Patent GB-1470452-A (GB)
Title: Doppler pulse radar systems
IPC: G01S13/70
Filed: 1974-06-03
Abstract: 1470452 Pulse Doppler radar SIEMENS-ALBIS AG 3 June 1974 [14 June 1973] 24443/74 Heading H4D A pulse Doppler radar system comprises suppression means for echo signals caused by moving and changing weather conditions, which are received within a selected range window in addition to echo signals caused by a moving target. The suppression means generates a measurement or sampling window which is distinct from the range window (although the two windows preferably adjoin). The location of the measurement window may be controlled, in distance by the range tracking means of the radar, and, angularly by deflection of the radiation beam of its aerial. The Doppler signals from the measurement window are analysed by a device which generates a signal when the net amplitude of detected components of frequencies below a set threshold frequency is greater than a set threshold amplitude and is less than the net amplitude of detected components of frequencies above said threshold frequency said signal from the analysing device apparatus during the period of the measurement window to transfer to a store a coded representation of that one of a plurality of cyclically switched quantized compensating quantities which is currently operative during the said period. The compensating quantity is utilized to change the frequency of the Doppler signal and remains effective during succeeding range window periods until replaced by another quantity during a measuring window period. In a first embodiment, a transmitter 7, Fig. 1, is modulated by an ocillator 9 to produce pulses of repetition frequency fR which are applied through a T/R switch 2 to an aerial 1. Echo signals are passed through the T/R switch to a mixer 4, which also receives an input from a local oscilltor 6. After amplification in an I.F. amplifier 5, the output of the mixer 4 is applied as a signal SA to one input of a demodulator 12. A portion of the output of the transmitter 1 is coupled to one input of a mixer 8, which also receives an input from the local oscillator 6. Pulses of I.F. from the output of the mixer drive a coherent oscillator 10, which stores the phase of the transmitted signal by continuing to oscillate until the end of the repetition period. At the output of the oscillator 10 a modulator 11 is connected, which provides a reference signal SB to the other input of the demodulator 12. A range gate 14 and a measurement gate 17 are connected to the output of the demodulator 12. A range tracking means 15 connected to the range gate 14 controls the latter such that the echo from an airborne target remains centered within the range window. The signals passed by the gate 14 contain the echo from the target and any weather echoes. An M.T.I. circuit 16 removes any fixed target echoes, and produces an output video signal S16. The tracking means 15 also controls the timing of the measurement window set up by the gate 17, such that it adjoins the range window. The signals passed by the gate 17 contain any weather echoes only, and are supplied to an analyser device 18, which is able to generate a signal S18 for application to the modulator 11. Said modulator comprises three phase shift devices 29, 30, 31, Fig. 2, which are arranged in series between the oscillator 10 and the demodulator 12 and which can be individually switched in by outputs g, h, i of a digital multiplier 27 to provide respective phase increments of 180, 90, 45 degrees. Thus, eight phase insertion steps are available, ranging from 0 to 315 degrees. By selecting one of said steps, and by advancing by it at each succeeding transmitted pulse, a compensating frequency modulation of ŒfR/2 in steps of fR/8 is available. The digital multiplier is cycled by coded outputs from an 8 : 1 counter 25, driven at the P.R.F., fR, and receives inputs from a change-over switch 37 in a code representing a compensating phase insertion step. Said switch is operated by the timing signals S15A, S15B from the range tracking means 15 for the range and measurement windows, respectively. During the range window a code from a store 24 is connected to the multiplier 27, and during the measurement window a code from a counter 23 is connected thereto. The counter 23 is driven by pulses S26 from a 256 : 1 counter 26 having input at the P.R.F., fR. The eight codes representing the different phase insertion steps appear successively at the outputs of the counter 23. Said outputs are also connected to a switch 38, which operates when there are output signals from both the counter 26 and the analyser 18, to write a new code into the store 24. In the analyser 18 the signal from the measurement gate is fed to the inputs of low- and high-pass filters 20, 21 which separate out components having frequencies less than, and greater than, a predetermined threshold frequency fg. The outputs from the filters 20, 21 are rectified and smoothed, and respective D.C. outputs S40, S41 are obtained. The two outputs are applied to respective inputs of a differential amplifier 44, and the output S40 is also applied to one input of a differential amplifier 42, a threshold voltage US being applied to the other input thereof. The outputs of the amplifiers are applied to the two inputs of an AND gate 43. If the voltages are such that S40 is greater than 541, and is greater than US, the gate 43 is enabled, and furnishes the signal 518. In another embodiment (Fig. 4, not shown) a measurement window is generated at each nth. transmitted pulse, which is angularly offset with respect to the range window. An aerial with electronic beam deflection arrangements is used, and during deflection the output of a common range gate is switched to an analyser. The operation otherwise is as described for the first embodiment. It is possible to arrange for a measurement window which is offset both angularly and in range. The frequency compensation means may be otherwise located than in the modulator 11, e.g., as indicated at points A to E in Fig. 1, or the frequencies of the oscillator 6, or the oscillator 10, or the transmitter 1, may be influenced.