ASCAT - Advanced Scatterometer

The ASCAT wind retrieval processing system used by NOAA was developed by the KNMI scatterometer team and adapted to the NOAA environment. The sigma-0 measurements are related to the ocean wind vectors and the radar measurement parameters via a geophysical model function (GMF). The GMF utilized in the ASCAT wind retrieval is CMOD5, and the wind retrieval is performed using the maximum likelihood estimation (MLE) principle. The harmonic characteristic of the GMF results in solutions is not unique which generally results in several ambiguities being found. The NOAA products currently include four ambiguity solutions. The NOAA Ice and Ultra High Resolution (UHR) processing algorithms were developed by Professor David Long, Brigham Young University.

The fan-beam ASCAT scatterometer provides measurements radar backscatter at 6 azimuth angles and a diversity of incidence angles. The wide swath and frequent overflights permit generation of a wide variety of ASCAT Image products. For highest possible spatial resolution, multiple orbit passes are combined. These are the 'all pass' images.

Enhanced resolution images made from ASCAT data are created from SZF files using the multi-variate the Scatterometer Image Reconstruction (SIR) algorithm with filtering (SIRF). Like previous fan-beam scatterometers, ASCAT collects measurements at multiple incidence angles and up to 6 azimuth angles. The effective image resolution varies depending on region and sampling conditions but is estimated to be 12-15 km in most areas. The SIRF algorithm makes "A" (sigma-0 normalized to a 40 deg incidence angle) images at 4.45 km pixel spacing for each beam. Multiple passes of the spacecraft are combined to produce a higher spatial resolution (at a cost of reduced temporal resolution). For ice imaging, the multiple beams are combined to create denser sampling, which reduces the noise level and provides the best spatial resolution. The resulting images show the normalized radar cross-section (sigma-0) in dB (10log10) at 40 degree incidence angle. No recalibration of the ASCAT data has been applied in this processing.

For Ultra High Resolution (UHR) wind retrieval the ASCAT SZF measurements are processed using the AVE algorithm (the first iteration of the SIR algorithm) from single passes, separately for each beam. The geometry information is retained. Then, the standard KNMI wind retrieval algorithm is applied at each pixel of the set of multi-azimuth images to estimate the wind speed and direction and each 4.45 km wind vector cell (pixel). The UHR winds are noisier than conventional 25 km winds, but have higher spatial resolution. Near the coastline, the estimated winds can be contaminated by proximity to land, causing the retrieved wind speeds to appear excessively high. Note that UHR winds are not currently rain flagged. Rain generally causes the wind speed to appear higher than it should but it can also cause the wind speed to appear reduced.