Scatterometers are unique among satellite remote sensors in their ability to determine the wind direction over water. Scatterometers can provide a wealth of wind velocity observations over the earth's bodies of water. These wind observations have a wind variety of applications including weather forecasting, marine safety, commercial fishing, El Nino prediction and monitoring, and long term climate studies. The exceptional accuracy of the recent NASA Scatterometer is leading to development of new applications.

At COAPS, we have found that examining animations of scatterometer winds can inspire new scatterometry applications. We have generated wind animations for 37 overlapping regions spanning the global oceans.

To date, all scatterometers have been active microwave sensors: they send out a signal and measure how much of that signal returns after interacting with the target. Microwaves are Bragg scattered by short water waves; the fraction of energy returned to the satellite (backscatter) is a function of wind speed and wind direction. The wind speed can be determined from the strength of the backscatter signal.

The wind direction is found by determining the angle that is most likely to be consistent the backscatter observed from multiple angles. In roughly 5 minutes, a satellite in a low polar orbit will move far enough to view a point on water surface from angles spanning 90^o. The mathematical function describing the fit of the observed backscatter (as a function of the wind direction) usually has multiple minima (ambiguities). Ideally, the best fit corresponds to the true direction of the wind. Typically, the next best fit is in approximately the opposite direction, and the next two minima are in directions roughly perpendicular to the wind direction. The process of selecting the direction from among the multiple minima is called ambiguity selection. Noise in the observations can change the quality of fit and thereby cause incorrect directions (also known as aliases) to be chosen. NSCAT ambiguity selection has proven to be much better than previous scatterometers, with roughly 90% successful selection of the correct ambiguity. Most of the problems with ambiguity removal occur for low wind speeds, where the signal is weak and easily confounded by noise. For wind speeds greater than 8 ms^-1 successful ambiguity removal is near certain.

NSCAT provided wind observations with a superb combination of unprecedented coverage, spatial and temporal resolution, and ease of processing. However, many applications require these winds in a regular grid, without gaps in coverage (see example of daily coverage over the Indian Ocean). The pattern of wind observations follows the satellite orbits rather than a regularly patterned grid. Therefore, the winds from orbital swathes have to be transferred to a grid, and gaps in the observations have to be filled in a reasonable manner. COAPS has produced several of these gridded products, as well as gridded products from other observations.

The NASA Scatterometer (NSCAT) was launched on the polar orbiting ADEOS satellite and measured wind speeds from Sept. 15, 1996 to June 30, 1997. The observational coverage was 600 km wide swaths on both sides of the satellite. The resolution within the swaths was approximately 25 km. Each swath was sampled by fore, mid, and aft beams; each of which sampled from different directions. One beam had two polarizations, which provided another independent backscatter observation that could be used in ambiguity selection. Wind speeds and directions are calculated where there were observations from each of these beams. Cloud cover has little impact on the backscatter, so there were few instances of dropout over water.

QuikSCAT is a new design of scatterometer developed for the SeaWinds project. It is tentatively scheduled to be launched on November 20, 1998. QuikSCAT is expected to have accuracy characteristics similar to NSCAT, and roughly double the coverage. QuikSCAT's coverage will extend an additional 100 km to each side of the NSCAT swaths, and the gap between the NSCAT swaths will be filled.