US 5,552,787 AGranted

Measurement of topography using polarimetric synthetic aperture radar (SAR)

AssigneeThe United States of America as represented by the Secretary of the Navy

InventorsDale L. Schuler, Jong-Sen Lee

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Abstract

The polarimetric technique of measuring azimuth direction terrain slopes utilizes a polarimetric synthetic aperture radar (SAR) to provide a direct measure of terrain azimuthal slopes and a derived estimate of terrain elevation. Utilizing this measure of the azimuthal slopes and estimated terrain elevations, a one-dimensional terrain slope map over a wide area may be produced without any prior knowledge of the terrain. Utilizing the method of steepest descent (or gradients) the polarimetric orientation of the peak (maximum) of the signature is determined for each image pixel. The terrain elevations are derived by integrating the slopes in the azimuthal direction and may be further refined so as to obtain absolute, rather than relative, elevation values by independently knowing at least one elevation "tie-point" along each slope profile being integrated. These orientations are proportional to terrain slope in the azimuthal direction. Processing of all the image pixels allows a complete two-dimensional topography elevation map of the terrain slopes can then be constructed from sets of elevation profiles spaced throughout the range direction.

A polarimetric processor for measuring azimuth direction terrain slopes comprised of:

a radar for transmitting and receiving analog electromagnetic signals of alternately horizontally and vertically polarization;
an analog-to-digital converter where the analog electromagnetic signals are converted into a digital data bit stream;
a formatter for formatting the digital bit stream;
means for recording and storing the digital data bit stream;
a playback unit for removing the digital data from the storage means;
a digital correlator for correlating the digital data bit stream;
a computer for receiving the correlated digital data bit stream and computing
(a) a Stokes-Mueller matrix, M, utilizing the digitized analog electromagnetic signals of alternately horizontally and vertically polarization extracted from the playback unit and correlated;
(b) a polarimetric signature, .sigma. (.PSI. .chi.), utilizing the Stokes-Mueller matrix, M;
(b) a first, a=a'=[-1,0,0], and second initial value, a'=[1,0,0], for a pixel or pixels;
(c) a first gradient utilizing the method of steepest descent for the first and second initial values;
(d) updating the first and second initial values utilizing the gradient;
(e) normalizing the first and second initial values;
(f) repeat steps (c)-(e) utilizing the normalized first and second values until the maximum has been found;
(g) .sigma..sub.1 and .sigma..sub.2 utilizing a new first and second values;
(h) a comparison of .sigma..sub.1 and .sigma..sub.2,
(i) a value for Ellipticity, .chi.=sin.sup.-1 (a.sub.3)/2, and Orientation, .PSI.=tan.sup.-1 (a.sub.2 /a.sub.1)/2;
(j) a combination of individual pixel maximum locations;
(k) a initialization of elevations for each row of azimuthal pixels by a known elevation point;
(l) an integration of slope values in an azimuthal direction;
and
means for plotting the ellipticity and orientation to produce a presentation that is a direct measure of terrain azimuthal slopes and a derived estimate of terrain elevation.

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Abstract

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