regions frequently overcast, this repeatitivity can only be achieved with geostationary satellites or microwave 
systems. 
2.3 Wavelength 
Most of operational applications are based on the use of vegetation indices, like the normalized difference 
vegetation index (NDVT) computed from red and near infrared reflectances. Some of them make use of the 
surface temperature obtained derived from thermal infrared data (drough detection) or of the mid infrared 
radiances (3.7 pm) (forest or bush fires), whereas the applications of the mid infrared channels (at 1.6 or 2.3 
pm) are still under study. 
Much is expected from the imaging spectroscopy in particular for retrieval of chlorophyll, water, but also lignin 
or nitrogen (Curran, 1989). In fact, this is still very experimental and has to be further studied. The interest of 
the red shift is still controversial (Baret et al, 1992) and remains to be demonstrated. Note that as the data rate 
is a major system constraint, a data compression has to applied. As it has been demonstrated by Price (1991), 
there is a high redundancy and a best choice of less than 15 channels may be envisaged. 
The application of SAR data for agriculture, forest, land use looks promising (i.e Wooding, 1993) but awaits 
confirmation on the full range of conditions. 
2.4 Methods in development 
These data .with different resolution, acquired at different time steps and measured at different wavelengths 
may be used consistently for the same application like in oceanography and meteorology where such 
techniques are used in forecast models. Data assimilation techniques are now proposed for use in models 
of crop production, vegetation functioning, etc. 
3 - THE EUROPEAN CONTEXT 
The European satellites with instruments of interest for vegetation monitoring are ERS 2 (1995) with the same 
payload than ERS-1 but ATSR-2, the payload being mainly designed for oceanographic applications. 
ENVISAT (1998) with MERIS, ASAR, ATSR-2 (Table 1) is mostly oriented towards global monitoring of the 
terrestrial environment when the objectives of METOP with AVHRR, MIMR and METEOSAT Second 
Generation, MSG, (Table 1) are oriented towards operational meteorology and GEWEX. All of them are 
designed by ESA but with a tight collaboration of EUMETSAT for METOP and MSG and of member states. If 
many of these systems may find applications in the domain of biosphere studies and vegetation monitoring 
none has been specifically designed for such a use. Moreover the instruments of interest for this purpose are 
experimental. The best method to use the data have to be defined and introduced in the operational procedures. 
Furthermore the continuity of data is not fully guaranteed. 
To summarize, the requirement of an operational instrument for long term vegetation monitoring is not 
fulfilled by ESA projects. 
4 - CNES PROJECTS 
4.1 High resolution optical imagery ° n SPOT4 and follow up mission 
SPOT-4, the launch of which is planned in 1997, will differ from SPOT-3 thanks to the presence of an 
additional Shortwave Infrared Channel (MIR) 1.5-1.7 pm on the twin radiometers now named HRVIR 
SPOT 5 design is still under study. The main improvement targetted concerns the spatial resolution and the 
possibility of along track stereoscopy. In the vegetation monitoring domain, the main impact would be on the 
applications based on texture analysis like forest characterization (forest/non forest, type, parameters) or crops 
discrimination.