Several issues remain unresolved, still. There has been so far very little validation of the results; the 
accuracy of the retrieved corrected reflectances should be carefully assessed with field and airborne 
experiments on selected sites. Whether or not atmospheric corrections should be performed prior to the 
application of a bidirectional reflectance model should be debated. A key criterion in this issue is the accuracy 
with which atmospheric corrections can be performed. The quality of atmospheric corrections can be expected 
to improve in the next decade, with the development of ground based Sun-photometer network (eg, Holben et 
al., 1994), General Circulation Models, and satellite data such as those of the POLDER/ADEOS instrument. It 
is our personal opinion, still little justified, that this should favor composition methods where atmospheric 
corrections are performed as a first step. An other debatable issue concerns the trade-off between the number 
of free parameters to be retrieved in the regression process, and the duration of the composition period. Several 
compromises have been presented in this paper, without providing conclusive evidence that a solution is by far 
better than the others. Finally, inversion of bidirectional reflectance models in terms of other parameters than 
the corrected reflectance appears difficult with AVHRR data. This latter conclusion should however be re 
examined in the case of future sensors, such as POLDER/ADEOS (Deschamps et al., 1994) or MISR/EOS 
(Diner et al., 1989), for which (i) the number of directional data available for each terrestrial target will be 
much larger than for AVHRR in a given period of time, (ii) the directional space, in particular in azimuth, will 
be much better covered than with AVHRR. 
ACKNOWLEDGMENTS 
The author thanks M. Ba, F. Cabot, G. Dedieu, P.Y. Deschamps, R. Frouin and J.L. Roujean for providing a 
copy of their manuscript prior to publication and allowing the author to comment their results in this paper. 
REFERENCES 
Ba, M.B., P.Y. Deschamps, and R. Frouin, 'Error reduction in NOAA satellite monitoring of the land surface 
vegetation during FIFE', Journal of Geophysical Research , submitted, 1994. 
Cabot, F., and G. Dedieu, 'Surface albedo from space: coupling bidirectional models and remotely sensed 
measurements'. Journal of Geophysical Research , in revision, 1994. 
Dee ring, D.W., T.F. Eck, and J. Otterman, 'Bidirectional reflectances of selected desert areas and theit 3- 
parameter soil characterization', Agric. Forest Meteorology, 52, 71-93,1990. 
Derrien, M., B. Farki, H. Legldau and A. Sairouni, 'Vegetation cover mapping over France using NOAA- 
11/AVHRR', Int. Journal of Remote Sensing, 13, no 9, 1787-1795,1992. 
Deschamps P.Y., F.M.Brdon, M. Herman, M. Leroy, A. Podaire, A. Bricaud, J.C. Buriez, J.L. Deuz6, and G. 
Sfeze, ‘The POLDER mission; Instrument characteristics and scientific objectives’, in press in IEEE Transactions on 
Geoscience and Remote Sensing, 1994. 
Dijk, A., S.L. Callis, C.M. Sakamoto, and W.L. Decker, 'Smoothing vegetation index profiles: an alternative 
method for reducing radiometric disturbance in NOAA/AVHRR data'. Photogram. Eng. Remote Sensing, S3, 1059-1067, 
1987. 
Diner D.J. et al, ‘A Multiangle Imaging SpectroRadiometer for geophysical and climatological research from 
EOS’, IEEE Transactions on Geoscience and Remote Sensing, 27, no. 2, 200-214,1989. 
Duggin M.J., D. Piwinski, V. Whitehead, and G. Ryland, ‘Evaluation of NOAA-AVHRR data for crop 
assessment'. Applied Optics, 21, 1873-1875, 1982. 
Duggin M J. and D. Piwinski, ‘Recorded radiance indices for vegetation moniyoring using NOAA AVHRR data; 
atmospheric and other effects in multitemporal data sets’, Applied Optics, 23, n° 15,2620-2624, 1984. 
Fung I.Y., C.J. Tucker, and K.C. Prentice, ‘Application of Advanced Very High Resolution Radiometer 
vegetation index to study atmosphere-biosphere exchange of CO 2 ’, Journal of Geophysical Research, 92, no. D3, 2999- 
3015, 1987. 
Go ward S.N., B. Markham, D.G. Dye, W. Dulaney, and J. Yang, ‘Normalized Difference Vegetation Index 
measurements from the Advanced Very High Resolution Radiometer’, Remote Sensing of Environment, 35, 257-277, 
1991. 
Gutman G.G., ‘The derivation of vegetation indices from AVHRR data’. International Journal of Remote 
Sensing, 8, 1235-1243, 1987. 
Gutman G.G., ‘Vegetation indices from AVHRR: an update and future prospects’. Remote Sensing of 
Environment, 35,121-136,1991.