Radar apparatus

Abstract: Radar apparatus provided with a signal generator (1), transmitter means (2), antenna means (3), receiver means (4), a video processor (6) and an indication device (7) for the generation of a radar picture of the surroundings of the radar apparatus with a first resolution. A selected target, designated with control unit (8), may be displayed on the indication device (7) with a second, higher resolution. To this end signal generator (1) generates in the direction of the selected target control signals for transmitter (2) with an increased bandwidth. Selection and conversion unit (5) matches echoes of these increased bandwidth signals to the bandwidth of video processor (6) and indication device (7).

Description: The invention relates to a radar apparatus for detecting targets, provided with transmitting means coupled to a signal generator, with receiver means coupled to a video processor, with antenna means coupled to the transmitting means and receiver means, with an indication device coupled to the video processor for displaying with a first range resolution a radar picture of targets in at least a part of the radar apparatus surroundings and with selection means for selecting a target. In a state-of-the-art radar apparatus, the bandwidth of the transmitted signal, the bandwidth of the receiver means and the bandwidth of the video processor and the indication device are usually identical. This bandwidth is determined by a range resolution desired for that radar apparatus. As known in radar technology, the bandwidth is inversely proportional to the theoretically feasible range resolution. The transmitted signal shall then be provided with some type of modulation which, in conjunction with a matched filter incorporated in the receiver means, realises the desired range resolution. If, according to this state of the art, a greater range resolution is to be realised, the transmitter means, receiver means, video processor and indication device shall be adapted accordingly. With the radar apparatus according to the invention, the bandwidth of the video processor and the indication device may be considerably narrower than the above-mentioned, theoretically required bandwidth for realising a certain range resolution. This entails that not the entire echo signal of a transmitting signal, but only a portion of this signal selected in time, is used. This may be of advantage when observing a target by means of a radar apparatus and when it is desirable for identifcation purposes to analyse the target and its immediately surrounding at an increased range resolution. To this end, the radar apparatus according to the invention is provided with means for displaying the selected target together with an area in range and azimuth around the selected target with a second range resolution which exceeds the first range resolution. The invention will be further explained with reference to the following figures, of which: Fig. 1 represents a block diagram of a radar apparatus according to the invention; Fig. 2 represents a block diagram of a first embodiment of the selection and conversion unit; Fig. 3 represents a block diagram of a second embodiment of the selection and conversion unit. Fig. 1 shows a block diagram of a possible embodiment of a radar apparatus according to the invention. Upon switch-on, a signal generator 1 generates a control signal for transmitter means 2, which are connected to antenna means 3. These antenna means 3 may either be a conventional, rotating type of antenna, or a phased array antenna which performs an electronically-steered search scan. Echo signals received by antenna means 3 are fed to receiver means 4 and subsequently applied to a video processor 6 by a selection and conversion unit 5, which is assumed to be initially inactive. Echo signals processed by video processor 6 are visualized on an indication device 7, which will generally be a display unit presenting the radar apparatus surroundings. In this configuration the radar apparatus operates entirely according to the state of the art. The radar apparatus according to the invention enables a target, observed on the indication device 7, to be monitored at an increased range resolution. To this effect, the radar apparatus is provided with a control unit 8, by means of which a certain target can be designated. Control unit 8 may be provided with a rolling ball or a joystick for correlating a symbol with the target on the indication device 7, or with a keyboard by means of which the target coordinates may be entered. Furthermore, control unit 8 receives the instantaneous azimuth direction from the antenna means 3 and receives a sync signal from signal generator 1, if a control signal for the transmitter means is delivered. If the instantaneous azimuth direction at least substantially coincides with the target direction, signal generator 1 receives a signal from control unit 8, and proceeds to generate control signals for the transmitter means at an increased bandwidth. The control signals are modulated such that the desired increased range resolution may be obtained by auto-correlation. Control signals with such a type of modulation are well-known in radar engineering and may be pulse-modulation, frequency-modulation, phase-modulation or pseudo noise-modulation. The control signal having a larger bandwidth may be transmitted for example in an azimuth sector of 5 degrees around the target to be observed. In this azimuth sector echoes will be produced, which contain, at least potentially, the desired increased range resolution. The measures required for converting this potentially increased range resolution into a radar picture with increased resolution on the indication device, are presented in Fig. 2. This figure represents a block diagram of a first embodiment of the selection and conversion unit 5. This unit comprises two branches. One branch consists of matched filter 9, optimized for echoes of standard transmitter signals, detector 10 for the generation of analog video and A/D converter 11 for the conversion of analog video to digital video. Detector 10 may either be a simple envelope detector, or in case of a radar apparatus whose video processor applies MTI or MTD processing, a quadrature detector. If a quadrature detector is used, A/D converter 11 is duplicated, all entirely compliant with the state of the art. In effect, the invention is embodied in the second branch. This branch comprises a matched filter 12, optimized for echoes of wideband transmitter signals, detector 13 for the generation of analog video, A/D converter 14, and data reduction unit 15, which contains a storage unit. Since wideband signals are processed in this branch, the sampling speed of A/D converter 14 shall be selected to be higher than the sampling speed of A/D converter 11. This means that between two transmitted signals, A/D converter 14 will yield more samples than A/D converter 11. Video processor 6, however, is equipped for the reception of the number of samples delivered by A/D converter 11. In view of this and according to the invention, only a part of the samples delivered by A/D converter 14 is stored in the memory circuit of data reduction unit 15 at the command of control unit 8 and is, after a subsequent transmitted signal, read at a sampling speed which is in line with the fixed sampling speed of video processor 6 and with the sampling speed of A/D converter 11. The samples thus delivered by the memory circuit can be processed by video processor 6 and present the radar picture with the desired increased resolution for display on indication device 7. Both branches are eventually combined by a first selection device 16 which, at the command of control unit 8, connects one of the two branches to video processor 6. In a specific embodiment of the first embodiment, signal generator 1 generates, in the first mode, pulsed control signals having a length of 1 µsec at a pulse repetition time of 200 µsec. Consequently the range resolution is approximately 150 m. Matched filter 9 consists of a bandpass filter having a bandwidth of 1 MHz. A/D converter 11 samples the output of detector 10 at a frequency of 2 MHz. This means that in the first mode, 400 digitized measuring values are fed to video processor 6 between two transmitter pulses. In the second mode, signal generator 1 generates pulsed control signals having a length of 0.1 µsec at a pulse repetition time of 200 µsec. The range resolution is now approximately 15 m. Matched filter 12 consists of a bandpass filter having a bandwidth of 10 MHz. A/D converter 14 samples the output of detector 13 at a frequency of 20 MHz. This means that in the second mode, 4000 digitized samples are generated between two transmitter pulses. Only 400 of these samples, successively obtained around the target, are selected and stored in the memory circuit of data reduction unit 15. After the next ensuing transmitter pulse these 400 samples are read from the memory circuit at a clock frequency of 2 MHz and are fed to the video processor via the first selection device 16. After processing by video processor6 the 400 range quants are applied to the indication device 7, which will generally be a PPI. This indication device displays the 400 range quants which represented a 30 km range in the first mode and represent a 3 km range in the second mode. Fig. 3 shows a block diagram of a second embodiment of the selection and conversion unit 5, in which only one detector 13 and one A/D converter 14 is used. In this case detector 13 is preceded by a second selection device 19 which, at the command of control unit 8, enables the output of one of the two matched filters 9, 12 to be connected to detector 13. The sampling speed of A/D converter 14 has been selected such that wideband signals, i.e. echoes of signals transmitted in a wide band, can be sampled without information loss. This entails that in processing narrow-band signals, these signals are oversampled by A/D converter 14. To cure this, a first data reduction unit 20 is connected between A/D converter 14 and video processor 6 to reduce the sampling speed of A/D converter 14 to the fixed sampling speed of video processor 6. If we assume the oversampling factor to be M, data reduction unit 20 will, in a most elementary embodiment, only pass each Mth sample. A better method, however, is to provide the data reduction unit 20 with a digital low-pass filter and to subsequently pass each Mth sample. This data reduction method, well known in the art, improves the signal-noise ratio and the dynamic range of the processed signal. For wideband signals a second data reduction unit 15 is provided, which operates entirely according to the description of Fig. 2. The combination of the outputs of the two data reduction units 15, 20 is effected by the first selection device 16 which, at the command of control unit 8, connects the output of one of the two data reduction units to video processor 6. Also as regards this embodiment the requirement prevails that in case of a radar apparatus whose video processor applies MTI or MTD processing, detector 13 is a quadrature detector and A/D converter 14 is duplicated. Both reduction units 15, 20 and the first selection device 16 will then be dual-channeled. As an alternative, the single video obtained with one detector and one A/D converter can after first selection device 16 be split up into quadrature video using a Hilbert filter, well known in the art. In a special embodiment of the second embodiment, signal generator 1 generates, in the first mode, pulsed control signals having a length of 1 µsec and a pulse repetition time of 200 µsec. The range resolution will then approximately be 150 m. Matched filter 9 consists of a bandpass filter having a bandwidth of 1 MHz. In the second mode, signal generator 1 also generates pulsed control signals having a length of 1 µsec and a pulse repetition time of 200 µsec, provided, however, with a linear FM modulation having a bandwidth of 10 MHz. Matched filter 12 consists of a pulse compression line which compresses the 1 µsec pulse with the linear FM modulation to a pulse having a length of 0.1 µsec. A/D converter 14 samples the output of detector 13 at a frequency of 20 MHz. In the first mode, 4000 digitized samples are generated between two transmitter pulses, which results in 20 samples per 150 m, the range resolution in this mode. The first data reduction unit limits this number to 400 samples, which consequently amounts to 2 per 150 m. To this end, said data reduction unit comprises a digital low-pass filter with a cut-off frequency of 1 MHZ, after which each tenth sample is passed to the first selection device, as a result of which the clock frequency is reduced to 2 MHz. In the second mode, 4000 digitized samples are generated between two transmitter pulses. The second data reduction unit 15 limits this number to 400 samples surrounding the target. These samples are stored in the memory circuit of the second data reduction unit 15. After the next ensuing transmitter pulse, these 400 samples are read from the memory circuit at a clock frequency of 2 MHz. In the first as well as in the second mode, video processor 6 shall be suitable for processing 400 digitized samples which are presented at a clock frequency of 2 MHz. Nowadays, radar apparatuses coupled to fire control equipment for which a short reaction time is required, do not always use an indication device. Instead of this device, the radar apparatus is equipped with a track radar which, should the occasion arise, is capable of independently activating weapons against targets. For identification reasons, it could be essential to observe a target at an increased range resolution. According to the considerations underlying the invention, the track computer, instead of the human operator, can then designate a target and subsequently request an increased range resolution for this target.

Claims: Radar apparatus for detecting targets, provided with transmitting means (2) coupled to a signal generator (1), with receiver means (4) coupled to a video processor (6), with antenna means (3) coupled to the transmitting means (2) and receiver means (4), with an indication device (7) coupled to the video processor (6) for displaying with a first range resolution a radar picture of targets in at least a part of the radar apparatus surroundings and with selection means for selecting a target, characterised in that the radar apparatus is provided with means for displaying the selected target together with an area in range and azimuth around the selected target with a second range resolution which exceeds the first range resolution. Radar apparatus according to claim 1, characterised in that said means comprise a control unit (8) coupled to the antenna means (3) and the signal generator (1), for switching the signal generator (1) from a first mode of operation, in which signals with a first bandwidth corresponding with the first range resolution are generated, to a second mode of operation, in which signals with a second bandwidth corresponding with the second range resolution are generated, if the azimuth direction of the antenna means (3) and the azimuth direction of a selected target at least substantially coincide. Radar apparatus according to claim 1 or 2, whose video processor (6) is equipped for the processing, at a fixed sampling speed, of signals having a bandwidth which at least substantially corresponds with the first bandwidth, characterised in that said means are furthermore provided with a selection and conversion unit (5) connected between the receiver means (4) and the video processor (6) for selecting echo signals of the transmitted signals of the second bandwidth of the surroundings of the selected target and for converting these echo signals into a signal which corresponds with the sampling speed and bandwidth of the video processor (6). Radar apparatus according to claim 3, characterised in that the selection and conversion unit (5) comprises a first A/D converter (11) equipped for sampling and digitizing signals of the first bandwidth; a second A/D converter (14) equipped for sampling and digitizing signals of the second bandwidth, followed by a data reduction unit (15) for converting signals of the target surroundings into a signal which corresponds with the video processor (6) bandwidth; and a first selection device (16) for selecting, in the first mode, the output of the first A/D converter (11) and for selecting, in the second mode, the output of the data reduction unit (15). Radar apparatus according to claim 4, characterised in that the data reduction unit (15) comprises a memory circuit for storing at least a portion of the signals generated by the second A/D converter (14) and for subsequently delivering at least a portion of the stored signals at a sampling speed which corresponds with the video processor (6) bandwidth. Radar apparatus according to claim 5, characterised in that the first A/D converter (11) is preceded by a matched filter (9) for signals of the first bandwidth and in that the second A/D converter (14) is preceded by a matched filter (12) for signals of the second bandwidth. Radar apparatus according to claim 3, characterised in that the selection and conversion unit (5) comprises an AID converter (14), equipped for sampling and digitizing signals of the second bandwidth, followed by a first data reduction unit (20) for converting digitized signals of the first bandwidth into a signal which corresponds with the fixed sampling speed of the video processor (6) and a second data reduction unit (15) for converting digitized signals of the second bandwidth from the target surroundings into a signal which corresponds with the video processor (6) bandwidth, and a first selection device (16) for the selection, at the command of the control unit, of the output signals of the first or the second data reduction unit for connection to the video processor (6). Radar apparatus according to claim 7, characterised in that the first data reduction unit (20) is provided with a digital low-pass filter. Radar apparatus according to claim 7, characterised that the second data reduction unit comprises a memory circuit for storing at least a portion of the signals generated by the A/D converter (14) and for subsequently delivering at least a portion of the stored signals at a sampling speed which corresponds with the video processor (6) bandwidth. Radar apparatus according to claim 7, characterised in that the A/D converter (14) is preceded by a first matched filter for signals of the first bandwidth (9) and a second matched filter (12) for signals of the second bandwidth, and a second selection device (19) for selecting, at the command of the control unit (8), the output signals of the first or the second matched filter for applying to the A/D converter (14).

Inventors: ZWARTS JOHAN MARTIN CAROL, ZWARTS, JOHAN MARTIN CAROL
Applicants: HOLLANDSE SIGNAALAPPARATEN BV, HOLLANDSE SIGNAALAPPARATEN B.V.
Application year: 1992
Main classification: G01S13
IPC codes: G01S13, G01S7