Calibration of HRVIR and vegetation cameras on SPOT4

The SPOT 4 satellite was launched on the 24th of March 1998 to ensure continuity of the SPOT program. It has extended capabilities with two upgraded HRVIR (High Resolution in the Visible and Infra-Red) instruments as well as a new payload based on a wide field-of-view, imaging radiometer (101 degrees) for monitoring the terrestrial biosphere on a global scale. This so-called VEGETATION payload has been specially designed for vegetation studies. It provides images with high radiometric accuracy and a 1 km spatial resolution, in 4 spectral bands: red, near infrared and short-wave infrared (SWIR) bands identical to those of the HRVIR instruments as well as a blue band for atmospheric measurements. Scale integration is thus made possible by combination with simultaneous HRVIR data. This paper presents the radiometric calibration results obtained with different methods based on experience with SPOT and POLDER, in: pre-launch measurements, onboard calibration systems, vicarious calibration over test sites, calibration over molecular scattering, calibration over sun glint, calibration over clouds and instruments intercalibration. The accuracy of these measurements is discussed. The measurements are combined in a model which gives the cameras' sensitivity as a function of time for HRVIR and VEGETATTON data processing algorithms. (C) 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.