Patent GB-1038632-A (GB)
Title: Radar apparatus
IPC: G01S7/02
Filed: 1963-05-21
Abstract: 1,038,632. Pulse radar. N.V. HOLLANDSE SIGNAALAPPARATEN. May 21, 1963 [May 23, 1962], No. 20184/63. Heading H4D. In a radar system of the type disclosed in Specification 944,098, wherein interference and jamming are overcome by altering the frequency and/or polarization of the carrier wave, the system causing this alteration is controlled by a signal from an automatic noise limiting device upon an upper limit of received noise being detected. A first embodiment comprises, at the output of gain controlled receiver 102, two parallel channels controlling the gain of the receiver in dependence on the absolute and relative strengths of the echo pulses and noise. Gate arrangement 114 allows that gain control signal through to the receiver which will cause the greatest decrease in gain. The echo channel comprises a video amplifier 115, feeding a display at 116 and a gate echo tracking system 117. The tracking system selects the echoes from a selected target which are used to control an aerial tracking system and to control an automatic gain control circuit 119, which, when operative, adjusts the gain of receiver 102, via OR gate 114, until the echoes have an amplitude equal to a chosen limit. The interference channel comprises a gate 104, closed during the transmission of a pulse and for a little time after that so that ground reflection is eliminated, and a noise level limiting arrangement comprising a Miller integrator 112 with feedback capacitor 111 and diode 110. The output of the integrator is normally fed via OR circuit 114 to reduce the gain of receiver 102 until the level of the interference falls to a limit much lower than that of the echoes. When both channels are producing outputs, OR circuit 114 passes on to the receiver that output which results in the lowest receiver amplification. The input of the Miller integrator is biased by a potential divider, such that when the interference level there just reaches the permitted maximum, the integrator output remains constant, and when this level is exceeded, the output immediately rises, said output not being allowed to fall below said constant value due to diode 110. Thus, in the absence of echoes, a high interference level, arising for example from jamming, results in the gain of receiver being reduced until the level of the interference at the output thereof falls to the chosen limit. An echo pulse, having an amplitude below the chosen limit, will produce no change. One above said limit however, causes the receiver amplification to be further reduced until the limit is reached. Increase of interference, causes further decrease in the receiver gain. A second Miller integrator 126 is provided and is normally fed with signals from the integrator 112. A diode 124 ensures that the output of the integrator remains at a constant quiescent level until the output of the integrator 112 reaches a value determined by a bias at the input 123, corresponding to the allowable limit of interference. When this limit is exceeded the output of the integrator rises and actuates a relay 128, which causes the change of the carrier frequency and/or polarization. To avoid " chattering " of the relay, relay contact A switches to feed the negative potential charge in condenser 133 into the input of the integrator, such that it is locked. When the charge on condenser 133 has gone, and assuming that, due to the charge of carrier frequency and/or polarization, the interference decreases and the output of integrator 112 is below the allowable limit, the output of the integrator falls to its quiescent value and relay 128 is deactuated. Contact A now switches to the left whereby a positive lock-on charge is fed into integrator 126 from condenser 131. To enable the detection of the interfering source, actuation of relay 128 may cause the transmitted pulses to stop. Thus the only incoming signals are the interference, and the aerial can thus track the interference source. However the gain of receiver 102 will be so reduced by the output of integrator 112, that the amplitude of the interference on lead 118, will be too weak for tracking. To overcome this, activation of relay 128 causes contact C to switch from the output of the interference channel to that of the echo channel. The receiver gain is then reduced to a lesser extent, such that the amplitude of the interference is large enough for tracking. An embodiment, dispensing with relay 128 is also described.