Spaceborne Synthetic Aperture Radar (SAR) imagery maps the surface microwave
radar reflectivity at resolutions from a sub-meter to 100 m depending on the
particular SAR satellite and mode. Since a radar provides its own
illumination, imagery is independent of the time of day. At typical radar
frequencies, SARs can image through clouds, so SARs are considered
For side-scanning SARs, the normalized radar cross section (NRCS) of the
surface is proportional to the surface roughness on the scale of the radar
wave length (from 3 to 30 cm). Ocean surface roughness on this scale is
dependent on the local wind speed and direction. The higher the wind speed the
larger the NRCS of the surface. The highest NRCS occurs when the radar look
direction and wind direction are aligned and smallest when the radar looks
The relationship between wind speed and direction and radar cross section is
called the "geophysical model function." These functions are generally
Although we can predict an expected NRCS value for a given wind speed and
direction, the reverse is not true. A specific NRCS value can be associated
with many pairs of wind speed and direction. If we know the wind direction a
priori, we can infer the wind speed.
For the operational SAR-derived wind speed products, we use the model
directions from NOAA’s Global Forecast System (GFS) model on a 0.5 degree,
latitude-longitude grid to initiate the retrievals. The wind speed retrievals
are provided at 500 m resolution. These retrievals agree with independent wind
speed estimates to better than 2 m/s.
Current users of these data are the US National Ice Center, the Alaska Weather
Service, and the Naval Oceanographic Office.