NSCAT normalized radar backscattering coefficient biases using homogenous land targets

The NASA scatterometer (NSCAT) is a spaceborne radar sensor designed to mea sure the normalized radar backscattering coefficient sigma(o) of the Earth' s surface. Over the ocean, backscatter measurements are used to infer surfa ce wind vectors. Wind retrieval is based on a statistical relationship betw een short-ocean wave roughness (that causes the backscatter) and the surfac e wind speed and direction. For NSCAT geometry, multiple antennas are used to provide backscatter measurements at several azimuth directions to resolv e wind direction ambiguities. To achieve the desired wind vector accuracy, these antenna beams must be calibrated within a few tenths of a decibel. A simple relative-calibration method is applied to the NASA scatterometer bac kscatter from homogenous, isotropic, large-area targets. These targets exhi bit both azimuth and time invariant radar response. A simple polynomial mod el for incidence angle dependence of sigma(o) is used, and the mean radar r esponse from all antenna beams is taken as the reference. Corrections (sigm a(o) biases) are calculated as differences tin log space) between measureme nts from particular beam and the reference. This simple model is applied to data from the Amazon rain forest and the Siberian plain. These areas are t ested for temporal stability within the calibration period (several weeks). High-resolution masks are applied to extract suitable calibration data set s. Calculated corrections for each antenna beam are added to NSCAT sigma(o) measurements as a function of incidence angle. The magnitudes of correctio ns show the necessity of on-orbit calibration.