FOAM: From Ocean to inland waters Altimetry Monitoring

Pascal Bonnefond (Observatoire de Paris - SYRTE, France)


Olivier Laurain (OCA-Geoazur, France); Damien Allain (LEGOS, France); Muriel Bergé-Nguyen (LEGOS/CNES, France); Stéphane Calmant (LEGOS/IRD, France); Jean-François Crétaux (LEGOS/CNES, France); Frédéric Frappart (LEGOS, France); Florent Lyard (LEGOS/CNRS, France); Valérie Ballu (LIENSs, France); Xavier Bertin (LIENSs, France); Etienne Poirier (LIENSs, France); Laurent Testut (LIENSs/LEGOS, France); José Darrozes (GET, France); Pierre Exertier (GET, France); Felix Perosanz (GET/CNES, France); Guillaume Ramillien (GET, France); Alvaro Santamaría-Gómez (GET, France); Cédric Brachet (DT-INSU, France); Michel Calzas (DT-INSU, France); Christine Drezen (DT-INSU, France); Lionel Fichen (DT-INSU, France); Antoine Guillot (DT-INSU, France); Mathilde Cancet (NOVELTIS, France); Flavien Mercier (CNES, France); Philippe Schaeffer (CLS, France); Daniel Medeiros Moreira (CPRM, Brazil); Joecila Santos da Silva (EST/UEA, Brazil); Rodrigo Abarca Del Rio (DGEO, University of Concepcion, Chile)

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

Session: Salient results from the 2017-2020 OSTST PIs

Presentation type: Type Forum

Current estimates of regional and global change in mean sea level are only possible through careful and continuous Calibration and Validation (Cal/Val) of the altimetry missions. Cross calibration of past, present and future altimetry missions will remain essential for the realization of a continuous and homogeneous series of sea level. There is no doubt, however, that calibration of an altimeter requires a multiple approach, including using both in situ calibration sites and global studies based on the global tide gauge network. The relative calibration between different missions flying on the same period through crossover analysis or by along-track comparisons during tandem phase of the missions is also an important contribution for the Calibration/Validation activities (Cal/Val). All these techniques are considered complementary and fundamental in oceanography.
The traditional concept of in situ calibration of an altimeter involves direct satellite overflight of a site equipped with dedicated instruments. If it is essential that such a calibration site has means of in situ sea-level observation — using for example a classic tide gauge, a mooring or a floating system equipped with GNSS —, it is fundamental, however, to link the observed sea level to a terrestrial reference frame comparable to that used to analyze altimetry satellite measurements. In an ideal situation, the site of the experiment is located on a repetitive ground track (or better still on a crossover point between ascending and descending tracks), and far enough off the coast to avoid contamination of the altimeter or radiometer by reflections on land.
The potential for a number of geographically correlated errors within the altimetry system underscores the need for calibration experiment to be placed at different locations across the globe. The ability to sample the various systematic errors and characterize them in an absolute sense is one of the important advantages of a set of well-distributed calibration sites on Earth. This ensures a diverse sampling of ocean, inland waters and atmospheric conditions, and allows the use of different methodologies and processing software to help isolate systematic errors in all geodetic techniques involved. We present the salient results of the FOAM project (2017-2020), funded in the framework of the Ocean Surface Topography Science Team (thought as a multi-mission approach), at various sites (Corsica, Kerguelen, île d’Aix, Pertuis Charentais, Arcachon Bay, Gironde Estuary, lakes and rivers…) where the local conditions are different from each other and where permanent instruments and infrastructures already exist and will be reinforced.
The proposed CAL/VAL activities are thus focused not only on the important continuity between past, present and future missions but also on the reliability between offshore, coastal and inland altimetric measurement. In addition, we enter a new era of altimetry (Synthetic Aperture Radar and wide swath altimetry) and our group will take a particular focus on the new measurement systems and their reliability with the past ones (Low Resolution Mode). Our objective is to aggregate the past and present effort and expertise of several groups, in order to notably establish a homogeneous and optimized network of calibration sites geographically distributed for more robust characterization of the existing and future radar altimeter system instrument biases and their drift.
Pascal Bonnefond
Observatoire de Paris - SYRTE