The vicinity of the French city of Marmande, located on the Garonne River, has been identified as a possible super-site for the Copernicus Sentinel-3 STM Cal/Val  activities, thanks to the favourable orientation and location of a Sentinel-3A ground track, and the location of a Sentinel-3A crossover point nearby. The site is also overflown by a Sentinel-6 track, which is of high interest for multi-mission cross-comparisons. Several in-situ instruments provide water levels along the river (Vigicrues stations and VorteX.io micro-stations) and the installation of new automatic sensors is planned, in order to refine the local comparisons with the satellite altimetry measurements.

To accurately compare the satellite altimetry measurements with the in-situ measurements, the river slope as well as the presence of waterfalls must be accounted for.

On 8 February 2022, a drone equipped with a LidAR instrument and a camera (to determine the characteristics of the scene) was deployed by VorteX.io to measure the surface water height on a section of about 20 km along the Garonne River. Overflights of the Vigicrues station in Marmande were also performed for comparison purposes. These measurements will be used to determine the river slope and identify local specificities (waterfalls, pools, etc…), which are crucial information to ensure accurate satellite altimetry calibration and validation over rivers.

The campaign occurred in a period of low water levels, which are quite common for this river. Two other campaigns are planned to estimate the river slope and its characteristics in different water level and river flow conditions.

These campaigns are supported by the French Space Agency (CNES) and contribute to the implementation of Sentinel-3 Cal/Val sites within the St3TART project.