Abstract: Spaceborne scatterometers are radar systems designed specifically to measure the normalized radar backscatter coefficient ($sigma$- 0$/) of the ocean's surface in order to determine the near- surface wind vector. Precise calibration of the instrument sensor is required to determine $sigma$-0$/ accurately (of the order of a few tenths of a dB). Although careful calibration of the instrument is performed before launch, a post-launch calibration must also be performed. Post-launch calibration may be performed using ground stations and/or extended-area natural targets. The most commonly used extended area target has been the Amazon tropical rainforest which exhibits a remarkably high degree of homogeneity in its radar response over a very large area. However, the rainforest does exhibit some spatial and temporal variability. In this paper we present a simple technique for post-launch calibration of spaceborne scatterometer data using tropical rainforests which accounts for the temporal and spatial variability of the forest response. We first illustrate the technique with Seasat scatterometer (SASS) data then apply the technique to ERS-1 Active Microwave Instrument (AMI) scatterometer data. Gains corrections of up to several tenths of a dB are estimated for SASS. ERS-1 data was found to be well calibrated so that no corrections are required.!5
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