Vicarious calibration as a method for inflight monitoring of atmospheric composition small-sat instrumentation

Due to their dimensions and weight constraints, atmospheric composition small-sat missions typically lack onboard calibration capabilities, forcing them to resort to vicarious calibration methods. The sole instrument on the Sentinel-5 Precursor mission, TROPOMI, has been performing excellently since its launch in 2017. The SWIR module has degraded less than 0.2%. TROPOMI provides an excellent test-case for the applicability of vicarious calibration for atmospheric composition small-sat application such as the upcoming TANGO mission. Here we present the ongoing efforts to validate both the relative radiometric calibration (i.e. instrument stability) using desert PICS, as well as the absolute radiometric calibration using ground-measurement of the Railroad Valley playa. Such independent validation constrains TROPOMI to 0.5% relative and 6% absolute. Several significant limitations have been identified; First, results of large pixel sizes deviate from the assumption of surface homogeneity, in particular smaller sites such as RRV. Modelling of the BRDF (Bidirectional Reflectance Distribution Function) needs to be accurate to account for non-Lambertian effects. For small-sat applications with much higher spatial resolution, homogeneity is much simpler to guarantee. We also recommend soundings be done as close to nadir-view as possible to minimize other uncertainties due to the BRDF.