A tiered approach to the validation of satellite constellations

2023-2024
Geophysical validation of satellite observations is essential to establish confidence in the data on the global scale and in the long term. This activity has typically focused on comparisons with ground-based reference measurements providing traceable quality metrics, but with a limited view on regional and global quality features.

The advent of satellite sounders coordinated in constellations requires the development of a tiered approach to their validation, addressing also the mutual consistency of multi-orbit, multi-scale data.

The best of two worlds

The CEOS constellations of satellite sounders for monitoring air quality and ozone combine global mappers measuring daily atmospheric composition from a low Earth polar orbit (LEO), with regional mappers measuring the diurnal cycle of atmospheric composition in the more limited field of view of a geostationary vantage point (GEO).

The highly successful Sentinel-5P TROPOMI and its awaited successor Sentinel-5 UVNS belong to the first category, while the Korean GEMS, the US TEMPO and the upcoming Sentinel-4 UVN belong to the second category.

Coordinating LEO and GEO sounders in such a constellation opens new perspectives on multi-mission monitoring of tropospheric trace gases and synergistic applications addressing the multi-scale nature of atmospheric composition. A prerequisite is to ensure confidence in the data not only for every single satellite mission, but also across the satellite constellation. The rigorous validation of these data and evaluation of their uncertainties become even more important.

Accordingly, the state of the art of geophysical validation has evolved. Classical comparisons to ground-based measurements remain the reference, since these provide quality metrics (e.g., bias, scatter, drift) traceable to community standards.

Complementary evaluation techniques have been developed to spot regional and global scale features (e.g., across-track biases, surface albedo related artefacts…) and evaluate the mutual consistency of the different satellites. These techniques rely among others on cross-satellite comparisons and on information content analysis methods, but with the drawback that there is no clear reference.

Therefore, the best of the two worlds is here combined: an operational validation environment performs cross-satellite comparisons with the TROPOMI LEO sounder selected as the travelling standard between the three geostationary sounders, this travelling standard being itself firmly anchored to the ground-based reference measurements.

Metrology-based data harmonization tools are applied to the datasets to mitigate differences in representativeness and comparison errors.

Application to tropospheric ozone

Figure 1 illustrates this tiered approach for tropospheric ozone column data from the OMI and GOME-2-B satellites, revealing respectively a North-South bias and an overall negative bias with respect to TROPOMI.

The observations of all three sounders are then anchored to SHADOZ tropical ozone sonde data used as a traceable reference. In figure 2, this approach is further extended to more tropospheric ozone datasets (Figure 2).

 

References

Tropomi-OMI validation focus image
TROPOMI - OMI - GOME-2B bias
Fig. 1. (Top) Tropospheric ozone bias between TROPOMI and OMI (left) and GOME-2B (right). (Bottom) Longitudinal and latitudinal sections of bias and dispersion between TROPOMI and OMI (blue) and GOME-2B (yellow), and the bias and dispersion between TROPOMI and SHADOZ ozone sonde data (black markers).
Mean bias TROPOMI 7 satellite or model datasets
Fig. 2. Mean bias with respect to TROPOMI of 7 satellite or model datasets.