Abstract

RapidScat is a pencil-beam wind scatterometer which operated from September 2014 to August 2016. Mounted aboard the International Space Station (ISS), RapidScat experiences significant altitude and attitude variations over its dataset. These variations need to be properly accounted for to ensure accurate calibration and to produce high resolution scatterometer images. Both the antenna pose and the one-way antenna pattern need to be validated. The spatial response function (SRF) is the two-way antenna pattern for a scatterometer combined with the processing and filtering done in the radar system electronics, and is dominated by the two-way pattern. To verify the pointing of the RapidScat antenna, the RapidScat SRF is estimated using on-orbit data. A rank reduced least squares estimate is used, which was developed previously for the Oceansat-2 (OSCAT) scatterometer [1]. This algorithm uses a small, isolated island as a delta function to sample the SRF. The island used is Rarotonga Island of the Cook Islands. The previously developed algorithm is updated to estimate the SRF in terms of beam azimuth and elevation angle rather than in kilometers on the ground. The angle-based coordinate system promotes greater understanding of how the SRF responds to biases and errors in antenna geometry. The estimation process is simulated to verify its accuracy by calculating the SRF for several thousand measurements in the region of Rarotonga. The calculated SRFs are multiplied by a corresponding synthetically created surface and integrated to yield simulated backscatter measurements, with added white noise. The SRF estimation algorithm is then performed. The results of the simulation show that the SRF estimation process yields a close estimate of the original SRF. The antenna pointing is validated by introducing a fixed offset in azimuth angle into the simulation and observing that the SRF is correspondingly shifted in the azimuth-elevation grid. The SRF computed from real data shows that there is an azimuth rotation angle bias of about 0.263 degrees for the inner beam and about 0.244 degrees for the outer beam. Since the SRF is dominated by the two-way antenna pattern, it can be modeled as the product of two identical one-way antenna patterns which are slightly offset from each other due to antenna rotation during the transmit/receive cycle. A method is developed based on this model to derive the one-way antenna pattern from the estimated SRF. Using a Taylor series expansion the one-way antenna pattern is computed from the SRF. The derived pattern recovers the SRF with small error, but there is significant error in the inferred one-way pattern when compared to the pre-launch estimated RapidScat one-way antenna pattern.

Degree

College and Department

Ira A. Fulton College of Engineering and Technology; Electrical and Computer Engineering

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