Scatterometer Climate Record Pathfinder
OSCAT Enhanced Resolution Image Product Users Notes
This page is designed as a dynamic
repository of relevant information to aid users of OSCAT enhanced
resolution image products.
The Indian Space Resarch Organization (ISRO) Oceansat-2 scatterometer (by convention, termed "OSCAT") began
operations in 2009. The sensor is very similar to the SeaWinds sensor flown on QuikSCAT and aboard ADEOS-II, but operates at a different incidence angle and at a lower orbit. While the images for OSCAT and QuikSCAT/SeaWinds are produced on the same grids, due to the differences in incidence angle, care should be taken when comparing the two data sets.
The same image formation algorithms used for QuikSCAT and SeaWinds are used on OSCAT. Enhanced resolution images made from OSCAT data use two
different forms of the single-variate Scatterometer Image
Reconstruction (SIR) algorithm: for egg sigma-0 measurements the SIR
algorithm is used while for slice sigma-0 measurements, the SIR with
filtering (SIRF) algorithm is used. For egg measurements the full
antenna/processing spatial response is used while a simplified
spatial response function in which the spatial response is assumed
to be 1 over the footprint and 0 elsewhere is used for slice
measurements. OSCAT observations of a particular incidence angle are at
(essentially) a single incidence angle for each beam so no incidence
angle dependence (B) estimates are made. The SIR algorithm with eggs
makes images of A (in this case, sigma-0 at the measurement
incidence angle) on an 4.5 km pixel grid. The effective resolution
varies depending on region and sampling conditions but is estimated
to be 8-10 km in most areas. The SIRF algorithm with slices makes A
images at 2.225 km pixel spacing with an estimated effective
resolution of 6-8 km. Slice measurements are much noisier than egg
measurements. Multiple passes of the spacecraft are combined to
produce a higher spatial resolution (at a cost of reduced temporal
resolution).
OSCAT is a dual-pencil-beam conically scanning scatterometer
with the outer beam V pol and the inner beam H pol. The operation
frequency is 13.1 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.
The current version of OSCAT data uses the sigma-0 data as supplied by ISRO. No corrections or recalibrations have 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.
Due to its polar orbit, the local-time-of-day of the 0SCAT
observations varies with latitude and direction of the orbit. They also differ from QuikSCAT/SeaWinds. At mid-
to low-latitudes, most areas are imaged at two different local
times-of-day -- one for the ascending (north-bound) pass and one for
the descending (south-bound) pass. Thus, by separating the data into
ascending and descending passes, images corresponding to two different
local times-of-day are created. In the polar regions, particular
locations are observed at multiple local times-of-day and so a
different method is required. Local time-of-day is considered in more
detail in MERS research report on
improving temporal resolution for QuikSCAT/SeaWinds at the poles
(pdf) and
improving temporal resolution for Tandem SeaWinds at the poles
(pdf).
|