Scatterometer Climate Record Pathfinder
QuikSCAT Enhanced Resolution Image Product Users Notes

SeaWinds-on-QuikSCAT (by JPL convention, termed "QuikSCAT") began operations in 1999. The sensor is identical to the SeaWinds sensor aboard ADEOS-II (by JPL convention, termed "SeaWinds"). This document applies to both.

Enhanced resolution images made from QuikSCAT 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. Unlike other instruments, QuikSCAT observations 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).

QuikSCAT is a dual-pencil-beam conically scanning scatterometer with the outer beam V pol and the inner beam H pol. The operation frequency is 13.4 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.

QuikSCAT data was obtained from QuikSCAT L1B data 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.

Due to its polar orbit, the local-time-of-day of the QuikSCAT observations varies with latitude and direction of the orbit. 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).