NRSC SAR Geocoding/Calibration

The NRSC's geocoding service allows SAR scenes from potentially any SAR
sensor to be spheroid and terrain corrected. This puts the image into the
map co-ordinates of a specified projection. A wide range of projections
can be catered for. The geocoded version of the scene has the appearance
of being taken vertically (as if the sensor is pointing down directly to
the earth's surface instead of viewing it from an angle) which makes
interpretation and further processing of the data easier and more natural.

In addition to the SAR images, a digital elevation model (DEM/DTM) of the
area is required, preferably at 100m or better resolution. If a DEM is
unavailable, a spheroid-corrected but not terrain-corrected image can
still be produced.

The process can either be provided for supplied data or as part of a
complete service of acquiring and processing imagery for requested
areas/dates.

The T-SAR (Topographic SAR) software used to process the SAR imagery is
capable of generating a number of additional products useful for
interpreting the data, including:

Radiometrically Corrected Image (Calibrated Image): the radiometrically-
corrected or calibrated image is a version of the SAR scene where the
values of each pixel have been corrected for range (in the original SAR
data pixels further from the sensor are dimmer), antenna pattern effects,
and the effect of the angle of incidence between the radar beam and the
earth's surface at the pixel.

Pixel Validity Mask (Mapped pixels, Shadow, Layover): this product
identifies which SAR pixels are valid/invalid. Pixels may be invalid for
several reasons. First, products based on the terrain will be invalid for
areas that fall outside the DEM. Second, due to the physics of the SAR
sensor, pixels that fall into areas of 'shadow' or 'layover' are invalid.
A combined Pixel Validity Mask is available showing which pixels are valid
after considering all three cases, or three separate masks can be
generated showing the pixels invalidated by each effect, the Mapped Pixel
Array (showing pixels which fall inside the DEM), the Layover Mask
(showing areas of layover) and the Shadow Mask (showing areas of shadow).

Pixel Geoidal Heights: this is effectively the DEM transformed into the
co-ordinate system of the original SAR image, i.e. the height of each SAR
pixel is recorded.

Angles of Slope/Incidence: these two products respectively record the
angle of slope of the earth's surface and the angle of incidence between
the radar beam and the surface for each SAR pixel.

Projected Pixel Area: this product gives the area (in square metres) of
the earth's surface contributing to each pixel in the SAR image.

These products can be generated in either the original SAR co-ordinates or
map co-ordinates for a given projection (i.e. geocoded co-ordinates).

Finally, an Image to Map Look-up Table is available which gives the map
co-ordinates of each pixel in the original SAR image.

The processed data can be supplied as either digital data (on Exabyte, CD-
ROM or CCT), or in various hard-copy forms. Further details and prices for
processing are available from the National Remote Sensing Centre (NRSC).