AU632280B2

AU632280B2 - A synthetic aperture radar

Info

Publication number: AU632280B2
Application number: AU59712/86A
Authority: AU
Inventors: Thomas Hair; David Edward Rice
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1985-07-02
Filing date: 1986-06-20
Publication date: 1992-12-24
Anticipated expiration: 2006-06-20
Also published as: FR2684767B1; SE466120B; IT1236504B; IT8648198A0; FR2684767A1; SE8602927D0; NL8601631A; NO862630L; SE8602927L; DE3622186A1

Description

Form COMMONWEALTH OF AUSTRALIA PATENTS ACT 195 COMPLETE SPECF

(ORIGINAL)

Class Application Number: Lodged: Int. Class Complete Specification Lodged: Accepted: Published: Priority: i d SRelated Art: o i Name of Applicant: o O4 Ao \d6'ess of Applicant: Q 64 o o 0003 Actual Inventor: o 0 °AtUiress for Service 0 4 0 t0 GEC AVIONICS LIMITED Airport Works, Rochester, Kent ME1 2XX, England DAVID EDWARD RICE and THOMAS HAIR EDWD. WATERS SONS, 50 QUEEN STREET, MELBOURNE, AUSTRALIA, 3000.

Complete Specification for the invention entitled: A SYNTHETIC APERTURE RADAR The following statement is a full description of this invention, including the best method of performing it known to US -la- I/7134/MB A SYNTHETIC APERTURE RADAR This invention relates to a synthetic aperture radar and arose in relation to two specific problems. The first problem is that a conventional synthetic aperture radar cannot detect a target whose velocity relative to the radar is outside the range of velocities relative to the radar of stationary targets illuminated by the real beam.

In practice this means that the radar might fail to a 4 oo respond to targets 'having a radial velocity of more than say im per second. The second problem is that the conventional radar assumes that all targets are stationary and, in making this assumption, targets which are moving slowly en.ough to be detected, are displayed at an ooIa incorrect broadside position i.e. an incorrect position in the direction of movement of the radar.

Figure 1 shows the relationship between Doppler frequency and time for the part of a signal received from a stationary target broadside at time t as it is swept 0 l during time T by the real radar beam. The synthetic aperture processing sums phase and amplitude weighted samples of the signal received during the period to +t I to give a synthesized response at time to which is assumed in conventional systems to come from a target on t boresight at time t 0 r i.

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itt It A slow moving target which is broadside at time A with a Doppler frequency of fA will have the same characteristics as shown on Figure 1 and will therefore not be distinguished from a target on boresight. A faster moving target B having a Doppler frequency of fB and also having the same characteristics is not detected at all because at the time t when the processor is matched to o the characteristics of the target, the target is not within the real radar beam.

This invention provides a synthetic aperture radar comprising a plurait!y of synthetic aperture processing channels matched to different target Doppler frequencies, for the detection of targets moving at velocities within respective predetermined velocity ranges, an additional processing facility and means responsive to a detection in any one of the said channels for causing the said facility to form further synthetic aperture processing channels matched to frequencies spaced within the channel in which the detection took place to determine with increased 'accuracy the position and/or velocity of the detected target.

A processing channel matched to a Doppler fp will give detection for any target having a Doppler frequency between fp VP/ where Vp is the transjerse platform velocity,9 is the real beam width and k is the wavelength. Channels provided in accordance with the invention and spaced evenly by 2VpQ/ over the Doppler

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3 band will thus provide full coverage, eliminating the first problem referred to. Figure 2 shows the relationship between Doppler frequency and time for three targets on the boresight at time t and having Doppler 0 frequencies of fPl, fP2 and fP3. This figure illustrates that processing channels matched to those frequencies will provide full cover over the band shown.

The second problem, namely that of the incorrect broadside position of targets whose Doppler is not matched to the processing, occurs for all processing channels.

The broadside positional uncertainty is equal to the real beamwidth in azimuth. Likewise there is an uncertainty in Doppler of Vp The invention resolves the broadside position and the Doppler of a detected target more accurately through the introduction of additional the processing capability.

Each processing channel is equivalent in complexity to a further synthetic aperture radar processor and to cover the likely Doppler range a number of channels, perhaps about 30, are required. A 30 fold increase in processing facilities would be prohibitive for a high litresolution synthetic aperture radar. However the resolution appropriate to the detection of moving targets which have a relatively high radial velocity and are therefore not associated with returns from stationary background detail is lower than is required for conventional synthetic aperture radar processing. Indeed

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ji 4 a lower resolution is essential for such faster moving targets since these will not otherwise remain within the synthetic aperture radar resolution cell during the synthetic aperture integration time. It is proposed therefore to detect the aforementioned higher radial velocity targets by means of the invention, as previously defined, using a coarse resolution mode whilst retaining a conventional high resolution mode for the detection of targets which are either stationary or have a small radial component of velocity. High resolution is essential for the detection of targets associated with background returns to maximise target to background contrast.

The use of the invention results in a number of detections in resoective channels spaced throughout the time of illumination, by the real beam, of the target.

The uncertainty in broadside position is reduced to half the time between detections and the uncertainty in Doppler is reduced to half the frequency separation *between channels Figure 3 illustrates the frequency and time of detections occurring at times tl, t2, t 3 and t 4 in the different channels matched to respective Doppler 4( frequencies of fA, fB, fC and fD. The best estimate of the time when the target is broadside is T mean which is the mean of times tl, t2, t 3 and t 4 In fact it is only necessary to use the times of the detections at t 1 and t 4 to obtain the optimum accuracy of broadside position.

Similarly the best estimate of Doppler is f mean, the average of the frequencies fA, fB, fC, fD associated with hose channels in which detections

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5 occur.

The restriction of this further processing capability to channels within which a target has been detected limits the processing requirement to a multiple of the number of targets rather than the number of resolution cells. This is achieved at the expense of storage of radar samples over the full real beamwidth rather than the synthesized beamwidth.

Calculation of the processing requirements for a particular case indicates that full moving target cover and accuracy improvement can be achieved at a cost of a 2- 41 3 fold increase in the processing requirement over that of 1 a conventional synthetic aperture radar which is matched 4 f to stationary targets only.

The slope of the time Doppler characteristics is a measure of the target velocity in the broadside direction.

When combined with the radial velocity obtained from the aforementioned measurement of Doppler this enables the target absolute velocity and track to be deduced if 4 I required.

One way in which the invention may be performed will now be described by way of example with reference to Figure 4 of the accompanying drawings which shows a radar constructed in accordance with the invention and mounted on an airborne platform moving relative to the earth's surface.

Referring to Figure 4 a transmitter 1 produces short I I J- 6 and long pulses corresponding in range resolution to 3m and 36m respectively. The short and long pulses are generated alternately with sufficient time being left after each pulse for returns from the swath being inspected to be received before transmission of the next pulse.

The pulses are transmitted via a duplexer 2 and antenna 3 and are received after reflection from the swath under inspection in a receiver 4 which is designed to process the long and short pulses respectively and to generate two respective outputs on lines 5 and 6.

Digitized samples of the received signal on line are stored at 7 throughout the time of illumination of a i* point on the earth's surface by the real beam. This is T in the example shown on figure 1. A conventional 4 ti synthetic aperture radar processor 8 produces, from the stored signal, outputs with high resolution (in this .4 example 3m x 3m) These outputs are applied to a threshcld detection device 9 which, on receiving a detection at say time t (Fig causes a read out device 10 to read from the store 7 the digitized radar signals received during the time interval T for the range gate in which the detecting is observed (on Fig These signals are passed to a processor 11 which is programmed to perform a number of aperture synthesizing processes as illustrated schematically at 11A matched to frequencies such as shown at fA, fB, fC and fD as shown on Fig 3 over 7 a range of Doppler frequencies Vp e8x. A calculating facility 12 calculates, from the times of detections in the various channels 11A, 11B etc., the values f mean, t mean and slope as previously described with reference to Fig 3. This indicates the position and velocity of the detected targets which information is presented on line 13 to a display 14. In this way targets which, because they are stationary or slow moving are received in conjunction with terrain reflections, are detected with reduced ambiguity of position and velocity as compared with conventional techniques.

Digitized samples on line 6 derived from the long i! pulses are stored at 14 and are processed in synthetic aperture processors 15, 16 etc. matched to frequencies fPl, fP2 etc. from targets of different radial velocities as shown on figure 2. These processors produce outputs having lower resolution (in this example 36m). These outputs are applied to threshold detectors 17, 18 etc.

any one of which, on receiving a detection, causes an interface device 19 to instruct a readout device 20 to read from the store 14 the digitized radar signals received during the time interval T for the range gate in which the detection is observed. The interface logic 19 communicates to a processor 21 the identity of the channel 16 etc. in which a detection was observed. The processor 21 is programmed to perform a number of aperture synthesizing functions 21A matched to frequencies such as 8-8shown at fA, fB, fC and fD (Fig 3) over a range of Doppler frequencies fp Vp eQ/ where fp is the frequency of the channel in which a detection is made, this frequency being identified on line 19A.

In practice a number of detections must be processed simultaneously and so the components 11 and 21 must include sufficient computing facilities for that purpose.

The output of the processor 21 is operatedton at 22 in a way identical to the operation peiformed at 12 to present on line 23, to the display 14, the position and velocity .il. of the detected targets. In this way targets which, because they are moving relatively fast in a radial ,n direction, are not indicated on line 13, are nevertheless di3played at 14.

In this particular embodiment of the invention an additional processor 24 is employed. This performs conventional synthetic aperture radar processing but at a 4i relatively low resolution of 36m to provide a coarse representation of the background. This representation is 0444, correlated at 25 with data from a store 26 and in particular with a part of that store containing a low resolution digital map 26A. This map 26A represents a relatively large area of ground over a particular part of which the aircraft can be assumed to be located. The output of the correlator 25 indicates the current position of the aircraft carrying the radar and this positional information is used to read out the appropriate part of a if 0 9detailed map 26B of the same area of ground. This information is displayed at 14 where it is superimposed on the targets indicated on lines 13 and 23. The store 26 could be located on the aircraft but it is preferred that it be based on the ground and linked to the aircraft by a suitable communications channel. In another embodiment of the invention the low resolution map 26A can be omitted.

In such an arrangement each frame of video information from the output of circuit 24 is compared in the correlator 25 successively with different parts of the map 26B. The part which gives the best correlation is then read out to the display 14.

Claims

1. A synthetic aperture radar comprising a plurality of synthetic aperture processing channels matched to different target Doppler frequencies for the detection of targets moving at velocities within respective predetermined velocity ranges, an additional processing facility and means responsive to a detection in any one of the said channels for causing the said facility to form further synthetic aperture processing channels matched to frequencies spaced within the channel in which the detection took place to determine with increased accuracy the position and/or velocity of the detected target.

2. A radar according to claim 1 mounted on a platform moving at velocity Vp in which the channels are matched to different frequencies spaced by not more than 2 Vp E/ where Vp is the transverse platform velocity, is the real beamwidth and is the wavelength of the radar signals.

3. A radar according to claim 1 or 2 comprising a second synthetic aperture processing channel having a higher resolution than the first mentioned channels and matched to stationary targets and designed to give an output of higher spacial resolution in the direction of movement of the radar than that given by any of the firstmentioned lower resolution channels.

4. A radar according to claim 3 comprising a transmitter which transmits long and short pulses and in which the I fir of cha pul cha sta con con rec cur ter rec dis inf tare

6. wit RESTRICTED RESTRICTED 11 firstmentioned channels are adapted to respond to returns of the long pulses and the second higher resolution channel is adapted to respond to returns of the short pulses. A radar according to claim 1 or 2 in which one of the channels is matched to a Doppler frequency appropriate to stationary targets and including correlating means connected to correlate the output of this channel with the contents of storage means defining a low resolution record of terrain so as to give an output identifying the current position of the radar relativd to the said terrain, further storage means defining a high resolution record of the said terrain; and display means for displaying the so identified part of the terrain from information in the further storage means together with targets detected by the radar. 6. A synthetic aperture radar substantially as described with reference to figure 4 of the accompanying drawings. DATED THIS 9TH DAY OF MAY, 1990 GEC AVIONICS LIMITED WATERMARK PATENT TRADEMARK ATTORNEYS THE ATRIUM 290 BURWOOD ROAD HAWTHORN, VICTORIA 3122, AUSTRALIA. RESTRICTED