SMAP Antenna Pointing Error Estimation Using GNSS-Reflectometry

This manuscript explores the use of Global Navi-gation Satellite System-Reflectometry (GNSS-R) data collectedby the Soil Moisture Active Passive (SMAP) mission to estimateantenna view angle offsets. A novel methodology for estimatingantenna view angle offsets using GNSS-R data is proposedby comparing the received signal-to-noise ratio (SNR) of thereflected GNSS signal with SMAP's antenna pattern measuredprior to launch. To properly compare them, observation anglesare defined with respect to the SMAP position and the specularpoint position, which are computed from the SMAP telemetryand the Global Positioning System (GPS) ephemeris data using a1-km resolution digital elevation model (DEM) map. A method-ology based on a second-order polynomial fit and a linear fitis proposed to estimate the offset by azimuthal sector and atdifferent times of the year. Results are obtained for a total of sixyears, showing a consistent dependence on the antenna azimuthallook angle and on the time of the year, with an average antennaoffset of 0.2(degrees )with a standard deviation of 0.06(degrees), which producesa geolocation error of similar to 3.6 km. Further analysis proposes anonlinear model 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 for descending and ascending passes, respectively. This model can beused by the SMAP team to further correct the antenna footprintlocation and pointing angle.