Prediction of the Error Induced by Topography in Satellite Microwave Radiometric Observations

A numerical simulator of satellite microwave radiometric observations of mountainous scenes, developed in a previous study, has been used to predict the relief effects on the measurements of a spaceborne radiometer. For this purpose, the trends of the error due to topography, i.e., the difference between the antenna temperature calculated for a topographically variable surface and that computed for a flat terrain versus the parameters representing the relief, have been analyzed. The analysis has been mainly performed for a mountainous area in the Alps by assuming a simplified land-cover scenario consisting of bare terrain with two roughness conditions (smooth and rough soils) and considering L- and C-bands, i.e., those most suitable for soil moisture retrieval. The results have revealed that the error in satellite microwave radiometric observations is particularly correlated to the mean values of the height and slope of the radiometric pixel, as well as to the standard deviations of the aspect angle and local incidence angle. Both a regression analysis and a neural-network approach have been applied to estimate the error as a function of the parameters representing the relief, using the simulator to build training and test sets. The prediction of the topography effects and their correction in radiometric images have turned out to be feasible, at least for the scenarios considered in this study.

I. Introduction

Microwave radiometry of land should carefully account for large-scale relief effects when applied to hilly or mountainous areas [1], which represent important targets within earth remote sensing from satellites, particularly for hydrological purposes. Indeed, variations in topography influence the upwelling brightness temperature measured by a satellite radiometer in several ways. The optical depth of the atmosphere is modified, the radiometer observation angle becomes a function of the surface slope, parts of the scene may be shadowed, radiation can be reflected from one tilted surface to another, and a depolarization effect occurs. Moreover, at microwave frequencies, the large variability of the mountainous scenario should be considered, if compared with the relatively large antenna footprint [2].