RETRIEVING SOIL-MOISTURE OVER BARE SOIL FROM ERS-1 SYNTHETIC-APERTURERADAR DATA - SENSITIVITY ANALYSIS BASED ON A THEORETICAL SURFACE SCATTERING MODEL AND FIELD DATA

In order to assess the retrieval of soil moisture from ERS 1 (European Remote Sensing Satellite) synthetic aperture radar (SAR) data, an inv ersion procedure based on the integral equation model (IEM) [Fung et a l., 1992] is developed. First, the IEM is used to analyze the sensitiv ity of radar echoes (in terms of the backscattering coefficient sigma( 0)) to the surface parameters (roughness and dielectric constant) unde r ERS 1 SAR configuration. Results obtained for random rough bare soil fields show that the effect of surface roughness is very strong, part icularly in the case of smooth surfaces, and that the sensitivity of s igma(0) to dielectric constant is independent of the radar configurati on and the roughness conditions. This means that the range of variatio n of backscattering with respect to the dielectric constant variation of dry to wet soil remains the same (about 5 dB) for any roughness con dition and radar configuration. The possibility of applying the invers ion procedure to retrieve soil moisture is investigated using a set of data collected in a test site situated near Naples, Italy, during the Sele Synthetic Aperture Radar experiment (SESAR) campaign (November 1 993). Simultaneous with ERS 1 overpasses, dielectric constant and roug hness measurements were taken over two flat bare fields. From this ana lysis it is found that the inversion of backscattering from ERS 1 SAR into soil moisture is not reliable without accurate information on rou ghness if the surface is smooth. In this case it is observed that the sensitivity to the roughness parameters is much higher than the sensit ivity to dielectric constant, so that even a small error in the measur ement of this parameter can affect the retrieved value of soil moistur e significantly. The inversion procedure provides more reliable soil m oisture estimates when surfaces rougher than those analyzed in the fie ld experiment are considered.