Three dimensional interferometric synthetic aperture radar terrain mapping employing altitude measurement and second order correction
First Claim
1. A method of terrain mapping employing a platform including first and second antennas, the first and second antennas each having a predetermined field of view and together forming an interferometer baseline, said method comprising the steps of:
- moving the platform substantially perpendicular to the interferometer baseline along a set of substantially parallel platform paths;
for each such platform path;
repeatedly detecting the position of the moving platform,repeatedly detecting the distance above the terrain of the moving platform via a radiant ranging sensor,repeatedly transmitting a radar signal via at least one of the first and second antennas,receiving reflections of each of said transmitted radar signals via the predetermined field of view of at least one of the first and second antennas,forming first synthetic aperture complex image data for a plurality of resolution cells in slant range and Doppler frequency from radar reflections received by the first antenna employing plural transmitted radiant signals;
forming second synthetic aperture complex image data for a plurality of resolution cells in slant range and Doppler frequency from radar reflections received by the second antenna employing plural transmitted radiant signals;
determining the phase difference between said first and second synthetic aperture complex image data for each resolution cell;
computing the elevation and ground range for each resolution cell within a selected one of said first and second synthetic aperture complex image data for each of said set of platform paths employing the detected position of the moving platform and the phase difference between said first and second synthetic aperture complex image data for that platform path;
computing a first order corrected elevation and ground range for each resolution cell for each of said set of platform paths corrected for interferometer baseline roll angle employing said detected distance above the terrain of the moving platform of a corresponding adjacent platform path where the moving platform passed above the terrain imaged in said selected one of said first and second synthetic aperture complex image data; and
computing a second order corrected elevation and ground range for each resolution cell corrected for the first return point of said radiant ranging sensor, employing said first order corrected elevation and ground range for resolution cells near each of said corresponding adjacent platform paths.
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Abstract
Synthetic aperture radar data is used in conjunction with altimeter data to produce a terrain map corrected for platform roll angle. The technique uses two synthetic radar antennas and a ranging altimeter placed on an aircraft. The aircraft is moved in a set of substantially parallel flight paths where each flight is directly over the strip of terrain viewed by the synthetic aperture radar of an adjacent flight. During each flight the at least one antenna repeatedly transmits radar signals whose return echoes are received by both the first and second antennas. Conventional synthetic aperture radar processing yields a terrain map uncorrected for roll angle. Altimeter data from an adjacent flight determines the first order corrected aircraft roll angle when the corresponding synthetic radar data was taken. This roll angle is used to correct the height and ground range of nearby points in the uncorrected terrain map. A second order corrected roll angle is determined using the first order corrected terrain map to identify the first return point of the altimeter measurements. The measured altitude is then realigned to the first return point to produce a second order corrected roll angle and a corresponding second order corrected terrain map. The result is a terrain map corrected for aircraft roll angle.
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Citations
33 Claims
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1. A method of terrain mapping employing a platform including first and second antennas, the first and second antennas each having a predetermined field of view and together forming an interferometer baseline, said method comprising the steps of:
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moving the platform substantially perpendicular to the interferometer baseline along a set of substantially parallel platform paths; for each such platform path; repeatedly detecting the position of the moving platform, repeatedly detecting the distance above the terrain of the moving platform via a radiant ranging sensor, repeatedly transmitting a radar signal via at least one of the first and second antennas, receiving reflections of each of said transmitted radar signals via the predetermined field of view of at least one of the first and second antennas, forming first synthetic aperture complex image data for a plurality of resolution cells in slant range and Doppler frequency from radar reflections received by the first antenna employing plural transmitted radiant signals; forming second synthetic aperture complex image data for a plurality of resolution cells in slant range and Doppler frequency from radar reflections received by the second antenna employing plural transmitted radiant signals; determining the phase difference between said first and second synthetic aperture complex image data for each resolution cell; computing the elevation and ground range for each resolution cell within a selected one of said first and second synthetic aperture complex image data for each of said set of platform paths employing the detected position of the moving platform and the phase difference between said first and second synthetic aperture complex image data for that platform path; computing a first order corrected elevation and ground range for each resolution cell for each of said set of platform paths corrected for interferometer baseline roll angle employing said detected distance above the terrain of the moving platform of a corresponding adjacent platform path where the moving platform passed above the terrain imaged in said selected one of said first and second synthetic aperture complex image data; and computing a second order corrected elevation and ground range for each resolution cell corrected for the first return point of said radiant ranging sensor, employing said first order corrected elevation and ground range for resolution cells near each of said corresponding adjacent platform paths. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17)
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18. A method of terrain mapping employing a platform including first and second antennas, the first and second antennas each having a predetermined field of view and together forming an interferometer baseline, said method comprising the steps of:
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moving the platform substantially perpendicular to the interferometer baseline along a set of substantially parallel platform paths; for each such platform path; repeatedly detecting the position of the moving platform, repeatedly detecting the distance above the terrain of the moving platform via a radiant ranging sensor having a predetermined beam width, repeatedly transmitting a radar signal via at least one of the first and second antennas, receiving reflections of each of said transmitted radar signals via the predetermined field of view of at least one of the first and second antennas, forming first synthetic aperture complex image data for a plurality of resolution cells in slant range and Doppler frequency from radar reflections received by the first antenna employing plural transmitted radar signals; forming second synthetic aperture complex image data for a plurality of resolution cells in slant range and Doppler frequency from radar reflections received by the second antenna employing plural transmitted radar signals; determining the phase difference between said first and second synthetic aperture complex image data for each resolution cell; computing the elevation and ground range for each resolution cell within a selected one of said first and second synthetic aperture complex image data for each of said set of platform paths employing the detected position of the moving platform and the phase difference between said first and second synthetic aperture complex image data for that platform path; computing a first order corrected elevation and ground range for discrete regions of terrain for each of said set of platform paths corrected for interferometer baseline roll angle including computing a first order corrected interferometer baseline roll angle at discrete points disposed at intervals which are short in relation to the quotient of the rate of change of the interferometer baseline roll angle with distance along the platform path divided by the speed of the moving platform employing said detected distance above the terrain of the moving platform of a corresponding adjacent platform path where the moving platform passed above the terrain imaged in said selected one of said first and second synthetic aperture complex image data, computing a first order corrected elevation for each resolution cell in a region surrounding each of said discrete points of first order corrected interferometer baseline roll angle computation employing said first order corrected interferometer baseline roll angle, computing a first order corrected ground range for each resolution cell in a region surrounding said discrete points of first order corrected interferometer baseline roll angle computation employing said first order corrected interferometer baseline roll angle; and computing a second order corrected elevation and ground range for each resolution cell corrected for the first return point of said radiant ranging sensor including determining the illuminated terrain within said predetermined beam width of said radiant beam for each of said discrete points of first order corrected interferometer baseline roll angle computation taking into account said first order corrected interferometer baseline roll angle, said region surrounding each of said discrete points of first order corrected interferometer baseline roll angle computation selected to include said illuminated terrain, determining the first return point of said illuminated terrain closest to said moving platform for each of said discrete points of first order corrected interferometer baseline roll angle computation, determining the elevation of said first return point of said illuminated terrain closest to the moving platform for each of said discrete points of first order corrected interferometer baseline roll angle computation, computing a second order corrected interferometer baseline roll angle for discrete points corresponding to said discrete points of first order corrected interferometer baseline roll angle computation employing said elevation of a corresponding first return point, computing a second order corrected elevation for each resolution cell employing a corresponding second order corrected interferometer baseline roll angle, and computing a second order corrected ground range for each resolution cell employing a corresponding second order corrected interferometer baseline roll angle.
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19. A terrain mapping apparatus used with a moving platform having an axis of motion, said apparatus comprising:
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a position detector disposed on the moving platform for repeatedly detecting the position of the moving platform; a radiant ranging altimeter disposed on the moving platform for repeatedly detecting the distance above the terrain of the moving platform; a first radar antenna disposed on the moving platform having a predetermined field of view off the axis of motion of the moving platform; a second radar antenna disposed on the moving platform a predetermined distance from said first radar antenna having said predetermined field of view, said first and second radar antennas forming an interferometer baseline substantially perpendicular to the axis of motion; a transmitter disposed on the moving platform and connected to at least one of said first and second antennas for repeatedly transmitting a radar signal via said at least one of said first and second antennas; a first receiver disposed on the moving platform and connected to said first antenna for forming first synthetic aperture complex image data for plural resolution cells in slant range and Doppler frequency from reflections received by said first antenna employing plural transmitted radar signals; a second receiver disposed on the moving platform and connected to said second antenna for forming second synthetic aperture complex image data for plural resolution cells in slant range and Doppler frequency from reflections received by said second antenna employing plural transmitted radar signals; a phase detection means disposed on the moving platform and connected to said first and second receivers for determining the phase difference between said first and second synthetic aperture complex image data for each resolution cell; a terrain map means connected to said position detector, said first receiver and said phase detection means for computing the elevation and ground range for each resolution cell within said first synthetic aperture complex image data for each of said set of platform paths employing the detected position of the moving platform and the phase difference between said first and second synthetic aperture complex image data for each resolution cell; and a terrain map first order correction means connected to said radiant ranging altimeter and said terrain map means for computing first order corrected elevation and ground range for each resolution cell for each of said set of platform paths employing said detected distance above the terrain of the moving platform for a corresponding adjacent platform path; and a terrain map second order correction means connected to said terrain map first order correction means for computing a second order corrected elevation and ground range for each resolution cell corrected for the first return point for said radiant ranging sensor, employing said first order corrected elevation and ground range for points of terrain near each of said corresponding adjacent platform paths. - View Dependent Claims (20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33)
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Specification