CoAuthors

Shailen Desai (Jet Propulsion Laboratory, California Insitute of Technology, USA); Robert Leben (Colorado Center for Astrodynamics Research, University of Colorado, Boulder, USA); Dallas Masters (Colorado Center for Astrodynamics Research, University of Colorado, Boulder, USA); Christian Meinig (NOAA Pacific Marine Environmental Laboratory, Seattle, USA); R. S. Nerem (Colorado Center for Astrodynamics Research, University of Colorado, Boulder, USA); Rashmi Shah (Jet Propulsion Laboratory, California Institute of Technology, USA)

We describe the latest satellite radar altimeter calibration/validation (CALVAL) results from data collected at the Harvest offshore platform. Located about 10 km off the coast of central California near Point Conception, Harvest has hosted a dedicated altimeter CALVAL facility since the launch of TOPEX/POSEIDON (T/P) in August 1992. Harvest is directly in the path of the 10-d repeat ground track for the primary reference (Jason-class) altimeter missions, enabling the development of a continuous calibration record based on direct (overhead) passes of the platform. The experiment has produced a CALVAL time series spanning 24 years, enabling improved understanding and connection of the sea-level records from the T/P, Jason-1, -2 and -3 missions.

Based on data collected from early overflights of the Jason-3 mission, we estimate the sea-surface height (SSH) bias is –16 ± 15 mm (one standard error with N= 9). In contrast, results for Jason-2 continue to yield a slight positive bias: +24 ± 2 mm (N=239). When we account for systematic error sources—such as uncertainty in the platform vertical position—only the Jason-2 bias is slightly significant. Based on the results from nine dual overflights, however, we conclude that Jason-3 measures SSH significantly lower (by 38±7 mm) than Jason-2, consistent with the relative bias from global analysis. This difference is due mainly to the altimeter range measurements.

In terms of characterizing SSH stability, only the Jason-1 and -2 missions have a long enough time series to support the monitoring of drift at the 1 mm/yr level or better. Results from the new Jason-1 GDR-E product yield a drift estimate of –0.5 ± 0.9 mm/yr (N = 206), while comparable results for the Jason-2 GDR-D show a drift of +0.6 ± 0.9 mm/yr (N = 239). Neither estimate is statistically distinguishable from zero.

We compare the results from Harvest with those from a new precise GPS buoy moored at a Jason crossover location (Daisy Bank) off the coast of Oregon. Data from the buoy have been collected continuously since May 2016, and have being used to monitor the SSH, significant wave height (SWH), and zenith troposphere and ionosphere delays from the Jason missions. Early results show excellent agreement with Harvest, and testify to the potential of the technique for expanding the calibration footprint of Harvest in order to support other missions (such as SWOT).