High-resolution Land/ice Imaging Using Seasat Scatterometer Measurements

Abstract

A B S T R A C TIn this paper we introduce a new method for obtaining high resolution images (to 4 km) of the land backscatter from low-resolution Seasat-A scatterometer (SASS) measurements. The method utilizes the measurement cell overlap in multiple spacecraft passes over the region of interest and signal processing techniques to generate high resolution images of the radar backscatter. The overlap in the 0 ' resolution cells is exploited to estimate the underlying high resolution surface radar backscatter characteristics using a robust new multivariate image reconstruction algorithm. The new algorithm has been designed to operate in the high noise environment typical of scatterometer measurements. The ultimate resolution obtainable is a function of the number of measurements and the measurement overlap. We describe the method and provide sample results based on SASS data. We describe modifications to future scatterometers which would permit the method to produce 1-2 km resolution over land and ice. The high-resolution scatterometer imaging technique, coupled with the frequent, multiple-incidence angle coverage afforded by the scatterometer, is expected to significantly enhance the utility of sca.tteroineter data in various land, ice, and vegetation studies. I N T R O D U C T I O NSpaceborne wind scatterometers are an important element in future remote sensing systems because of their proven ability to make all-weather measurements of vector winds over the ocean. Current and planned scatterornetser designs have resolutions in the 25-50 km range which is sufficient for the study of oceanic winds. While scatterometers make measurements of the normalized radar backscatter coeficient (a') over land, these measurements have been used primarily for calibration of the instrument [l].Spaceboriie wind scatterometers observe U' over a wide range of incidence angles. Over an incidence angle ( 8 ) range of [20",57"], the dependence of o ' on the incidence angle is linear, i.e., [1] 1Ologl0 ~" ( 0 )where A and B are constants independent of 0. A is the value of 0 ' at an incidence angle of 40". A can thus be considered to be the "incidence angle-normalized" 0". Because of the wide range of measurement incidence angles used by wind scatterometers, the A and B coefficients of this model are used in lieu of go.The coarse resolution (nominally 50 km) of the SASS measurements is a significant limitation to the application of the SASS data in land and ice studies. To ameliorate this difficulty we have developed a new method for obtaining high-resolution estimates of A and B from SASS-class scatterometer measurements. The technique can a.chieve even higher resolution for future scatterometers such as the NASA Scatteroineter (NSCAT). This paper describes our approach a,ird results. H I G H -R E S O L U T I O N I M A G I N G M E T H O DSASS obtained two measurements of 8 at different azimuth angles using multiple anteiliias. The 0 ' measurement resolution ''cells" were (approximately) six-sided polygons with variable size on an i...