Patent GB-1492429-A (GB)
Title: Multibeam synthetic array mapping system
IPC: G01S13/90
Filed: 1975-06-13
Abstract: 1492429 Synthetic aperture radar HUGHES AIRCRAFT CO 13 June 1975 25439/75 Heading H4D In an airborne multibeam synthetic array radar where Doppler echo returns are continuously recorded on to a moving film strip, a real time twodimensional display is provided by a mapping system which includes a processor which reads the recorded Doppler information at successive ranges over distances equivalent to large radar motions and compares the read out signals at successive ranges with a reference function to simultaneously form a multibeam synthetic array and separates out predetermined Doppler centre frequency portions of the recorded echo signal, each Doppler centre frequency corresponding to the boresight of a respective synthesized beam, objects at various instantaneous azimuthal angles producing a respective Doppler echo signal frequency. These signals are combined and fed to a display means; e.g. a C.R.T., or to a recording means. The system comprises, Fig. 1, sidelooking antenna 1, transmit/ receiver 16, raw data recorder 18, data processor 12 and output device 10. In transmit/receiver 16, Fig. 2, reference oscillator 20 generates an IF signal f 0 . The R.F. signal is generated as (a-1)f 0  at the output of single sideband modulator 24, from the multiplied I.F. frequency signal a(f 0 ) at 22. Synchronizer 28 produces shaped synchronizing pulses 29 which control gate pulsed amplifier 34 to output the pulsed transmit signal which is fed through duplexer 36 to antenna 14. Received echo signals from irradiated targets, e.g. terrain, are received, and include a Doppler frequency shift f d . The received signals at frequency (a - 1 )f 0  + f d  are mixed with the multiplied I.F. signal a(f 0 ) at 30 and a Doppler I.F. signal f 0 -f d  is obtained and fed via amplifier 38 to mixer 40. A composite signal containing f 0  and a reference offset frequency f r  is mixed with the Doppler I.F. signal in mixer 40 to produce a bipolar video output signal f r +f d . This signal and pulses from synchronizer 28 are fed to raw data recorder 18 wherein a range gate generator 48, connected to synchronizer 28, gates the video signal to be displayed in C.R.T. 50 during a predetermined interpulse interval, and also controls the timebase of C.R.T. 50. Film transport mechanism 60 is controlled by a radar motion adjust device 69 and motor 68 controls passage of film strip 66 which is exposed to the C.R.T. display image via lens system 58 and is developed in processing unit 70. Data processor 12, Fig. 3, reads the thus recorded Doppler information by flying spot scan converter 85 comprising a read scanning C.R.T. 80 which illuminates the film 66 in a raster pattern 65 in the direction of film movement whereby each scan line thereof reads a particular range element of the Doppler information scan lines successively recorded perpendicular thereto. The vertical timebase of C.R.T. 80 is controlled by radar motion sweep generator, and the horizontal timebase is controlled by range sweep generator 84. As the film strip is moved past the read C.R.T. 80, the Doppler history or information signal recorded thereon modulates the light passing therethrough to be detected in photomultiplier tube 90, each raster forming a successive reading frame. As the slope of the Doppler history or information signal frequency versus time is a function of range, the signal from video amp. 92 is mixed at 93 with a linearly frequency varying reference signal of predetermined slope for a respective range element, the start and end points and slope being adjusted accordingly to match the slope of the Doppler shift during computation over successive ranges. Radar motion sweep generator produces a ramp function which is modified to such an end, by the instantaneous range information in range sweep gen. 84 which controls variable gain amp. 86, the output voltage of which controls the frequency output from oscillator 88 which is fed to mixer 93. If the synthesized antenna boresight is squinted off broadside, at an angle #, Fig. 9, for example corresponding adjustments are made to this circuitry, to modify the reference frequency function. The mixture signal from mixer 93 contains components representing the difference frequency between the Doppler history and the reference frequency function which are selected by bank of filter units 95. Each filter unit, Fig. 4, comprises narrow band filter 97, amp. 98, envelope and box car detector 99 and commutator 100 which outputs the envelope voltage stored capacitively at 99 when gated by gate generator 103 which sequentially commutates the filter unit outputs under the control of radar motion sweep generator 82. The actual frequency to which each integrating narrow band filter 97 is tuned determines the angle # with respect to the flight path at which the readout target Doppler history cross-correlates to a maximum with the reference frequency functions whereby the tuning of each of filters 97 determines the exact azimuthal pointing direction of the corresponding synthetic array beams, e.g. 131-135, Fig. 9, all of which lie within the real antenna illumination 130. The stored or boxcarred outputs of filter bank 95 represent the correlation amplitudes at one range element over a multiplicity of azimuthal pointing directions. Successive range elements are azimuthally scanned on successive scans of raster frame 65, and, due to the relative movement of film 66 and raster 65, a target, as its azimuthal position changes due to the relative motion between radar and target, migrates through successive filter channels and/or range increments. The filter outputs are commutatively applied to an output device which may be a moving strip recorder or may comprise real time display system 10 comprising display C.R.T. 110. Radar motion sweep generator 82 synthesizes an azimuthal sweep for a given range increment by causing gate generator 103 to sequentially gate through filter units 95, which it is stated, may total 100 units and additionally to trigger azimuth sweep generator 112 which deflects the C.R.T. electron beam in arrow direction 115, in synchronism with the synthesized azimuthal direction corresponding to each filter unit, which provides an output signal dependent on echo signal strength and also deviation of the target from the boresight of the synthesized beam. Range sweep generator 84 provides the vertical timebase voltage which synchronizes the display with the range increment being read on raster frame 65, e.g. R2 in Fig. 9. A two dimensional real-time high resolution picture in range and azimuth of the region illuminated by the real antenna is thereby realized.