Ant Sibthorpe

Sub-Centimeter Precision Orbit Determination with GPS for Ocean Altimetry

We assess the accuracy of JPLs estimated OSTM/Jason-2 Global Positioning System (GPS)-determined orbits based on residuals to independent satellite laser ranging (SLR) data, compared with orbits produced by different software from different data (SLR/DORIS), Geophysical Data Record version C (GDR-C) orbits, and altimeter crossover tests. All of these tests are consistent with sub-cm radial accuracy: high elevation SLR residual standard deviation lies at 6.8 mm, RMS differences from GDR-C in the radial component typically fall below a cm, and altimeter crossovers from JPL orbits have a variance 89 mm2 smaller than altimeter crossovers from GDR-C orbits. Although RMS differences between radial components of different orbit solutions typically lie below a cm, we observe systematic dependences on both time and geography. The improved precision and accuracy of JPLs OSTM/Jason-2 orbit solutions rely on a new algorithm for applying constraints to integer carrier phase ambiguities. This algorithm is sufficiently robust to improve solutions despite half-cycle carrier phase identification issues in OSTM/Jason-2s BlackJack receiver. Although Jason-1 receiver performance differs, our algorithm should extend to Jason-1 processing (during the time span of nominal GPS receiver operations).

Calibration and Validation of the Jason-2/OSTM Advanced Microwave Radiometer Using Terrestrial GPS Stations

We use estimates of zenith tropospheric delay from terrestrial Global Positioning System (GPS) stations to validate the wet path-delay measurements from the Jason-2/Ocean Surface Topography Mission (OSTM) Advanced Microwave Radiometer (AMR). We validate the AMR wet path-delay measurements provided in the version “T” Geophysical Data Records (GDRs) and those generated using a recently developed coastal algorithm that will be applied to the version “C” GDRs. The new coastal algorithm reduces the variance of the GPS-AMR differences by as much as 28% within 25 km of land. This algorithm also facilitates open-ocean measurement accuracies as close to land as 15 km (as opposed to 25 km). We perform AMR/GPS comparisons with three different troposphere-mapping functions for the GPS-based estimates: Niell, Global, and Vienna. The choice of mapping function has minimal impact on our results, although the Vienna mapping function provided the smallest variance in the AMR–GPS differences.