503 
4.2. The continuous approach 
For the forest pixel (target 9 in Table 2), the h-Q approach is not adequate since vegetation is included in the soil 
emission whereas soil is unlikely to contribute significantly to the surface emission in the case of a forest. 
Assuming a typical value of 1.5/1000 for vegetation volume fraction, the forest correlation lengths could be 
obtained (given the METEOSAT temperature). We find values of about 8 and 2 mm in the vertical and 
horizontal directions, respectively, at 19 GHz. Lower values are determined at 37 GHz: about 4 and 1 mm. 
These values are consistent with the results of Calvet et al (1994) over the Amazon forest. 
5 - CONCLUSIONS 
Using PERIDOT outputs for atmospheric corrections, the Li and Becker (1993) method to retrieve surface 
temperature and emissivities from NOAA/AVHRR channels 3,4 and 5 was implemented successfully. It is 
shown that these temperatures can be used to calibrate the METEOSAT/IR channel emissivity and to obtain a 
METEOSAT-derived surface temperature. The obtained temperature is validated against surface measurements 
at the EFEDA Tomelloso test-site. Using this temperature, as well as an a priori knowledge of the surface-type 
(NOAA/AVHRR-NDVI classification), it is possible to calibrate microwave emission models of the surface. 
Preliminary results are presented over selected targets in Spain. Other-than-forest targets display a strong diurnal 
variation of the structure parameters which may be due to variations in vegetation water status (plant water 
content, leaf orientation, etc.) in semi-arid areas. Furthermore, the link between what is known about the 
considered surfaces and the derived microwave structure parameters is demonstrated. Finally, a field experiment 
showed that, in semi-arid conditions, it is possible to retrieve near-surface soil moisture accurately from high 
frequencies and, to a lesser extent, surface temperature. The application to SSM/I data is under way at Meteo- 
France/CNRM. 
ACKNOWLEDGEMENT 
The authors wish to thank BJ. Choudhury for providing SSM/I data; H.A.R. de Bruin for providing ground 
temperature data; and F. Becker, M.-P. Stoll and Y.H. Kerr for their helpful comments. They also wish to thank 
J.-P. Wigneron, A. Chanzy and Y.H. Kerr for inviting the CNRM to participate to the PORTOS-93 experiment. 
REFERENCES 
Bolle H.-J. et al., 1993. EFEDA: European Field Experiment in a Desertification-threatened Area. Ann. 
Geophysicae, 11:173-189. 
Bruin (de) H.A.R., 1993. Personal communication. 
Calvet J.-C., J.-P. Wigneron, E. Mougin, Y.H. Kerr and J.L.S. Brito, 1994. Plant water content and temperature 
of the Amazon forest from satellite microwave radiometry. IEEE Trans. Geosci. Remote Sensing, accepted. 
Champeaux J.-L. and J.-P. Jullien, 1993. Cartographie de la végétation sur l'Espagne à partir des données 
NOAA/AVHRR. Report CNRM/GMME-19, Météo-France/CNRM, Toulouse, France. 
Chanzy A. et al., 1994. Radiométrie micro-onde sur divers types de couverts végétaux: influence de l'humidité 
du sol. In: Proceedings of the Sixth international symposium on physical measurements and signatures in remote 
sensing. January 17-21, 1993. Val d'Isère, France. Accepted. 
Choudhury BJ., TJ. Schmugge, A. Chang and R.W. Newton, 1979. Effect of surface roughness on the 
microwave emission from soils. J. Geophys. Res., 84:5699-5706. 
Choudhury BJ., 1992. Personal communication. 
Hofer R. and E.G. Njoku, 1981. Regression techniques for oceanographic parameter retrieval using space borne 
microwave radiometry. IEEE Trans. Geosci. Remote Sensing, GE-19:178-189. 
Kerr Y.H. and E.G. Njoku, 1990. A semiempirical model for interpreting microwave emission from semiarid 
land surfaces as seen from space. IEEE Trans. Geosci. Remote Sensing, 28:3:384-393. 
Kneizys F.X., E.P. Shettle, W.O. Gallery et ai, 1983. Atmospheric transmittance/radiance: computer code 
Lowtran 6. Report AFGL-TR-83-0187, U.S. Air Force Geophysics Laboratory, Hanscom AFB, MA-U.S.A.. 
Li Z.-L. and F. Becker, 1993. Feasibility of land surface temperature and emissivity determination from AVHRR 
data. Remote Sens. Environ., 43:67-85.