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  Conformal range migration algorithm (crma) karmaThe Patent Description & Claims data below is from USPTO Patent Application 20060028371. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] This application is a continuation in part of U.S. Patent and Trademark Office application Ser. No. ______, titled Mensuration for the Conformal Range Migration Algorithm (PD 03W139) and Ser. No. ______ titled Motion Compensation for Convolutional SAR Algorithms (PD 03W137). FIELD OF INVENTION [0002] This invention is in the field of motion compensation of radar returns for use with Synthetic Aperture Radar imaging. DESCRIPTION OF THE RELATED ART [0003] 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. [0004] The plurality of returns creating a SAR image generated by the transmitted pulses along a known path of the platform make up a frame length. During the frame length, 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 frame length. [0005] The plurality of pulses transmitted during an SAR frame length, 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 frame length. [0006] The clarity of a SAR image is in many respects is dependent on the quality of the motion compensation applied to each radar return prior to SAR image computation. The SAR process depends on the coherent, phase accurate summing of radar returns. Slight deviations in the accuracy of the motion compensation of incoming pulses introduces distortions over the whole SAR image, reducing its utility. [0007] The Range Migration Algorithm (RMA), a popular way to focus the radar returns forming a SAR image, is derived assuming straight line motion over a flat earth. Inaccuracies arise because the earth is not flat, being in fact ellipsoidal. Some of these inaccuracies are corrected in an ad hoc fashion, but become less accurate as the range and squint angle become larger. SUMMARY OF THE INVENTION [0008] Motion compensation for coherent combination of pulses is improved by a method for acquiring a synthetic aperture image of a scene on earth's surface. A great circle is centered with respect to the earth's center, The great circle has an axis perpendicular to a first plane. The axis passes through the earth's center. The first plane contains the great circle and includes the earth's center. The great circle has a first center defined by an intersection of the first plane and the axis. The scene has one or more radar scatterers and is located on a surface. The radar system is mounted on a moving platform moving with a component of motion in a direction along the great circle. The radar comprises a radar receiver for digitizing the radar returns having a phase from scatterers on the surface, and a computer for focusing the phase of said radar returns from the scatterers on the surface. The surface is located on a local scene centerline circle, the local scene centerline circle defining a second plane. This second plane is parallel to the first plane. The local scene centerline circle is centered on the axis at a second center, where the second center is displaced with respect to the first center along the axis by a distance. The phase of the radar returns received from the scene is compensated for the motion of the moving platform along the great circle using a cylindrical coordinate system referenced with respect to the second center to yield a clearer SAR image. BRIEF DESCRIPTION OF THE DRAWING [0009] In the Drawing: [0010] FIG. 1 is a SAR spotlight configuration of the prior art; [0011] FIG. 2 shows a path of a moving platform (flying aircraft) defining an ideal, straight and level path along the centerline of a scene to be acquired for a SAR image, a baseline case for the acquisition of said SAR image of a scene in spotlight mode; and [0012] FIG. 3 shows a typical SAR processing sequence for generating a SAR image from radar returns; [0013] FIG. 4 is shows the path of a moving platform (flying aircraft) following a great circle path for acquiring a SAR image, the phase deviation of the radar returns from a surface due to the curvature of the earth being corrected by the present invention for a clearer SAR image. DETAILED DESCRIPTION OF THE INVENTION [0014] The present invention describes a method for motion compensating pulses for clearer SAR images using concepts related to the Range Migration Algorithm (RMA). The structure of the RMA is based on a convolution. Generally, it consists of a two dimensional Fourier transform, then a multiply operation, a change of variables to get to rectilinear coordinates, and finally an inverse Fourier transform. [0015] FIG. 1 shows the typical prior art geometric relationship between a moving platform carrying a radar transmitter/receiver using Synthetic Aperture (SAR) spotlight methods and target area 101 to be imaged by said radar transmitter/receiver. The moving platform is initially at position 103, travels with velocity V in the direction shown to position 105. In SAR spotlight mode, the SAR antenna is actively oriented towards scatterer 101 as the platform moves with respect to scatterer 101 with velocity V. The moving platform moves from position 103 to position 105, while adjusting the side looking angle from .phi. to .phi.+.DELTA..phi. for spotlight mode so that the antenna keeps illuminating target area 101. Antenna illumination with radar energy covers area 107 during the frame length, and includes target area 101. Similarly, the antenna receive pattern covers area 107, and includes target area 101. Radar pulses are transmitted and corresponding returns received at many points during the frame length between position 103 and position 105. SAR radar is well known in the art and, for example, is described by W. Carrara, et al, in Spotlight Synthetic Aperture Radar, Artech house, 1995, incorporated herein be reference in its entirety. [0016] For this invention, motion compensation is the process of digital correction of radar phase error for each radar return in a SAR frame forming a SAR image due to the change in position of scatterers along the curved surface of the earth. Motion compensation is performed in the airborne processor on each radar return. The exact form of motion compensation depends on the method used to compile the SAR image from the radar returns. This invention is compatible with the Range Migration Algorithm (RMA), a convolutional algorithm. RMA is preferred because it is better suited to many advanced image analysis techniques such as coherent change detection. The CRMA method of this invention re-casts the convolution which forms the image from a straight line track of the moving platform to a circular track. This allows the pulses gathered during a frame length to conform to the curvature of the earth. A specialized expansion is used to construct the along track matched filter. The method separates typically coupled variables for a more efficient computation. [0017] One assumption in RMA is that the nominal flight path of the moving platform (flying aircraft) during the SAR frame is in a straight line parallel to the centerline of the scene being acquired, where the scene is flat. This condition is not necessarily true at long ranges, resulting in unfocused SAR images. Aircraft ideal track is a straight line, the scene is flat, in a plane parallel to the flight track, as shown in FIG. 2. Aircraft 202 is following flight path 204 in the x direction, at an altitude h above flat area 200 and acquiring SAR image of scene 206. Scene 206 has a centerline 208, where centerline 208 is parallel to flight path 204. In FIG. 2, [0018] x direction is the direction of flight. [0019] y direction is perpendicular to x, parallel to the plane of the scene being imaged. [0020] z direction is normal to the plane of the scene, defined by {circumflex over (z)}={circumflex over (x)}.times.y [0021] With these definitions, the nominal flight track of the moving platform (aircraft) is x.sub.ac,o(s)=s{circumflex over (x)}+y.sub.ac,oy+z.sub.ac,o{circumflex over (z)} [0022] where y.sub.ac,o and z.sub.ac,o are constants as it is assumed that the moving platform flies generally straight and level with minor deviations during the duration of the SAR frame resulting in a SAR image of scene 206. The deviations from the straight and level are due to buffeting, pilot inputs, atmospheric conditions or the like. I. The RMA Method--Baseline Example. Continue reading... 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Start now! - Receive info on patent apps like Conformal range migration algorithm (crma) karma or other areas of interest. ### Previous Patent Application: Motion compensation for convolutional sar algorithms Next Patent Application: Mensuration for the conformal range migration algorithm Industry Class: Communications: directive radio wave systems and devices (e.g., radar, radio navigation) ### FreshPatents.com Support Thank you for viewing the Conformal range migration algorithm (crma) karma patent info. IP-related news and info Results in 0.20311 seconds Other interesting Feshpatents.com categories: Software: Finance , AI , Databases , Development , Document , Navigation , Error |
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