Calibration

Radiometric Calibration Information

The Image Quality Center  (IQC) continuously monitors the instrument radiometric performance to be able to compensate for drifts caused by systematic changes, such as ageing of the instruments in order to ensure the delivery of stable, reliable, and consistent data over its full mission lifetime. 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

The instrument stability over time is furthermore 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.  

This table gives an overview of the different updates to the radiometric ICP files.


Geometric Calibration Information

Imaging satellites are subjected to several factors that may reduce the post-launch sensor geo-pointing accuracy. Some of these are: launch shock, precipitation of water vapour from the launch at the instrument once in space, and gravity release. Therefore in-orbit geometric calibration is a key technology to continuously guarantee and monitor the geometric quality of satellite imagery.

To maximise PROBA-V product’s geolocation accuracy, a continuous and highly autonomous in-flight geometrical calibration/validation software has been implemented in the mission’s User Segment (US). It processes large data volumes with limited human intervention in order to estimate and monitor the Exterior Orientation parameters (boresight angles) and Interior Orientation deformations (CCD line of sight vectors) of each camera of the PROBA-V Vegetation Instrument (VI) on a regular basis. The geometrical parameters are estimated using a large number of globally distributed Ground Control Points (GCP), a priori extracted from the Landsat Global Land Survey GLS 2010 cloud-free dataset (https://landsat.usgs.gov/science_GLS.php). The estimated geometrical calibration parameters are then used to update the Instrument Calibration Parameter (ICP) file that is used by the US Processing Facility.

In addition to the calibration step, the Level 2 data produced by the Processing Facility are continuously checked and validated by cross-correlation with the Landsat Global Landsat Survey GLS 2010 dataset. Since Landsat observations have a finer spatial resolution than PROBA-V, Landsat images have been rescaled to 300 m (the PROBA-V product resolution) by means of bilinear interpolation. Three types of geometric errors (absolute, inter-band, and multi-temporal) are monitored on a regular basis to ensure the highest geometric accuracy at any ground location.

After more than one year of calibration monitoring, PROBA-V shows an impressive geometric performance stability.

Geometric Calibration Overview

Figure 1:  PROBA-V Geometric calibration overview


Geometric Instrument Calibration Parameter files

The values of the different geometric parameters are stored in the Instrument Calibration Parameters (ICP) files. For each camera there is a separate ICP file. Initial values for parameters are given by the on-ground calibration. In-flight updates of the parameters are computed by the Geometric Calibration and delivered to the User Segment Processing Facility (PF) for the direct geo-referencing of the PROBA-V products.

This table gives an overview of the different updates to the geometric ICP files.