New upgrades of Open-Loop Tracking Command (OLTC) tables of nadir altimeters in 2020 and benefits for inland waters users

Sophie Le Gac (CNES, France)

CoAuthors

Denis Blumstein (LEGOS/Univ. Toulouse/CNES/CNRS/IRD, France); Léa Lasson (OceanNext, France); Simon Boitard (NOVELTIS, France); François Boy (CNES, France); Nicolas Picot (CNES, France); Pierre Féménias (ESA/ESRIN, Italy)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Science IV: Altimetry for Cryosphere and Hydrology

Presentation type: Type Forum

In times of ever decreasing amount of in-situ data for hydrology, altimetry can provide global and continuous datasets of water surface height. Indeed, studying lakes, reservoirs and rivers water level at global scale is of prime importance for the hydrology community to assess the Earth’s global resources of fresh water.
Much progress has been made over the past decade in the altimeters’ capability to acquire quality measurements over inland waters. In particular, the Open-Loop Tracking Command (OLTC) represents a major evolution of the tracking function. This altimeter on-board tracking mode’s efficiency has been proven on past missions and it is now stated as operational mode for current Sentinel-3 and Jason-3 missions. It has benefited from improvements brought to onboard tables contents in 2017 (Jason-3), 2018 (Sentinel-3B) and 2019 (Sentinel-3A).

In 2020, new upgrades have been performed on the onboard OLTC tables of the Jason-3, Sentinel-3A and Sentinel-3B missions.
The number of hydrological targets used to define the tracking command has been increased by a factor of 5 for Jason-3 and by a factor of 3 for both Sentinel-3, bringing the number of hydrological targets defined in the onboard tables to about 30,000 for Jason-3 and more than 70,000 for each Sentinel-3 unit. This further major step is made possible by the analysis and merging of the most recent digital elevation models (SRTM, MERIT and ALOS/PalSAR) and water bodies databases (HydroLakes, GRaND v1.3, SWBD, GSW). For example, special attention has been brought to introducing the most recent reservoir databases. This methodology ensures coherency and consistent standards between all nadir altimetry missions and types of hydrological targets.
Finally, additional efforts have been carried out to define a relevant tracking command even in areas where no hydrology targets are found in the current input databases, in order to keep continuous tracking of the continental surface while optimizing the OLTC onboard memory.
A detailed description of these upgrades will be given as well as measurements first validation results obtained since their upload.

These 2020 OLTC upgrades constitute a great asset for building a valuable and continuous record of the water surface height of worldwide lakes, rivers, reservoirs, wetlands and even a few continental glaciers.
This work is also essential while preparing for the upcoming launch of the Sentinel-6 mission (which will hold the same OLTC tables than Jason-3) and its tandem phase with Jason-3, as well as for the preparation of the calibration and validation of the Surface Water and Ocean Topography (SWOT) mission.

 
Sophie Le Gac
CNES
France
sophie.legac@cnes.fr