FDR4ATMOS (Task B): FDR Long time series for spectral imagers

Kurzfassung

The Fundamental Data Record for ATMOSpheric Composition (FDR4ATMOS) project is part of the ESA Long Term Data Preservation (LTDP) programme. A Fundamental Data Record (FDR) is a long-term record of selected Earth observation Level 1 parameters (radiance, irradiance, reflectance), possibly multi-instrument, which provides improvements of performance with respect to the individual mission data sets. The aim of task B of the project is to be a pathfinder for future harmonisation of spectrally highly resolved data from other instruments, starting with 2 well known instruments GOME-1 and SCIAMACHY and the spectral ranges for the retrieval of SO2, O3 (UV), NO2 (VIS) and cloud properties (NIR). The FDR will contain harmonised irradiances and reflectances with associated uncertainties. The GOME-1 and SCIAMACHY instruments together span 17 years of spectrally highly resolved data essential for air quality, climate, ozone trend and UV radiation applications. We plan to generate harmonised data sets that allows to directly use it in long-term trend analysis, independently of the instrument. Since this was never done for highly resolved spectrometers, new methods have to be developed that e.g. take into account the different observation geometries for the Earth measurements: GOME-1 and SCIAMACHY had different orbits with a local descending node crossing time of 10:30 and 10:00 respectively resulting in different solar zenith angles. The spatial resolution also differs with GOME-1 having a resolution 40 × 320 km^2 and SCIAMACHY a resolution of 32 × 233 km^2 to 26 × 30 km^2, depending on the spectral range and orbit phase. Since the retrieval of atmospheric trace gas content and other parameters depends on the relative structures of the spectrum, any harmonisation must keep these structures and at the same time take care of broader band differences between the instruments. The data will also contain uncertainties that are based on metrological principles. For that purpose, we reviewed the error sources and error correlations of the original Level 1 data. These uncertainties will flow into the error propagation, leading to final uncertainties of the FDR. The resulting algorithms will be implemented into a processing system using the DLR developed GCAPS framework (Generic Calibration and Processing System). The purpose is to keep the methods and the implementation as generic as possible to be able to easily transfer the methodology to other wavelength ranges and to other instruments (e.g. GOME-2 and Sentinel-5p) for a future extension of the time series. Starting with the solar irradiance as the simpler problem (compared to the Earth radiance measurements) we investigated different methods for the harmonisation and will present the results in the paper. We will also describe the overall validation concept and the status of the harmonisation of the reflectances.