Patent GB-1408064-A (GB)
Title: Radar apparatus
IPC: G01S7/42
Filed: 1973-06-13
Abstract: 1408064 Radar FUJITSU Ltd 13 July 1973 [14 July 1972] 33635/73 Heading H4D Radar apparatus having an aerial arrangement whereby the apparatus can emit simultaneously first and second signals that are respectively polarized in mutually distinctive manners and can receive simultaneously first and second reflected signals that are respectively polarized in the said mutually distinctive manners, includes phase detector means arranged to provide, from the said first and second reflected signals, a detected signal which varies with the phase difference between those reflected signals, whereby frequency components in the detected signal that are due to clutter, and have frequencies in the frequency range of frequency components in the detected signal that are due to an object to be located, are of reduced amplitude as compared with those frequency components in the detected signal that are due to such an object. The apparatus further includes signal processing circuitry connected to receive the said detected signal, the circuitry being adapted to suppress preferentially, from the detected signal received thereby, those frequency components that are due to clutter and have frequencies above the said frequency range. As described, a radar apparatus comprises an aerial 1, Fig. 5, for horizontally planepolarized waves, an aerial 2 for vertically planepolarized waves and duplexers 3, 5, which are supplied with R.F. power, when a pulse is transmitted, by a transmitting section 6 through a variable-ratio distributer 4. Received signals are fed from the duplexers 3, 5 to respective mixers 7, 9, which also receive inputs from a local oscillator 8. Outputs from the mixers 7, 9 are amplified in respective logarithmic linear I.F. amplifiers 10, 11. Output from the amplifier 10 is connected directly to a first input of a homodyne detector 13 and, through a 90 degree phase shifter 12, to a first input of a homodyne detector 14. Output from the amplifier 11 is connected directly to second inputs of the detectors 13, 14. The detected signals from the homodyne detectors 13, 14 respectively pass into two signal channels I and Q. The target signal component of the signal in the channel I is dependent upon the amplitudes and phases of the respective target signal components in the reflected horizontally- and vertically-polarized signals received by the aerials 1, 2, and remains substantially unchanged with time. However, the clutter signal components in the reflected signals received by the aerials 1, 2 have rapidly changing amplitudes and phases, and so the clutter signal component in the signal in the channel I appears as a noiselike component of rapidly changing amplitude and polarity, which may be suppressed by filtering. Any Doppler effect phase shift owing to movement of the target is experienced equally in the horizontally- and vertically-polarized signals, and so the signals from the homodyne detectors 13, 14 can be handled in the same manner as in the case of a fixed target. The detector 14 acts analogously to the detector 13, but the introduction of a 90 degrees phase shift between its inputs ensures that it will provide an output in the event that the reflected signals received by the aerials 1, 2 should differ in phase by 90 degrees. The detected signals in the I and Q channels are processed in respective gated target detection circuits 15, 16 and are combined in an adder 17, which supplies the resulting composite signal to an indicator 18. Each gated target detection circuit comprises a distributer 19, Fig. 6, feeding a plurality of range channels which are controlled by a pulse distributer 20. Each channel comprises a boxcar detector 21, a low-pass filter 22, which allows the passage of the target signal component, a polarity converter 23 for converting all signals to the same polarity, and a sampling circuit 24 which is connected to a signal adder 25. Other forms of mutually distinctive polarization may be used for the signals handled by the aerial arrangement instead of horizontally and vertically plane-polarizations, for example, circular polarizations.