WO2016005738A1

WO2016005738A1 - Method and system for surveillance using synthetic aperture radar images

Info

Publication number: WO2016005738A1
Application number: PCT/GB2015/051967
Authority: WO
Inventors: Nicholas Giacomo Robert Colosimo
Original assignee: Bae Systems Plc
Priority date: 2014-07-09
Filing date: 2015-07-07
Publication date: 2016-01-14
Also published as: GB2528089A; GB201412198D0

Abstract

A method and system of surveillance using a synthetic aperture radar system, in which a plurality of datasets are captured, each dataset comprising a plurality of images (1A, 2A, 3A, 4A, 5A) captured at a respective plurality of imaging positions along a path relative to an area of interest. Each image of a dataset is compared with a respective corresponding image of the preceding dataset to produce respective coherent change detection images [1(A-B), 1(B- C), 1(C-D). 1(D-E)] for each of said plurality of imaging positions. Then, the coherent change detection image for each imaging position is compared with the change detection image for the immediately preceding imaging position on said path to produce incoherent change detection images [1(A-B) –2(A-B)] representative of changes between consecutive imaging positions along said path.

Description

METHOD AND SYSTEM FOR SURVEILLANCE USING SYNTHETIC APERTURE RADAR IMAGES

This invention relates to a method and system for surveillance using synthetic aperture radar (SAR) images. Synthetic aperture radar systems are known, and have been used for many years to generate high resolution images of the ground. The synthetic aperture is produced by recording a series of radar returns taken from a moving platform over a period of time, applying a phase correction factor to each one that corrects for the platform movement, and integrating them over a time period. Images may be generated from the processed returns that have an equivalent resolution dependent upon the aperture size, i.e. the distance moved by the platform over the time period, or by the aperture beamwidth of the radar's antenna.

SAR systems usually operate in one of two modes of operation. These are called strip-map mode, and spotlight mode. In strip-map mode, the antenna pointing direction is fixed relative to the flight path providing an area of illumination that sweeps along a strip of terrain. In spotlight mode, the sensor steers the antenna to continuously illuminate an area of interest allowing a smaller area to be imaged at a higher resolution. The latter approach also provides, for a single pass of the moving platform, a set of images of the same region, albeit that each are from a different aspect, whereas a plurality of different passes are needed to produce multiple images of the same region using the former, strip-map, approach.

With each approach, therefore, a set of images of a region of interest may be built up. By comparing two or more images of the same region, taken at different times, and looking for differences between them, changes in the scene, such as may be caused by an object which has been relocated, can be identified. Various ways of doing this are known.

The most simple is known as incoherent change detection (ICD), and uses changes in image intensity for detecting changes in the interval between the collection of two SAR images of the same region. As the technique operates on intensity information then complex image data (i.e. phase information) is not required.

Another technique is known as coherent change detection (CCD). This technique exploits changes in both amplitude and phase content between image pairs (i.e. images of the same scene taken at different times). It relies on the image generation process being coherent, wherein each pixel of the image contains an amplitude and a phase, or alternatively a real and imaginary value. As the phase is measured in fractions of a wavelength of the radar signal, which may typically be around 3cm, CCD has then potential to detect very subtle scene changes that may remain undetected using incoherent techniques.

Thus, the output of a known SAR CCD process is a set of still images showing the changes that have occurred in a scene between two consecutive passes of the moving platform. It is usually sufficient for static objects and their tracks to be detected and identified within the images. However, SAR systems are not usually capable of effectively imaging objects that are moving, as movement within SAR images is either filtered out of the resultant images by processing, or blurred or otherwise represented by some artefact in the image. Thus, conventional SAR systems are not suitable for tracking moving objects.

GMTI (Ground Moving Target Indication) systems are known which enable moving objects to be detected and tracked, and can provide a velocity component of the object along the radar beam sight line (radial velocity). These systems take advantage of the fact that the object moves with respect to stationary clutter. Successive return radar pulses are sampled at the same distance and each sample is rotated by 180° and added to the next sample. This is known as destructive interference. If an object is moving in the location corresponding to both samples, the signal reflected back from the object will survive this because of constructive interference. If all objects are stationary, the two samples will cancel out and very little signal will remain, In other words, moving objects can be detected and tracked using GMTI, but they cannot be identified, nor can static objects within the area be detected.

It would be desirable, therefore, to provide a surveillance system which allows moving objects to be both tracked and identified. In accordance with the present invention, there is provided a method of surveillance using a synthetic aperture radar system, the method comprising receiving first and second datasets, each dataset comprising a plurality of images captured at a respective plurality of imaging positions along a path relative to an area of interest; comparing each image of said second dataset with each respective corresponding image of said first dataset to produce respective first change detection images for each of said plurality of imaging positions; and comparing the change detection image for each imaging position with the change detection image for the immediately preceding imaging position on said path to produce second change detection images representative of changes between consecutive imaging positions along said path.

Thus, the present invention provides an improved surveillance system that can track moving objects, including slow-moving and/or small objects, and reveal the type of track made by the moving objects such that it can be classified or otherwise identified provided that the movement causes measurable changes to the surface on which the object has travelled. It can also be appreciated that the gathered datasets could also be used to produce conventional SAR images of the scene.

In a preferred exemplary embodiment of the invention, the first change detection images are coherent change detection images, and the second change detection images are preferably incoherent change detection images.

Also in accordance with the present invention, there is provided a synthetic aperture surveillance system, comprising a receiver for receiving first and second datasets, each dataset comprising a plurality of images captured at a respective plurality of imaging positions along a path relative to an area of interest; a first change detection module for comparing each image of said second dataset with each respective corresponding image of said first dataset to produce respective first change detection images in respect of said plurality of imaging positions; a second change detection module for comparing the change detection image for each imaging position with the change detection image for the immediately preceding imaging position on said path to produce second change detection images representative of changes between consecutive imaging positions along said path; and an output for consecutively displaying said second change detection images.

In a first exemplary embodiment of the present invention, the first change detection module is a coherent change detection module, and the second change detection module may be an incoherent change detection module.

In a further aspect, the present invention provides a program or plurality of programs arranged such that when executed by a computer system or one or more processors it/they cause the computer system or the one or more processors to operate in accordance with the method according to the first aspect.

In a further aspect, the present invention provides a machine readable storage medium storing a program or at least one of the plurality of programs according to the preceding aspect.

These and other aspects of the present invention will become apparent from the following description of embodiments of the present invention, which will now be described by way of examples only and with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram illustrating a process for capturing a first dataset for use in a method and system according to an exemplary embodiment of the present invention;

Figure 2 is a schematic diagram illustrating a process for capturing a second dataset for use in a method and system according to an exemplary embodiment of the present invention;

Figure 3 is a schematic block diagram illustrating the output of a first change detection algorithm employed in a method and system according to an exemplary embodiment of the present invention;

Figure 4 is a schematic block diagram illustrating a change detection process employed in a method according to an exemplary embodiment of the present invention; and Figure 5 is a schematic block diagram of the principal elements of a SAR surveillance system according to an exemplary embodiment of the present invention.

Referring to Figure 1 of the drawings, in a SAR surveillance system according to an exemplary embodiment of the present invention, an aircraft 10, having an onboard SAR imaging device (not shown) travels along a path 12 in the vicinity of a region of interest (Rol) 13. In the example shown, the spotlight mode of operation, as described above, is employed to capture SAR images, but the strip-map method could equally be used. Thus, a radar beam 14 in the SAR imaging device continuously projects a footprint over the Rol 13 from the aircraft 10, irrespective of its position along the path 12. Image data is gathered at each of a plurality of positions 1 , 2, 3, 4, 5. In reality, many more image capturing positions will tend to be defined along the path 12, but only five are shown in Figure 1 for simplicity. Thus, at the end of the aircraft's first complete pass along the path 12, first dataset has been gathered which comprises a plurality of SAR images captured at a respective plurality of positions along the path. For the purposes of this description, these images will be referred to as 1A, 2A, 3A, 4A and 5A corresponding to positions 1 , 2, 3, 4 and 5 respectively along the path 12. Referring to Figure 2 of the drawings, this process is then repeated to generate a second dataset: the aircraft 10 again travels along the path 12 and image data is gathered at each of the same plurality of positions 1 , 2, 3, 4, 5 as before. The images of the second dataset will be referred to herein as 1 B, 2B, 3B, 4B and 5B corresponding to positions 1 , 2, 3, 4 and 5 respectively along the path 12.

It will be appreciated that the above-described process can be repeated for any number N passes, depending on user requirements.

Next, referring to Figure 3 of the drawings, each image of the second dataset is coherently compared with each corresponding image of the first dataset, and each pair of images is used to create a corresponding CCD image in a conventional manner. As is well known in the art, coherent change detection offers the capability for detecting changes between imaging passes. To detect whether or not a change has occurred, two images are taken of the same scene at different times, e.g. 1A and 1 B, and these images are geometrically registered so that the same target pixels in each image align. After the images are registered, they are cross correlated pixel by pixel. Where a change has not occurred between the imaging passes, the pixels remain correlated, whereas if a change has occurred, the pixels are uncorrelated. The resultant CCD image, representative only of the changes, is thus output, e.g. 1 (A-B).

Referring now to Figure 4 of the drawings, once a set of CCD images has been generated in this manner, CCD images from adjacent positions can be compared, in this case incoherently, although coherent change detection could once again be used, to create a corresponding ICD image in a conventional manner, representative only of the changes occurring in the scene between adjacent image capture positions. In ICD, only the amplitude of the images is used in a straightforward subtraction algorithm to produce an output representative of the changes, e.g. {[1 (A-B)] - [2(A-B)]}, {[2(A-B)] - [3(A-B)]}, etc. and these ICD images are successively output in real-time to provide a moving update, i.e. near real-time tracking by means of moving surface depressions.

Referring to Figure 5 of the drawings, a SAR surveillance system 300 according to an exemplary embodiment of the present invention comprises a receiver 301 for receiving images from the SAR imaging system onboard an aircraft. The image datasets NA, NB, NC, ND, NE, etc. are fed to a CCD module 302 configured to perform the coherent change detection process described above with reference to Figure 3, in respect of successive pairs of images captured at the same position along the path but at different times. The CCD images thus generated may be output to a display 303, if required, to provide a conventional SAR system output.

The CCD images are then fed to an ICD module 304, configured to perform the incoherent change detection process on each successive pair of CCD images, as described above with reference to Figure 4 of the drawings. The ICD images thus generated are then output to a display 305 in real time, thereby providing the near real-time tracking functionality. It will be appreciated by a person skilled in the art that modifications and variations can be made to the described embodiments without departing from the scope of the invention as claimed.

Claims

1 . A method of surveillance using a synthetic aperture radar system, the method comprising receiving first and second datasets, each dataset comprising a plurality of images captured at a respective plurality of imaging positions along a path relative to an area of interest; comparing each image of said second dataset with each respective corresponding image of said first dataset to produce respective first change detection images in respect of said plurality of imaging positions; and comparing the change detection image for each imaging position with the change detection image for the immediately preceding imaging position on said path to produce second change detection images representative of changes between consecutive imaging positions along said path.

2. A method according to claim 1 , wherein said first change detection images are generated by means of a coherent change detection process.

3. A method according to claim 1 or claim 2, wherein said second change detection images are generated by means of an incoherent change detection process.

4. A synthetic aperture surveillance system, comprising a receiver for receiving first and second datasets, each dataset comprising a plurality of images captured at a respective plurality of imaging positions along a path relative to an area of interest; a first change detection module for comparing each image of said second dataset with each respective corresponding image of said first dataset to produce respective first change detection images in respect of said plurality of imaging positions; a second change detection module for comparing the change detection image for each imaging position with the change detection image for the immediately preceding imaging position on said path to produce second change detection images representative of changes between consecutive imaging positions along said path; and an output for consecutively displaying said second change detection images.

5. A system according to claim 4, wherein said first change detection module is a coherent change detection module.

6. A system according to claim 4 or claim 5, wherein said second change detection module is an incoherent change detection module.

7. A program or plurality of programs arranged such that when executed by a computer system or one or more processors it/they cause the computer system or the one or more processors to operate in accordance with the method of any of claims 1 to 3.

8. A machine readable storage medium storing a program or at least one of the plurality of programs according to claim 7.