In order to ensure the delivery of stable, reliable, and consistent data over its full mission lifetime, the Image Quality Center (IQC) continuously monitors the radiometric performance to be able to compensate for drifts caused by systematic changes, such as ageing of the instrument. To this end, a fully operational and automated vicarious Cal/Val facility dedicated to the routine calibration of the PROBA-V instrument was developed.
The absolute and inter-band accuracy of the absolute calibration coefficient is evaluated through the use of three independent vicarious calibration methods. These are:
- Absolute calibration over the bright Libya-4 desert site
- Absolute calibration over Rayleigh calibration zones
- Inter-band calibration over Deep Convective Clouds (DCCs)
Furthermore , the instrument stability over time is assessed through monthly lunar observations.
The performance of the equalization coefficients to correct for inter-pixel variations over the Field Of View (FOV) is regularly analysed through acquisitions over large homogeneous snow and desert sites.
The stability of the dark current values is monitored through nighttime observations over dark oceans in a prolonged image capture mode.
Finally, the linearity of the response has been verified using the special Linearity Calibration Mode. In the Linearity Check Calibration Mode, a set of 5 integration times is commanded to the spacecraft, and repeated in a periodic sequence.
Radiometric Instrument Calibration Parameter files
The values of the different radiometric parameters are stored in the Instrument Calibration Parameters (ICP) files. For each camera there is a separate ICP file. Initial values for these parameters are given by the on-ground calibration. In-flight updates of the parameters are computed by the IQC and delivered to the User Segment Processing Facility (PF) for the processing of the raw DN images.
The latest information on Radiometric ICP files is given in the Quarterly Image Quality Reports.