Acquisition processing

The acquisition process is twofold:

• straightforward retrieval and reformatting of altimeter data
• synchronization process.

The measurements ([O/I]GDR or equivalent) from different altimeters are retrieved. DUACS system takes in input L2P altimeter products.

In NRT processing, the acquisition step uses two different data flows: the OGDR/NRT flow (within a few hours), and the IGDR/STC flow (within a few days). For each OGDR/NRT input, the system checks that no equivalent IGDR/STC entry is available in the data base before acquisition; for each IGDR/STC input, the system checks and delete the equivalent OGDR/NRT entry in the data base. These operations aim to avoid duplicates in DUACS system. This processing is summarized in Figure 3.

The acquisition software detects, downloads and processes incoming data as soon as they are available on remote sites (external database, FTP site). Data are split into passes if necessary. This processing step delivers “raw” data, that is to say data that have been divided into cycles and passes, and ordered chronologically.

In NRT processing, the acquisition step uses two different data flows: the OGDR/NRT flow (within a few hours), and the IGDR/STC flow (within a few days). For each OGDR/NRT input, the system checks that no equivalent IGDR/STC entry is available in the data base before acquisition; for each IGDR/STC input, the system checks and delete the equivalent OGDR/NRT entry in the data base. These operations aim to avoid duplicates in DUACS system. This processing is summarized in the following figure.

 

The number of altimeter processed varies with time. An overview of the evolution of the number of altimeter available is presented in the QUID document.

Input data quality control

The L2 Input Data Quality Control is a critical process applied to guarantee that DUACS uses only the most accurate altimeter data.DUACS system is supplied with L2p altimeter products that include a quality flag for each measurement. The valid data selection is directly based on this quality flag. Thanks to the high quality of current missions, this process rejects a small percentage of altimeter measurements, but these erroneous data could be the cause of a significant quality loss.

The L2p quality control relies on standard raw data editing with quality flags or parameter thresholds, but also on complex data editing algorithms based on the detection of erroneous artifacts, mono and multi-mission crossover validation, and macroscopic statistics to edit out large data flows that do not meet the system’s requirements. Details of threshold editing can be found in the handbook of each altimeter mission [e.g. Aviso/SALP 2016a, 2016b, 2016c] as well as Cal/Val reports [e.g. Aviso/SALP (2015)].