Scatterometer Climate Record Pathfinder
ERS Enhanced Resolution Image Product Users Notes
This page is designed as a dynamic repository of relevant
information to aid users of ERS-1/2 enhanced resolution image
products.
Enhanced resolution images made from ERS-1/2 data use the
Scatterometer Image Reconstruction (SIR) algorithm. This version of
the algorithm does not incoporate a median filter and, for
ERS-1/2, uses a 2-d Hamming window as the spatial response function
for each beam. 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. This simple linear model is
used in place of the gamma=sigma-0/cos(theta) as it more accurately
represents the sigma-0 versus backscatter response over a wider
range of surface and volume scattering conditions. The SIR
algorithm makes images of A and B on an 8.9 km pixel grid. The
effective resolution is estimated to be 20-30 km resolution,
depending on region and sampling conditions. Raw ERS measurements
have a quoted nominal resolution of 50 km on a 25 km sampling
grid.
In producing ERS-1/2 SIR images, multiple passes of the
spacecraft are combined to produce a higher spatial resolution (at
a cost of reduced temporal resolution). The overlapped processing
enables true resolution enhancement of the images with any
"excess sampling" contributing to an improve SNR. Because
the AMI SAR and scatterometer modes are mutually exclusive, the
resulting coverage gaps, produce reduced quality images when the
SAR mode is operated over the study area. 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.
ERS-1/2 data is obtained from fast-delivery products sent to
JPL. No recalibration has been applied. However, several times
throughout the mission dataset, the calibration parameters were
changed prior to in the tapes delivered to JPL. These can be
observed as "steps" or "jumps" in the A or B
time series. In order to make a more uniform data set, calibration
corrections for these steps are being developed and will be
available soon.
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.
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