On the SEASAT-A satellite, a microwave scatterometer was used to determine the vector wind over the world's oceans. The technique is based on the sensitivity of microwave radar backscatter to the centimeter length ocean waves created by the action of the surface wind. This paper describes the algorithm used to convert the scatterometer' s measurements of ocean normalized radar cross section, •, to the neutral stability vector wind at 19.5 m height and the comparison of these winds with high quality surface observations. The wind vector algorithm used an empirical • model function to describe the dependence of the ocean • on the 19.5-m neutral stability wind vector. Two model functions, developed from a limited base of aircraft and satellite o • measurements, were evaluated by using an independent set of in situ surface wind observations from the Joint Air Sea Interaction Experiment (JASIN). Although these model functions were found to have some weaknesses, the results of these comparisons produced better results than the SEASAT specifications of wind speed accuracy of +-2 m/s and wind direction accuracy of +-20 ø over the 0-16 m/s range of winds observed during JASIN. An improved model function was later developed by 'tuning' to these JASIN data so that the remaining biases between the observed surface winds and the scatterometer-derived winds were minimized. Results are presented for this model function compared against other surface wind observations from the Gulf of Alaska SEASAT Experiment and the SEASAT Storms (Hurricane) Experiment. INTRODUCTION On June 28, 1978, •he National Aeronautics and Space Administration (NASA) launched SEASAT, the first satellite dedicated to establishing the utility of microwave sensors for remote sensing of the earth's oceans [Born et al., 1981]. This concept had its beginning in the mid-1960's when a conference called 'On the Feasibility of Conducting Oceanographic Explorations from Aircraft, Manned Orbital and Lunar Laboratories' was held at Woods Hole Oceanographic Institute, Woods Hole, Mass., in August 1964 [Ewing, 1965]. At this conference, the rudiments of many of the remote sensing systems for measuring oceanographic parameters were described that eventually were orbited on Skylab, Geos-3, and SEASAT. A few years later, a second conference sponsored by the National Academy of Sciences at Woods Hole made a broader study of potential areas of activity for NASA, including the study of the oceans. The concepts of high precision radar altimetry and of using radar backscatter to measure the winds both received considerable attention [National Research Council, 1970]. A third conference at Williamstown, Mass. [Kaula, 1970] also investigated the general subject; and ocean and atmospheric scientists postulated that satellite technology could provide the mecha-nism for monitoring the world oceans on a scale appropriate to the requirements of their research communities. Thus came SEASAT with its compliment of four microwave sensors; namely, a radar altimeter, a multifrequency radiom...
The dependence of ocean normalized radar cross section (NRCS or tr ø) on the 19.5 m neutral stability wind vector for the SEASAT-A Satellite Scatterometer (SASS) is described. For the SASS wind vector algorithm this dependence is specified in the form NRCS = f(0, X, U), where 0 is the radar incidence angle, X is the angle between wind direction and radar azimuth, and U is the neutral stability wind speed in meters per second at a height of 19.5 m. The development of models to express this relationship and to provide the basis for inversion of NRCS to SASS winds is traced from its initial form based on aircraft scatterometer measurements through the SEASAT field-validation experiments (GOASEX, STORMS, and JASIN) to its final form. Comparison of the model NRCS versus surface wind speed with SASS data from these experiments are made. INTRODUCTION The SASS (SEASAT-A Satellite Scatterometer) model function is an empirical relationship between the ocean normalized radar cross section (NRCS or o 'ø) of the ocean and the wind vector at a height of 19.5 m above the surface, assuming neutral stability. For the wind-vector algorithm, the relationship is specified in the form of a table that gives two coefficients G and H in the equation NRCS (dB)= 10[G(0, X) = H(O, X) logt0 U](1) or its equivalent in ratio form, with G' = 10 c(ø'x) NRCS = G'(O, X) Un(ø'x) (2)where 0 is the radar incidence angle, X is the angle between wind direction and radar azimuth, and U is the neutral stability wind speed in meters per second at a height of 19.5 m. The NRCS is a function of the radar parameters: incidence angle, azimuth angle, and polarization; therefore, the G and H coefficients are tabulated separately for V and H polarizations every 2 ø in incidence and every 10 ø in azimuth. These tables thus relate backscatter to wind velocity, given the aspect and incidence angles. The backscatter varies harmonically with relative azimuth, with maxima at upwind and downwind and minima at crosswind; therefore, the values of the coefficients at these directions are the most important. The inversion algorithm that uses these tables to convert SASS NRCS measurements to winds is described in Jones et al. [this issue]. BACKGROUNDThe model function is not known theoretically, nor has it been completely defined in experimental studies; however, there are considerable measurements at the Ku-band frequency which demonstrate that ocean radar backscatter is highly sensitive to surface winds. A good review is contained in Moore and Fung [1979]. Prior to the SEASAT-A launch, the SASS model function was developed by using airborne radar measurements obtained after demonstration of the ,sensitivity of NRCS to wind direction (relative to radar look direction) [Jones et al., 1977]. The most important of these measurements were made by using the Advanced Applications Flight Experiment-Radiometer-Scatterometer (AAFE RADSCAT) in the North Sea (JONSWAP 1975 mission) and the Atlantic Ocean (East Coast mission of 1976). During these missions, banked circles were flown so...
The Seasat microwave scatterometer was designed to measure, globally and in nearly all weather, wind speed to an accuracy of +/- 2 meters per second and wind direction to +/- 20 degrees in two swaths 500 kilometers wide on either side of the spacecraft. For two operating modes in rain-free conditions, a limited number of comparisons to high-quality surface truth indicates that these specifications may have been met.
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