ADEOS attitude determination from NSCAT measurements

The NASA Scatterometer (NSCAT) is a spaceborne scatterometer which flew aboard the National Space Development Agency of Japan´s ADvanced Earth Observing Satellite (ADEOS). The NSCAT instrument has proven to be remarkably sensitive and accurate, achieving a calibration of within a few tenths of a dB. The key source of calibration error for NSCAT σ⁰ measurements is the spacecraft attitude knowledge. Unfortunately, the attitude of the ADEOS spacecraft as determined by its onboard sensors is not as accurate as desired. Further, there appear to be biases in the spacecraft attitude between the ascending and descending portions of the orbit. The accuracy and stability of the NSCAT instrument makes it possible to use the σ⁰ measurements to infer the mean spacecraft attitude. This approach models the observed σ⁰ as a polynomial function of incidence angle and determines gain corrections to ensure consistency between different antennas (beam balance) while estimating the orbit-varying spacecraft attitude such that the beam balance correction is fixed over the orbit and with time. The result is improved accuracy and consistency of the σ⁰ measurements