Surface wind speed, significant wave height, sea surface height, etc... in OPR product are computed from ERS-1 and ERS-2 radar altimeter measurements, whereas vapour content and atmospheric water content in VLC product are computed from their microwave sounder (ATSR/MW). Both OPR and VLC product are distributed after data quality assessment. Since both products are jointly processed, both are featured in this section.
Principle
The raw data are acquired and controlled within 2 days after their reception at CERSAT.
The Radar Altimeter and Microwave radiometer raw data are processed in 3 steps.
- The first step of the processing consits in transforming the raw data (recorded on-board the satellite) in physical quantities, time tagged, located and corrected for the instrumental effects. It generates the OIP and MBT intermediate products. This processing is done within 2 weeks after the raw data reception at CERSAT.
- The second step processes the intermediate products and uses the preliminary orbit to generate a first version of the OPR distributed to the German PAF for the precise orbit generation.
- The third step is performed as soon as the precise orbit is received (6 month for ERS-1 and 2 month for ERS-2 after the raw data acquisition in the stations). The intermediate products are again processed to obtain the second version of the OPR intended for users. The use of the precise orbit improves the altimeter range accuracy.
The second and third steps uses the same geophysical processing but with two distinct accuracies. This processing mainly consists in computing the environmental corrections to enhance the intermediate product with all geophysical correction and orbit altitude.
Quality control
Each processed pass is automatically controlled to verify mainly the validity bounds of all physical quantities. In case of an invalid processed pass, a set of graphics and text files are generated to identify the anomaly(ies). Only the passes declared valid or corrected for detected anomalies, are archived and can be distributed (OPR and VLC generated with the precise orbit).
Computing control
This first level is performed at CERSAT and is aimed to fully control the format and range of products going into the OPR CD-ROM/exabyte or VLC exabyte.
The general controls are with respect to chronology, software versions and product measurements quality checks performed on revelant flags of MCD (measurement confidence data) and parameters. Graphics and statistics reporting the controlled flags and parameters are created.
The following graphics show two examples of the mapped results.
An example on OPR flag control from 18/06/95 to 24/07/95
The above maps identify the invalid measurements and three of the causes of invalidity :
- acquisition mode
- not acquisition mode, but over land
- not acquisition mode, not over land, but not "ocean" measurements
An example on OPR parameter control from 18/06/95 to 24/07/95
The above maps show the measurements that are out range for four OPR parameters :
- Standard deviation of range
- Sea surface height
- Standard deviation on significant wave height
- Standard deviation on sigma naught
Scientific control
This second level of control is related to the scientific quality of the products. It is performed by a team from CLS Argos (in Toulouse) only if no anomalies were detected by the first level of quality control. The "Quality Assessment" report is then produced within three days. This control gives the following information :
- quality assessments in terms of statistics, events (orbit manoeuvres), absence of data.
- repeat-track analysis
- crossover analysis and orbit error calculation