The radiometric and geometric accuracy of the instrument is routinely validated using a combination of vicarious calibration methods (e.g., Desert, Rayleigh, Deep Convective Clouds). A detailed description of some of these methods is available here.
The results of the routine vicarious calibration processing are available on a quarterly basis in the "Image Quality Reports" generated by VITO, see the full list of reports here.
The current results for geometric and radiometric accuracy verification are summarized here below.
Radiometric accuracy: The absolute and relative radiometric accuracy is periodically monitored using a combination of vicarious calibration approaches: i.e., Rayleigh, Desert and Deep Convective Clouds methods. Long-term radiometric stability is additionally verified using pseudo-invariant calibration sites (Libya-4 desert), which are acquired as part of the nominal planning, and special moon observations, which are monthly acquired with a special pitch manoeuver (360° rotation). The radiometric performances retrieved with these methods are excellent and very stable along the mission: absolute and relative radiometric accuracies are respectively better than 5% and 3%, radiometric stability is well within 3%. Cross-comparison with Landsat-8 show an agreement of better than 2% for all bands as summarized in Fig.1, see detailed results of this inter-comparison here.
Fig.1 - Fraction of the averaged calibration parameters for Proba-V with respect to Landsat-8 OLI for the 4 Proba-V bands. The calibration parameters were derived by applying the same desert calibration method over Libya-4 site. (Image credits: VITO)
Geometric accuracy: The absolute and relative (inter-band) geo-location accuracy is daily monitored using ground control points, extracted from the Landsat GeoCover dataset. This monitoring allowed to detect an issue with the on-board handling of the star tracker attitude data during the first months of 2014, owing to this anomaly a significant degradation of the geometric accuracy was observed. After a long investigation, the root cause of the problem was found and the issue was solved on 10 March 2014 by applying an on-board patch. Since that date the geometric performances of Proba-V remain at an excellent level and very stable (see Fig.2), with an average absolute geo-location error of better than 50m and a relative error as low as 32 m for the RED-NIR bands.
Fig.2 - Proba-V Average Location Error (ALE) in meters from Aug 2013 to Apr 2015. The ALE is determined using a set of GCP extracted from Landsat GeoCover database. The on-board star-tracker anomaly at the beginning of the mission is clearly visible, it was solved with an on-board patch during March 2014. Since then the ALE is stable around a mean value of 60m. (Image credits: VITO).