controlled by an out-of-phase square wave 34, Fig. 3, derived from generator 13 so that the envelope 36, Fig. 5, of the echo pulses received by the mixer has a duration proportional to the range of the reflecting object. These pulses are heterodyned at 22 with a local oscillation produced by modulating at 25 the carrier from 11 with an oscillation of true intermediate frequency from generator 24 and selecting one side band by filter 26 and the output from 22 is applied to a narrow-band I.F. amplifier 27 which is gated by the square wave 34, Fig. 3. The I.F. pulses 36, Fig. 5, due to a stationary reflecting object may be analyzed as an I.F. carrier having an amplitude proportional to the pulse width and side bands separated from the carrier by the fundamental and harmonics of the switching frequency and the pulses due to a moving object may be analyzed as a similar series shifted in frequency by the Doppler effect. The band width of amplifier 27 is sufficiently narrow so that in either case only the carrier is amplified and the output carrier is then heterodyned with an oscillation of true intermediate frequency applied from 24 via an attenuator 29 so that when the reflected signal is produced by a moving object a beat frequency is produced in the detector 28 equal to the Doppler shift frequency which is amplified at 31 and applied to a frequency meter 32 which may be of the vibrating reed type capable of indicating the velocities of a plurality of moving objects. The duration of the pulses will vary with the range of the object, as shown in Fig. 6, and since the amplitude of the beat frequency is proportional to the amplitude of the carrier output from 27 and hence to the duration of the pulses 36, the amplitude of the beat frequency will vary in a similar manner, the space between the radar system and the reflecting object being in effect divided into a number of " amplitude zones." This property is utilized to determine the range by periodically varying the frequency of the switching wave generator 13 in a linear manner according to the waveform 41, Fig. 7, produced by generator 38, so that the curve of Fig. 6 is alternately compressed and expanded and the " amplitude zones " are swept back and forth past the object. The more distant the object is from the radar system the more zones are swept past it during each cycle of waveform 41 and the Doppler beat-frequency output is thus amplitude-modulated at some harmonic of the frequency of waveform 41 which harmonic characterizes the range of the object. This amplitude modulation is detected at 42 and applied to a range-indicating frequency meter 43 and also to the Y plates 49 of a cathode-ray tube 44, the waveform 41 being applied from the generator 38 to the X-plates 48 to produce a stationary pattern having maxima with a spacing indicative of range. Specification 654,226 is referred to.