GB680704A - Improvements in or relating to radar systems - Google Patents

Abstract

680,704. Radiolocation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. April 22, 1949 [July 7, 1948], No. 18302/48. Class 40 (vii): In a frequency-modulated radar in which the direct and reflected signals are heterodyned to give a beat note having a frequency dependent on the distance of the target, means are provided for discriminating between targets having different relative radial velocities or for determining the magnitude and sense of the relative velocity. The invention utilizes the fact that the beat note produced by a stationary target comprises a group of harmonics of the modulation frequency whereas a moving target produces corresponding components which are displaced by the Doppler shift frequency and according to the invention the beat note is applied to a " comb " filter having a plurality of pass or rejection bands, the frequency spacing between mid-points of adjacent bands being equal to the modulation frequency. Thus, when the pass or rejection bands are centred at harmonics of the modulation frequency, the filter will pass frequency components produced by targets moving at less than or greater than a predetermined velocity which depends on the bandwidth. When asymmetrical modulation is employed, e.g. sawtooth, the components of the beat note are shifted in a direction dependent on the sense of the relative velocity and this may be utilized to determine the magnitude and sense of the relative velocity by analysing the frequency components by means of a variable comb filter or by heterodyning the components with a variable frequency and applying the resultant to a fixed comb filter. Range indication may be provided by heterodyning the filter output with a cyclically varying frequency and applying it through a bandpass amplifier and a detector to a cathode-ray tube display, e.g. P.P.I., Fig. 1 (not shown). Due to the finite response time of the comb filter, the heterodyne frequency variation should comprise spaced sawteeth, Fig. 2. Alternatively, each band of a fixed comb filter may be constituted by a plurality of separate elements each having a single narrow pass or rejection sub-band, the sub-bands of each group of elements being substantially contiguous in the frequency spectrum and the elements being connected to individual detectors indicating range, speed and sense. When using a rejection comb filter the modulation frequency should not be less than the maximum Dopple shift frequency. Specification 595,063 is referred to.