233. SSM/I data. Four channels of the SSM/I instrument are considered: 19.35 and 37 GHz, V and H 
polarizations. The data consist of quarter-degrees TBs maps covering the Iberian penisula during june 1991 at 
0630 and 1830 +/- 30' UTC (Choudhury 1992). The first treatments consisted in: 
- selecting targets as homogeneous as possible in terms of vegetation type at the SSM/I resampled pixel 
size (about 25x25 km) over all Spain and EFEDA central-sites. This objective could be achieved through a 
classification of surface types obtained from the temporal analysis of the NDVI computed from NOAA-11 
AVHRR channels 1 and 2 after atmospheric corrections (Champeaux and Jullien 1993). 
- checking the consistency of SSM/I data with the NDVI classification. Figure 3 shows a map of the 
Iberic peninsula obtained from a simple thresholding on average TBs and Polarization Ratio (PR = (TBv- 
TBh)/(TBv+TBh)). The emerging regional structures are similar to those obtained with the NDVI. 
- performing atmospheric corrections on TBs. Atmospheric radiation components are computed for 
each frequency following Waters (1976) and Hofer and Njoku (1981). 
From METEOSAT data, atmospheric emission, and extrapolated emissivities (see above), it is possible 
to obtain an integrated black-body radiance at SSM/I pixel size. Inversion of METEOSAT-window Planck law 
gives a reference surface temperature Ts for interpreting SSM/I data. The value of Ts can be used to calibrate 
and/or validate microwave soil-vegetation emission models. 
SSM/I thresholds — June 1991 
Figure 3 - Surface-types regional structure of Spain as obtained from SSM/I by simple thresholding. 
Selected targets are indicated: S stands for "summer crops", G for "grass and shrubs", O for "olive groves", V for 
"vineyard", F for "forests", D for "dry crops", and C for "spring crops”. 
3 - GEOPHYSICAL PARAMETERS RETRIEVAL AT HIGH FREQUENCIES: EXPERIMENTAL 
RESULTS 
Some results from the PORTOS-93 experiment (Chanzy et al. 1994) are summarized here. This experiment 
provided helpful information about the way high frequencies (SSM/I frequencies in particular) can be used. As 
far as high frequencies are concerned, the PORTOS field-data include the 23.8, 36.5 and 90 GHz frequencies, V 
and H polarizations. Measurements were performed on several types of surfaces at Montfavet (43°58 N, 4°48 E) 
under a large range of moisture conditions (Chanzy et al. 1994, Wigneron et al. 1994). The downwelling 
atmospheric emission is computed by using (again) PERIDOT outputs and cloud coverage surface-observations. 
In this study, we focussed on a smooth bare soil surface and a Sorghum sparse canopy. 
3.1. High frequencies dielectric constant for a silt-loam 
After adequate calibration, PORTOS measurements over a smooth silt-loam were used (along with 
measurements of soil surface temperature and moisture) to calibrate for high frequencies the Wang and 
Schmugge (1980) empirical soil-dielectric-constant model. The considered silt-loam was artificially crushed and 
compressed to obtain a surface as smooth as possible (Chanzy et al. 1994). Therefore, Fresnel coefficients- 
derived surface reflectivities can be used to model the signal. At high frequencies, it is shown that the near- 
surface 5-mm soil layer must be considered. In the Wang and Schmugge (1980) model, two parameters can be 
calibrated: the transition volumetric moisture Mv t and the ^coefficient. Calibrated values of these coefficients 
are presented in Table 1. 
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