Background

Wind scatterometers such as SASS observe over a wide range of incidence angles, at several azimuth angles, and with both horizontal and vertical polarizations. The SASS measurement cells had nominally 50 km resolution on an irregular 50 km grid; however, they varied in size and shape across the measurement swath and along the orbit track (see Fig. 1). Further detail regarding the SASS measurement method and the measurement geometry are contained in LHW.

For the purposes of applying our resolution enhancement method, we have adopted a simple model describing the dependence of on the incidence angle , i.e., [4]

where is the incidence angle of the observation and and are constants dependent only on the observed surface characteristics. For the experiments described below this model is applied only over the relatively narrow incidence angle range of . Bracalente and Sweet determined this linear model to be accurate with 0.5 dB over this range. In this model is the value of at an incidence angle of . can thus be considered to be the ``incidence angle-normalized'' . Because of the diversity of measurement incidence angles used by wind scatterometers, the and coefficients of this model are more useful in application than the direct measurements [6][5].

Data from the first spaceborne scatterometer, the Skylab S-193 experiment, showed that the 14.6 GHz values over land were sensitive to vegetation cover, surface water, soil moisture, and physiography [13]. Because the Amazon rainforest appeared to be homogeneous, azimuthally isotropic, and relatively insensitive to polarization with a small diurnal variation, measurements of the Amazon rainforest were used for calibration of SASS [10][4][3][1]. Later, Kennett and Li [6][5] conducted a study of SASS measurements over land at a resolution of by in a search for additional homogeneous areas for future scatterometer instrument calibration. They found remarkable correlation between land cover types and the values of and with dB/ and dB depending on the surface type and vegetation cover. While the early demise of Seasat precluded study of the seasonal change of the backscatter versus land type, they found significant variability in the Arctic region and other areas undergoing high seasonal change during the abbreviated three-month mission (July through September 1978). However, the Amazon and Congo rainforests and various desert regions (mapped at a somewhat higher resolution of ), proved to be remarkably stable and were thus deemed suitable targets for future scatterometer calibration activities.