Patent GB-585158-A (GB)
Title: Improvements relating to radio object detecting and locating systems
IPC: G01S1/02
Filed: 1944-02-21
Abstract: 5 8 5, 1 5 8. Radiolocation. SPERRY GYROSCOPE CO., Inc. Feb. 21, 1944, No. 3225. Convention date, Feb. 23, 1943. [Class 40 (v)]- In a radiolocation system of the type in which a Doppler beat frequency between transmitted and reflected waves is obtained due to moving objects, distance is determined by modulating the transmitted wave with the modulation frequency of the received Doppler beat frequency, the transmitter and receiver together with the space coupling through a moving object forming a system which oscillates at the modulation frequency indicative of the range of the object. The continuous wave detecting and locating system of Fig. 1 comprises a V.H.F. oscillator 11 and amplifiermodulator 12, the output of which is fed to a radiator 13, and a superheterodyne receiver comprising local oscillator 16, mixer 17, I.F. amplifier 18 and detector and A.D. amplifier 19 and 21. The receiving antenna 14 is responsive to energy from reflecting objects as well as to a portion of the transmitted carrier which leaks directly from radiator 13. If the source of reflected waves is moving radially with respect to the system, a Doppler frequency shift fd of the reflected waves occurs. The local oscillator 16 may be made to maintain a constant frequency separation from the carrier frequency f by automatic control or by the use of a crystal oscillator and multiplier chains, and two intermediate frequencies separated by the Doppler frequency fd are produced. The detector 19 produces the Doppler beat frequency fd which is fed through the A.F. amplifier to a frequency meter 22 which thus measures the radial velocity of the obstacle, producing the reflected waves. In order to measure range, the Doppler beat wave from the amplifier 21 is fed through a switch 23 and feed-back means 24 which does not pass the Doppler shift frequency, so that all fluctuations superimposed upon the Doppler beat wave are fed through an amplifier 25 to produce amplitude modulation of the transmitted wave in amplifier-modulator 12. If there is sufficient amplification in this feed-back path oscillations will be set up in the system at a frequency inversely proportional to the transit time around the entire system, the frequency of this oscillation which appears as an amplitude modulation of the Doppler beat wave being measured on a frequency meter 26 which can thus be calibrated in terms of distance. The envelope frequency fe of the Doppler beat wave may be higher or lower than the Doppler frequency, and in the case when fe is always lower than fd, the feed-back means 24, Fig. 2 (not shown), may comprise a detector preferably of the full-wave type followed by a low-pass filter which blocks the Doppler frequency and passes the envelope frequency. A delay network may be connected into the feed-back path to increase the transit time around the system and so maintain fe substantially below fd for reflecting objects at short ranges, and to determine the value of fe corresponding to zero .distance. When the envelope frequency is always higher than the Doppler frequency, the feed-back means 24, Fig. 3 (not shown), may comprise a balanced square-law detector followed by a high-pass filter and optional delay network. When the ranges of variation of the Doppler and envelope-frequencies overlap the feed-back means 24, Fig. 4, comprises a frequency adder 35 which with the aid of an audio oscillator 36 transposes the Doppler beat to a frequency always appreciably higher than the modulation envelope and permits a low-pass filter 28 following a detector 27, to pass only the envelope frequency to modulate the transmitted wave. A delay network 31 brought into operation by switch 29 may be used to modify the range of fe. The frequency adder 35, Fig. 9 (not shown), is a single side-band modulator, the Doppler frequency being fed through a frequency distortion correcting network and phase shifters in quadrature to two balanced modulators, while the output of the audio oscillator is also fed in phase quadrature to the modulator signal inputs. The modulator outputs are connected in phase opposition and produce only the upper side-band-frequency. The frequency adder may be replaced by an automatic frequency shifter, Fig. 8 (not shown), in which the frequency of the Doppler beat is changed in a balanced modulator fed also with the output of an audio oscillator so that a band-pass filter passes only the required upper side-band frequency. The frequency-shifted Doppler beat is fed to a frequency correcting path comprising high- and low-pass filters whose cut-off frequencies correspond to the limits of the band-pass filter and which produce equal attenuation at the mid-frequency of the band-pass filter range. The outputs of the high- and low-pass filter are fed to a balanced rectifier and D.C. amplifier which feeds a correcting voltage to control the frequency of the audio oscillator in any suitable manner. The distance-controlled self-oscillation of the system may be established by frequency modulation of the transmitted waves, Figs. 6 and 7 (not shown), the detector and filter in the feed-back means being replaced by an F.M. demodulator and the V.H.F. oscillator and amplifier modulator of Fig. 1 being replaced by a V.H.F. oscillator modulator capable of frequency modulation.