CoAuthors

Shailen Desai (Jet Propulsion Laboratory, California Institute of Technology, USA); Jean-Damien Desjonqueres (Jet Propulsion Laboratory, California Institute of Technology, USA); Adam Gourmos (University of Colorado, Boulder, USA); Robert Leben (University of Colorado, Boulder, USA); Christian Meinig (NOAA Pacific Marine Environmental Laboratory, USA); Scott Stalin (NOAA Pacific Marine Environmental Laboratory, USA); Matthieu Talpe (Jet Propulsion Laboratory, California Institute of Technology, USA)

As the Sentinel-6 Michael Freilich (S6) mission is prepared for launch at Vandenberg Air Force Base, we describe the latest satellite radar altimeter calibration/validation (CALVAL) results from the Harvest offshore platform. Located 10 km off the coast of central California near Point Conception, Harvest has served as the NASA prime verification site for the TOPEX/POSEIDON (T/P) and Jason series of altimeter reference missions for nearly three decades. The T/P repeat ground track was designed to take the satellite directly over Harvest every ten days, enabling the development of a continuous calibration record based on direct, overhead passes of the platform. The crucial role of T/P in developing a climate-quality record of sea level and ocean circulation has been inherited by the Jason series of reference missions, which have traced out the same 10-d repeat ground track passing by the platform. With its planned November 2020 launch, the S6 mission will assume the mantle of collecting this valuable scientific data set.

After another update to the Harvest seafloor motion model (from GPS)—which shows the platform rebounding after years of subsidence—we estimate the Jason-3 sea-surface height (SSH) bias is –12 ± 3 mm (one standard error with N= 132). The Jason-1 and -2 SSH biases remain quite consistent (+5 and + 6 mm respectively) but with sign opposite to Jason-3. Accounting for systematic errors, none of these SSH bias estimates are statistically different from zero. The platform systems have been serviced and are ready for the upcoming S6 verification phase.

We also describe emerging results from regional campaigns and new observing systems in the Harvest vicinity. These initiatives have two important purposes: 1) to address risk that the platform, which is no longer actively drilling, may be permanently shut down and dismantled; and 2) to prepare for the increasing demands of future altimeter missions for denser in-situ monitoring. The results from a 2018–19 tandem GPS buoy campaign at Harvest suggest RMS accuracies of <2 cm and <1 cm were achieved for absolute and relative SSH respectively. Following on this successful campaign, we deployed (October 2020) a GPS buoy to start permanent occupation of the site (with yearly servicing trips). The adjacent (ascending) S6 repeat ground track passes bisect nearby Santa Catalina Island. A facility to evaluate new, low-cost tide gauge technologies (e.g., lidar, radar) has been established there at the University of Southern California (USC) Wrigley Marine Science Center. On a nearby bluff, we plan to install a new dual-frequency transponder. Together, these sensors will provide important new insights on the accuracy and stability of the S6 data.