Patent GB-1426143-A (GB)
Title: Target detection system in a radar system employing main  and guard channel antennas
IPC: G01S7/292
Filed: 1974-06-17
Abstract: 1426143 Radar HUGHES AIRCRAFT CO 17 June 1974 [15 June 1973] 26728/74 Heading H4D  A radar system which has first and second target echo signal receiving aerials, and associated respective main and guard signal channels, comprises first means responsive to the outputs from said channels for providing a selected output only when the ratio of the main channel output to the guard channel output exceeds a selected threshold value. The system also comprises second means responsive to the outputs from said channels and from said first means fr providing a target indicator signal either when said main channel output exceeds a selected threshold value and said guard channel output is less than a selected threshold value or when the outputs of both channels exceed their respective threshold values, and said first means provides said selected output. As described, the system is installed in an aircraft (Fig. 1, not shown) and the main aerial 12, which is used to transmit and to receive, has a narrow main lobe, whilst the guard aerial 13, which is used only to receive, has a wide beam radiation pattern. The two aerials are angularly offset, so that when the axis of the main aerial is horizontal, the axis of the guard aerial is directed to the ground. The purpose of the invention is prevent false target indications, which might be caused by reflection from the ground of energy directed thereto in a side lobe of the main aerial radiation pattern. The radar signals to be transmitted from a transmitter 22 are coupled through a transmit/receive (TR) unit 23 to the main aerial 12. Signals received by the main and guard aerials 12, 13 are coupled to first inputs of respective mixers 24, 31 (in the case of the aerial 12, via the TR unit 23) A local oscillator 25 is coupled to second inputs of the mixers 24, 31, whose outputs are amplified in amplifiers 26, 32 and are processed in range/Doppler processors 28, 33. An automatic gain control unit 35 controls the gain of both amplifiers, in accordance with the signal amplitude in the main channel. The output a M  of the processor 28 for each range/Doppler cell of interest represents the main channel output, and, similarly, the output a G  of the processor 33 for corresponding range/ Doppler cells represents the guard channel output. The outputs a M , a G  are supplied to respective detection logic units 40, 42, and to respective inputs of a main-to-guard ratio unit 41. The units 40, 42 are comparators, which give respective outputs V M , V G  when their inputs a M , a G  exceed threshold levels T M , T G . The output R of the unit 41 is fed to a detection logic unit 44( a comparator) which gives an output V R  if R exceeds a ratio threshold K. The outputs V M , V G , V R  are supplied to a target discrimination unit 45 which provides an output D only when V M  is present and V G  is absent, or when V M , V G , V R  are all present. Suitable logic circuitry is described with reference to Fig. 7 (not shown). The functions of the units 41, 44 may be combined by providing a comparator which is either supplied with the output a M  and with the output a G  multiplied by K (i.e. Ka G ) or is supplied with the output a M  divided by K (i.e. a M /K) and with the output a G  (Figs. 3a, 3b, not shown). It is stated that the threshold levels T M , T G  are advantageously selected as functions of, and greater than, the background noise levels in their respective channels. The background noise level in each channel may be obtained by connection a constant false alarm circuit to the output of the processor (28 or 33) and this may be followed by a multiplier to obtain the threshold level (Fig. 6, not shown). The multiplying factor may be 1À5 in the case of T G , and 3 in the case of T M .