1,120,293. Pulse radar. NORTH AMERICAN AVIATION Inc. 5 Oct., 1965, No. 42295/65. Heading H4D. In an airborne moving target indicator radar, using a non-coherent pulse system, the radar return pulses are applied to a unipolar video detector to provide a beat frequency between the ground clutter and moving target spectra. A filter rejects the non-moving target signals and accepts signals from targets within a selected range of ground velocities. Range gating of the received signals is effected by a scan converter, which feeds the band-pass filter, the output from which controls the display device. The scan converter may provide either optical or electrostatic storage and has orthogonal read and write modes. The system provides clutter-referenced signals, independent of the radar motion and requires only a single Doppler band-pass filter. The non-coherent radar 10, Fig. 1, feeds the scan-converter 12 through the diode detector 11. The band-pass Doppler filter 13 is supplied from 12 and controls the indicator 14. The transmitted spectrum, Fig. 2a, of carrier frequency f o , produces a received spectrum. Fig. 2b, having two envelopes, 16a and 16b corresponding to clutter and moving target returns respectively, the latter being Doppler-shifted by the amount f TD . The effect of diode 11 is to provide the folded spectrum, Fig. 2c, having the clutter envelope 16c and the target components 17, corresponding to the beat frequency between the moving target spectrum and the ground clutter spectrum. The ground clutter is represented by the zero frequency signal 15c regardless of the radar motion. The target beat frequency is passed through the band 18, Fig. 2d, of the filter 13 and the clutter, 15c is rejected. The variation of signal intensity at range R 2 , Fig. 3, indicates the presence of an A.C. signal having a spectral content corresponding to 17 of Fig. 2c. In a modification of the band-pass filter 13, to prevent the spread of the clutter spectrum into the band-pass zone due to increasing radar velocity, two filters having contiguous pass bands 18a, 18b, Fig. 8, are employed. A separate threshold control is associated with each filter so as to relate the threshold level of each component filter to the clutter spectral remnant 115a spreading into the band-pass.
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