Calibration and Validation of the RapidScat Scatterometer Using Tropical Rainforests

Launched in September2014, RapidScat is currently operating on the International Space Station (ISS). RapidScat estimates ocean vector winds via the measurement of the normalized radar coefficient (σ
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) of the ocean's surface. Measurements are also collected over land. The ISS orbit permits, for the first time, the observation of the diurnal variation in Ku-band σ
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at midto high-incidence angles. To complement calibration efforts over the ocean, in this paper the calibration and validation of the σ
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measurements are performed using natural land targets, namely the Amazon and Congo rainforests. The diurnal σ
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cycle of the targets is estimated with respect to incidence angle, azimuth angle, and season using measurements from previous sensors. Understanding this diurnal backscatter response enables the comparison of RapidScat measurements with measurements from the QuikSCAT, NASA Scatterometer, SeaWinds, and Oceansat-II scatterometers. RapidScat σ
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measurements are found to be consistent but biased low compared to those of QuikSCAT by up to 0.3 dB. The effectiveness of slice balancing is evaluated and found to be dependent on the pitch of the ISS. Extreme pitches of the ISS are also found to introduce azimuth dependencies in egg measurements. By accounting for seasonal and diurnal cycles, we find that the rainforests are well suited for scatterometer sensor cross-calibration, even for disjoint years.