ELIMINATION IN A PULSE RADAR RECEIVER OF INTERFERENCE SIGNALS STEMMING FROM N DISTINCT SOURCES

Abstract: In a method for the cancellation of interference signals, originating from a number n of sources, a radar receiver containing a main channel and a number n of auxiliary channels for the reception, i.f. detection, sam­ pling and digitising of signals E m E₁, E₂ . . . , E n , is used, which signals are composed of target return signals and interference signals. From these signals weighting factors W₁, W₂, . . . W n are obtained in accordance with the matrix equation: The target return signals E/ m processed in the main channel are then determined in accordance with the equation: E′ m = E m - W₁E₁ - W₂E₂- . . .-W n E n .

Description: Method and apparatus for the cancellation of interference signals from a number n of sources during the reception of return signals by a pulse radar. The invention relates to a method for the cancellation of interference signals from a number n of sources during the reception of target return signals by a pulse radar, of which the receiver comprises a main channel for the reception, i.f. detection, sampling and digitising of signals (Em), composed of target return signals (elm) and of interference signals, and n auxiliary channels for the reception, i.f. detection, sampling and digitising of signals (E1, E2, ..., En) composed of target return signals and interference signals. The invention further relates to the pulse radar apparatus employed to implement said method. Such a method and a similar pulse radar apparatus provided with a digital side-lobe canceller are known from an article of IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-14, No. 1, January 1978, pp. 165-171, "A digital open-loop adaptive Processor", by Frank F. Kretschmer 3r and Bernard L. Lewis. This article describes a digital side-lobe caaceller for the case when n=1, i.e. the unit is suitable for the cancellation of interference signals originating from one interference source and the receiver therefore comprises one auxiliary channel. To cancel interference signals from n sources, the desired interference cancellation is obtained by applying the latter canceller in a cascaded configuration, as described in U.S. Patent No. 4,222,051. However, in such a cascaded configuration the cancellation of interference signals appears to be strongly dependent upon the phase relations between the signals from one and the same interference source in the different channels (primary and auxiliary channels) of the receiver. The present invention has for its object to considerably reduce this phase dependency, and thereby to enhance the interference cancellation. According to the invention, the method as set forth in the opening paragraph comprises the steps of: (a) determining from the received, detected, sampled and digitised signals Em, E1, E2, ..., E quantities corresponding with the n valuesE1* Em, E2* Em, ..., En# Em; E1* E1, E2* E1, ..., En* E1; E1*E2, E2*E2, ..., En* E2; ..... ; E1* En, E2* En, ..., En* En; ------ ------ ------ ------ ----- ------ (b) averaging the values in (a) over several samples to the values E1* Em, E2*Em, ..., En# Em; E1* E1, E2* E1, ..., En*E1; ------ ----- ------ ------ ----- ------- E1* E2, E2*E2, ..., En*E2; .....; E1* En, E2*En, ..., En* En; (c) solving the weights W1, W2, ..., Wn from the matrix equation: EMI2.1 (d) determining from the signals Em, E1, E2, ..., E delayed over n the total processing time of steps (a), (b) and (c) and from weights W1, W2, ..., W the target return signals n Em = Em - W1E1 - W2E2@ ... - WnEn processed in the main channel. The weights W1, W2, ..., W determined by this method are n equal to those obtained in the analogue side-lobe canceller after the settling process. The analogue side-lobe canceller is described in U.S. Patent No. 3,202,990. The settling process however appears to be again strongly dependent upon the phase relations between the signals of one and the same interference source in the receiver; the settling process could therefore be extended considerably, while the extent of interference cancellation could remain far from its optimal value. For the case when n = 1, the method here applied appears to be equivalent to that described in the aforementioned article in "IEEE Transactions on Aerospace and Electronic Systems". A preferred embodiment of a pulse radar apparatus suitable for the application of the method according to the invention will be dependent upon n. For n = 1 or 2,a hardware implementation is expected to receive preference, whereas for larger values of n the pulse radar apparatus is to contain a programmable signal processor to execute the processing steps of the method according to the invention. The invention will now be described with reference to the accompanying drawings, of which: Fig. 1 is a block diagram showing the operating principle of the pulse radar apparatus for the cancellation of interference signals originating from a number n of sources, according to the invention; Fig. 2 is a more detailed block diagram of the digital side-lobe canceller for the case when n = 1; and Fig. 3 is a more detailed block diagram of such a digital side-lobe canceller for the case when n=2. Like parts in the figures are denoted by like reference numbers. In Fig. 1 the receiver of a pulse radar apparatus is denoted by the numeral 1. Receiver 1 comprises a main channel 2 and n auxiliary channels 3a, 3b, ..., 3n. Both the main and auxiliary channels are suitable for the reception, i.f. detection, sampling and digitising of target return signals and interference signals originating from a maximum number n of sources. The number of auxiliary channels determines the maximum number of interference sources, of which the interference signals can be cancelled. The main channel comprises an antenna directed at the target; this antenna receives in the side lobes signals from interference sources, apart from the target return signals (Em) in the major lobe. m The target return signals processed in the main channel and interference signals are jointly denoted by Em. The n auxiliary channels comprise in general omni-directional antennas; the target returns processed in these channels and interference signals are jointly denoted by E1, E2, ..., E . In both the main channel n and each of the auxiliary channels a quadrature detector is employed, being a combination of two phase-sensitive detectors used to detect the signals, processed in the channels, with a mutual phase shift of 900. In this way the real and imaginary components of signals Em, E1, E2, ..., E are obtained. These n components are conventionally sampled and digitised by AD converters contained in each of the channels. Signals Ek, where k=m, 1, 2, ..., n,from channels 2, 3a, 3b, ..., 3n are thus composed of two digitised signals: Re(Ek) and Im(Ek), per sample. The pulse radar apparatus contains first means to determine from signals Em, E1, E2, ..., En the quantities corresponding with values E1* E@m E21* Em,@ ..., En*@Em; E1*E1, E2*E1, ..., En* E1; E1*E2, E2*E2, ..., En* E2; .....; E1* En, E2*En, ..., En*En These means are constituted by logical unit 4. Each of the abovementioned quantities is composed of two numerical values, representing the real and the imaginary components. The quantity Ei* Ej thus comprises the numbers Re(Ei*Ej)= Re(Ei). Re(Ej) + Im(Ei). Im (Ej) and Im (Ei* Ej) = Re(Ei). Im (Ej) - Im (Ei). Re(Ej) for i = 1,2, ..., n and j = m, 1, 2, ..., n. The pulse radar apparatus also comprises second means to average the quantities E1*Em, E2* Em, ..., En*Em; E1* E1*E1, E2*E1, .., En*E1; E1*E2, E2*E2, ..., En*E2; .....; E1*En, E2*En, ..., En*En ----- ----- ------ ----over several samples to values E1*Em, E2* Em, ..., En*Em; E1*E1, ----- ------ ----- ----- ------ ----- ----- E2*E1, ..., En* F1; E1*E2, E2*E2, ..., En* E2; .....; E1*En, E2*En, -- E*E . These means are constituted by averaging circuit 5. n*En Also in this circuit the real and the imaginary components of EiX EJ are averaged separately ; the quantity Ei*Ej is thus composed of --------- ---------two digital numerical values Re(Ei* Ej) and Im (Ei*Ej). The pulse radar apparatus further comprises third means to determine the values W1, W2, ..., W from the matrix equation: EMI5.1 These third means are constituted by a second logical unit 6, in which the real and the imaginary components of Wi, where i =1, 2, ..., n, are determined separately. Splitting into real and imaginary components allows the above matrix equation to be written as: EMI5.2 This matrix equation can be solved in a known manner. Logical unit 6 supplies the W. signals, consisting of two components, Re(Wi) and 1 1 Im(W). The pulse radar apparatus still further comprises fourth means to determine from signals Em, E1, E2, ..., En, delayed over the total processing time of the aforementioned steps, and from weights W1, W2, ..., Wn, the target return signals E' =E Em ¯ W1E1 - W2E2- ... -1E , processed in the main channel. m m 11 22 nn The fourth means are constituted by a third logical unit 7. Logical unit 7 consists of multipliers 8a, Sb, ..., 8n, an adder 9 and a subtractor 10. Logical unit 7 is supplied with signals, provided these signals are delayed over a time T corresponding with the time needed for the determination of W1, W2, ..., Wn from Em, E1, E2, ..., E . The pulse radar apparatus thereto comprises delay elements 11a, 11b, ..., 11n and 12, each consisting of two parts, namely for the real and the imaginary components of the signal supplied to the delay element. Multipliers 8a, 8b, ..., 8n determine the products WiEi, i = 1,2, ..., n. The quantities WiE again consist of two numerical values Re(WiEi) and Im (WiEi). Adder 9 determines the va of two numerical values: EMI6.1 this sum value also consists and EMI6.2 Subtractor 10 finally establishes the value EMI6.3 which consists of the numerical values Re(E'm) and Im (E'm). The above first, second, third and fourth means together form the digital side-lobe canceller. The circuit as shown in Fig. 1 schematically is in practice useful only for small values of n, in particular when n = 1 or 2. For these two cases, the digital side-lobe canceller is illustrated in greater detail in Figs 2 and 3. In Fig. 2 the logical unit 4 consists of multipliers 13 to 18, adders 19 and 20 and subtractor 21. These circuits, connected as shown, supply the following output signals: EMI7.1 These signals are fed to averaging circuit 5, which comprises, for each signal supplied, an adder 22 and a register 23. Iterative addition at the sampling frequency establishes, short of a scale factor, the average value of the signals supplied during a certain time. Processing of the scale factor occurs with the readout of register 23. The average values Re(E1* Em), Im(E1*Em) and E1@2 are supplied to logical unit 6. This unit comprises two dividers, 24 and 25, connected as shown in the figure. The dividers supply the signals: EMI7.2 these signals are fed to logical unit 7. Logical unit 7 comprises four multipliers, 26-29, an adder 30, and subtractors 31, 32 and 33; these circuits are connected as shown in the figure. Logical unit 7 is also supplied with signals Re(Em ) Im(Em), Re(E1)v and Im(E1)v; these are the signals supplied to logical unit 4 after being delayed over a time corresponding with the processing time of these signals in logical unit 4, averaging circuit 5 and logical unit 6. The output signals of logical unit 7 are: Re(Em') = Re(Em) - [ Re(W1).Re(E1)-Im(W1) Im(E1)J Im (E@) = Im(Em)v - [Re(W,). Im(E1)v + Im(W1). Re(E1)v] Fig. 3 illustrates the digital side-lobe canceller in greater detail for the case when n=2. Logical unit 4 here receives signals Re(E ), Im(Em), Re(E1), Im(E1), Re(E2) and Im(E2). Logical unit 4 comprises multipliers 34-49, adders 50-54 and subtractors 55-57, connected as shown in the figure. The output signals of logical unit 4 are: EMI8.1 These signals are fed to averaging circuit 5. Circuit 5 again comprises for each signal supplied an adder 58 and a register 59, whose function is similar to that of elements 22 and 23 of Fig. 2. The output signals of averaging circuit 5 are: Re(E1*Em), Im(E1*Em), RE(E2*Em), Im(E2*Em), Re(E1*E2), $Im(E1*E2), $#E1#ê and #E2#ê. These signals are fed to logical unit 6. Unit 6 comprises multipliers 60-74, combining circuits 75-79 and dividers 80-P3, connected as shown in the figure. The output signals of logical unit 6 are: EMI8.2 EMI9.1 These signals are fed to logical unit 7. Unit 7 comprises multipliers 84-91, adders 92-95 and subtractors 96-99, connected as shown in the figure. Logical unit 7 is also supplied with sig nalsRe(Em v' Im(E,)V' Re(E1), Im(E1)v, Re(E2)v and Im(E2)v; these are the signals fed to logical unit 4 after being delayed over a time corresponding with the processing time of these signals in logical unit 4, averaging circuit 5 and logical unit 6. The output signals of logical unit 7 are now: EMI9.2 In the case when n is greater than 2, it is preferred to replace logical units 4, 6 and 7 by a programmable signal processor for implementing the process steps of the methods according to the invention under the control of a suitable program. Obviously, this program will be written on the same mathematical basis as that of the hardware described. The result of the method here described is such that the interference cancellation obtained is highly independent of the phase relations between the signals of the same interference source in the different channels. Omnidirectional antennas are utilised in the auxiliary channels; should these antennas be replaced by directional antennas, each of which to be directed to an interference source, a still better interference cancellation and a still greater independence of the phase relations would be obtained.

Claims: Claims: 1. Method for the cancellation of interference signals, originating from a number n of sources during the reception of target return signals in the receiver of a pulse radar apparatus, whereby the receiver comprises a main channel suitable for the reception, i.f. detection, sampling and digitising of signals (E ), which signals are composed of target return signals (E'm) and interference signals, and a number n of auxiliary channels suitable for the reception, i.f. detection, sampling and digitising of signals (E1, E2, ..., En), which signals are composed of target return signals and interference signals, characterised in that the method comprises the steps of: : (a) determining from the signals Em, E1, E2, ..., En quantities corresponding with the values E1#Em, E2* Em, ..., En#Em; E1*E1, E2*E1, ..., En* E1; E1* E2, E2* E2, ..., En* E2; ... E1* En, E2*En, ..., En* En; ----- ----- ------ ----- ----- ----- ----- (b) averaging said values over a plurality of samples to values E1*Em, E2*Em, ..., En*Em; E1*E1, E2*E1, ..., En*E1; E1*E2, ----- ------ ----- ----- ----- E2*E2, ..., En*E2; ......;E1*En, E2*En, ..., En*En; (c) solving the weights W1, W2, ..., Wn from the matrix equation: EMI10.1 (d) determining from the signals Em, E1, E2, ..., En delayed over the total processing time of steps (a), (b) and (c) and from the weights W1, W2, ..., Wn the target return signals E'm = Em - W1E1 - W2E2 - ... - WnEn processed in the main channel. 2. Pulse radar apparatus for cancelling interference signals, originating from a number n of sources, during the reception of target return signals in the receiver of the pulse radar apparatus, which receiver comprises a main channel suitable for the reception, i.f. detection, sampling and digitising of signals (Em), which signals are composed of target return signals (Em) and interference signals, and a number n of auxiliary channels suitable for the reception, i.f. detection, sampling and digitising of signals (E1, E2, ..., En), which signals are composed of target return signals and interference signals, characterised in that the pulse radar apparatus comprises: first means for determining, from the signals Em, E1, E2, ..., En, quantities corresponding with the values E1*Em, E2* Em, ..., En#Em; E1*E1, E2*E1, ..., En*E1 ; E1*E2, E2*E2, ..., En* E2; ..... ; E1*En, E2*En, ..., En*En; second means for averaging said values over a plurality of samples to values E1*Em, E2*Em, ..., En*Em; ----- ----- ----- ----- ----- ----- E1*E1, E2*E1, ..., En*E1; E1*E2, E2*E2, ..., En* E2; ...... ; ----- ----- ----- E1*En, E2*En, ..., En*En; third means for solving the weights W1, W ..., Wn from the matrix equation: EMI11.1 and fourth means for determining from the signals Em, E1, E2, ..., E delayed over the total processing time of said steps, n and from the weights W1, W2, ..., Wn the target return signals Em = Lm - W1L1 - W2t2 - ... - Wntn processed in the main channel. 3. Pulse radar apparatus as claimed in claim 2, characterised in that said first means are constituted by a first logical unit for determining from the real and the imaginary components of the input signals Em, E1, E2, ..., En the real and the imaginary components of the output signals "E1*Em, E2*Em, ..., En*Em E1*E1, E2*E3, ..., En#E1; E1*E2, E2*E2, ..., En#E2 ; ...... 1 1' 2 @ E1 En, E2 n En 4. Pulse radar apparatus as claimed in claim 2, characterised in that said second means comprises for each of the real and the imaginary components of the output signals of said first means an averaging circuit for averaging the supplied signals over several samples. 5. Pulse radar apparatus as claimed in claim 2, characterised in that said third means are constituted by a second logical unit for determining from the real and the imaginary components of the output signals of said second means the real and the imaginary components of the weights W1, W2, ..., n 6. Pulse radar apparatus as claimed in claim 2, characterised in that the pulse radar apparatus comprises for each of the real and the imaginary components of the signals Em, E1, E2,..., En a delay element whose delay time corresponds with the processing time of said signals in the first, second and third means, and that said fourth means are constituted by a third logical unit for determining from the output signals of the delay elements and of said third means the real and the imaginary components of the signal E'm. 7. Pulse radar apparatus as claimed in claim 2, characterised in that said first, second, third and fourth means jointly are constituted by a programmable signal processor for implementing the process steps of the method as claimed in claim 1 under the control of a suitable program.

Inventors: VAN MEURS, EINAC
Applicants: HOLLANDSE SIGNAALAPPARATEN B.V.
Application year: 1981
Main classification: G01S7
IPC codes: G01S7, G01S13, H01Q3, H04B1