AIRPORT SURFACE DETECTION RADAR

Abstract: An Airport Surface Detection Radar (ASDR) including a transmitting antenna (116), a transmitter (110), which transmits a substantially linearly frequency modulated CW signal via the transmitting antenna (116), a monopulse receiving antenna (118) and a monopulse receiver which receives at least one response from a target on an airport surface via the receiving antenna (118), generates azimuthal sum and difference signals and determines the position of the target responsive to the instantaneous sum and difference signals and orientation of the antennas.

Description: AIRPORT SURFACE DETECTION RADAR FIELD OF THE INVENTION This invention relates to the field of radar and more particularly to an airport surface detection radar utilizing high range-resolution. BACKGROUND OF THE INVENTION Airport surface detection radars are well known and have been in operation in the United States for well over 35 years. Such systems present a radar map of an air field to a controller who may use the radar map to determine the position and heading of planes on the ground. In general, planes which are in the air are not detected by these radars except when they are very close to the ground, during takeoff and landing. The resolution requirements for such radars are very severe since it is generally desirable that the mapping of the plane on the radar show both the existence of the plane and its orientation. Thus such systems generally have sufficient resolution to show the actual shape of aircraft on the field. Furthermore, other vehicles, such as cars, must also be resolved on the screen. A further requirement of the systems is that they operate in rain and fog, since this is when they are generally needed most. In good whether, visual surveillance of the airport is often thought to be sufficient. In order to meet these requirements, which translate into a technical requirement of very narrow angular discrimination (about 0.1°-0.2°) and very fine range discrimination, high power Ku band nanosecond long pulses are used. Thus, extremely large antennas are generally used (4 to 5 meter) and these antennas are rotated at a high rate to provide a short update period. Angular discrimination is dependent on the design of the radar antenna, on the frequency used and on the characteristics and separation of the objects being mapped. It should be understood, however, that poor range resolution may also affect the ability of the radar system to discriminate between objects whose angular spacing is greater than the angular discrimination of the antenna. In some radar applications, for example in tracking isolated targets, angular discrimination may be enhanced by using monopulse reception of the reflections. However, in ASDR applications where the objects being mapped are often very closely spaced, monopulse reception is generally unreliable due receptions from multiple targets at a given range. FM/C radar systems are known in the art. For example, FM/CW is used in the Improved CW Radar (ICWR) of Hawk missile systems. These radars transmit a CW signal whose frequency is varied linearly with time. The range of an object is determined by measuring the difference in frequency between the signal being transmitted and the signal being received from the target at a given time. If the slope of the frequency variation is known (for example, Δf Hz/sec), the resulting frequency difference δf is equivalent to a range of (δ/Δ)*c. SUMMARY OF THE INVENTION It is an object of some aspects of the invention to provide a radar system, having a high angular and depth discrimination. Preferably, the present invention employs an FM/CW radar system. It is an object of some aspects of the invention to provide an improved Airport Surface Detection Radar (ASDR) having a lower power, improved reliability and smaller size than prior systems. The present invention is based on the use of an improved, high range-resolution, preferably FM/CW, radar to determine the angular position and range of an object. Since an ASDR target generally gives a complex return, the present invention, by its very high position discrimination allows for the separation of returns from adjacent objects and the identification of groups of returns as coming from the same object. The FM/CW radar which is the subject of one aspect of the present invention, utilizes a very stable, preferably very linear FM on a CW signal. Such signals are generated from a digital synthesizer. This results in a high range discrimination between returns from objects. Angular discrimination is enhanced by using monopulse reception of the reflections. The present radar system is adapted to have a sufficiently high range-resolution, so as to prevent the monopulse angular discrimination from being contaminated by returns of multiple targets at a given range. This allows the system to use a smaller antenna than conventional systems and to operate at a lower power, resulting in a smaller, solid state, highly reliable system. There is thus provided, in accordance with a preferred embodiment of the present invention, an Airport Surface Detection Radar (ASDR) including: a transmitting antenna; a transmitter which transmits a substantially linearly frequency modulated CW signal via the transmitting antenna; a monopulse receiving antenna; and a monopulse receiver which receives at least one response from a target on an airport surface via the receiving antenna, generates azimuthal sum and difference signals and determines the position of the target responsive to the instantaneous sum and difference signals and an instantaneous orientation of the antennas. In a preferred embodiment of the present invention, the ASDR includes a direct digital synthesizer (DDS) waveform generator. Preferably, the DDS waveform generator generates an FM/CW signal which is transmitted by the transmitting antenna such that the at least one response received by the monopulse receiver comprises a response to the FM/CW signal. In a preferred embodiment of the present invention, the monopulse receiver has an angular discrimination finer than about 0.5 degrees. Additionally or alternatively, the monopulse receiver has a range resolution finer than about 4 meters, preferably, finer than about 2 meters. Further, in accordance with a preferred embodiment of the present invention, there is provided an FM/CW radar including: a direct digital synthesizer (DDS) waveform generator which generates an FM/CW signal; an antenna system which transmits the FM/CW signal; a receiver which receives a reflected FM/CW signal from a target ; and a signal analyzer which compares the instantaneous frequencies of the transmitted and received signals and determines the range of the target therefrom. In a preferred embodiment of the present invention, the signal analyzer is a monopulse analyzer which receives sum and difference signals from the receiver. Additionally, in accordance with a preferred embodiment of the present invention, there is provided a radar system for mapping targets within a predetermined field-of-view, including: a transmitting antenna; a transmitter which transmits a signal via the transmitting antenna; a monopulse receiving antenna; and a monopulse receiver which receives at least one response from a target in the field-of-view via the receiving antenna, generates azimuthal sum and difference signals and determines the position of the target responsive to the instantaneous sum and difference signals and orientation of the antennas, wherein the position of the target is determined with a range resolution finer than approximately 4 meters. In a preferred embodiment of the present invention, the position of the target is determined with a range resolution finer than approximately 2 meters. Additionally or alternatively, the position of the position target is determined with an azimuthal angle discrimination finer than about 0.5 degrees. In a preferred embodiment of the present invention, the predetermined field-of-view includes an airport surface. In a preferred embodiment of the present invention, the signal transmitted by the transmitter is frequency modulated CW signal, preferably a substantially linearly frequency modulated CW signal. In a preferred embodiment of the present invention, the radar system includes a direct digital synthesizer (DDS) waveform generator which generates the frequency modulated CW signal. SHORT DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the following detailed description of the preferred embodiments of the invention, in conjunction with the following drawings in which: Fig. 1A shows the transmitted and received waveforms in a linear FM/CW radar, illustrating a range determination method in accordance with a preferred embodiment of the present invention; Fig. IB is a simplified block diagram showing an FM/CW transmitter/receiver radar system in accordance with a preferred embodiment of the present invention; Fig. 1C shows a typical output of the system illustrated by the block diagram of Fig. IB; Fig. 2 is a simplified block diagram of an Airport Surface Detection Radar (ASDR) in accordance with a preferred embodiment of the present invention; Fig. 3 is a schematic block diagram of a DDS based oscillator as used in the ASDR of Fig. 2, in accordance with a preferred embodiment of the present invention; and Fig. 4 is a schematic block diagram of a radar signal and data processor, in accordance with a preferred embodiment of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig. 1A shows, in simplified form, the transmitted and reflected waveforms of a FM/CW radar system according to a preferred embodiment of the invention. A CW signal having a frequency modulation which varies as a ramp is transmitted by an antenna. The frequency variation of this signal is shown in Fig. 1A by a solid curve 100. The frequency variation of a reflection from a simple target, having a single return, situated at a given distance from the transmitter is shown in Fig. 1 by dashed curve 102. Since, for this simplified example, only one reflection is present, the frequency of a received reflected signal will track that of the transmitted signal with a time delay, . δt=2r/c, where r is the range of the object and c is the speed of light. If the frequency of the transmitted and reflected signals are compared at any point in time (except near the frequency discontinuities shown) the difference in frequency between the two signals is given by δf=sδt, where s is the rate of change of frequency with time, i.e., the slope of the curve of Fig. 1A. Fig. IB shows a simple transmitter/receiver for a FM/CW radar system operating in accordance with the principle of Fig. 1A. A generator 110 generates a signal having the frequency variation such as that shown in Fig. 1A. This signal is amplified by a power amplifier 112 and transmitted via the direct path of a pick-off 114 to a transmitting antenna 116. A sample of the transmitted signal, from pick-off 114, and a return signal received from a receiving antenna 118 are mixed in a mixer 120 and filtered by a band pass filter (BPF) 122 which removes undesired returns signals, e.g. return from objects outside a given distance range. The frequency of the mixed and filtered signal is substantially linearly dependent on the range of the object being mapped. The signal from BPF 122 is then digitized by an analog to digital (A/D) converter 124 and the digitized signal, which includes LFM frequency components from both transmitted and received signals, is analyzed by a Fast Fourier Transform (FFT) or other frequency analyzer 126 to produce the spectrum shown in Fig. 1C. It should be understood that Fig. IB is an extremely simplified version of a FM/CW radar and that many variations of portions of the circuit are possible. For example, a circulator and a single antenna may be used in place of the pick-off and dual antenna system shown in Fig. 1 and/or the digitization may take place earlier in the signal path or the system may be completely analog. Furthermore, a monopulse receiver is preferably used to improve the angular discrimination of the receiving antenna, as described below. Other variants in the circuit shown in Fig. IB are also possible, as known in the art. Fig. 1C shows the output of frequency analyzer 126 for the simple reflection illustrated in Fig. 1A. A signal output representing the transmitted signal is shown as a blip 125 at a time corresponding to a frequency If(f 0 ). This signal acts as a reference for reflected signals, one of which is shown as a blip 127 at a frequency δf greater than that of the reference. If more than one reflection are received simultaneously from one or more targets they would be represented by separate spectral components at various frequencies corresponding to their distances. The ability of a FM/CW radar to resolve different components of the overall reflection depends on both the range resolution and the angular discrimination of the system. If the angular discrimination is not sufficient, then the frequency components of two targets which are angularly spaced will overlap and it will be impossible to discriminate between them. Similarly, if the range resolution is not sufficient, then it will be impossible to discriminate two radially displaced objects. When mapping closely spaced objects, very large antennas are generally required to obtain high angular discrimination capability because other discrimination techniques, such as monopulse reception, are generally not practical in such cases. However, the present invention utilizes a particularly high range-resolution to enable angular discrimination by a monopulse receiver, as described in detail below, obviating the need for large antennas. Such a system is not useful in the absence of high range resolution. The present invention is described above and below in the context of an FM/CW radar system, as is preferably the case. However, it should be appreciated that other methods and systems, for example short-pulse radar systems, may alternatively be used for providing the high range resolution required by the present invention. Nevertheless, the FM/CW embodiment is preferred because of its generally simpler and less expensive implementation. A basic requirement for high range-resolution when using FM/CW radars is extremely accurate linearity of signal source 110, or at least a variation with time which is very stable and predictable, so that corrections may be made for any non- linearity of the frequency sweep. As shown below, such a system is achievable in accordance with one aspect of the invention. As indicated above, prior art systems have not used FM/CW systems for ASDR systems due to the seeming incompatibility of target discrimination requirements of the ASDR systems with the capabilities of the FM/CW methodology. This has resulted in large, high power, generally unreliable, very short pulse systems for ASDR. A simplified block diagram of an ASDR system in accordance with a preferred embodiment of the invention is shown in Fig. 2. A DDS based waveform generator 130, described more fully below, generates a basis FM/CW signal such as that shown in Fig. 1. This waveform is up-converted to, for example, Ku band by an up- converter 132, of conventional design, which receives a reference frequency from an exciter 134 via a line 136. Exciter 134 also generates one or more fixed frequency signals which are received by waveform generator 130 on line 138 and which are used together with control signals received from a controller 140 on a line 142, to produce the basic FM/CW signal. The output of up-converter 132 is transmitted by a transmitting antenna 144 after being amplified by a transmitter 146. Signals reflected by targets in the beam of antenna 144 are received by a monopulse receiving antenna 148 which rotates, together with antenna 144, at a relatively high rate, for example at a rate of 60 RPM corresponding to an update rate of 1 Hz. The signals received by monopulse antenna 148 are converted, by a front end 150 of conventional design, into sum and difference signals. The sum and difference signals are down-converted by down-converter 152, which receives a reference signal from up- converter 132 via a line 154, to intermediate frequency (IF) signals. The IF sum and difference signals are converted to video sum and difference signals by an IF converter 156 which receives the IF reference signal from exciter 134 via a line 157. The video sum and difference signals are converted into a high angular discrimination reflection signal by a video unit 158 of conventional design and sent to a processor 160. Front end 150, down converter 152, IF converter 156 and video unit 158 may all be embodied in superheterodyne receiver, as is well known in the art. Processor 160 has a number of tasks. First, the processor analyzes the reflection signals and transforms them into range signals, for example, using a FFT. Second, processor 160 communicates with controller 140 providing the controller with a clock frequency, for example a 24 MHz clock frequency, as is known in the art. These requirements are transmitted to exciter 134 as control signals on a line 162. Processor 160 also controls the rotation of the antennas via a pedestal controller 164, for example at approximately 60 RPM, and receives information on the angular position of the antennas from a shaft encoder 166. In a preferred embodiment, encoder 166 has a high resolution and accuracy, desirably a 14 bit resolution and accuracy. Using this information and the signals received from video unit 158, processor 160 displays on a display 165 a map of the reflections from targets on the surface. A user interface 168 allows an operator to turn on the system and to change certain parameters such as overall range of the system and rotation rate. A preferred embodiment of digital waveform generator 130 is shown in Fig. 3. In the preferred embodiment of Fig. 3, waveform generator 130 includes a direct digital synthesizer (DDS) 170, such as a PLESSEY SP 2002, controlled by an arithmetic logic unit (ALU)/counter 172 which provides DDS 170 with control signals via a bus 174. These control signals designate the instantaneous frequency output of DDS 170 with respect to the signal received from exciter 134 via line 138. ALU/counter 172 generates the control signals in response to an FM start signal which designates the starting frequency of the DDS and an FM rate signal which designates the frequency change with each clock pulse. These two signals are received by ALU/counter 172 from controller 140 via lines 142. As indicated above, a stable linear waveform such as that produced by generator 130 allows for fine range discrimination and thus makes it possible to produce a compact relatively low power, all solid state ASDR system. In particular, transmitter 146 can generate 10 watts of output power using any suitable solid state amplifier. In one preferred embodiment of the invention, exciter 134 supplies a 24 MHz and a 1.44 GHz signal to waveform generator 130, a 1.44 GHz and a 1.68MHz signal to up-converter 132, a 1.68 GHz signal to down converter 152 and a 1.656 GHz signal to IF converter 156. All of these frequencies are generated based on a master crystal oscillator in the exciter operating at 240 MHz. The output of waveform generator 130 is in the 360 MHz range and is swept over a range of between 240 MHz and 480 MHz depending on the desired resolution. The output of the up-converter is at 15,960 MHz, so that the transmitted signal is in the 16 GHz range and is swept over a range of ±120 MHz. Antenna 144 is a feed- horn/parabolic antenna having a horizontal extent of 0.4-0.6 meters and a vertical extent of 1.2-1.5 meters. In a preferred embodiment of the present invention, receiving antenna 148 is a dual feed-horn/parabolic antenna of similar dimensions and has an intrinsic angular discrimination of approximately 1 degree. As a result of the monopulse reception, this discrimination is improved to 0.3 degrees or better. This angular discrimination results in a linear azimuth discrimination of approximately 2.5 meters at a 500 meter range. The range of this system is normally between 100 meters and 4,000 meters. This allows a range resolution finer than 4 meters, typically 1-2 meters. The elevation beam-width of the antenna system is preferably on the order of 25 degrees, e.g. between the horizon and 25 degrees below the horizon, which ensures exclusion of flying objects. A preferred embodiment of the invention utilizes a first IF frequency of 1,680 MHz and a second IF frequency of 24 MHz. The DDS-based waveform generator 130 described above, which preferably employs a precision crystal oscillator source as known in the art, provides an FM/CW signal having an extremely accurate linearity which is sufficient for the range-resolution requirements of the present invention. Fig. 4 schematically illustrates a block diagram of a radar signal and data processor 190, in accordance with a preferred embodiment of the present invention, which combines the functions of video unit 158 and processor 160. Sum and difference signals, Σ and Δ, are digitized by a dual A/D 192 and sent to two FFT converters 194 and 196, respectively. For example, the digitized sum and difference inputs received by FFT converters 194 and 196 may include 10 MHz signals, as indicated in Fig.4. FFT converter 194 generates a FFT-converted digitized sum signal, for example a 5MHz signal as indicated in Fig. 4, which is filtered by a magnitude thresholder 197. FFT converter 196 generates a FFT- converted digitized difference signal, for example a 5MHz signal as indicated in Fig. 4, which is filtered by a sum-to-difference comparator 199. Filtered sum and difference signals, 198 and 200, respectively, are received by a fast input/output interface unit 202 which also receives an indication of the azimuth from the shaft encoder 166 and produces an azimuth signal 204 which is sent to a fast computer 206 such as a DEC 2100 computer. Computer 206 converts the digitized video from polar coordinates to longitude/latitude coordinates, provides a digital Sensitivity Time Control equivalent capability, eliminates multi-path signals, stores clutter returns from previous scans, creates a dynamic type clutter map to facilitate clutter suppression, provides video encoding, video correlation and video integration and provides tracking (i.e. following) capability for a large number of targets. It should be appreciated that computer 206 may be adapted to perform any other desired functions as may be required. It should be further appreciated that the functions attributed to computer 206 may be implemented in hardware, software or a combination of both, in accordance with specific design considerations, as known in the art. Information generated by computer 206 is transferred to a display processor and to a VME I/F 208 via ethernet. Most of the elements of radar signal and data processor 190 communicate via a VME bus 210. Based on information generated by computer 206, the display processor adds data tags, alarms for runway incursion, advisories of cautions and warnings, vehicle obstructions on active runways and on approach paths, etc., to the display. It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather, the scope of the present invention is defined only by the following claims:

Claims: CLAIMS 1. An Airport Surface Detection Radar (ASDR) comprising: a transmitting antenna; a transmitter which transmits a substantially linearly frequency modulated CW signal via the transmitting antenna; a monopulse receiving antenna; and a monopulse receiver which receives at least one response from a target on an airport surface via the receiving antenna, generates azimuthal sum and difference signals and determines the position of the target responsive to the instantaneous sum and difference signals and an instantaneous orientation of the antennas. 2. An ASDR according to claim 1 comprising a direct digital synthesizer (DDS) waveform generator. 3. A system according to claim 2 wherein the DDS waveform generator generates an FM/CW signal, wherein the transmitting antenna transmits the FM/CW signal and wherein the at least one response received by the monopulse receiver comprises a response to the FM/CW signal. 4. An ASDR according to any of claims 1-3 wherein the monopulse receiver has an angular discrimination finer than about 0.5 degrees. 5. An ASDR according to any of the preceding claims wherein the monopulse receiver has a range resolution finer than about 4 meters. 6. An ASDR according to claim 5 wherein the monopulse receiver has a range resolution finer than about 2 meters. 7. A radar system for mapping targets within a predetermined field-of-view, comprising: a transmitting antenna; a transmitter which transmits a signal via the transmitting antenna; a monopulse receiving antenna; and a monopulse receiver which receives at least one response from a target in the field-of-view via the receiving antenna, generates azimuthal sum and difference signals and determines the position of the target responsive to the instantaneous sum and difference signals and orientation of the antennas, wherein the position of the target is determined with a range resolution finer than approximately 4 meters. 8. A system according to claim 7 wherein the position of the target is determined with a range resolution finer than approximately 2 meters. 9. A system according to claim 7 or claim 8 wherein the position of the position target is determined with an azimuthal angle discrimination finer than about 0.5 degrees. 10. A system according to any of claims 7-9 wherein the predetermined field-of-view comprises an airport surface. 11. A system according to any of claims 7-10 wherein the signal transmitted by the transmitter is a frequency modulated CW signal. 12. A system according to claim 11 comprising a direct digital synthesizer (DDS) waveform generator which generates the frequency modulated CW signal. 13. A system according to claim 11 or claim 12 wherein the frequency modulated CW signal is a substantially linearly frequency modulated CW signal. 14. An Airport Surface Detection Radar (ASDR) comprising: a direct digital synthesizer (DDS) waveform generator which generates an FM/CW signal; an antenna system which transmits the FM/CW signal; a receiver which receives a reflected FM/CW signal from a target; and a signal analyzer which compares the instantaneous frequencies of the transmitted and received signals and determines the range of the target therefrom. 15. An FM/CW radar according to claim 14 wherein the signal analyzer is a monopulse analyzer which receives sum and difference signals from the receiver.

Inventors: LONGMAN DAVID, LONGMAN, DAVID
Applicants: EL AR ELECTRONICS, HELFGOTT & KARAS P C, LONGMAN DAVID, EL-AR ELECTRONICS, HELFGOTT & KARAS, P.C., LONGMAN, DAVID
Application year: 1996
Main classification: G01S13
IPC codes: G01S13