CALIBRATION OF SPACEBORNE SCATTEROMETERS USING TROPICAL RAIN-FORESTS

Wind scatterometers are radar systems designed specifically to measure the normalized radar backscatter coefficient (sigma degrees) of the o cean's surface in order to determine the near-surface wind vector. Pos tlaunch calibration of a wind scatterometer can be performed with an e xtended-area natural target such as the Amazon tropical rain forest. R ain forests exhibit a remarkably high degree of homogeneity in their r adar response over a very large area though some spatial and temporal variability exist. In this paper we present a simple technique for cal ibrating scatterometer data using tropical rain forests. Using a polyn omial model for the rolloff of sigma degrees with incidence angle, the technique determines gain corrections to ensure consistency between d ifferent antennas and processing channels. Corrections for the time va rying instrument gain are made consistent with a seasonally fixed rain forest response; however, without ground stations or aircraft flights , it is difficult to uniquely distinguish between seasonal variations in the rain forest and slow variations of the system gain. Applying th e corrections, the intrinsic variability of the sigma degrees of the r ain forest is estimated to be +/-0.15 dB, which is the limit of the ac curacy of calibration using the rain forest. The technique is illustra ted with Seasat scatterometer (SASS) data and applied to ERS-1 Active Microwave Instrument scatterometer (Escat) data. Gain corrections of u p to several tenths of a decibel are estimated for SASS. Corrections f or Escat data are found to be very small, suggesting that Escat data i s well calibrated.