NASA’s Physical Oceanography Distributed Active Archive Center (PO.DAAC) has released a value-added SWOT Level 2 Near Real Time GPS Nadir Altimeter Operational Geophysical Data Record (NRT GPS OGDR). The dataset produced by the Surface Water and Ocean Topography (SWOT) mission provides sea surface height and significant wave height and wind speed measurements from the Poseidon-3C nadir altimeter—a Jason-class dual-frequency (Ku/C) altimeter. SWOT is a joint mission between NASA and CNES that launched in December 2022. It measures ocean surface topography with unprecedented resolution and accuracy, while also mapping inland water bodies globally.
This value-added, best-efforts product supplements the standard SWOT Nadir Altimeter OGDR dataset, another near real-time product at nadir from the SWOT mission. The NRT GPS OGDR dataset has slightly longer latency (3-5 hours vs 1-3 hours for the standard OGDR), but provides higher-accuracy sea surface height measurements comparable to those in the next-day SWOT Nadir Altimeter Interim Geophysical Data Record (IGDR). The higher accuracy is achieved by utilizing a GPS-based orbit ephemeris instead of the onboard DORIS orbit ephemeris and by accounting for radiometer beam pointing and spacecraft attitude when computing the radiometer wet troposphere correction.
Aside from latency and accuracy differences, the data file structures between the standard OGDR and this dataset are otherwise identical. Both consist of discrete measurements along the nadir track with posting intervals of approximately 6 km and 300 m at 1Hz and 20Hz, respectively. The data are distributed as one file per data downlink in netCDF-4 file format. Additionally, this NRT GPS OGDR collection also has no sub-collections, unlike the standard OGDR collection.
The dataset is described and discoverable via Earthdata Search.