This paper reviews the calibration techniques used for 19 years of SPOT satellite exploitation:
- from the decision to fly a calibration system (involving a lamp and a sun sensor) on SPOT1 to its suppression on SPOT5 and to the emphasis placed on the development of calibration methods over natural targets like oceans or deserts. Oceans provide low reflectance targets for short wavelengths calibration over the atmospheric molecular scattering, while deserts (warm over North Africa and Middle East, cold over Antarctica) provide stable references for sensors cross-calibration and temporal monitoring.
- from vicarious calibration campaigns performed by scientific teams over test sites like White Sands (USA) or La Crau (France) to an autonomous ground based station. An automatic radiometer continuously characterizes the reflectance and the atmosphere of the French test site and thanks to its original calibration procedure provides the top of atmosphere radiance needed for in-flight calibration.
The results provided by these different methods are discussed. We show that the on-board calibration unit used to monitor with time SPOT1, 2, 3 and 4 cameras sensitivity is loosing sensitivity, justifying the overall calibration update that was proposed to users in October 2004 for all SPOT since their beginning of life.