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Manfred Schroeder 
  
Fig. 1: Enlarged MOMS-2P-scene (2 July 1998) with 400 x 400 sqm calibration area. 
The La Crau test site is used on routine basis for SPOT calibration. Therefore it is equipped with a CIMEL 
automatic radiometer operated by the CNRS LISE French laboratory originally developed for SPOT absolute 
calibration [1]. The instrument is a sunphotometer mounted on a 10 meter high mast and consists of two 
collimators, one for IR channels (Germanium detector) and one for visible bands (Silicon detector). The 
instrument delivers data in nine spectral bands 10 nm wide. The central wavelengths are 380 nm for molecular 
scattering, 440 nm, 550 nm, 670 nm, 870 nm and 1600 nm for aerosols in the SPOT and VEGETATION spectral 
bands, 936 nm for the water vapour, 1020 nm to complete the aerosols knowledge. The radiance at 870 nm is 
acquired both with the IR and the visible collimators in order to intercalibrate them. The automatic operating 
procedure includes 3 sequential modes repeated all the day long for optical airmasses up to 5. In the first mode, 
the sun collimator points at the sun and measures direct solar irradiance in order to retrieve the water vapour 
abundance, aerosol optical depths and the Angstróm coefficient. The second mode consists in sky radiances 
measurements in the almucantar and principal plane and is used for phase function retrieval. In the last mode, the 
collimator scans the ground both in azimuthal and zenital directions and measures the surface BRDF. This 
instrumentation enables autonomous self-sufficient measurements for SPOT calibration. As the instrument had 
power unit and data transmission troubles on the 7/2/98, it has been replaced by the previous operational 
instruments, formerly used on the test site for vicarious calibration. First a classical automatic CIMEL 
sunphotometer with the commonly used six filters centered at 440 nm, 670 nm, 870 nm, 1020 nm for the 
aerosols characterisation, and 937 nm (10 nm wide) and 940 nm (50 nm wide) for the water vapour was operated 
instead. As a redundancy, a portable CIMEL 318 sunphotometer equipped with the same bands was operated 
simultaneously in manual mode [8]. These instruments are not fully equipped for all the Spot spectral channels 
and can only be used for the atmospheric characterization. 
Therefore, a ground radiometer, based on a standard photodiode equipped with 3 bands centered at 550, 650, 850 
nm provides the ground reflectance in the 3 SPOT multispectral channels. For MOMS bands, shifted with 
regards to SPOT, the spectrometer ZEISS-MCS[2] was used to measure the ground reflectance, the total and 
diffuse downwelling irradiance. The last two parameters lead to the atmospheric direct transmittance from which 
the aerosol optical properties will be derived. This method although indirect, offers the advantage to give 
spectral information and thus to separate the contribution of most important gazeous constituents and aerosols. 
This technique was assessed for the first time during this campaign, and the proven method based on the 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part Bl. Amsterdam 2000. 279