Die Operational data processing is designed to provide users with Level 1 and Level 2 data continuously over the entire lifetime of a mission. The levels are defined as follows:
There are two processing stages – Level 0-1b with an optional Level 1c, and Level 1b-2.
In general, the recording of the calibration data required for Level 0-1b processing does not take place at the same time as the atmosphere measurements themselves. It is therefore necessary to process both types of data separately. With the SciCal software package developed by the IMF Atmospheric Processors department the calibration data can be processed with the specified algorithms and then centrally managed. For any given atmosphere data set, appropriate calibration data—usually that which is closest in time—can be defined and incorporated in a Level 1b data set along with geolocation information. The calibration algorithms were developed together with SRON (Netherlands Institute of Space Research), whereas the operational processor was provided by industry (Jena Optronik) according to IMF specifications.
Many users require Level 1c data for their own purposes. This requirement can be met with the software package SciaL1c, which was developed for ESA by the IMF Atmospheric Processors department. It permits the selection of all, individual, or any combination of calibration steps for Level 1b data in order to generate a customized Level 1c product.
Operational Level 1b-2 processing begins with internal processing to Level 1c, in other words, to calibrated spectra. After deriving parameters for use when later calculating trace gas concentrations (for example, the extent of cloud cover), a classification is made according to the observation geometry—limb or nadir. Only then does the actual retrieval (determination of trace gas concentrations) take place, with the results being collected in a Level 2 data set for each orbit. The new Version 5 of the data processor, which became operational in September 2009, covers the atmosphere parameters listed in the table. The table also includes an overview of the algorithms employed.
The processor runs in a so-called offline mode at the German Processing and Archiving Center for Envisat (D-PAC) and supplies data about ten days after they are recorded. A time offset of this duration assures that optimal calibration parameters are always available. An additional fast delivery service which provides data within 24 hours of recording is planned.
There is more information about calibrating absorption spectrometers on the calibration page of the "Team Sensor-specific Methods" of the "Atmospheric Processors Department".