ALTEMETERS EXPERTISE

Poseidon-1 & Poseidon-2 altimeters expertise

Since the first Poseidon-1 altimeter faisability studies in 1987, CLS has been involved in the altimetric data processing and in particular in the instrumental expertise tasks. Since this date, CLS expertise has been reinforced by the in flight Poseidon-1 data analysis and by the analysis of Poseidon-2 data and algorithms before and after launch. Within the scope of the Topex/Poseidon and Jason-1 missions, CNES has given to CLS instrumental expertise responsibilities. In particular, CLS is in charge of the continuous monitoring of the instrumental behavior of the altimeter and is in charge of the analysis of any problem corrupting the nominal operating of the mission.

CLS tasks:

A continuous instrumental monitoring of the Poseidon-1 and Poseidon-2 altimeters is performed at CLS. The aim of this monitoring is to verify the correct operating mode of the altimeter, to estimate the drift of its components and their consequences on quality of the user data products, to compute the instrumental corrections that have to be applied on the raw altimetric values and to analyze the exceptional events on the altimeter (Single Event Upsets). CLS analyses cycle by cycle the instrumental information like the temperatures of the various equipments of the altimeter, the running mode, the internal calibration data (low pass filter of the receiving chain and point target response) and the altimeter housekeeping data. A cyclic monitoring of some geophysical parameters like sigma-0 blooms is also performed. All these analysis are reported in a cyclic bulletin called " Instrumental Quality Assessment Report ". Exceptional events are also reported in a special issue of this cyclic bulletin.

A Poseidon-1 altimeter simulator of performances has been developed from 1985 by CNES teams. Since then, this simulator has been improved to be more representative of the embarked instrument (Poseidon-1 and then Poseidon-2). This simulator is a key tool used during the whole life of the altimeter, from feasibility studies, definitions, specifications and development phases to "expertise" tasks on the behavior of the instrument during operating phase. In particular, this simulator is used to compute the Poseidon-2 look up tables that are taken into account in the scientific ground algorithms to compute user data products. In this context, CLS has the responsibility of development, maintenance and implementation of the evolutions of this simulation tool.

Algorithm studies are also preformed at CLS. The aim of these studies is to improve the user product quality and to get a better understanding of the Jason-1 data issued from the ground segment processing chains. Examples of studies :

• Improvement of the retracking algorithm (Maximum Likelihood Estimator with 4 and 5 parameters)
• Poseidon-1 and Poseidon-2 waveforms retracking using Kalman filtering technique.
• Sigma0 blooms characterization
• Jason-1 coastal product definition and realization

As the same time as this expertise on Poseidon-1 and Poseidon-2 altimeters, CLS is also involved in the instrumental expertise of other altimeters like ERS and ENVISAT.

Users:

Studies are performed with the collaboration of CNES team in charge of the instrument and with CNES team in charge of data production and diffusion to the users.

More:

• AVISO: http://www-aviso.cls.fr
• CNES: http://www.cnes.fr