GB671461A - Improvements in or relating to radar systems - Google Patents

Abstract

671,461. Radiolocation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. April 22, 1949 [July 7, 1948], No. 18300/48. Class 40 (vii). In a frequency-modulated radar system in which the direct and reflected signals are heterodyned to give a beat frequency indicative of the target distance, means are provided for indicating the distance of targets having a relative radial velocity within a predetermined range of velocities or for indicating the relative radial velocity. The invention utilizes the fact that the beat frequency produced by a stationary target comprises a group of components, Fig. 1 (not shown), which are harmonics of the modulating frequency whereas a moving target produces corresponding components, Figs. 2 and 34 (not shown), which are displaced by the Doppler shift frequency and according to the invention the Doppler shift frequency is extracted by heterodyning the beat frequency with harmonics, Fig. 3 (not shown), of the modulating frequency; alternatively both the beat frequency and the harmonics may be frequency-changed by a common local oscillator before being combined. The invention is described as applied to a known system in which the beat frequency output from 5 produced by heterodyning the direct output from transmitter 2 and the reflected signal received at 5 is analysed by heterodyning it at 6 with a cyclically varying frequency from oscillator 7 and applying the resultant sum or difference signal to a band-pass I.F. amplifier 8 whose output comprises short trains of oscillations occurring at a time relative to the variation period of oscillator 7 which is dependent on the beat-frequency and hence on the target distance. Harmonics of the transmitter modulating frequency produced at 16 are applied to mixer 17 and amplifier 18 which are similar to mixer 6 and amplifier 8 and the outputs from amplifiers 8 and 18 are heterodyned at 9 to give short trains of oscillations of Doppler frequency which are applied through a filter 13 passing the desired range of Doppler frequencies to an amplifier 15 and indicator 14, e.g. frequency meter or loud-speaker. In order to avoid ambiguities from targets having different velocities within the predetermined range, the modulation frequency should be at least twice the upper limit of filter 13; saw-tooth or triangular modulation may be employed. The output from filter 13 may also be applied to produce a P.P.L. or type-A display of all targets within the predetermined range of speeds. Since the distance search by oscillator 7 is comparatively slow, a long time is required to complete a P.P.I. display and this may be reduced by combining the outputs from several systems searching different limits of distance.