FDR4ATMOS: A data set for harmonised solar irradiance and Earth reflectances and future plans

Abstract

The Fundamental Data Record for ATMOSpheric Composition (FDR4ATMOS) project is part of the European Space Agency (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 beyond the individual mission data sets. The aim of the project is to be a pathfinder for future harmonisation involving spectrally highly resolved data from other instruments. Initial focus was placed on well known instruments ERS-GOME-1 and ENVISAT/SCIAMACHY in the spectral ranges for the retrieval of SO2, O3 (UV), NO2 (VIS) and cloud properties (NIR). The combined data set from the GOME-1 and SCIAMACHY instruments spans a 17 year period offering spectrally highly resolved data essential for air quality applications, climate, ozone trend and UV radiation studies. We generated harmonised data for solar irradiances and reflectances that can be combined in a timeseries. Since harmonisation was never applied for highly resolved spectrometers, new methods had to be developed, e.g. accounting for the different observation geometries between the two instruments. Specifically, GOME-1 and SCIAMACHY operated on different orbits with a local descending node crossing time of at 10:30 and 10:00 respectively resulting in different solar zenith angles. Additionally, the spatial resolution also differs with GOME-1 having a resolution 40 × 320 km^2 and SCIAMACHY providing a resolution ranging from 32 × 233 km^2 to 26 × 30 km^2, depending on the spectral region and orbit phase. Since the retrieval of atmospheric trace gas content and other parameters depends on the relative structures of the spectrum, any harmonisation effort must preserve these structures while also addressing broader band differences between the instruments. The uncertainties on the original calibration data and Level 1 processing algorithms of GOME-1 and SCIAMACHY were reviewed. An uncertainty propagation model was built that allowed individual uncertainty components and their correlations to be combined, to calculate the total uncertainty on FDR irradiance and reflectance. The harmonisation algorithms were implemented into a processing system using the DLR developed GCAPS framework (Generic Calibration and Processing System), resulting in the generation of a FDR data set spanning the mission times of GOME-1 and SCIAMACHY. The data will be released in the 3rd quarter of 2024. A follow-on phase 2 of the project was initiated by ESA in 2024 to incorporate GOME-2 data and to employ SCIAMACHY and GOME-2 to develop a spectrally high resolved lunar data model. Building upon the experience of the first phase we plan to improve the harmonisation algorithms further and establish a time series of solar irradiances and reflectances spanning GOME-1, SCIAMACHY and GOME-2 A-C. This paper will give an overview of the FDR dataset and the harmonisation algorithms as well as give an outlook into the upcoming phase of the project.