Abstract's details

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

Sophie Le Gac (CNES, France)


Florian Wery (CNES, France); Alexandre Homerin (NOVELTIS, France); Jean-Baptiste Barneix (NOVELTIS, France); Malik Boussaroque (HydroMatters, LEGOS, France); François Boy (CNES, France); Nicolas Picot (CNES, France); Pierre Féménias (ESA/ESRIN, Italy)

Event: 2023 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 Copernicus Sentinel-3 and Sentinel-6 missions, as well as the nadir altimeter onboard the SWOT mission. Altimeters have benefited from iterative improvements brought to onboard tables contents, repeatedly since 2017.

In 2023, no less than five nadir altimetry missions hold up-to-date OLTC tables with a very satisfactory data acquisition success rate over inland waters (i.e. more than 90% for inland water targets at nadir). Each mission has its own specificities on how it handles the OLTC command and tracking mode switch, and its own database of targets (depending on the orbit).
The number of hydrological targets used to define the tracking command currently reaches almost 75,000 for each Sentinel-3 and about 30,000 for Sentinel-6A and Jason-3 on its interleaved orbit. The nadir altimeter of the SWOT Mission, also holds OLTC tables with about 60,000 targets.
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.
This year, a special experiment is also carried out over land ice margins in several areas such as Antarctica or Greenland, to improve the tracking of the surface by the altimeter and we expect to improve by at least a factor of 30% the number of relevant data compared to the Closed-Loop mode.

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 be used for the validation of the Surface Water and Ocean Topography (SWOT) mission. In this context, we will present an overview of OLTC achievements and perspectives for future altimetry missions.

Sophie Le Gac