SAR radar system

• US 8,698,668 B2
• Filed: 11/11/2008
• Issued: 04/15/2014
• Est. Priority Date: 11/11/2008
• Status: Active Grant

• Current Assignee: Saab AB
• Original Assignee: Saab AB
• Inventors: Hellsten, Hans
• Primary Examiner(s): Sotomayor, John B
• Assistant Examiner(s): WINDRICH, MARCUS E
• Application Number:
  US13/128,642
• Publication Number:
  US 20110298654A1
• Time in Patent Office: 1,981 Days
• Field of Search: 342 25 R- 25 F, 342/109, 342/191
• US Class Current: 342/25.F
• CPC Class Codes: G01S 13/72 for two-dimensional tracking, e.g. combination of angle and range tracking, track-while-scan radar
• CPC Class Codes: G01S 13/9017 with time domain processing of the SAR signals in azimuth G01S13/9005 takes precedence
• CPC Class Codes: G01S 13/9029 specially adapted for moving target detection within a single SAR image or within multiple SAR images taken at the same time
• CPC Class Codes: G01S 13/9092 combined with monopulse techniques

Abstract

A method for detecting targets including moving and stationary targets with a radar system equipped with Synthetic Aperture Radar (SAR) onboard a SAR platform including navigation equipment for accurate determination of the position of the SAR platform. The SAR platform is transversing a stationary ground region and targets in the ground region, in which the SAR platform obtains radar data utilizing at least one antenna. A SAR processor records the radar data and the position of the antenna or antennas for each transmitted radar pulse. Radar data within synthetic sub-apertures, are successively merged in N iteration steps into SAR images of increasing resolution of the surveyed region and wherein each iteration step includes forming a new SAR image at a new iteration level by a linear combination of neighboring SAR images in the previous iteration step. A radar system and a SAR processor used for calculating the detection and positioning of targets including moving and stationary targets.

Claims

1. A method for detecting targets comprising moving and stationary targets with a radar system equipped with synthetic aperture radar onboard a synthetic aperture radar platform, comprising navigation equipment for accurate determination of the position of the synthetic aperture radar platform, and which is transversing a stationary ground region and targets in the said ground region, in which the synthetic aperture radar platform obtains radar data utilizing at least one antenna, the movement of the synthetic aperture radar platform during recording of the data being essentially rectilinear and uniform in an azimuth direction, and a velocity of the synthetic aperture radar platform being significantly larger than those of the moving targets, the radar system further comprising a synthetic aperture radar processor recording the radar data and the position of the at least one antenna for each transmitted radar pulse, and wherein within synthetic sub-apertures, each sub-aperture covering a surveyed region being a part of the stationary ground region, radar data are:

successively merged in N iteration steps into synthetic aperture radar images of increasing resolution of the surveyed region, and

wherein each iteration step comprises forming a new synthetic aperture radar image at a new iteration level by a linear combination of neighbouring successively merged synthetic aperture radar images,

wherein the linear combination being obtained by a relation such that related coordinates indicates the same target position at a given reference time for all targets having a specific relative velocity with respect to the moving synthetic aperture radar platform and a specific acceleration in the direction orthogonal to the movement of the synthetic aperture radar platform,

wherein m iteration steps, m<

N, are performed covering the surveyed region where data from one velocity computation at each iteration level using velocity increments Δ

W_l of a processing velocity W, and data from one acceleration computation for acceleration increments Δ

ÿ

_l are used in a motion insensitive process comprising an A-loop for producing one synthetic aperture radar-image of the stationary ground points and the targets per iteration level,

wherein N−

m further iteration steps are performed for a selected patch being a part of the surveyed region, each iteration step using a target detection process at each iteration level with data from at least three computations, comprising at least one velocity computation with the velocity increments Δ

W_l and at least one acceleration computation with the acceleration increments Δ

ÿ

_l for producing one synthetic aperture radar-image per combination of velocity and acceleration computation and for detecting targets and target tracks as well as calculating target parameters R,Φ

, a processing velocity W and an acceleration ÿ

in a direction perpendicular to the azimuth direction, the R,Φ

,ÿ

,W-parameters defining an R,Φ

,ÿ

,W-space for each target,

wherein in a target positioning process, the target position within the patch is calculated by determining the φ

-parameter representing the relative velocity of the target in relation to the moving synthetic aperture radar platform, and

wherein each combination of velocity and acceleration computations represents an assumption of possible target parameters.

2. The method according to claim 1, wherein the synthetic aperture radar images in each iteration are formed for all relative velocities which are conceivable for the moving targets and all such small accelerations, which are considered consistent with keeping the velocity of the moving target constant over time, and wherein this is performed by letting the synthetic aperture radar image resolution and the density of velocity and acceleration assumptions increase with increasing iteration level to an extent such that synthetic aperture radar image pairs will remain related by related coordinates corresponding to the same target position at the synthetic aperture radar resolution of any iteration level by selecting the closest assumption within the set of assumptions for velocity and acceleration for each target made at this particular iteration level.

3. The method according to claim 1, further comprising:

a φ

determination process being accomplished by said at least one antenna having a first and a second phase center displaced in the azimuth direction by a distance larger than the characteristic wavelength of the radar system and smaller than the range resolution and where each phase center is feeding data into a separate receiver channel and where by forming synthetic aperture radar images from data from each channel, these two synthetic aperture radar images being related by related coordinates corresponding to the same target position by selecting the closest assumption for velocity and acceleration for each target, a phase difference between the two synthetic aperture radar images for each target position represents an angle φ

, defined as the φ

-parameter, between a true and an apparent position of the target, and thus enables a complete determination of the state of motion of the target.

4. The method according to claim 1, wherein in an extended surveillance process, the surveillance area is extended to the surveyed region by using outputs from the motion insensitive process, the target detection process and the target positioning process by calculating synthetic aperture radar-images in the immediate surroundings in R,Φ

,ÿ

,W-space of each track.

5. The method according to claim 4, wherein the motion insensitive process is continuously repeated over the surveyed region, each cycle defined as a loop A.

6. The method according to claim 5, wherein the moving targets for a selected patch are detected by first conducting all iterations in the A-loop requiring only just one velocity and acceleration assumption for all conceivable target velocities and accelerations and that the information from the A-loop is fed into two parallel processing paths:

in a first path comprising ex-clutter targets, i.e. targets which have an apparent position outside an antenna footprint defining the boundaries of the synthetic aperture radar image of stationary ground but a true position within the selected patch, the ex-clutter targets being possible to detect through a CFAR thresholding scheme in processing step ex-clutter CFAR, prior to further motion sensitive iteration steps which then are conducted in processing step ex-clutter iteration only for the synthetic aperture radar image fragments and velocity and acceleration parameter assumptions which are in the immediate vicinity of the position, velocity and acceleration parameters of the detected targets

in a second path comprising in-clutter targets, i.e. targets which have an apparent and true position inside the antenna footprint defining the boundaries of the synthetic aperture radar image of stationary ground, and for which further in-clutter iterations, involving processing of the entire synthetic aperture radar image of the surveyed region must be performed, thus improving resolution

in which the two paths are followed by a fusion of data and further CFAR evaluation in processing step CFAR resulting in the complete determination of state of motion for detections forming moving target candidates.

7. The method according to claim 6, wherein one cycle of the processing steps ex-clutter and ex-clutter-iteration is defined as a D-loop, wherein one cycle of the processing steps in the first and the second paths together with the processing step CFAR is defined as a C-loop, and wherein the C-loop is repeated at least for two cycles, the C-loop defining the target detection process.

8. The method according to claim 7, wherein the C-loop is repeated for 10 cycles.

9. The method according to claim 7, wherein a B2-loop comprises:

the target positioning process comprising processing information from the C-loop in a processing step, track, where detected target candidates are analysed according to a track validation algorithm in order to detect valid target tracks, target parameters for valid tracks being fed to processing step, target parameters, and invalid target tracks causing the selection of a new patch and initiation of a new C-loop for this new patch and

wherein the detected target parameters comprising R,Φ

,ÿ

,W from processing step, target parameters, are fed to processing steps, target data and calculation of φ

, wherein in processing step, calculation of φ

, target positioning for detected targets are calculated by determining the φ

-parameter according to a φ

determination process, the processing results from processing step, calculation of φ

, are fed to processing step, target data, and further

wherein the processing results comprising the detected target parameters R,Φ

,ÿ

,W and φ

available in processing step, target data, thus being the result of a target detection and positioning process comprising information processing in the C-loop and in the processing steps, track, target parameters and calculation of φ

one cycle of which is defined as the B2-loop and

wherein the completion of the B2-loop initiates the start up of a new C-loop for a new selected patch.

10. The method according to claim 9, wherein the radar system has an ordinary radar channel with an antenna having the first phase center and an adjacent radar channel with an antenna having the second phase center, both radar channels receiving simultaneous data which are fed to the synthetic aperture radar processor producing one synthetic aperture radar image for each channel, the phase centers being displaced a distance Δ

x along the azimuth direction and further wherein the φ

determination process comprises the calculation of φ

through calculation of the angular phase difference between the two synthetic aperture radar images by multiplying the synthetic aperture radar image ƒ

′

(R,Φ

) from the adjacent radar channel in the coordinates of the ordinary channel with the synthetic aperture radar image conjugate ƒ

(R,Φ

)* from the ordinary channel according to;

11. The method according to claim 9, wherein the obtained detections are represented as 5-tuples (p,n_R,n_Φ

,n_ÿ

,n_W), where p is a batch number ranging from 0 to P, n is the discrete representation of the state of motion parameters R,Φ

,ÿ

,W representing a particular target, a chain of detections constituting a validated target track defined by a track validation algorithm as;

12. The method according to claim 9, wherein the extended surveillance process comprises processing in a continuously repeated B1-loop information available from outputs of the B2-loop and the A-loop and where one cycle of the B1-loop comprises the following processing steps performed in series:

in a processing step further iteration the outputs of the B1- and B2-loop are used to extract out of the output of the A-loop further motion sensitive iteration steps, from the synthetic aperture radar image fragments and velocity and acceleration parameters assumptions which are in the immediate vicinity of the position, thus obtaining velocity and acceleration parameters of the detected targets, the further iteration then being conducted only for validated moving targets

CFAR thresholding is performed in processing step CFAR B1

validated target tracks are generated in processing step target tracks corresponding to the processing steps track and target parameters of the B2 loop and forms the output of the B1-loop

wherein by changing to a new patch in the B2-loop, the previously surveyed patches will remain updated in the B1-loop and thus validated targets tracks for the entire surveyed region will be updated by the B1-loop, whereas the B2 loop will be used to initiate new target tracks for one patch a time.

13. The method according to claim 1, wherein a GPS system is used to calculate the position of the synthetic aperture radar platform and the antenna or antennas.

14. The method according to claim 1, wherein N=9 and m=6 wherein there are 9 iteration steps and 10 iteration levels where the first iteration level is the start-up level for the first iteration corresponding to l=0.

15. The method according to claim 1, wherein the radar system operates at a frequency of around 10 GHz.

22. A synthetic aperture radar processor used for calculating the detection and positioning of targets comprising moving and stationary targets according to the method according to claim 1.

16. A radar system equipped with synthetic aperture radar onboard a synthetic aperture radar platform, comprising navigation equipment for accurate determination of the position of the synthetic aperture radar platform, the radar system being arranged for detecting targets comprising moving or stationary targets and for transversing a stationary ground region and targets in the said ground region, in which the synthetic aperture radar platform is arranged to obtain radar data utilizing at least one antenna, the synthetic aperture radar platform being arranged to move essentially rectilinear and uniform in an azimuth direction with a velocity significantly larger than those of the moving targets during recording of the data, the radar system further comprising a synthetic aperture radar processor arranged to record the radar data and the position of the antenna or antennas for each transmitted radar pulse, and wherein within synthetic sub-apertures, each sub-aperture arranged to cover a surveyed region being a part of the stationary ground region, radar data are:

successively arranged to be merged in N iteration steps into synthetic aperture radar images of increasing resolution of the surveyed region,

wherein each iteration step consisting of arrangement for forming a new synthetic aperture radar image at a new iteration level by a linear combination of neighbouring synthetic aperture radar images in the previous iteration step,

wherein the linear combination is arranged to be obtained by a relation such that related coordinates are arranged to indicate the same target position at a given reference time for all targets having a specific relative velocity with respect to the moving synthetic aperture radar platform and a specific acceleration in the direction orthogonal to the movement of the synthetic aperture radar platform,

wherein m iteration steps, m<

N, are arranged to be performed covering the surveyed region where data from one velocity computation at each iteration level using velocity increments Δ

W_l of a processing velocity W, and data from one acceleration computation for acceleration increments Δ

ÿ

_l are arranged to be used in a motion insensitive process comprising an A-loop for producing one synthetic aperture radar-image of the stationary ground points and the targets per iteration level,

wherein N−

m further iteration steps are arranged to be performed for a selected patch being a part of the surveyed region, each iteration step being arranged to use a target detection process at each iteration level with data from at least three computations, comprising at least one velocity computation with the velocity increments Δ

W_l and at least one acceleration computation with the acceleration increments Δ

ÿ

_l for an arrangement to produce one synthetic aperture radar-image per combination of velocity and acceleration computation and for detecting targets and target tracks as well as calculating target parameters R,Φ

, a processing velocity W and an acceleration ÿ

in a direction perpendicular to the azimuth direction, the R,Φ

,ÿ

,W-parameters defining an R,Φ

,ÿ

,W-space for each target,

wherein in an arrangement for a target positioning process, the target position within the patch is calculated by determining the φ

-parameter representing the relative velocity of the target in relation to the moving synthetic aperture radar platform, and

wherein each combination of velocity and acceleration computations represents an assumption of possible target parameters.

17. The radar system according to claim 16, wherein the synthetic aperture radar images in each iteration step are arranged to be formed for all relative velocities which are conceivable for the moving targets and all such small accelerations, which are considered consistent with keeping the velocity of the moving target constant over time, and wherein this is arranged to be performed by letting the synthetic aperture radar image resolution and the density of velocity and acceleration assumptions increase with increasing iteration level to an extent such that synthetic aperture radar image pairs will remain related by related coordinates corresponding to the same target position at the synthetic aperture radar resolution of any iteration level by arranging the selection of the closest assumption within the set of assumptions for velocity and acceleration for each target made at this particular iteration level.

18. The radar system according to claim 16, further comprising:

a φ

determination process being accomplished by said at least one antenna having a first and a second phase center displaced in the azimuth direction by a distance larger than the characteristic wavelength of the radar system and smaller than the range resolution and where each phase center is arranged to feed data into a separate receiver channel and where by arranging the formation of synthetic aperture radar images from data from each channel, these two synthetic aperture radar images being related by related coordinates corresponding to the same target position by arranging for the selection of the closest assumption for velocity and acceleration for each target, a phase difference between the two synthetic aperture radar images for each target position representing an angle φ

, defined as the φ

-parameter, between a true and an apparent position of the target, and thus enables a complete determination of the state of motion of the target.

19. The radar system according to claim 16, wherein in an arrangement for an extended surveillance process, the surveillance area is extended to the surveyed region by using outputs from the motion insensitive process, the target detection process and the target positioning process by calculating synthetic aperture radar-images in the immediate surroundings in R,Φ

,ÿ

,W-space of each track.

20. The radar system according to claim 16, wherein the radar system comprises a synthetic aperture radar and an antenna upholding one or multiple simultaneous beams and also providing dual phase centers.

21. The radar system according to claim 16, wherein the radar system operates at a frequency of around 10 GHz.