Abstract's details

Nadir altimetry over land: achievements using the Open-Loop Tracking Command (OLTC) and benefits for inland water users

Sophie Le Gac (CNES, France)


Simon Boitard (NOVELTIS, France); Denis Blumstein (CNES/LEGOS, France); Malik Boussaroque (HydroMatters/LEGOS, France); François Boy (CNES, France); Nicolas Picot (CNES, France); Pierre Féménias (ESA/ESRIN, Italy)

Event: 2022 Ocean Surface Topography Science Team Meeting

Session: Science IV: Altimetry for Cryosphere and Hydrology

Presentation type: Type Forum only

Contribution: PDF file


In times of ever decreasing amount of in-situ data for hydrology, satellite altimetry has become key to 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 in the altimeters’ capability to acquire quality measurements over inland waters. In particular, the Open-Loop Tracking Command (OLTC) now represents an essential feature of the tracking function. This tracking mode’s efficiency has been proven on past missions and it is now stated as operational mode for current Sentinel-3 and Sentinel-6 missions. It has benefited from iterative improvements brought to onboard tables contents repeatedly since 2017.

In 2022, new updates will be performed on the onboard OLTC tables of the Sentinel-3A and Sentinel-3B missions, as well as Sentinel-6A and Jason-3 following their successful Tandem Phase.
The number of hydrological targets used to define the tracking command currently reaches an unprecedented number of targets of almost 100,000 for each Sentinel-3 and about 30,000 for Sentinel-6A. We expect to define a similar number of targets in the interleaved orbit of Jason-3, previously flown by Jason-2, although mostly in Closed-Loop Mode.
These major improvements over the last few years have been made possible by the analysis and merging of the most up-to-date digital elevation models (SRTM, MERIT and ALOS/PalSAR) and water bodies databases (HydroLakes, GRaND v1.3, SWBD, GSW, SWORD). In addition, special effort is put into 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 outside of hydrological areas, in order to keep track of the continental surface and enabling potential other land applications, while optimizing the OLTC onboard memory.

The OLTC function of nadir altimeters constitutes 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 essential at institutional and scientific levels, to make the most of current altimeters coverage over land and to prepare for the upcoming calibration and validation of the Surface Water and Ocean Topography (SWOT) mission. In this context, we will show an overview of OLTC achievements and perspectives for future altimetry missions.

Sophie Le Gac