PULSE RADAR APPARATUS EMPLOYING A PLURALITY OF PULSE REPETITION PERIODS

Abstract: In a pulse radar apparatus the transmitter is designed to generate similar transmitter pulses employing a plurality of pulse repetition periods, and the receiver, designed to transform detected echo signals, comprises: a switching-pulse generator (2) suitable for generating in several pulse repetition periods, at least a portion of a switching-pulse series each pulse repetition period, which portion is associated with switching intervals related to certain successive range sectors; a distributor (3) controlled by said switching-pulse generator (2) for selecting video signals in range sectors; and MTI filter units (4A-4B) connected separately to the distributor (3) for generating Doppler signals, representative of moving targets, from the video signals supplied by selection.

Description: Pulse radar apparatus whose transmitter is designed to generate similar transmitter pulses employing a plurality of pulse repetition periods. The invention relates to a pulse radar apparatus, whose transmitter is designed to generate similar transmitter pulses employing a plurality of pulse repetition periods, and whose receiver is designed to transform detected echo signals to video signals. Such a pulse radar apparatus is known from the U.S. Patent Specification 3,855,593. The pulse radar apparatus disclosed in this patent however has the disadvantage that through the addition of longer pulse repetition periods to the transmitter system the data rate from the range sectors corresponding with the shorter pulse repetition periods is strongly reduced. Just for the short-range sector of a pulse radar apparatus a great need for a higher data rate is felt to obtain a better distinction between clutter and video signals from stationary targets on the one hand and video signaLs from moving targets on the other hand. The present invention has for its object to provide a pulse radar apparatus of the type set forth in the opening paragraph, whereby the above disadvantage is obviated to a significant extent, which apparatus operates in accordance with the need expressed above. According to the invention, the receiver of the pulse radar apparatus of the type set forth in the opening paragraph is provided with: a switching-pulse generator suitable for generating, in several pulse repetition periods, at least a portion of a switching-pulse series each pulse repetition period, which switching pulses are associated with switching intervals related to certain successive range sectors; a distributor controlled by said switching-pulse generator for selecting video signals in range sectors; and MTI filter units connected separately to the distributor for generating Doppler signals, representative of moving targets, from the video signals supplied by selection. The invention will now be explained with reference to the accompanying figures, of which: Fig. 1 is a schematic diagram of an embodiment of a pulse radar apparatus; Figs. 2A-C are timing diagrams pertaining to a first embodiment of a pulse radar apparatus shown in Fig. 1; Fig. 3 shows an embodiment of a filter configuration for an embodiment of a pulse radar apparatus shown in Fig. 1; Figs. 4A-C are timing diagrams pertaining to a second embodiment of a pulse radar apparatus shown in Fig. 1; Figs. 5A-D are timing diagrams pertaining to a third embodiment of a pulse radar apparatus shown in Fig. 1; Figs. 6 and 7 are characteristics illustrating the advantages obtained with the invention when using a pulse radar apparatus as shown in Fig. 1. Fig. 1 illustrates a pulse radar apparatus provided with a transmitting and receiving unit 1, whose transmitter is designed to generate similar transmitter pulses employing a plurality of pulse repetition periods, and whose receiver is designed to transform detected echo signals to video signals to be delivered as output signals by the receiver. Such a transmitting and receiving unit is described, for example, by M.I. Skolnik, in "Introduction to Radar Systems", McGraw-Hill Kogakusha, Ltd., second edition, Chapter 4. For simplicity, the operating principle of the pulse radar apparatus is described for the case as shown in Fig 2A, whereby transmitter pulses with two pulse repetition periods T1 and T2, where T2 = 2T1, are generated alternately. The invention aims at including the video processing of the range interval, associated with pulse repetition periods T1 and covered during the longer pulse repetition period T2, under that of the short pulse repetition period T1 to achieve the contemplated increase of the data rate for the short range interval. In accordance with this concept, the receiver is provided with: a. a switching-pulse generator 2, producing a switching signal t1 coinciding with the transmitter pulse, and - within the pulse repetition period T2 - a switching signal t2 appearing a period T1 after the transmitter pulse; the switching-pulse generator 2 is thereto aligned with the transmitter signal and connected, for instance, to a pulse modulator 7 (or a trigger-pulse genera tor activating this modulator); b. a distributor 3, controlled by the switching-pulse generator 2; in the embodiment here described the distributor 3 has two switching positions S1 and S2 for selecting in range sectors the video signals supplied by the receiver 1. Distributor 3 will be in position S? during pulse repetition period T1 and during the period T1 directly following the transmitter pulse within pulse repetition period T2 and in position S2 during the remaining part of pulse repetition period T2; c. a plurality of MTI filter units 4A-4B, in the case in question two, separately connected to the distributor outputs (3) associated with switching positions S1 and S2. Filter unit 4A, connected to the distributor output (3) associated with switching position S1, will therefore provide for the processing of the video signals, obtained during each pulse repetition period T1 and during the time intervals T1 directly following the transmitter pulses within pulse repetition periods T2. Filter unit 4B, on the other hand, provides for the processing of the video signals supplied within each pulse repetition period T2, but after time interval T1 situated therein. A feasible embodiment of the switching pulse generator 2 is obtained with the aid of a clock generator, which is reset at each generation time S of a transmitter pulse (e.g. through the signal supplied by unit 7), thereby itself producing another switching pulse t as reset pulse for the distributor 3 to reset or to hold the distributor 3 in position S1. The clock generator counts the time interval T1 and produces a switching signal t2 to set distributor 3 to switching position S2. Switching signal t2 however is not to be fed to distributor 3 at the end of pulse repetition period T1. To this effect the clock generator in switching pulse generator 2 is provided with logic means supplying distributor 3 with a switching signal t2 for switching position S2, only if the Boolean formula µ 8s t2 is logically true. The time Lapse in Fig. 2B for switching positions S and that in Fig. 2C for switching positions S2 are part of the time lapse in Fig. 2A for pulse repetition periods T1 and T2. Fig. 2B shows that filter unit 4A processes video signals during each pulse repetition period T1 and during subsequent time intervals T1 of the pulse repetition periods T2, but not during the remaining part of pulse repetition periods T2. Filter unit 4A should therefore be adapted to this situation. With the use of delay line cancellers the triple canceller configuration obtained for m=2 in Fig. 3 can be successfully applied as filter unit 4A. The units denoted by T in Fig. 3 represent delay lines, while a denotes a weighting factor, to which the video signals supplied are subjected. The method of calculating weight factors in such filters is known from the European Patent Application published under number 0 069 415. Accordingly, a suitable canceller for the time intervals shown in Fig. 2B is obtained with the weighting factor a=-2. For the processing of the video signals supplied by distributor 3 in switching position S2 a double canceller can be successfully used as filter unit 4B, whereby delay lines of 3T1 are employed. Since the data rate for filter unit 4A is twice as large as that of unit 4B, the average output signal strength of filter unit 4A will differ from that of unit 4B. A correction must therefore be made for this difference; this is achieved, for instance, by subjecting the output signals of filter unit 4B to a certain standard correction. This correction may be effected as a separate process in a unit 5B after the particular canceller 4B, but can also be performed in the weighting factors of this canceller. Finally, the filtered and possibly corrected output signals of filter units 4A and 4B are combined in a circuit 6 and utilised for presentation, recording, processing or the like. For For the short range sector the data rate is stimies greater in comparison with the existing situation (with one single pulse 2. repetition period T2) and for the longer-range sector stimies greater; this fits in with the objective. It is however possible to select a different interrelation for pulse repetition periods T1 and T2. Pulse repetition periods T1 and T2 can be chosen in accordance with the relation T2 = m T1, where m > 2, while for the processing of video signals from the short-range sector corresponding with the period T1 the canceller configuration shown in Fig. 3 is applicable as filter unit 4A. In such a case, the video signals within the switching pulse intervals determined by switching position S2 are supplied to filter unit 48, functioning as a single canceller and including delay means with a delay time (m+1)T1. It is also possible to present the pulse repetition periods in a different sequence, for example in accordance with a repeating series T2, T2, T1, where T2 = 2T1, as shown in Fig. 4A. Fig. 4B illustrates that for the time intervals, in which the video signals from the range sector corresponding with pulse repetition period T1 are detected, the switching pulse generator 2 produces the switching pulse t1 to set the distributor 3 to switching position S1. In Fig. 4B the time intervals are grouped by means of accolades. In these time intervals the distributor 3 set to switching position S1, supplies the filter unit 4A with the video signals; for the video signals from the time intervals grouped in succession by means of accolades, a filter unit 4A can be designed as a double canceller to effect the filter process each time interval. The remaining time intervals having a length of T2 - T1 with pulse repetition periods T2 are also grouped by means of accolades in Fig. 4C, showing that in this case a single canceller can be applied for filter unit 4B. 6 For the short range sector the data rate has increased Ttimes in comparison with the existing situation, and for the more distant sector times. It is further possible to use a transmitting unit suitable for the generation of similar transmitter pulses employing more than two pulse repetition periods. If the generation of the transmitter pulse occurs with three pulse repetition periods T1, T2 and T3, with the interrelations T2 = 2T1 and T3= 3T1 according to the repeating series T1, T2, T1, T3 (see Fig. 5A) or T2, T3, T2, T1, then it is possible to process the supplied video signals in combination during pulse repetition periods T1 and the periods T1 following the transmitter pulses of pulse repetition periods T2 and T3. The number of filter units connected to the distributor 3 is then three, viz. 4A, 48 and 4C. During the time intervals, in which the video signals fro the range sector corresponding with pulse repetition period T1 are detected, the switching pulse generator 2 has to generate the switching pulse t1 whereby the distributor 3 is set to switching position S1; in Fig. 5B the time intervals are grouped by eans of accolades. During these time intervals the filter unit 4A is supplied with the video signals via the distributor 3 in the switching position S1. A geasibLe embodiment of a fitter unit 4A is obtained with the configuration of Fig. 3, where I = 2. The first range interval determined by pulse repetition period T1 is followed by a second range interval, whose magnitude is determined by a time interval of T2-T1. Such tine intervals are formed by the remaining parts of pulse repetition periods T2 and by parts in pulse repetition periods T3, which follow the tile intervals T1, as shown in Fig. 5C. The time intervals coibined for the video processing in filter unit 4B are grouped by leans of accolades in Fig. 5C. For filter unit 4B a single canceller can be used. The third range interval is determined by the remaining part having a Length of T3 - T2 - T1 within pulse repetition period T3, as shown in Fig. 5D. Also in this case, a single canceller can be used for filter unit 4C, processing the grouped time intervals in combination. Further a correction unit SC is connected to filter unit 4C to normalise the video signal strength. The required switching pulses t1 t2 and t3 have to be provided by switching pulse generator 2, namely at the instant of generation of the transmitter pulse, as well as after time intervals T1 and T1+T2 following the generation of this transmitter pulse. The switching pulses should be omitted at times when a following transmitter pulse is generated; therefore, switching pulse generator 2 is set to switching positions S2 and S3 at the instant the Boolean functions * t2 and what3, respectively, are logically true. The invention is not confined to the division of the longest pulse repetition period into maximum three time intervals (and a corresponding number of parallel grouped filter units); also other divisions of the longer pulse repetition periods (and a corresponding number of parallel grouped filter units) are possible. However, as already described in the cited European patent application, a choice should be made for symmetric or antisymmetric transversal filter configurations, implying restrictions as to the choice in the arrangement of the pulse repetition periods to be used, especially with analogue filter systems. With digital filters however these problems are present to a less extent, since video signals received at an inopportune instant with respect to the selected filter configuration can be stored temporarily and-utilised at the instant the filter process permits this. Besides the above restrictions (symmetric or antisymmetric transversal filter), there are other restrictions as to the choice of a filter configuration; as already stated in the cited European patent application, there are symmetric or antisymmetric transversal filter configurations with z-transformed transfer functions comprising, in addition to one or several roots z=1 suiting a canceller, also a root z=-1, belonging to a zero velocity filter. For this reason, such a filter configuration would not be suitable for application as a canceller. Such a filter configuration has a filter characteristic showing several very sharp, narrow dips. For the 7000Hz frequency such a characteristic is shown in Fig. 6 by curve I, depicting the response (R) in dB against the target speed in m/sec. By replacing the appropriate series of pulse repetition periods by a staggered version (e.g. k = 0.8) of these pulse repetition periods, a filter characteristic is obtained, where the above-mentioned dips have disappeared to a significant extent. Certain series of pulse repetition periods permit the use of more complex filter configurations instead of the more simple ones. For example, the series of time intervals in Fig. 2B permits the processing of video signals by a single canceller as well as by a triple canceller according to Fig. 3, while the video signals of the time intervals in Fig. ZC can be processed in a single cancelled, as well as in a double canceller. The above concepts will now be checked against the embodiment of a pulse radar apparatus shown in Fig. 1 and operating in accordance with the repeating series of two pulse repetition periods T1 and T2 in Fig. 2A. For the video processing of the range interval corresponding with pulse repetition period T1 an antisymmetric triple canceller is used as shown in Fig. 3, where m=2; here the z-transformed transfer function is expressed by: Eo.E; 1 = z4tz4 + &alpha;z - az - 1} , where E. and Eo are the input 0 Iz crt 1 1 and output, respectively. For a = -2 and a shift frequency of 7000 Hz in the delay Lines of this filter, curve I in Fig. 6 shows the characteristic of this filter. The z-transformed transfer function comprises, in addition to the roots z = 1, also a root z = -1, which is characteristic of a zero velocity filter. The transfer characteristic in question thus contains zero points, which are characteristic of a canceller or a zero-velocity filter. With the introduction of a stagger factor k = 0.8 (so that the pulse repetition periods are 5600 Hz and 7000 Hz) a characteristic according to curve II of Fig. 6 is obtained for the triple canceller; it is notable that the dips at the zero points are not so steep anymore. For the remaining part of the range interval of the pulse radar apparatus and hence for the time intervals in Fig. 2C a double canceller is used; the transfer characteristic of this canceller is shown by curve III in Fig. 7. Also curve II is included once again in Fig. 7. For comparison, this figure also shows the characteristic IV, pertaining to a pulse radar apparatus using only one canceller, whereby the pulse radar apparatus inclusive of the canceller operates in accordance with one single pulse repetition frequency of 3000 Hz with a stagger factor k = 0.8. It can clearly be seen that, through the wider fixed clutter region and its greater edge steepness, the characteristic of curve II presents a better suppression of the fixed targets and clutter signals in the near environment of the pulse radar apparatus than the characteristic of curve IV. On the other hand, for the more distant radar ranges, the concession made is that the fixed clutter region is smaller than that of the characteristic of curve IV; in practice however, this is considered permissible. Furthermore the resolution of video signals into I and Q components and the subsequent filter process disclosed in Fig. 1, can be applied for each component. Finally, it should be noted that the invention is not limited to the above embodiments. Repeating series of a plurality of staggered pulse repetition periods T1, T2, ..., TN are feasible in a certain combination. This requires a distributor having a corresponding number of switching positions S1, S2, ..., SN and, connected thereto, the same number of filter units 4A-4N (including correction units).

Claims: Claims: 1. Pulse radar apparatus, whose transmitter is designed to generate similar transmitter pulses employing a plurality of puLse repetition periods and whose receiver is designed to transfor detected echo signals, characterised in that the receiver comprises: a switching-pulse generator suitable for generating in several pulse repetition periods, at least a portion of a switching-pulse series each pulse repetition period, which portion is associated with switching intervals related to certain successive range sectors: a distributor controlled by said switching-putse generator for selecting video signals in range sectors; and MTI filter units connected separately to the distributor for generating Doppler signals, representative of moving targets, from the video signals supplied by selection. 2 Pulse radar apparatus as claimed in claim 1, characterised in that each filter unit comprises means for deLaying video signals over a time interval at least approximating a length of time corresponding with a multiple of the least switching pulse interval. 3. Pulse radar apparatus as claimed in claim 1, characterised in that the transmitter is designed for generating pulse repetition periods varying over a relatively short length of time.

Inventors: BERGMAN, JAN
Applicants: HOLLANDSE SIGNAALAPPARATEN B.V.
Application year: 1984
Main classification: G01S13
IPC codes: G01S13, G01S7