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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).