Validation of the database of hydrology targets for DEM onboard altimeters (Jason3, Sentinel-3A and Sentinel-3B)

Léa Lasson (Ocean Next, LEGOS, France)

CoAuthors

Denis Blumstein (Centre National d'Etudes Spatiales, LEGOS, France); Sylvain Biancamaria (LEGOS, France); Jean-François Cretaux (Centre National d'Etudes Spatiales, LEGOS, France); Stéphane Calmant (LEGOS, France); Sophie Le Gac (Centre National d'Etudes Spatiales, France); François Boy (Centre National d'Etudes Spatiales, France); Nicolas Picot (Centre National d'Etudes Spatiales, France)

Event: 2019 Ocean Surface Topography Science Team Meeting

Session: Science IV: Altimetry for Cryosphere and Hydrology

Presentation type: Type Poster

Initially designed to observe the oceans, the satellites radar altimetry are used now to measure river and lake water levels with large successes for the last 20 years. Despite this, it is well known that the altimeters have difficulties to track the surface of the water bodies especially in cases where the surrounding topography is rugged. To overcome this issue, recent radar altimeters can be operated in two tracking modes: the Close Loop mode (CL) and the Open Loop mode (OL). In OL mode the position of tracking window is not defined autonomously by the altimeter but is imposed by internal tables that are built from a database of Hydrology Targets (HT). This database provides position, approximate width and elevation of the water bodies. Many times the Open Loop mode has shown its benefits for inland water bodies, it now remains to enrich the database in order to still increase the amount of valid data.

A first validation on Jason3 was carried out on French rivers (around 100 targets) and has already shown good results. For 2 years now Jason3 had a new global dem with more than 4300 HT. During this time, many methods of validation have been developed or improved in order to automatically validate a large number of HT. Now, after 2 years of data with the update, we can validate the HT with these new tools.
Recently, the number of available HT has increased thanks to new automatic method of HT definition, especially for Sentinel-3A (33000 HT) and Sentinel-3B (32500 HT). We are very interested about Sentinel satellites for its complementarity with Jason, especially because Jason is limited at 66° of latitude while Sentinel can acquire data until latitude of 82°.
Therefore, all of the methods developed for the validation have been tested and validated for the first time on Sentinel 3B. Effectively, it was crucial to have an automatic method to validate this large number of Hydrology Targets. The process of validation is based on several criteria mainly based on two raw data produced by the altimeter: the Automatic Gain Control and the waveforms. These criteria have been defined to classify the overpasses for which the altimeter tracks a waterbody and those where it fails to do that.
We currently have a first encouraging result of validation for Sentinel-3B: 97 % of HT set has been classified as water by our process. This first validation was done this winter, so there was a risk of snow on waterbodies and that can skew the results. Consequently, we created a snow mask to reduce the number of HT that could be on snow, finally we have practically removed half of the targets for this first validation. That's why, it is crucial today to wait for the summer to renew validation on all HT.

In the poster, we will show the results of the validation obtained after the summer period which should be quite similar for Sentinel 3B and we will do the same validation process and show the results for Sentinel 3A and Jason 3 worldwide.
 

Poster show times:

Room Start Date End Date
The Gallery Tue, Oct 22 2019,16:15 Tue, Oct 22 2019,18:00
The Gallery Thu, Oct 24 2019,14:00 Thu, Oct 24 2019,15:45
Léa Lasson
Ocean Next, LEGOS
France
lea.lasson@legos.obs-mip.fr