## 1 Introduction  
Satellite remote sensing of the atmospheric composition has developed over the last decades with a number of missions such as GOME on ERS-2 [1], SCIAMACHY on ENVISAT [2], the Ozone Monitoring Instrument (OMI) on EOS-Aura [3], the Global Ozone Monitoring Experiment-2 (GOME-2) onboard MetOp A, B and C [4] and the TROPSpheric Monitoring Instrument (TROPOMI) onboard Sentinel-5 Precursor (S5p) [5], the latter to be followed in 2025 by Sentinel-4 and Sentinel-5, respectively, onboard the EUMETSAT platforms MTG-S and METOP-SG. Sentinel 4, 5 and S5p missions are an essential component of the Copernicus Atmosphere Monitoring Service (CAMS, [[https://atmosphere.copernicus.eu/](https://atmosphere.copernicus.eu/)]([https://atmosphere.copernicus.eu/](https://atmosphere.copernicus.eu/)), accessed on 27 November 2024), one of the six operational services of the EU's Copernicus Programme ([[https://www.copernicus.eu/en](https://www.copernicus.eu/en)]([https://www.copernicus.eu/en](https://www.copernicus.eu/en)), accessed on 27 November 2024). The CAMS relies on the Integrated Forecast System (IFS), a complex suite of atmospheric models that assimilate satellite data to produce forecasts and analyses of various reactive gases as well as aerosols and greenhouse gases. The calibration and validation (Cal/Val) of satellite data depends largely on the availability of high-quality independent ground-based reference datasets representative of the various measurement conditions sampled by satellite sensors. With time, Cal/Val has progressively evolved from a purely research-driven activity to a more coordinated and comprehensive assessment and reporting of the quality, biases and residual uncertainty in the satellite observations, being performed in operational environments such as, e.g., the ESA SSP Mission Performance Centre (MPC) Validation Data Analysis Facility (VDAF) ([[https://mpc-vdaf.tropomi.eu/](https://mpc-vdaf.tropomi.eu/)]([https://mpc-vdaf.tropomi.eu/](https://mpc-vdaf.tropomi.eu/)), accessed on 27 November 2024). A key requirement, formulated within the Quality Assurance Framework for Earth Observation (QA4 EO), is that the independent dataset itself must be fully characterised in a manner consistent with the QA4 EO process, i.e., that it has documented evidence of its level of internal consistency. This led the ESA to develop the concept of Fiducial Reference Measurements (or FRM), a label aiming to create a class of observations optimised to meet the needs of the satellite community. The Pandonia global network (PGN, [[https://www.pandonia-global-network.org/](https://www.pandonia-global-network.org/)]([https://www.pandonia-global-network.org/](https://www.pandonia-global-network.org/)), accessed on 27 November 2024) is an example of a recently developed atmospheric composition monitoring network based on the FRM concept. The PGN was built around the Pandora instrument [6] that was designed in the late 2000 to operationally monitor ozone and other trace gases using direct sun UV-Visible spectroscopy. Next to Pandora systems, many alternative instruments exploiting the UV-Visible Differential Optical Absorption Spectroscopy (DOAS) technique have also been developed by various research institutes and deployed worldwide for the monitoring of atmospheric trace gases such as nitrogen dioxide (NO\({}_{2}\)), ozone (O\({}_{3}\)), the collisional complex of oxygen (O\({}_{4}\)), formaldehyde (HCHO), bromine oxide (BrO) and several other species. A number of these instruments are operated within the Network for the Detection of Atmospheric Composition Change (NDACC, [[https://www.ndacc.org/](https://www.ndacc.org/)]([https://www.ndacc.org/](https://www.ndacc.org/)), accessed on 27 November 2024). Initially constituted in the 1990s to observe and understand the physical and chemical state of the stratosphere, the NDACC (formerly the NDSC, Network for the Detection of Stratospheric Changes) has gradually expanded its focus to the study of the tropospheric composition.