GB648881A - Improvements in or relating to object detecting and locating systems utilizing reflection of electromagnetic waves - Google Patents

Abstract

648,881. Automatic direction - finding. SPERRY CORPORATION. Dec. 4, 1942, No. 17312. Convention date, Jan. 16, 1942. [Class 38 (iv)] [Also in Groups XL (b) and XL (c)] Relates to apparatus for indicating the bearing, distance and relative velocity of conducting bodies reflecting electromagnetic waves. The invention is described as applied to a system in which the beat frequency (produced by the relative velocity of the reflecting body) is utilized to control the position of a directive transmitter and receiver having overlapping beam characteristics, so as to give the bearing. The transmitter radiates a carrier wave modulated by an audio-frequency note (i.e. the carrier frequency and a single side frequency, in effect two carriers of slightly different frequencies) and since the phase displacement of the received reflected wave depends upon its frequency as well as upon the distance of the reflecting body, this distance may be indicated by comparing the phase of the two reflected waves. In the system shown, oscillations from a generator 1 are multiplied in frequency at 2, 5, producing a carrier frequency f 1 which is modulated at 8 by an audio frequency # 3' thus producing two frequencies f 1 and f 1 +f 3 which are fed to the transmitter 12. The receiver comprises two aerial systems 13, 131 and 14, 14<SP>1</SP>, directional in azimuth and elevation respectively. The two aerials 13, 13<SP>1</SP> are slightly spaced from the focus of the reflector 183 so that the reception patterns of the two aerials overlap, the energy received by the two aerials being fed alternately at a low frequency f 5 by a commutator 16 to a mixer 18 which receives a local heterodyne frequency f 2 derived from generator 1 through multipliers 3, 20. The output of mixer 18 thus includes frequencies (f 1 +Df-f 2 ) and (# 1 +# 3 +D#-# 2 ), where Df is the Doppler frequency effect due to the relative velocity of the reflecting object, these frequencies being passed to selective intermediate-frequency amplifiers and detectors 23, 24, 38, the output of which consists of the Doppler-shift frequency Df. These Df voltages have a phase relation determined partly by the distance of the reflecting object and partly by the different frequencies (f 1 and f 1 +f 3 ) from which they are derived. This distance may be indicated by the phase comparator 29, which may be of the kind described in Specification 609,298, [Group XXXVI]. In order to reduce noise effects, the outputs of the detectors 23, 24 are fed to balanced modulators 25, 26, which are also fed from an audio-frequency oscillator which is adjusted by hand or automatically to such a frequency that the outputs of the modulators 25, 26 can pass through narrow-band filters 27, 28 to the phase comparator 29. The oscillator 35 is tuned to cause a maximum indication in a meter 311 across the output of filter 28 and the frequency of the oscillator 35 is then an indication of the frequency Df, i.e. of the relative velocity of the reflecting object. The bearing in azimuth and elevation of the reflecting object is indicated by the position of the transmitting and receiving reflectors 182, 183, 184 which are caused to follow automatically the reflecting object. For control in azimuth, the output from the mixer 18 is also led through devices 38, 40, 42, 44 to one input of a balanced rectifier 47, also supplied with current of frequency f 5 (ref.) from a reference generator 46 on the same shaft as commutator 16. The antennae 13, 13<SP>1</SP>, having overlapping directive reception patterns, are made effective alternately by the commutator 16, and if the energy received by the antennµ 13, 13<SP>1</SP> is unequal, the frequencies led to the mixer 18 and devices 38, 40, 42, 44 will be modulated at the commutation frequency f 5 and the phase of the output from detector 42 will be determined by the direction of the received waves. The direction of the output of the balanced rectifier 47 is determined by the phase of the currents f 5 relative to the reference phase f 4 (ref.) produced by generator 46 and thus causes motor 50 to rotate the aerials 13, 13<SP>1</SP> in azimuth until they receive equal energy. The movement of the aerial 14, 141 in elevation is similarly, effected by motor 51 controlled by the balanced rectifier 48 and units 32, 37, 41, 43, 45. In a modified system, Figs. 1A, 1B (not shown), generally resembling that shown in Fig. 1, the frequency of the audiooscillator 35 is automatically controlled by the incoming signals. The oscillator 35 may be a pentode oscillator, the plate-grid circuit of which includes a phase-shifting network comprising three similar capacity-coupled meshes including valves, whose grids are connected via a potentiometer to the output of a balanced rectifier connected to high- and low-pass filters. In a further modification (Fig. 2, not shown), overlapping beams are produced by the transmitter, two aerials slightly displaced from the focus of a reflector being rendered effective in turn by a commutator. The position of the combined transmitter and receiver is automatically controlled, somewhat as in Fig. 1, the control in azimuth being effected by a single receiving aerial and the control of elevation by a pair of receiving aerials. The transmitted carrier may be modulated by either of two audio frequencies, one ten times the other, so as to provide varying sensitivity of the distance indicator, the readings of which depend upon the difference in frequency of the two transmitted carriers. In order to maintain the heterodyne oscillator at a constant frequency difference from the transmitting oscillator, the beat frequency is rectified and passed through a frequency-discriminator producing a positive or negative voltage which controls the resistance of a valve in series with the accelerating voltage of the local oscillator, which is of the velocitymodulation type. Specifications 523,712, [Group XL], 648,882, 663,611 and 670,551, [all in Group XL (c)], are referred to.