used in field experiments. In its airborne configuration PORTOS operates at the frequencies 
of 5.05, 10.65, 23.8, 36.5, and 90 GHz. It is dual polarized and has a single beam with a field 
of view of 11 °. The incidence angle can be varied from 0 to 50° (Grosjean and Sand, 1994). 
Two flight patterns were performed with PORTOS also. One was at low altitudes to map 
completely the supesites. The ground resolution for these flights was 300m. and the 
incidence angle 45° so that polarization differences could be obtained. These flights were 
coordinated with those of the PBMR, to provide near simultaneous T B coverage over a wide 
spectral range. Again a good range of moisture conditions were observed as well as most of 
the vegetation growth cycle. The second pattern was at a high altitude (15,000m) and at nadir 
to yield a ground resolution of 2 km. These were mainly performed over the west central site 
and were designed to address scaling issues. 
For both radiometers flights over wide portions of the Niger river flown for calibration 
purposes. In general the results were positive indicating stable performance by both systems. 
Both sensors show a large dynamic range of T B on each day and between different days in 
response to variations in rainfall and drying conditions. 
These aircraft measurements were supported by ground based observations at the 3 central 
sites. These data were obtained on fallow and millet fields in order to establish the relationship 
between the surface soil moisture and T B . These data were collected on several days 
providing a good range of soil wetness conditions. The effects of vegetation on the 
microwave emission are being analyzed. The accuracies of the microwave determinations of 
surface soil moisture content are being assessed for both vegetated and non vegetated areas 
These joint flights of the PORTOS and PBMR microwave systems in HAPEX-Sahel will 
provide one of the first data sets covering a wide microwave spectral range over land surfaces. 
These data will enable us to study the possible the polarization difference at the higher 
frequencies, 10 to 36 GHz, to estimate vegetation amounts for correcting the soil moisture 
estimates made with the 21-cm data. 
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