SMAP Antenna Pointing Error Estimation Using GNSS-Reflectometry

This manuscript explores the use of Global Navigation Satellite System–Reflectometry (GNSS-R) data collected by the Soil Moisture Active Passive (SMAP) mission to estimate antenna view angle offsets. A novel methodology for estimating antenna view angle offsets using GNSS-R data is proposed by comparing the received signal-to-noise ratio (SNR) of the reflected GNSS signal with SMAP’s antenna pattern measured prior to launch. To properly compare them, observation angles are defined with respect to the SMAP position and the specular point position, which are computed from the SMAP telemetry and the Global Positioning System (GPS) ephemeris data using a 1-km resolution digital elevation model (DEM) map. A methodology based on a second-order polynomial fit and a linear fit is proposed to estimate the offset by azimuthal sector and at different times of the year. Results are obtained for a total of six years, showing a consistent dependence on the antenna azimuthal look angle and on the time of the year, with an average antenna offset of 0.2° with a standard deviation of 0.06°, which produces a geolocation error of ~3.6 km. Further analysis proposes a nonlinear model to estimate the antenna offset directly from an azimuthal look angle and the time of the year. Model results show a Pearson correlation coefficient of
$R$
= 0.85 and
$R$
= 0.80 for descending and ascending passes, respectively. This model can be used by the SMAP team to further correct the antenna footprint location and pointing angle.