Scatterometer Climate Record Pathfinder
SASS Enhanced Resolution Image Product Users Notes
This page is designed as a dynamic repository of relevant information to aid
users of SASS enhanced resolution image products.
Enhanced resolution images made from SASS data use the Scatterometer Image
Reconstruction with Filtering (SIRF) algorithm. This version of the algorithm
incoporates a median filter and a simplified spatial response function in which
the spatial response is assumed to be 1 over the footprint and 0 elsewhere.
In the processing, a linear model relating sigma-0 and incidence angle is assumed,
i.e. sigma-0(db) = A + B (theta - 40) where A is the "incidence angle normalized
sigma-0" at 40 deg incidence in dB, B is the effective incidence slope
of sigma-0 versus incidence angle in dB/deg, and theta is the incidence angle
of the observation. The SIR algorithm makes images of A and B on a 4.5 km pixel
grid. The effective resolution varies depending on region and sampling conditions.
Multiple passes of the spacecraft are combined to produce a higher spatial resolution
(at a cost of reduced temporal resolution) and fill in coverage gaps between
the individual measurement footprints. SASS measurement footprints were not
contiguous, had irregular six-sided shapes, and varied in size depending on
beam and location on the earth.
Capable of dual-pol operation over a single 500 km wide swath, it normally
operated in V-pol, dual-sided swath mode over the ocean with the mode frequently
switched over land, resulting in very little H-pol coverage during most of the
abbrievated mission. SASS operated at 14.6 GHz. In combining the multiple passes,
sigma-0 is assumed to be independent of azimuth angle. While true for most areas,
some azimuth dependence in sigma-0 has been observed in Antarctic firn, presumably
due to sastrugi or snow dunes.
SASS data was obtained from the SASS GDR datasets archived at the PO.DAAC.
No recalibration has been applied.
Images are produced in the BYU .SIR file format
using the standard naming scheme. The files are
gzipped to minimize storage and transfer requirements. The standard images are
designed for land and ice observation and so are landmasked. However, .SIR format
land mask files (containing 0 for ocean and 1 for land) are available for each
standard region. .SIR format images containing "images" of the latitude and
longitude of each pixel for each region are also available.