(ATBD) SMAP Level 1 Radar Data Products - NASA

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Figure 12: Processed image (azimuth) of a point target near (18 ◦ ) with and without
range walk correction.
scan angle (near 18 ◦ ) as seen on Figure 12 and Figure 13. The samples used for
azimuth compression could then be extracted column-wise in the complex matrix
containing the range-compressed signal.
7.8 Terrain-Corrected Geolocation
The direct correlation appoach will automatically geolocate the output image pixels
because the reference functions are formed for a specific output grid cell. The
Rectangular and SPECAN algorithms form the image in the range Doppler domain
which then needs to be transformed to image coordinates. As mentioned in the
prior section, this step could be done during azimuth compression to improve the
precision on the Doppler centroid. Geolocation consists of finding the location on
Earth of a pixel specified by a particular slant range and Doppler shift. This is done
separately for each range line by using orbit data and topography data. The procedure
consists of finding the simultaneous solution for the 3 cartesian coordinates
(x,y,z) of three equations: slant range equation, Doppler equation, and Earth model
equation (including height h above the reference spheroid). Solving this nonlinear
system (with some iteration needed for topographic corrections) for every output
pixel may be prohibitively time consuming, so it may be necessary for the L1C processor
to precompute and tabulate values for interpolation. More study is needed
here to see what the tradeoffs between speed and location accuracy will require
while still meeting the basic location accuracy of 1 km.