PULSE RADAR APPARATUS

Abstract: A pulse radar apparatus is provided with a coherent transmitting and receiving unit, including a transmitter (1) and a transmitting antenna (2) for the transmission of radar pulses and the generation of test signals, N juxtaposed receiving antennas (A₀, A₁, ..., A N ) and receivers (B₀, B₁, ..., B N ) connected thereto for the reception of echo signals and the processing thereof in each of the receivers into two orthogonally phase-detected and digitised video signal components, and with a DFT beamformer (4) to derive from said components, after multiplication, in a transformation circuit (C₁, C₂, ..., C N ) by a corresponding weighting factor, the orthogonal components of the video signal determined jointly by the N receivers. The pulse radar apparatus com­ prises a processor (5) to derive amplitude- and phase-­ correction signals from the beamformer input signals ob­ tained through the test signals. The amplitude- and phase-­ correction signals are supplied to the transformation cir­ cuits (C₁, C₂, ..., C N ) to compensate for mutual differences in gain and phase shift between the receivers.

Description: Pulse radar apparatus The invention relates to a pulse radar apparatus, provided with a coherent transmitting and receiving unit, including a transmitter and a transmitting antenna for the transmission of radar pulses and the generation of test signals, N Juxtaposed receiving antennas and receivers connected thereto for the reception of echo signals and the processing thereof in each of the receivers into two orthogonally phase-detected and digitised video signal components Ii and Qi' where i = 0, 1, 2, ..., N-i, and with a DFT beamformer to derive from said components, after multiplication of each Ii, Qi pair in a transformation circuit by a corresponding weighting factor Wi, the orthogonal components 1k and Qk of the video signal determined Jointly by the N receivers, and to supply the Ik and Qk components via beamformer output channel k corresponding with the angle of incidence of the received echo signals. Such a pulse radar apparatus is known from: Abraham E. Ruvin and Leonard Weinberg: "Digital Multiple Beamforming Techniques for Radar", IEEE EASCON '78 Record, pp. 152-163, Sept. 25-27, 1978, IEEE Publication 78 CH 1352-4 AES. In this pulse radar apparatus, errors in the beamformer output signals will be incurred, because of mutual differences in gain and phase shift in the various receivers. Differences in gain disturb the applied weighting factors and hence the desired reduction of sidelobes. A difference in the phase shift has the effect that the beamformer regards the target to be coming from a direction other than the true direction. Moreover, phase errors disturb the desired sidelobe suppression. The present invention has for its obJect to eliminate this kind of errors as much as possible. According to the invention, the pulse radar apparatus thereto comprises a processor to derive at any desired moment amplitude- and phase-correction signals from the beamformer input -signals obtained through- the test signals under the condition that all weighting factors be of the same constant value, which amplitudeand phase correction signals, or at least a portion thereof, are supplied to the transformation 'circuits to compensate for mutual differences in gain and phase shift between the receivers. In this way a correction of the beamformer input signals is obtained. In the operational mode a weighting function varying in vertical direction is applied, while during a gain- and phasecorrection cycle, which may be executed once or several times per radar sweep, a constant is introduced as weighting function. The invention will now be explained in more detail with reference to the accompanying figures, of which: Fig. 1 shows a first embodiment of the pulse radar apparatus according to the invention; Fig. 2 is a diagram illustrating the beam directions, as determinable by the beamformer. Fig. 3 illustrates a diagram of a specific beam pattern, as determined by the beamformer with the application of equal and different weighting factors; and Fig. 4 shows a second embodiment of the pulse radar apparatus according to the invention. The pulse radar apparatus of Fig. 1 comprises a coherent transmitting and receiving unit, including a transmitter 1 and a transmitting antenna for the transmission of radar pulses and the generation of test signals T, and N vertically disposed receiving antennas Ag, A1, ..., AN 1 with receivers Bos B1, ..., BN 1 connected thereto for the reception of echo signals and the processing thereof ineach of the receivers into two orthogonally phase-detected and digitised video signal components Ii and where i = 0, 1, ..., N-i. If the transmitter signal be represented by F(t).e2wJfot, where f is the transmitter frequency and F(t) the envelope of the transmitter pulse, the echo signal received by antenna A. can be expressed by EMI2.1 where G(t) is the envelope of the echo signal, fd the Doppler frequency, d the mutual distance of the receiving antennas, a the elevation angle at which wavefront 3 from the target is incident on the receiving antennas, c the velocity of light, and ç a phase angle depending on the target range. The echo signal transformed to intermediate frequency in receiver B. and subsequently coherently phase-detected is expressed by EMI3.1 or, with c=X.f and d = iA, by g(t,fd).e#jicos@; the orthogonal components of this signal are Ii and Qi. Receiver B. supplies these components in digital form. The Ii and Qivalues can be supplied directly to a DFT beamformer 4. The orthogonal components of the video signal determined Jointly by the N receivers are delivered by beamformer 4 via output channel k, the video signal being proportional to EMI3.2 The absolute value of the video signal passed via output channel k is proportional to EMI3.3 That is, each output channel of beamformer 4 corresponds with a receiving beam pattern, whose main direction is determined by 2k the relationship cosa= N. Possible beam directions which are thus determinable are shown in Fig. 2. If, for example, N=16, so that an FFT can be used as a special form of a DFT for beamformer 4, the main directions of the receiving beam patterns are obtained with a=0, 28.960, 41.410, 51.32 , 600 ..., 128.680, 138.590, 151.040 and 1800 for k= 8, 7, 6, 5, 4, ..., -5, -6, -7, -8. If the antenna system of the pulse radar apparatus is arranged at an angle of 430, say, only the receiving beam patterns are of concern, where the main directions are obtained with 01=51.320, 600, ..., 128.680 for k = 5, 4, ..., -5. In such a case, the main directions are of course -at--an-g-Iee-o# a -47 with respect to the horizontal plane (earth's surface). Therefore, of the 16 beam former output channels available in this case, only 11 are used. A target situated at an elevation of, say, 500, produces a video signal in the output channel denoted by k=-1; the video signal falls within the receiving beam pattern whose main direction 01=97.180, or 50.180 with respect to the horizontal plane. The pulse radar apparatus depicted in Fig. 1 is further provided with transformation circuits C0, C1, ..., CN 1 for multiplying the I. and Q. by a weighting factor W.. These factors 1 1 1 form a weighting function varying in vertical direction, the purpose of this function being to reduce sidelobes. The effect thereof is illustrated in Fig. 3. Following the given example, the continuous curves in Fig. 3 are representative of the receiving beam pattern, whose main direction is obtained with &alpha;=97.18 for k=-1; this pattern gives the normalised absolute values expressed in dB's for the video signal supplied via output channel k =-1 without the application of a spatially varying weighting function. The dashed curves give these values with a favourably selected weighting function and illustrate the effect of this function on the reduction of sidelobes. Because of mutual differences in gain and phase shift in receivers Bo, B1, ..., BN,1, errors will occur in the output signals of beamformer 4. Differences in gain disturb the applied weighting function and hence the desired reduction of sidelobes. Differences in phase shift may have the effect that the beamformer regards the target to be coming from a direction other than the true direction and/or disturb the desired reduction of sidelobes. To prevent such errors, the pulse radar apparatus comprises a processor 5 for deriving at any desired moment amplitude and phase correction signals from the beamformer input signals obtained through the test signals under the condition that all weighting factors be of the same constant value, which amplitude and phase correction signals, or at least a portion thereof, are supplied to the transformation circuits COS C07 ..., CN,1 to compensate for mutual differences in gain and phase shift between the receivers Bo, B1, ..., By 1 In the embodiment of Fig. 1,-both amplitude correction signals K. and phase correction signals ais ssi are fed to the transformation circuits C0,C1, ..., CN,1, In the embodiment of Fig. 4 described hereinafter the transformation circuits C0, C1, ..., l are fed with amplitude correction signals K. and fine phase-correction signals and the receivers Bos B1, ..., BN 1 with coarse phase-correction signals. The T test signals can be either injected into the antenna elements or the HF channels of the receivers Bob B1, ..., BN-1, or received by the receiving antennas A07 A1, ...,an 1 via a slot radiator placed in front of these antennas. The test signals are processed in receivers B0, B1, ..., BN 1 in the conventional way, but all are subsequently multiplied by the same weighting factor. The then obtained signals are not processed in beamformer 4, but supplied to processor 5 via a buffer circuit 6. From the amplitude and phase differences of these signals the processor 5 derives amplitude and phase correction signals Ki and ai, ssi' respectively. The amplitude- and phase-correction signals are supplied to the corresponding transformation circuits. In transformation circuit Ci, the supplied Ii, Qi signals are converted into Ii, Qi signals according to: EMI5.1 Ii' = Wiki(&alpha;iIi+ssiQi), Qi' = WiKi(-ssiIi+&alpha;iQi). If Ii and Qi are the orthogonal components of the EMI5.2 signal, then Ii and Qi are the orthogonal components of the EMI5.3 signal. That is, the weighting factors Wi are corrected by a factor K., while the signal receives a phase correction arctan i. 01i In the embodiment of Fig. 4 a distinction is made between coarse and fine phase corrections. Processor 5 thereto comprises means for determining coarse and fine phase-correction signals from the signals supplied via the buffer circuit 6. In a coho phase correction circuit 8 the-phase of the coho signal fcoho required for the coherent phase detection in the receivers is corrected for each receiver by the coarse phase-correction signals. Moreover, the fine phase correctionf as well as the amplitude correction, can be made in the above way in transformation circuits COS C1, ..., Con 1 However, the fine phase correction can then be applied more simply through multiplication by a phase factor cossi according to: EMI6.1 Ii' = WiKi cos#i Ii Qi' =WiKi cos#iQi.

Claims: Claims: 1. Pulse radar apparatus provided with a coherent transmitting and receiving unit, including a transmitter and a transmitting antenna for the transmission of radar pulses and the generation of test signals, N Juxtaposed receiving antennas and receivers connected thereto for the reception of echo signals and the processing thereof in each of the receivers into two orthogonally phase-detected and digitised video signal components Ii and Qi' where i= 0,1, 2, ..., N-i, and with a DFT beamformer to derive from said components, after multiplication of each Ii, Qi pair in a transformation circuit by a corresponding weighting factor Wi, the orthogonal components Ik and Qk of the video signal determined Jointly by the N receivers, and to supply the 1k and Qk components via beamformer output channel k corresponding with the angle of incidence of the received echo signals, characterised in that said pulse radar apparatus comprises a processor to derive at any desired moment amplitude- and phase-correction signals from the beamformer input signals obtained through the test signals under the condition that all weighting factors be of the same constant value, which amplitude- and phase-correction signals, or at least a portion thereof are supplied to the transformation circuits to compensate for mutual differences in gain and phase shift between the receivers. 2. Pulse radar apparatus as claimed in claim 1, characterised in that both the amplitude-correction signals Ki and the phasecorrection signals ais ssi are supplied to the corresponding transformation circuits, whereby the transformation circuit concerned converts the supplied orthogonal components Ii, Qi into the orthogonal components Ii, Qi' according to: EMI7.1 Ii' = Wi Ki (&alpha;i Ii + ssi Qi) Qi' = Wi K (ssi Ii +&alpha;i Qi). 3. Pulse radar apparatus as claimed in claim 2, characterised in that the processor comprises means for determining coarse and fine phase-correction signals and that further a coho phasecorrection circuit is provided to correct, for the coherent phase detection in the receivers, the phase of the coho signal for each receiver in response to the coarse phase-correction signals, while both the amplitude correction signals K. and the fine phasecorrection signals cossi are supplied to the corresponding transformation circuits, whereby the transformation circuit concerned converts the supplied orthogonal components Ii, Qi into the orthogonal components Ii', Qi' according to: EMI8.1 Ii = WiKi c S@i co Qi' = wiKi cos#i Qi.

Inventors: GELLEKINK, BERNARD, HOL, WILLEM ANDRIES
Applicants: HOLLANDSE SIGNAALAPPARATEN B.V.
Application year: 1983
Main classification: G01S7
IPC codes: G01S13, G01S7, H01Q3