RapidScat Backscatter Measurements Calibration
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Scatterometers are microwave radars deployed to estimate global wind speeds and directions over sea surface by measuring Normalized Radar Cross Section (σᵒ) of the sea. Measured is determined by the sea surface roughness that is modulated mostly by wind, thus establishing a relationship between and wind conditions. Wind vectors are retrieved by inverting an empirical Geophysical Model Function that relates measured to a wind vector (speed and direction) and radar measurement configuration (polarization, incidence, and azimuth angle). Careful scatterometer calibration is required for accurate wind retrieval. RapidScat is a National Aeronautics and Space Administration (NASA) Ku-Band scatterometer that was operated onboard the International Space Station (ISS) between September 2014 and August 2016 when the mission effectively ended after an irrecoverable instrument failure. A unique non-Sun-synchronous orbit facilitated global contiguous geographical sampling between the ±56° latitude. For the first time, such an orbit enabled an overlap with other scatterometers flying in Sun-synchronous orbits. To assure measurement accuracy, careful scatterometer calibration and validation is required. Cross-calibration among overlapping instruments is a common calibration method. This Dissertation explores the implementation of a cross calibration method between the RapidScat and QuikScat scatterometers taking advantage of their overlapping orbits. QuikScat is Rapidscat’s parent instrument whose spare parts were used to build the RapidScat. The biases of the two instruments have been calculated from observations collected during a 20-month period between Jan. 2015 and August 2016. Deviations from the common reference model are computed for both datasets, as a function of wind speed, relative wind direction, and incidence angles averaged for both polarizations over 1000 pairs of collocated QuikScat/RapidScat revolutions. For more accurate alternative to the classic single difference approach, the double difference technique was deployed to compare measurements from these two scatterometers. This Dissertation presents the first extension of the double difference methodology to scatterometry. The methodology has been adopted for the cross-instrument calibration between RapidScat and QuikScat scatterometers simultaneously orbiting the Earth on-board two independent satellite platforms. The initial results of the statistical analysis and biases between the two scatterometers are presented. Calculated biases may be used for measurement correction and reprocessing.