Abstract's details

Absolute calibration results from Bass Strait, Corsica, and Harvest facilities

Pascal Bonnefond (Observatoire de Paris - SYRTE, France)


Bruce Haines (Jet Propulsion Laboratory, United States); Benoit Legresy (Climate Science Centre, Oceans and Atmosphere, Commonwealth Scientific and Industrial Research Organisation, Australia); Christopher Watson (School of Technology, Environments and Design, University of Tasmania, Australia)

Event: 2022 Ocean Surface Topography Science Team Meeting

Session: Regional and Global CAL/VAL for Assembling a Climate Data Record

Presentation type: Type Oral

Bass Strait:
The Bass Strait altimeter validation facility (~ 40° 39'S, 145° 36' E) provides cycle-by-cycle estimates of absolute altimeter bias for the Jason-series, Sentinel-3A and Sentinel-3B missions. The facility uses an integrated geometric approach involving a suite of in situ sensors including moored oceanographic sensors, episodically deployed GNSS equipped buoys, a coastal tide gauge and continuously operating GNSS reference stations. This year we present the results for the 4 missions with a focus on Jason-3 GDR-F and the Sentinel-6 1-Hz data.With the advance of fully SAR capable altimetry (Sentinel-6) and in preparation for swath-based interferometric altimetry (SWOT), several enhancements to the Bass Strait facility have been progressed. We detail our ongoing use of 5-beam ADCP instruments for the determination of shallow water SSH, current and wave field information using a pressure inverted echo sounder concept (CWPIES). We review our progress in understanding small but significant systematic errors in SSH derived from GNSS equipped buoys. Together, advances in our in situ instrumentation seek to address an improved determination of along-track SSH (and SSH gradient) for the benefit of validating the next generation of altimeter missions. We present results from these investigations together with latest absolute bias results from the Bass Strait comparison points.

We describe the latest satellite radar altimeter calibration/validation (CALVAL) results from the Harvest offshore platform and vicinity. 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 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 verification 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 the beginning of the routine operations phase in November 2021, the Sentinel-6 Michael Freilich (S-6 MF) mission will assume the mantle of extending this valuable scientific observation record.With the completion of the year-long S-6 MF commissioning phase, we estimate the sea-surface height (SSH) bias is +13 ± 3 mm (one standard error with N = 29) for the low-resolution mode (LRM) altimeter data. In terms of this bias, the current non-time critical (NTC) and short-time critical (STC) products are indistinguishable, as are the A and B sides of the Poseidon-4 altimeter. The corresponding Jason-3 (J-3) SSH bias is very consistent (+11 ± 2 mm with N = 174). Data from legacy Jason mission also yield slightly positive SSH biases: +6 ± 2 mm (N = 206) and +5 ± 2 mm (N = 249) for J-1 and J-2 respectively. Accounting for systematic errors, none of these SSH bias estimates are statistically different from zero. However, the relative (inter-mission) SSH biases carry greater statistical significance. If the comparison between J-3 and S-6 MF is restricted to common cycles, the result at Harvest indicates the new mission is measuring SSH higher than its predecessor by +5 ± 3 mm (N = 29), in keeping with preliminary results from global analysis. We also provide preliminary results for reprocessed data from the legacy T/P mission, and describe developments from GPS buoy campaigns at Harvest. Results from a 2018-19 tandem GPS buoy campaign suggest RMS accuracies of <2 cm and <1 cm were achieved for absolute and relative SSH respectively. Following this successful campaign, we deployed in October 2020 a GPS buoy to start permanent occupation of the site (with yearly servicing trips). We report early results from this year-long deployment.

Initially developed for monitoring the performance of TOPEX/Poseidon and follow-on Jason legacy satellite altimeters, the Corsica geodetic facilities that are located both at Senetosa Cape and near Ajaccio have been developed to calibrate successive satellite altimeters in an absolute sense. Since 1998, the successful calibration process used to calibrate most of the oceanographic satellite altimeter missions has been regularly updated in terms of in situ instruments, geodetic measurements and methodologies. In this study, we present an assessment of the long-term stability of the in situ instruments in terms of sea level monitoring that include a careful monitoring of the geodetic datum. Based on this 20+ years series of sea level measurements, we present a review of the derived absolute Sea Surface Height (SSH) biases for the following altimetric missions based on the most recent reprocessing of their data: TOPEX/Poseidon and Jason-1/2/3, Envisat and ERS-2, CryoSat-2, SARAL/AltiKa and Sentinel-3A&B. For the longest time series the standard error of the absolute SSH biases is now at a few millimeters level which is fundamental to maintain the high level of confidence that scientists have in the global mean sea level rise.
Launch in November 2020, Sentinel-6 Mickael Freilich flew in tandem with Jason-3 during its first year of mission. We will present in detail the analysis of this tandem phase. Preliminary results shows that the absolute SSH bias for both missions are very close at the 1 cm level for the LRM mode and both very close to 0. Improvement thanks to SAR will be also presented: measurements are valid (and accurate) up to the coast (few hundred meters); on the whole set of cycles, the standard deviation of 20Hz data is improved by a factor close to two: 33 mm compared to 56 mm with LRM (69 mm for Jason-3).
In preparation of SWOT, an extension of the "geoid" over the Ajaccio and Senetosa area has been realised during 2 campaigns in 2021 and 2022. Prelimininary results will be also presented.

Oral presentation show times:

Room Start Date End Date
Sala Grande Wed, Nov 02 2022,09:00 Wed, Nov 02 2022,09:15
Pascal Bonnefond
Observatoire de Paris - SYRTE