Patent GB-1247710-A (GB)
Title: Improvements in the echo processing apparatus of side looking detection systems operating with frequency modulated transmitted pulses
IPC: G01S13/28
Filed: 1968-12-12
Abstract: 1,247,710. Radar. THOMSON-CSF. 12 Dec., 1968 [13 Dec., 1967], No. 59254/68. Heading H4D. [Also in Division G2]  Specification describes a sideways looking radar employing "chirp" type transmitted pulses wherein by applying a degree of echo pulse compression at the receiver dependant on the target range before the coherent detection process, the film record is in the form of a hologram representation of the target, whereby an observer can view a normal image of the target by illuminating the film with coherent light. In the radar arrangement of Fig. 6, which is carried by an aircraft; the output of I.F. oscillator 61 at f 0  is fed to a frequency modulator and dispersive filter arrangement 62 whereby long within-pulse frequency modulated pulses are produced. These are heterodyned up to a centre frequency of f 0  + F 0  by means of oscillator 66 and mixer .64 and are transmitted from antenna An in a beam extending laterally of the aircraft O, Fig. 2, and having an angular width 26 o . With aircraft travelling with the velocity V a target at range Yo = OMo enters the beam at B and leaves at A. Describing now the simpler case of transmitting none modulated short pulses. If the transmit and receive phase difference of a pulse along the shortest (perpendicular) path OMo is # 0 , then that along path OM is # given by where k is a constant and At is the time for the target to pass from M to Mo. The output of a coherent phase detector comparing the transmitted and received pulses (carriers) would thus vary from pulse to pulse as E.Cos k. #t<SP>2</SP> and this function is shown in Fig. 3a, where Applying the above signal as brightness modulation to a cathode ray display tube having a linear sweep synchronized with the pulse transmission and projecting the displayed varying intensity line on to a film moving at a speed proportional to V in a direction orthogonal to the line, a line is produced along the film positioned from one edge by a distance representing Yo and varying in brightness according to the curve of Fig. 3a. In the present case however, the pulses applied to the coherent phase detector 91 are IF " chirp " pulses whereby each such pulse produces a detector output also varying as shown in Fig. 3a but of length T 1  equal to the pulse width. A corresponding variation results in the displayed line on the tube 88. The result of this variation in combination with the variation, also according to Fig. 3a, of corresponding parts of the line from pulse to pulse, produces, on the film, a series of concentric ellipses, the dimension L 1  of the series, in a direction laterally of the film, remaining constant (proportional to T 1 ) independant of the target's range, but the dimension L 2 , longitudinally of the film, varying according to the target range as varies, from when L 2  > L 1  through L 2  = L 1 , to L 2  < L 1 . To convert all these varying ellipses to circles, the I.F. " chirp " pulses are fed, via sequentially opening range gates 81/85, to filters 71/75 which compress or expand the pulse as necessary according to the range, whereby L 1  is varied to be always equal to L 2 . The resulting representation on the film of each target as a series of concentric circles or " Newton rings ", is equivalent to a hologram and thus by illuminating the film at 100 with coherent laser light from laser 96, a viewer at 101 may observe a normal image of the targets at the virtual focal point F 1 . By illuminating the film off-axis the real image is projected to one side of the viewer. A camera 102 may also photograph said virtual image.