Patent GB-735941-A (GB)
Title: Improvements in or relating to radar systems
Inventor: RUST NOEL MEYER
IPC: G01S7/28
Filed: 1952-07-24
Abstract: 735,941. Radar. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. June 1, 1953 [July 24, 1952], No. 18820/52. Class 40 (7). A radar system enabling discrete targets hidden in or beyond a non-discrete masking reflector (such as " window ") to be indicated is provided with at least two spaced aerials giving two different paths from station to target and back and the relative lengths of the two paths are periodically varied, means being provided to separate returned signals having a modulation dependent on the periodicity of variation from those not so modulated. It is stated that if two " go and return" paths which are periodically varied in relative length are used and the echo signals 'combined the resultant will contain for an individual target a modulation component at the frequency of variation. Such modulation component is also present in echoes from individual elements in the masking but as its phase differs for different elements there is, on summation, an overall resultant in which the modulation frequency is cancelled out. For scanning round a complete 360 degrees it is preferred to use two pairs of spaced aerials at the ends of mutually perpendicular base lines, using each pair for the quadrants in which it is most effective. In one embodiment of a pulse radar five directional aerials A, A1, B, B 1 and C rotated in azimuth in unison are used. The central aerial C is used for transmission and all five for reception when A, Al and C and B, B1 and C are coupled to the indicator whilst scanning alternate quadrants. Each of the outer aerials is coupled through a T.R. box and phase-changer to combining units CA and CB to each of which the aerial C is also coupled through a T.R. box. All the phase changers are driven at the same speed, those that are associated with the same combining unit being driven in opposite directions. The signals from the units CA and CB are fed over switch SW whose operation is interlocked with the aerial rotation to mixer M fed by local oscillator L.O. to give intermediate frequency signals amplified at I.F.A. The signals at the output of I.F.A. may be fed to a standard type of P.P.I. display device P.P.I.T. scanned by time bases T.T.B. and also to the special display arrangement of this invention. In the latter an adjustable annular ring of display of constant range width is formed on the display tube TP by selecting a variably positioned but constant width section of the radial scan in unit RC and feeding it to the tube. The I.F. signals are fed to a series of gates G1, G2 ... GN which are successively opened for equal periods of time during each radial scan of the annular ring by means of gating pulses derived from the transmitter, delayed at PH in accordance with the setting of unit RC, and fed directly to G1 and through delay devices D2 ... DN of successively increasing values to gates G2 ... GN. Each gate circuit includes a frequency selective filter which passes only those signals modulated strongly at the frequency of variation of the phase changers. These filters eliminate echo signals due to masking and echoes due to discrete objects are passed and fed to signal stroing devices ST1, ST2 ... STN from whence they are picked off by an electronic switch SSW (shown as mechanical in the drawing) operated in synchronism with the range scan. The signals so picked off intensify modulate the annular section of tube TP. The invention can be applied to radar systems of other than pulse type and a frequency-modulated system is described with reference to Fig. 2 (not shown). In this system the aerial and phase-changing arrangements as far as switch SW are similar to those described above except that the transmitter is a frequency-modulated transmitter. After switch SW the signals received are mixed with the radiated signals and applied to two indicators to produce three possible diaplays. The first indicator may be used either to show targets with a component of radial velocity picked out by determination of the constance of a Doppler shift or stationary (and orbiting) targets. The second indicator shows discrete targets within a variably positioned constant width annulus such as that described with reference to tube TP of Fig. 1.