GB680548A - Improvements in or relating to radar systems - Google Patents

Abstract

680,548. Radiolocation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. July 7, 1950 [Aug. 15, 1949], No. 21235/49. Class 40 (vii). Relates to frequency-modulated radar systems in which the beat note produced by heterodynizing the transmitted and received waves is frequency analysed by applying it to a fixed band-pass filter, the frequency of the beat note being cyclically varied at a comparatively slow " analysing" " rate by cyclically varying the rate of change of the transmitter frequency as disclosed in Specification 620,568 or by maintaining said rate of change constant and heterodynizing the beat note with a local oscillation whose frequency is cyclically varied at the " analysing " rate as disclosed in Specification 621,846 so that the filter produces an output at a time in the " analysing " cycle which is dependent on the range of the reflecting object. In order to obtain a more accurate range indication in such systems, the transmitter frequency is modulated with an additional recurrent sinusoidal or sawtooth " ranging " waveform as disclosed in Specification 622,101 whose recurrence frequency is adjusted to maximize the output from the filter. When this condition obtains the echo time is equal to the " ranging " period on an integral multiple thereof and the " ranging " frequency control may be calibrated directly in terms of range, but when the " ranging " frequency control is incorrectly set the beat note will be frequency modulated at the " ranging " frequency as shown in Fig. 2b which illustrates the conditions obtained with a sinusoidal " ranging " modulation having a frequency deviation less than the average frequency of the beat note. Under such conditions the filter is excited only when the frequency of the beat note sweeps rapidly through the pass band FB of the filter and the resultant output from the filter is reduced, but the presence of such an output tends to make the setting of the " ranging " frequency control rather flat and according to the present invention the resultant output from the filter when the " ranging " frequency control is incorrectly set is reduced by reducing the amplitude of the beat note as its frequency sweeps through the pass band of the filter recurrently at the ranging frequency. This may be achieved by applying the beat note to the filter through an amplifier whose grain is varied cyclically at double the ranging frequency by a sinusoidal or peaky waveform, Figs. 3 and 4; alternatively the amplitude of either the received echo wave or the transmitted wave applied directly to the receiver may be varied cyclically. When a sawtooth " ranging " waveform is employed, Fig. 5 (not shown), the gain control waveform is preferably sinusoidal and should reduce the gain to zero during the sawtooth flyback time.