Presentation
As part of the ERS ground segment, the french PAF, so-called CERSAT, is in charge of :
- The archiving and processing the Radar Altimeter and micro-wave radiometer (ATSR/MW) products.
- The distribution of the Radar Altimeter and micro-wave Radiometer products processed at CERSAT.
- The archiving and distribution of the fast delivery products generated by ESA in the ground stations.
CERSAT houses a wind scatterometer processing and distribution facility not part of the PAF. It is operated under IFREMER responsability.
The launch of the ERS-2 satellite in 1995, taking over ERS1, led to the upgarde of the all ground segment to ensure an operationnal service during the all ERS-2 mission (5 years). For the CERSAT, the system of the facility was completely redesigned.
Data management
The data handling at CERSAT is performed in 5 steps :
- The registration of the raw data media received at CERSAT : each medium is registered if it is readable and its format is correct.
- The acquisition of all the passes present on the media (raw data) or received by networks (real time products) : the format and the data quality of each pass is controlled. Each pass is referenced in the catalogues managed in an Oracle database.
- The data processing (raw data or real time products) resulting in geophysical products for users.
- The product quality control.
- The product distribution to users.
Acquisition
The acquisition (or reception) facility consists of a set of elementary controls, after what a cataloguing in the Oracle database is performed. A catalogue item contains a short data description and a data reference.
All data received at CERSAT are acquired :
- The raw data recorded as passes on exabyte or optical disk.
- The real time products processed in the stations and received at CERSAT by networks for the RA, AMI-Wind and AMI-Wave products, or on exabyte for the IWA intermediate products of the AMI-Wave.
- The auxiliary data received by networks that are used by the processing, including :
- Restitued orbits and on-board/on-time relationships transmitted by the MMCC.
- Preliminary and precise orbits generated by the German PAF.
- Meteorological data (pression, troposphere, wind and temperature) provided by METEO-FRANCE
Archiving
The archiving of all received data and generated products must guarantee the durability of all information up to 12 years after the satellite launch. The raw data and IWA imagettes are not archived but kept on exabyte.
The following data are archived :
- The auxiliary data (orbits, meteo, on-board/on-time relationships),
- The real time products,
- The products generated by CERSAT.
The data are archived as disk files referenced in the database and can be easily retreived thanks to a naming convention.
Processing
The processing generates geophysical products dedicated to users, either from the raw data or the real-time products, for which ones an off-line processing can provide better and new information. The processing unit corresponds to an acquisition pass.
The following products are generated on a systematic baseline :
- Radar Altimeter (RA) and Microwave (ATSR/MW) : OPR and VLC products issued from raw data.
- Scatterometer : WNF products issued from WSC.FDC real-time products (IFREMER responsability).
- SAR imagettes : UWA products issued from IWA real-time products.
The OPR, VLC and WNF product generation consists of an algorithmic processing but also of an automatic quality control that performs elementary checks on pass measurements. This control enables an early detection of data anomalies as well as software bugs.
Distribution
The distribution facility includes a quality control task, so that the distributed products are fully validated. It is complementary of the quality control performed on each pass in the processing facility, because it concerns a set of user defined products elaborated from archived passes.
The quality control is performed in two steps corresponding to a two level control.
- The first level is done at CERSAT to verify :
- The data completness and coherence : this ensures that all data are fully and correctly processed. Any anomaly must be justify.
- The data quality and homogeneity by performing an automatic control similar to the one done on each processed pass, but applied on all distributed products.
- The format validity.
- The second level is entrusted to a scientific team in charge of analysing the products according to more elaborated criteria for a scientific validation. A report named "Quality Assessment" is produced and distributed together with the media to users.
The terms of product distribution by CERSAT are :
- The Radar Altimeter OPR products are distributed on CD-ROM and also on exabyte cassette to users which have no means to read CD-ROM.
- The Microwave radiometer VLC products are distributed only on exabyte cassette.
- The Scatterometer WNF products are distributed only on CD-ROM.
- The FDP products processed in the stations are only distributed on exabyte.