Technique for low grazing angle 3D SAR target recognition

A radar on a moving platform for three dimensional target recognition of a target on a flat or sloping terrain is described. The target is illuminated from a plurality of locations to generate images at many aspect angles. The radar is positioned at a low grazing angle with respect to the target for generating a shadow of the target on the flat or sloping terrain for each aspect angle of the plurality of aspect angles. The radar comprises an analog to digital converter for converting reflections from the target induced by radar illumination into target digital data and for converting reflections induced by the illumination from the flat or sloping terrain into terrain digital data. The radar further comprises a computer for extracting radar images of the target and its shadow(s) at the plurality of aspect angles at low grazing angles; computing the slope of the terrain from the terrain digital data; correlating a plurality of the radar images to compute a three dimensional image of the target from the shadow of the target upon the flat or sloping terrain; and classifying the three dimensional image for target recognition using a target recognition algorithm.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention is in the field of radar imaging at low grazing angles for use with target identification.

2. Description of the Related Art

Synthetic Aperture Radar (SAR) radar is used for ground mapping as well as target identification. The general principle behind SAR is to coherently combine the amplitude and phase information of radar returns from a plurality of sequentially transmitted pulses from a relatively small antenna on a moving platform.

The plurality of returns creating a SAR image generated by the transmitted pulses along a known path of the platform make up an array. During the array, amplitude as well as phase information returned from each of the pulses, for each of many range bins, is preserved. The SAR image is formed from the coherent combination of the amplitude and phase of return(s) within each range bin, motion compensated for spatial displacement of the moving platform during the acquisition of the returns for the duration of the array.

The plurality of pulses transmitted during a SAR array, when coherently combined and processed, result in image quality comparable to a longer antenna, corresponding approximately to the “length” traveled by the antenna during the array.

However, sometimes the ground targets of interest are relatively far away, and the radar platform is required to fly at low altitude, forcing the imaging radar to operate at shallow grazing angles. In such conditions, the ground targets cast long shadows. These long shadows may be insufficient to discern a 3D image of the target with a typical imaging process. The complexity of the imaging process may be compounded by the changes in elevation of a sloping terrain in the vicinity of the target of interest. The height of the target as compared to that of the sloping terrain is not directly discernible in the image. Therefore, interaction between elevation changes of a sloping terrain and details in the shadow cast by the target further hinder the imaging and target identification process.

SUMMARY OF THE INVENTION

Above limitations are avoided by a radar on a moving platform for three dimensional target recognition of a target on flat or sloping terrain. The sloping terrain has a slope. The target is illuminated from a plurality of aspect angles by the radar on the moving platform. The radar is positioned at a low grazing angle with respect to the target for generating a shadow of the target on the flat or sloping terrain for each aspect angle of the plurality of aspect angles. The radar comprises an analog to digital converter for converting reflections from the target induced by radar illumination into target digital data and for converting reflections induced by the illumination from said flat or sloping terrain into terrain digital data.

A computer is provided for:

a) extracting radar images of the target and its shadow at the plurality of aspect angles from the target digital data acquired at the low grazing angles;

b) computing the slope of the terrain from the terrain digital data;

c) correlating a plurality of the radar images to compute a three dimensional image of the target from the shadow of the target upon the flat or sloping terrain;

d) classifying the three dimensional image for target recognition using a target recognition algorithm.

The slope of the sloping terrain is computed from terrain digital data acquired using interferometric SAR or from a terrain database containing the slope.

The timeline for acquiring a plurality of aspect angles of the target is reduced by using interleaved SAR or a spoiled transmit beam with multiple simultaneous high gain independent receive apertures.

DETAILED DESCRIPTION OF THE INVENTION

The present invention avoids above limitations of the prior art by using a radar and method of radar operation for 3 dimensional target recognition from a plurality of SAR images acquired at different aspect angles of the target. The different aspect angles viewed from the radar illuminate the target at low grazing angles for lengthy shadows. The target is identified from the lengthy shadows cast upon uneven terrain, such as a sloping surface. The plurality of images generated while different aspect angles of target are being acquired are combined and processed using Automatic Target Recognition (ATR) methods for target recognition.

FIG. 1shows the typical top view of the geometric relationship between a moving platform carrying a radar transmitter/receiver using Synthetic Aperture (SAR) spotlight methods and target101to be imaged by said radar transmitter/receiver. Target101is a radar scatterer having an elevation above a ground surface, where the surface may be sloping at an elevation above sea level. The moving platform is initially at position103where it takes a first image of target101at a first aspect angle while moving with velocity V1. The moving platform then travels to position105where it acquires another image of the target101at a second aspect angle, while moving with velocity V2. Antenna illumination with radar energy covers area107during an array length, and includes target101at both positions103and105. Similarly, the antenna receive pattern covers area107, and includes target101. While only two illumination points are shown, yielding two aspect angles of the target, a plurality of such aspect angles are envisioned. SAR radar is well known in the art and, for example, is described by W. Carrara, et al, inSpotlight Synthetic Aperture Radar, Artech house, 1995, incorporated herein by reference in its entirety.

In addition to the top view shown inFIG. 1, other definitions apply for the operation of this invention, as shown inFIG. 2. The X,Y,Z Cartesian (orthogonal) coordinates provide reference. The X,Y plane defines a plane, best understood as a sea level. Moving platform202and moving platform204are at different heights along the Z coordinate above the X,Y plane. Target206has an extent in the Z direction perpendicularly above plane X,Y and above sloping terrain212. Target206is illuminated by a radar from platform204, generating a shadow210on sloping terrain212for a first aspect angle. Similarly, target206casts a shadow208on sloping terrain212at a second aspect angle when illuminated from a radar on moving platform202.

Target206is illuminated from moving platform204at a grazing angle ε with respect to the X,Y plane. Sloping terrain212slopes at an angle α with respect to the X,Y plane and contains target206.

Shadow210arises from illumination radiated from radar on platform204impinging onto target206, projected along sloping terrain212. Target206is perpendicularly positioned with respect to the X,Y plane. Its shadow, Laz, not shown, if projected onto the X,Y plane, has a length given by

where hazis the height of the platform relative to the target.

However, shadow210cast by target206upon sloping terrain212will be shorter than that predicted by equation 1 because sloping terrain212will intercept the shadow cast by target206induced by illumination from radar on platform204sooner, as compared to the X,Y plane.

The radar cross range resolution provides measurements of the shape of the shadow as projected on the sloping terrain212. The shape of the shadow is used to infer the 3 dimensional extent of the target.

A plurality of SAR images at different aspect angles are acquired to achieve a robust image of the features of the target. The plurality of SAR images, typically three or more, are combined to generate a three dimensional image of the target.

Thus, this invention is directed towards 3D target recognition. In its simplest form, the invention ascertains the height of target206above sloping terrain212from radar images acquired from a plurality of aspect angles, such as those acquired using moving platforms202and204and respective shadows210and208cast upon sloping terrain212.

FIG. 3in conjunction withFIG. 2describes a radar300on a moving platform for three dimensional target recognition of a target206on a terrain212having a slope α. α can be zero in the case of flat terrain. The target206is illuminated from a plurality of locations, creating different aspect angles by the radar on the moving platform, for example located as shown by platform202and platform204. The radar on the moving platform204is positioned at a low grazing angle ε with respect to the target206for generating a shadow208of target206on sloping terrain212for each aspect angle of the plurality of aspect angles. The radar comprises an analog to digital converter305for two purposes:

a) converting reflections induced by said illumination from said target into target and shadow (digital) data using SAR imaging303and

b) optionally, converting reflections induced by said illumination from said sloping terrain into terrain digital data in Terrain Data301. The terrain data is generated using various techniques to determine the slope α, such as Interferometric SAR, monopulse ground measurements, spoiled beam techniques and 3D SAR methods. Details of some of these techniques are described in U.S. Pat. No. 6,741,202, issued May 25, 2004.

In another embodiment, a terrain database309is used to provide terrain slope in digital format directly to computer307. The use of a database is applicable where the degree of accuracy required is met by the database. Target geographic (geo) location is input into terrain database309, and slope (terrain elevation), on a range bin by range bin basis is extracted for use by computer307.

a) extracting radar images of target206and the shadow(s) of said target at the plurality of aspect angles from the target digital data acquired at said low grazing angles;

b) computing said slope α of the sloping terrain212from the terrain digital data;

c) correlating a plurality of radar images extracted in (a) above to compute a three dimensional image of target206from the shadow of target206upon sloping terrain212and the slope α of the sloping terrain212;

d) classifying the three dimensional image computed in (c) for target recognition using a target recognition algorithm.

A typical target recognition algorithm applicable herein compares a collection of images with the unknown, three dimensional image. MSTAR is a popular such algorithm.

In one embodiment, the slope α of sloping terrain212is computed from terrain digital data acquired using interferometric SAR.

In another embodiment, a plurality of aspect angles are acquired using interleaved SAR, thereby saving time in acquiring the multiple images necessitated by this invention. In agile beam radar systems, intra PRI beam switching for simultaneous imaging in multiple directions can be employed.

In another embodiment, the timeline for SAR image acquisition for a plurality of aspect angles is reduced by using a spoiled transmit beam with multiple simultaneous high gain independent receive apertures.

All references cited in this document are incorporated herein in their entirety by reference. Specifically,Synthetic Aperture Radarby John J Kovaly, ISBN 0-89006-056-8, Artech House, andRadar Technologyby Eli Brookner, ISBN 0 89006 0215, Artech House, are incorporated herein in their entirety by reference to provide a background for this invention and definition of variables used herein.

Although presented in exemplary fashion employing specific embodiments, the disclosed structures are not intended to be so limited. For example, although various methods of terrain elevation measurements are outlined, others are also applicable. For example, placing a transponder, or other active device such as a GPS receiver capable of accurate measurements on sloping terrain212in the vicinity of target206, will assist in obtaining the needed elevation measurement.

Those skilled in the art will also appreciate that numerous changes and modifications could be made to the embodiment described herein without departing in any way from the invention.