Radar system

Abstract: To obtain digitized samples of an analog signal at a selectable rate, R, the system includes: a timing generator (15) producing a train of sampling pulses at a predetermined sampling rate, f s ; an analog to digital converter (28) producing a digitized sample of the analog signal in response to each one of the sampling pulses; a digital processor (32): and a selector (30) passing each Nth one of the digitized samples produced by the analog to digital converter (28), where N = f s /R, to the processor (32). The selector (32) includes a decimator (42), responsive to a signal, N = f s /R, which produces pulses at the rate, R = f s /N; and a register (40) for storing each Nth one of the digitized samples produced by the analog to digital converter (28), in response to a corresponding one of the pulses produced by the decimator (42). In a radar system, range cells signals can be produced at a rate, R, related to the range cell resolution selected by the radar system. The analog to digital conversion system is fed by the radar video signals (25,27).

Description: This invention relates to a radar system for determining to a predetermined range cell resolution the range of a target. As is known in the art, various systems, such as radar systems, require, in applications where digital signal processors are used to process return signals, conversion of the analog return signals into corresponding digital signals. For example, radar signal processors are used for determining the range and/or relative velocity, or velocities, associated with one, or more, objects. Examples of such radar systems are described in US patents 4 783 660 and 5 389 931. Systems in which colour image video signals in the form of analog luminance and chrominance signals are converted into digital signals for processing or transmission are also known. For example, EP 0 496 000 A describes several circuits in which such an analog signal is initially converted into digital signals by an analog to digital converter driven by a clock signal at a clock frequency f s , and the resulting train of digital samples is sub-sampled at lower rate obtained by frequency division of the clock signal, the division in the examples given being either division by 2 or division by 3. In each example a flip-flop is used as the subsampling circuit. In the operation of a pulse radar system, radar pulses are transmitted at a rate referred to as the pulse repetition frequency (PRF). During a predetermined time after pulse transmission, i.e., the expected time during which return pulses arrive, radar return signals are sampled. From a consideration of the difference in time between the time of pulse transmission and the time at which the sample is taken, it will be understood that each one of the samples corresponds to a range, or distance between the radar system antenna and the object producing the sampled return. The rate at which the samples are taken following each transmitted pulse is directly related to the desired range resolution. The process is referred to as range gating, where each one of the times at which a sample is taken represents a range cell, or gate, of the return produced by the object at the range corresponding to the time the sample is taken. In some applications it is necessary to have the radar system operate over a broad range of pulse repetition frequencies. Further, in some applications, it is necessary to have the samples taken at many different sampling rates for a broad range of range resolutions. One system used to produce the digitized samples includes an analog to digital converter. With such converter, a sample of the video signal is taken and converted into a corresponding digital sample in response to a sampling signal. In order to accommodate the variation in sampling/conversion rates, the analog to digital converter has to be capable of operating over this relatively large range of sampling/conversion rates. There is accordingly a need for a radar system in which there can be provided digitized samples of an analog signal over a wide range of selectable rates. According to the present invention there is provided a radar system as defined hereinafter by claim 1. In a preferred embodiment of the invention, an analog to digital converter operates at a continuous, relatively high, constant sampling rate, f s , while a processing section processes each Nth one of the digitized samples, where N = f s /R, and R is the selectable sampling rate. Thus, a wider selection of analog to digital converters, including relatively inexpensive analog to digital converters, may be used as compared with analog to digital converters which must digitize analog samples over the range of selectable rates, R. The preferred system includes: a timing generator for producing a train of sampling pulses at a predetermined sampling rate, f s ; an analog to digital converter for producing a digitized sample of the analog signal in response to each one of the sampling pulses; a processor; and a selector for passing each Nth one of the digitized samples produced by the analog to digital converter to the processor, where N = f s /R. The selector includes: a decimator, responsive to a signal, N = f s /R, representative of the ratio of the sampling date, f s , to the digitized signal obtaining rate, R, for producing pulses at the rate, R = f s /N; and, a register for storing each Nth one of the digitized samples produced by the analog to digital converter, each one of such digitized samples being stored in response to a corresponding one of the pulses produced by the decimator. The samples stored by the register are then fed to the processor. In a preferred radar system embodying the invention, range cell samples of the video signals produced in response to radar return pulses are obtained at a selectable rate, R, related to the range cell resolution selected by the radar system. An analog to digital conversion system is fed by the video signal. The analog to digital conversion system includes: a timing generator for producing a train of sampling pulses at a predetermined, continuous, constant, sampling rate, f s ; an analog to digital converter for producing a digitized sample of the analog signal in response to each one of the sampling pulses; and, a processor section for processing each Nth one of the digitized samples produced by the analog to digital converter following each transmitted radar pulse, where N = f s /R, and for determining Doppler frequency of energy in the range cells from the processed digitized samples. Each transmitted pulse is produced in response to a radar transmit trigger pulse. A gating window pulse is produced a predetermined time after each trigger signal. The processor section processes each Nth one of the digitized samples produced by the analog to digital converter during the time duration of the gating window to determine the Doppler frequency of energy in the range cells from the processed digitized samples produced during the time duration of the gating window. The invention will now be described by way of example with reference to the accompanying drawings, in which:- Fig. 1 is a block diagram of a radar system according to the invention; Figs. 2A - 2G are timing diagrams useful in understanding the operation of the radar system of Fig. 1; and, Fig. 3 is a functional block diagram of a decimator used in the radar system of Fig. 1. Description of the Preferred Embodiments Referring now to Fig. 1, a coherent Doppler radar system 9, is shown. The radar transmitter/receiver section 10 of the radar system 9 includes an antenna 11 coupled through a circulator 12 to an amplifier, here, for example, a klystron amplifier 13, a pulse modulator 14 and a timing and control unit 15 arranged in a conventional manner whereby a train of pulses of radio frequency (RF) energy is transmitted at a desired pulse repetition frequency (PRF). Here, the radar system 9 is adapted to produce transmitted pulses with different pulse repetition frequencies. Each one of the pulses in the train of transmitted pulses is reflected by various objects which are dispersed over various ranges from the antenna 11. A portion of the reflected energy produced in response to each transmitted pulse is received by antenna 11. The time at which a portion of such energy is received relative to the time at which the transmitted pulse associated therewith is transmitted corresponds to the range of the object causing such portion of the received energy. The energy received by the antenna 11 passes through circulator 12. The signal at the output of circulator 12 is heterodyned in mixer 17 in a conventional manner by a stable local oscillator (STALO) 18. The signal produced by STALO 18 is heterodyned with a signal produced by a coherent local oscillator (COHO) 19 in a mixer 20 to produce a signal which is amplified by klystron amplifier 13 in a conventional manner. The signal produced at the output of mixer 17 is passed through IF amplifier 21 to a conventional quadrature phase detector 22. Quadrature phase detector 22 responds in a conventional manner to the signals fed thereto from IF amplifier 21 and a reference signal supplied by the COHO 19 to produce a pair of video signals on lines 25, 27, generally referred to as the "in phase" video and "quadrature" video signal, respectively. The "in phase" and "quadrature" video signals are therefore representative of the phase shift of the received signals after being heterodyned to intermediate frequency (IF) signals by mixer 17 and the reference signal produced by the COHO 19. The rate of change in the levels of the "in phase" and "quadrature" signals at any selected time after a pulse is transmitted varies from pulse to pulse at a frequency related to the Doppler frequency of the object at the range corresponding to the selected time. The "in phase" and "quadrature" video signals are digitized and fed to a processing section 26, the details of which will be described. Suffice it to say here, however, that the processing section 26 is used, among other things, to determine the Doppler frequency of various objects illuminated by the radar system 9 and also to estimate the range to a desired object, i.e., a target. Once the desired target is located, the timing and control unit 15 predicts from information determined by the processing section 26, the expected range of the target for the next transmitted pulse. That is, a prediction is made of the expected range of the target and, hence, the approximate return time of each target return. The timing and control unit 15 thereby produces a dwell, or gating window pulse on line 29 after each transmitted pulse, i.e. after each trigger signal, XMT, on line 31, during which returns from the target are expected. The processing section 26 incudes a selector 30, and a processor 32, arranged as shown. Briefly, an analog to digital converter section 28 produces digitized samples of video signals at a continuous, relatively high, constant rate, f s . That is, the sampling frequency remains constant, independent of the pulse repetition frequency and independent of the range resolution cell. Only each Nth one of the digitized samples is processed by the processor section 26, where N = f s /R and R is the rate at which the range cells of the video signal are to be processed; i.e., R is selected in accordance with the desired range cell resolution. Thus, only each Nth one of the digitized samples is passed to the processor section 26 for processing therein, such processed samples being passed to the processor at the rate, R, selected in accordance with the range cell resolution desired by the radar system 9. The value of N, and consequently the range resolution cell, is selected in any conventional manner by the timing and control unit 15. Here the range resolution may be at a rate, R, from f s to f s /N, where N is an integer. That is, during a target acquisition mode, the range cell resolution may be large, i.e., N is a large integer; whereas, once the target has been acquired and the radar system enters a target tracking mode, a small range resolution may be desired, in which case N = 1. More particularly, and referring also to Figs. 2A - 2F, each transmitted radar pulse is produced in response to a radar transmit trigger signal, XMT, on line 31 as shown in Fig. 2A. Digitized samples of the video signals on lines 25, 27 produced by the quadrature phase detector 22 are produced by the analog to digital conversion section 28 in response to a train of sampling pulses, f s , on line 35. The sampling pulses, i.e., here, for example, the rising edges of the pulses from the master clock in the timing and control unit 15 are represented as pulses, shown in Fig. 2C. The sampling pulses are produced on line 35 and have a sampling frequency, f s . The sampling pulses are produced continuously at a fixed sampling frequency, f s , such frequency, f s , being high enough to provide the highest expected range cell resolution desired by the radar system. Thus, rather than sampling only in short bursts of varying sampling rates, the sampling pulses fed to the analog to digital converter section are continuous in time and at a constant rate, here f s . A gating window pulse (Fig. 2D) is produced by the timing and control unit 15 on line 29 a predetermined time after each transmit pulse trigger signal on line 31, XMT. The digital processing section 26 processes each Nth one of the digitized samples produced by the analog to digital converter section 28 during the time duration of the gating window pulse on line 29 to determine the Doppler frequency of energy in the range cells from the obtained digitized samples during the time duration of the gating window. Further, because of the gating window signal on line 29, digital samples obtained during periods of time outside of the gating window are discarded by the processing section 26. More particularly, the selector 30 includes a register section 40 and a decimator 42. The decimator 42 (functional diagram in Fig. 3) includes a counter 50. The counter 50 counts signals fed to the clock input thereof via line 35, i.e., the sampling pulses on line 35 having the rate f s . The counter 50 is enabled during the time duration of the gating window on line 29. The decimator 42 includes a register 52 for storing the integer, N, produced by the timing and control unit 15 on line 53. As noted above, the integer, N, is related to the range resolution desired by the radar system 9. The count of the counter 50 is initially reset to a count of zero. When enabled, the count of counter 50 increments in response to each one of the sampling pulses on line 35. When the count increments to N, a comparator 54 produces an output pulse on line 56 (Fig. 2F) to: reset the count of the counter 50 to zero; and enable register section 40 to store (i.e. load, LD), after a time delay, Δ, the digitized sample available at the output of the analog to digital converter section 28 (i.e., the digitized samples available on lines 58, 60, as shown in Figs. 2E and 2F). Thus, pulses, R 1 , R 2 , ... (Fig. 2F) are produced by the decimator 42 on line 56 at the selected rate, R, where R = f s/ N. Because of the gating window signal on line 29, digital samples obtained during periods of time outside of the gating window are discarded by the processing section 26. Further, it should be noted that the delay, Δ, is provided at the output of the decimator 42 because of the time delay, A, in the analog to digital converter section 28 converting the samples on line 25, 27 into digital samples. Thus, when a pulse R 1 , R 2 , ... (Fig. 2F) is produced by the decimator 42 on line 56, a pulse R' 1 , R' 2 , ... (Fig. 2G) is produced on line 59 after a time delay, Δ. The pulses on line 59 are used to enable the register section 40 to store (i.e., load, LD) the digitized samples available on lines 58, 60, as shown in Figs. 2E, 2F and 2G. Thus, the digital processor 32 has passed to it only each Nth one of the digitized samples produced by the analog to digital converter section 28 during the time duration of the gating window to determine, among other things, the Doppler frequency of energy in the range cells from the processed digitized samples produced during the time duration of the gating window.

Claims: A radar system for determining to a predetermined range cell resolution the range of a target, the system having: receiver means (11, 12, 17, 21, 22) for deriving analog signals representative of radar return signals; analog-to-digital converter means (28) for converting the analog signals into digital signals by producing a digitized sample of an analog signal in response to each one of a train of sampling pulses supplied at a constant rate f s ; and processor means (32) for obtaining range-related data from the digital signals, characterised by selector means (30) for passing each Nth one of the digitized samples produced by the converter means (28), the converter means (28) being coupled to the processor means (32) by the selector means (30), whereby digitized samples are selected and passed by the selector means (30) to the processor means (32) at a rate R where R = f s /N whereby the range cell resolution of the system depends on the value of N, and a timing generator (15) for producing the train of sampling pulses and supplying the train of sampling pulses to the converter means (28) and to the selector means (30), and for supplying a selectable value of N to the selector means (30). A radar system according to claim 1, characterised in that the selector means (30) comprises: a decimator (42), responsive to a signal (53) supplied thereto by the timing generator (15) and representative of the value of N, for producing pulses at the selecting rate R; and a register (40) for storing each Nth one of the digitized samples produced by the analog-to-digital converter means (28), each one of such digitized samples being stored in response to a corresponding one of the pulses produced by the decimator (42). A radar system according to claim 1 or 2, characterised in that the analog signals are video signals produced in response to a series of transmitted radar pulses, and in that the timing generator (15) generates radar transmit trigger signals, each transmitted radar pulse being produced in response to a radar transmit trigger signal, and gating window pulses each produced a predetermined time after each trigger signal, and in that the processor means (32)processes each Nth one of the digitized samples produced by the converter means (28) during the time duration of the gating window following each one of the transmitted pulses, to determine the Doppler frequency of energy in the range cells from the obtained digitized samples during the time duration of the gating window.

Inventors: BOUCHARD GEORGE A, CARMELLA JEFFREY E, DEMILIA MICHAEL P, HAZARD ROBERT W, BOUCHARD, GEORGE A., CARMELLA, JEFFREY E., DEMILIA, MICHAEL P., HAZARD, ROBERT W.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1996
Main classification: H03M1
IPC codes: H03M1, G01S7, G01S13