DEVICE FOR THE IDENTIFICATION OF UNDESIRABLE ECHOES IN RADAR SYSTEMS

Abstract: Device for the detection of undesirable echoes which have significant correlation in azimuth, applicable to surveillance or tracking radars. The device works through autocorrelation function estimate over a pair of pulses received by averaging, over N range cells, the complex product of the signal received ZK (t1), relevant to the K-th range bin and time instant t1, with the conjugated complex value ZK (t1 + T), still relevant to the K-th range bin, at instant t1 + T (where T is the pulse repetition period, PRT).

Description: "DEVICE FOR THE IDENTIFICATION OF UNDESIRABLE ECHOES IN RADAR SYSTEMS" The present invention takes regard for a device for the iden- tification of undesirable echoes in radar systems. Such invention belongs to the radar field and more exactly to the field of surveillance or tracking radars. At present, undesirable echoes are recognized by relying upon two distinct techniques: a) Estimate of the power at the radar receiver input by means of the overage of the power level over a group of echoes relevant to the same impulse transmitted and further com¬ parison with a fixed precalculated threshold; b) Measurement of the number of times a precalculated thre- shold is crossed by the receiver input power into a group of echoes relevant to the same impulse transmitted. Such techniques, however, suffer poor sensitivity, and present poor discrimination between echo sources (clutter, jammer, asynchronous pulses and targets). Object of the present invention is therefore a device which can detect the presence of undesirable echoes having large azimuth correlation coef¬ ficient (clutter) and discriminate them against echoes having small correlation coefficient (jammer, asynchronous pulse). The device operates on an estimate of the auto correlation function C (which here coincides with the autocovariance fun¬ ction as the mean value is nil, as shown- by Papoulis: Pro¬ bability, Random Variables and Stoctrastic Processes, Inter¬ national Student Edition Mc Grow Hill paragraph 9-3) by ave¬ raging, over N range bins, the complex product of the signal received Z (t ) related to the K-th range bin and to generic K 1 instant and the complex conjugate value Z* (t + T) still 1 1 relevant to the K-th range bin, but at time t + T (where T is the PRT pulse ripetition period): N N C ( T) = - Z rr . ( t, ) x Z jr . ( t + T ) = - C . ( T ) ( 1 ) K+j 1 K+j 1 K+ j=ι ' j=ι The crossing of a threshold S by the modulus /C (T)/ of the estimated value, indicates the presence of a correlated di¬ sturbance in azimuth (clutter) in the examined range bins (i.e. range cells). The threshold S is suitably calculated to obtain the desired false detection probability. The present invention will now be described with reference to some of its actually preferred forms of implementation and these are reported as an illustration, are not limited to these and make reference to the drawings attached where: Figure 1 shows the block diagram of the device made up of: (I, Q = the two in phase & in quadrature components of the input signal) . Sign inverter of the component Q of vector representing si- gnal , 1; Delay element having duration T, 2; Complex multiplier, 3; Delay line having N intermediate taps, with elementary delay (resolution cell of the radar), 4; N input adder, 5; Modulus extractor, 6; Divider by N, 7; Comparator with the prefixed threshold, 8. Figure 2 shows the block diagram of a variant to the present invention, where blocks 1,2,3,4,8 have already been described in Figure 1. The further blocks are meant to make the device insensitive to the interference, particularly to the pulsed one. They are: two adders, 9; two modules extractors, 10; two divide by N/2, 11. Blocks A & B within dashed lines are two alternative ways of selecting the smaller of two inputs and further comparing it with the prefixed threshold. Block A shows the selector of smaller input 12 and the compa¬ rator with prefixed threshold 8 (as already seen in Figure 1). • Blo.ck B shows two comparators 14 and an AND logic gate 15. The present invention will now be described with reference to the two preferred implementations. ( ote that I and Q are the two components, in phase & in quadrature, of the input signal Z (t), represented in complex form: Z (t) = I (t) + j Q (t) (2)) In Figure 1, the two components I & Q of input signal Z (t) to - A - be analyzed are sampled at instants t , t + ...t + n , where t is the initial time instant and is the sampling period. These samples are delayed by time (PRT: pulse repetition period) and are suitably processed to obtain their complex conjugate. Thereafter the complex product is carried out to obtain autocorrelation function C (T) to be averaged K+J over N samples. This value is sent to a delay line with N taps (having elementary delays ) the outputs of which are added, divided by N and sent to a modulus extractor to obtain /C (T)/. The value obtained is compared to threshold S, settable, which discriminates between disturbance detection or not. The second diagram, provided in Figure 2, shows an input pro¬ cessing sequence which is identical to the one performed by the first schematic, up to tapped delay line output signal C . (T). K+J In the new diagram these outputs are split into two groups and in turn are sent to two adders, to two divide by N/2 circuits and to two modulus extractors. Values /C (T)/ and /C (T)/ obtained may be processed in two perfectly equivalent ways to obtain detection. The first way is to select the minor between /C (T)/ and /C (T)/ and to compare it with threshold S. The second way is to compare /C (T)/ and /.C (T)/ with the threshold S and then to send the output to a ' logic "AND" for. detection. Modifications made to diagram 2 of Figure 2 compared to dia¬ gram 1 (Figure 1) have been introduced to obtain better di¬ scrimination in presence of a strong asynchronous impulse (which we do not wish to detect). To sketch how the device object of this invention works we may separate three cases: a) presence of clutter echoes: - samples are present in more than one sweep in areas extended in range and are strongly correlated in azimuth; b) presence of jammer echoes: - samples are present in more than one sweep over the whole range and are not correlated in azimuth c) presence of asynchronous impulse echoes: - samples present at random over a few isolated range cells. In case a: clutter is detected by the device described in figures 1 & 2 respectively with probabilities P de- pending on the value S of the threshold, on the clutter power and on the correlation coefficient. In case b: the jammer, which is not correlated in azimuth ' , is detected with probability P T1 , by the device depected in Figure 1 and with probability P by the device in Figure 2. J-1 For a fixed value of S these probabilities are much less than those for clutter P and P . Therefore the discrimination Cl C2 between clutter and jammer is achieved, which is a characte- ristic of the present invention with respect to previous techniques. In case c, the asynchronous pulse, which is usually very high in power, will be detected with a good probability by means of the solution given in Figure 1, while it will cause only slight performance degradation of the device in Figure 2, which will drop the estimate, either C (T 1 ) or C (t) affected by the error due to the asynchronous impulse.

Claims: CLAIMS 1. Device for the detection of undesirable echoes in radar sy¬ stems, characterized by being made up of: an element which changes sign of the quadrature component of the vector of complex signal at input 1; an element introducing delay T, which resets in time the input present to that present during previous sweep, 2; a complex multiplier 3 which multiplies the outputs of blocks 1 and 2; a Tapped delay line, 4, with elementary delay (radar re¬ solution cell) ; an adder, 5, of the N outputs from delay line 4; a modulus extractor, 6, of adder output 5; a divide by N, 7, of. the modulus extractor output; a comparator, 8, which by comparing the divide by N output with a settable threshold, generates a binary indication related to clutter detection. 2. Device according to claim 1 characterized by being fur¬ thermore built up of: two adders, 9, of delay line outputs 4; two modulus extractors, 10, of adder outputs 9; two divide by N/2, 11, of modulus extractors 10; a selector of the minor, 12, of the two inputs from the mo¬ dulus extractor. 3. Device as per claims 1 and 2 characterized by furthermore including two comparators, 14, which, through comparison with a settable threshold of the dividers two outputs, give two binary indications; an AND logic gate 15, fed by the comparator's outputs which gives a binary indication of undesirable disturbance de- tection. 4. Device for the detection of undesirable echoe detection in radar signals as per illustrations in description and dra¬ wings provided, as also any part of it in isolation or com- bination.

Inventors: GALATI GASPARE, ORSINI MARIO, GALATI, GASPARE, ORSINI, MARIO
Applicants: ORSINI MARIO, SELENIA IND ELETTRONICHE, ORSINI, MARIO, SELENIA, INDUSTRIE ELETTRONICHE ASSOCIATE, SPA
Application year: 1985
Main classification: G01S7
IPC codes: G01S7