Abstract's details

In-situ measurements for altimetry cal/val: overview of the H2020 CCVS project

Céline Tison (CNES, France)


Sylvie Labroue (CLS, France); Thierry Guinle (CNES, France); François Bignalet-Cazalens (CNES, France); Matthias Raynal (CNES, France); Francesco Nencioli (CLS, France); Stefanie Holzwarth (DLR, Germany); Martin Ligi (University of Tartu, Estonia); Sébastien Clerc (ACRI-ST, France)

Event: 2022 Ocean Surface Topography Science Team Meeting

Session: Quantifying Errors and Uncertainties in Altimetry data

Presentation type: Type Poster

The objective of the H2020 project “Copernicus Cal/Val Solution (CCVS)” (https://ccvs.eu/) is to define a holistic Cal/Val strategy for all ongoing and upcoming Copernicus Sentinel missions. CCVS will identify gaps and propose long-term solutions to address currently existing constraints in the Cal/Val domain and exploit existing synergies between the missions. The project started end of 2020 for two years.
An exhaustive review of all CAL/VAL aspects has been made to identify the major gaps. In this presentation, we will focus on the in-situ data and campaigns for altimetry topics.
First, the main results of the survey on the in-situ measurements available for the altimetry CAL/VAL will be presented. Secondly, the gap analysis will be performed.
In-situ measurements and campaigns are used to better characterize some errors of the instruments and, mostly, to qualify the level 2 products. The ground algorithms are improved by comparing their results with these measurements. They also play an important part for climate monitoring. For this topic, the determination of the systematic and random uncertainties is a key issue. These measurements enable also to prepare for future missions.
The in-situ networks, used for Cal/Val of altimetry sensors, are of three main kinds:
• In-situ facilities deployed specially to qualify the altimetry products, deployed under satellite tracks, mainly with multi-missions’ coverage (for instance, tide gauges or GNSS-based instruments for Sea Surface Height),
• In-situ facilities deployed specially to characterize instrumental error sources (for instance, transponders for altimeters),
• In-situ facilities dedicated to other purposes and used as opportunities (this is the case for many networks).
Some campaigns are deployed to prepare future missions or to better characterize surfaces with few observations (ice sheet, hydrology).

Several key sites have been identified around the world. They are located on cross-overs of altimetry mission tracks and often overflown by more than two different missions. Most of them are designed for ocean variables (Corsica, Crete for Europe) but there are also hydrological ones such as the Parintins (Brazil) site over the Amazon river and the Issykkul site (Kirghizstan) which has been used for over 15 years. These sites can be qualified as “FRM sites for altimetry”. Note that an ESA project, FRM4ALT, covers the CRETE site.
These sites are hosted by different organizations (Universities, Research Labs, etc.) and receive funding from national agencies, space agencies or European entities. They often rely on national partnerships to support in-situ equipment (one country provides the equipment; another country hosts them). The processing is performed by the entities in charge of the sites but collaborative efforts have been made over many years to homogenize the processing systems into a standard system (e.g. considering geodetic corrections etc) even though realized at best effort.

A few sites use active ground sensors or reflection signals pointing at the satellite to calibrate some physical measurements. In the altimetry field, we can identify the following:
• The ILRS (laser reflectometry) network which provides very accurate measurements on satellite orbits, a great validation for the POD solutions.
• Radar transponders designed especially for the altimeters.
The transponders have very specific design adapted to each altimeter. Located at the nadir below the satellite, they re-emit the altimeter waveforms once received. Its own calibration remains difficult but it is operational for several missions (JA3, S3, S6-MF) and ESA aims at having a new location to complement the CRETE site.

At large scale, the Cal/Val of altimetry missions rely mainly on “opportunity networks” where in-situ data which are not collected for space purposes become key data for Cal/Val activities (e.g. buoy networks).
The in-situ networks are often gathered in “networks of networks”. Countries aggregate their in-situ facilities, often under the UN aegis. The data are open access but the processing may depend on each country, as well as the design of the sensors and the calibration process. Some national entities gather the available data and use their own harmonized process (for instance, the CMEMS in-situ TAC for all oceanic observations or the NDBC for buoys). Some confusion can result from the same original measurements being accessible via several routes and with different processing pathways.

The presentation will detail the main networks we are using for altimetry CAL/VAL. Our objective is to underline the gaps but also the assets to propose the best CAL/VAL solutions for the Copernicus system.
This project has received funding from the European Union’s Horizon 2020 research and innovation program under the grant agreement

Poster show times:

Room Start Date End Date
Mezzanine Tue, Nov 01 2022,17:15 Tue, Nov 01 2022,18:15
Mezzanine Thu, Nov 03 2022,14:00 Thu, Nov 03 2022,15:45
Céline Tison