The PICS approach relies on the long-term stability of the site, and does not require simultaneity. The Libya 4 PICS was chosen for this calibration study as investigations have indicated that the site has been stable to within 2% for the last decade [22,28]. ETM+ has made over 200 nadir cloud-free acquisitions over Libya 4 since its launch in 1999. Similarly OLI has acquired more than 20 cloud-free images since its launch. Figure 4 shows a RGB color composite Landsat image over Libya 4 PICS. The red rectangle shows the region of interest (ROI) which is about 45 × 28 km2 [14]