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Title
Mesures physiques et signatures en télédétection

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.
REFERENCES
Goutorbe, J.P., Lebel.T., Tinga,T., Bessemoulin, P., Brouwer, J., Dolman, A.J., Engman,
E.T., Gash, J.G.C., Hoepfther, M., Kabat, P., Kerr, Y.H., Monteny, B., Prince, S.D., Said, F.,
Sellers, P., Wallace, J.S.(1994). HAPEX-Sahel: A large scale study of land-atmosphere
interactions in the semi-arid tropics, To be published in Annales Geophysicae.
Grosjean, O., and Sand, A., (1994). Etalonnage du Radiometre Hyperfrequence PORTOS et
Methode de Correction de Mesures. Proc this symposium.
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measurements of surface soil moisture in FIFE. IEEE Trans Geosci. Remote Sens,
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