SMAP Antenna Pointing Error Estimation Using GNSS-Reflectometry

This manuscript explores the use of Global NavigationSatellite System – Reflectometry (GNSS-R) data collected by theSoil Moisture Active Passive (SMAP) mission to estimate antennaview angle offsets. A novel methodology for estimating antennaview angle offsets using GNSS-R data is proposed by comparingthe received Signal-to-Noise Ratio (SNR) of the reflected GNSSsignal with SMAP’s antenna pattern measured prior to launch. Toproperly compare them, observation angles are defined withrespect to the SMAP position and the specular point position,which are computed from the SMAP telemetry and the GPSephemeris data using a 1-km resolution digital elevation modelmap. A methodology based on a 2nd order polynomial fit and alinear fit is proposed to estimate the offset by azimuthal sector andat different times of the year. Results are obtained for a total of 6years, showing a consistent dependence on the antenna azimuthallook angle and on the time of the year, with an average antennaoffset of 0.2° with a standard deviation of 0.06°, which produces ageolocation error of ~ 3.6 km. Further analysis proposes a nonlinearmodel to estimate the antenna offset directly from anazimuthal look angle and the time of the year. Model results showa Pearson correlation coefficient of R = 0.85 and R = 0.80 fordescending and ascending passes, respectively. This model can beused by the SMAP team to further correct the antenna footprintlocation and pointing angle.