Patent GB-1429013-A (GB)
Title: Pulse radar arrangement
IPC: G01S7/292
Filed: 1973-08-25
Abstract: 1429013 Pulse radar MARCONI CO Ltd 14 Aug 1974 [25 Aug 1973] 40356/73 Heading H4D A pulse radar arrangement has a clutter switch comprising means for dividing an area to be surveyed into a plurality of predetermined annular sectors defined by azimuths and ranges, means for correlating the number of received signals having an amplitude greater than a predetermined value in a first sector with received signals in annular sectors adjacent all the sides of said first sector also having an amplitude greater than said predetermined value so as to declare the presence or absence of clutter in said first annular sector, which declaration in operation is utilized to determine which of a plurality of video displays is to be employed for the said first annular sector. As described, the video output signal from the radar receiver is first compared in a comparator 1, Fig. 1, with a signal from a reference source 3 having a level slightly above the mean noise level of the receiver. The output of the comparator 1 is supplied to a range quantizer 4, which in turn supplies a range integrator 5, both the quantizer 4 and the integrator 5 being synchronously clocked by pulses from a clock pulse generator 6. The range integrator 5 feeds into an azimuth integrator 7, which is stepped by pulses from a generator 8, said pulses serving to divide the surveyed area azimuthally. The azimuth integrator 7 provides output to a comparator bank 9 comprising three paralleled comparators each having a different reference signal, supplied by sources 10. The comparator bank 9 supplies output to a first delay 11, and to a decoder 12. The first delay 11 supplies output to a second delay 13, and to the decoder 12. The second delay 13 supplies output to the decoder 12, which itself supplies output to a store 14 for utilization on the next subsequent sweep of the P.P.I. As shown in Fig. 2, in which 21 represents clutter and 22 moving targets, the surveyed area is effectively divided into annular sectors 23 by range clock pulses from the generator 6 to the integrator 5 (providing the boundaries 24) and by azimuthal clock pulses from the generator 8 to the integrator 7 (providing the boundaries 25). Typically, there might be 64,000 sub-areas 23. As shown in Fig. 3, an annular sector 23 of interest is further subdivided into sixteen range increments by further pulses from the generator 6 supplied to the range quantizer 4, and into sixteen azimuthal increments 27a to 27p by the transmitted radar pulse repetition frequency, thus forming 256 zones 28. In operation, radar echoes received at the comparator 1 are compared with the signal from the reference source 3 and the number of zones 28 in each azimuth increment 27a or 27b or ... 27p having received echoes greater in amplitude than the signal from the source 3 are summed and stored by the range integrator 5. Thus the integrator 5 contains a number (from 0 to 16 since there are sixteen range increments 26) for each of the azimuth increments 27a to 27p and these numbers are then summed by the azimuth integrator 7 thereby providing a number for each annular sector 23 representative of the amount of clutter therein. In practice these numbers may be divided by a suitable factor dependent upon the operational requirements. The number for each annular sector 23 is sequentially passed to the comparator bank 9 where it is compared with each of the three reference sources 10. The value of each of the three reference sources 10 will vary from one radar site to another since the expected degree of clutter varies from site to site. The reference sources 10 may produce outputs which are linearly spaced or which are spaced in a logarithmic or some other suitable fashion. The reason for providing more than one reference source 10 will be seen with reference to Fig. 4 which shows, hatched, a large area of clutter 30 extending over several annular sectors 23 and an area of broken clutter 31, also hatched, in which each piece of clutter is approximately the size of a target. The annular sector 23a contains a very large proportion of clutter so that the number representative of clutter for that annular sector will be near the maximum, whilst the sector annular 23b contains only a small proportion of clutter so that its number representative of clutter will be smaller. If only a single reference source 10 were to be used then so as to avoid a target being recognized as clutter the reference source 10 would have to provide a signal that was so high that many small clutter echoes 31 would be undetected as clutter. The outputs of the comparator bank 9 are passed to the decoder 12 which receives as a parallel input the comparator bank 9 output for the annular sector of interest and for the previous and succeeding azimuthal annular sectors, the azimuthal annular sector of interest having been delayed by delay 11, the previous azimuthal annular sector having been delayed by both delays 11 and 13 and the succeeding azimuthal annular sector being undelayed. If the annular sector of interest is annular sector 23a in Fig. 4 and since it is a very large area of clutter it exceeds the level of the largest reference source of the sources 10 then clutter is declared to be present in the annular sector 23a by the decoder 12. If, however, the output from the comparator bank 9 indicates a small clutter area, such as in annular sector 23b of Fig. 4, then the decoder 12 investigates whether or not clutter has been declared to be present in annular sectors adjacent the sides of the annular sector of interest, i.e., annular sector 23b. Thus, referring to Fig. 5, annular sectors with which correlation is made are shown cross-hatched in broken and solid lines. Returning to Fig. 4, it will be seen that the adjacent further range annular sector 23c contains a considerable proportion of clutter so that clutter will be declared to be present by the decoder in annular sector 23b even though the proportion of clutter in annular sector 23b is smaller than a target. Similarly the small clutter echoes 31 will be cross-correlated for each annular sector such that the decoder 12 will declare clutter to be present. When clutter is declared to be present in an annular sector by the decoder an output signal is passed to the store 14 so that during the next scan of the radar antenna, MTI mode operation of the radar arrangement is switched-in for that particular annular sector; thus only moving targets will be observed in that particular annular sector and with the clutter being substantially reduced. The output from the store 14 may also be used to generate a low level signal which can be mixed with the display video to "ghost in" the areas of clutter or to pass the clutter information over a narrow bandwidth link so that the clutter areas alone may be plotted. In the example described above the decoder 12 was used to correlate with annular sectors adjacent to the sides of an annular sector of interest, however, it may, in some circumstances, be desirable to increase the area of correlation to include those annular sectors at the corners of the annular sector of interest. Thus, referring to Fig. 5, the correlation area may be as shown hatched in broken lines.