Synergistic use of active and passive microwave measurements for sea/atmosphere parameters retrieval

S. Boukabara* , L. Eymard, C. Guillou, D. Lemaire and P. Sobieski**

*CETP 10-12 avenue de l'Europe. 78140 Velizy FRANCE
**Universite Catholique de Louvain La Neuve, Belgique

ABSTRACT
        Several satellites as ERS1-2 and TOPEX-POSEIDON carry both active and passive
microwave sensors. In particular, on these platforms, a microwave radiometer has been 
specially designed to provide the tropospheric path delay due to humidity, in order to 
correct the altimeter path measurement. Both instrument signals are sensitive to the 
sea surface and the atmosphere properties (surface wind, cloud, absorption,....). 
However, the altimeter and radiometer data processing has always been performed 
independently. Because emissivity and reflectivity are not really independent 
parameters, we know that the measurements from active and passive microwave modes are 
related to each other, therefore the coupling of these two types of microwave 
measurements should improve the retrieval of all the geophysical parameters obtained 
separately with no any interaction. We have undertaken to examine the feasibility and 
performances of a coupled retrieval of surface and atmosphere parameters of both 
instrument couples.
        A double scattering model has been developed at Louvain-La-Neuve university 
(Belgium), taking into account the idea of coupling active and passive modes. A first 
step of this work is the validation of direct results of this model (brightness 
temperatures and scattering coefficients). This is done using comparisons with several 
satellite measurements (ERS1-2, TOPEX-POSEIDON, SSM/I) and ECMWF meteorological 
coincident fields over which we apply the model. Improvements are being introduced and 
tested in this model. The atmosphere modelisation is based on the Liebe atmosphere 
absorption model. It also contains Monahan & Lu foam coverage model and Droppleman 
foam emissivity model. A new dielectric constant model has been developed in order to 
improve the surface model. Comparisons with other surface models used in radiometry  
have been undertaken. Verification is done using satellites measurements: ERS1-2, T/P, 
SSM/I. 
        The retrieval process is done by iterations in this model. Surface effect is 
detected first, then used as a first guess for the atmosphere parameters processing, 
the results are then used again in order to determine with more accuracy the surface 
parameters. The iteration is repeated until we reach convergence of the results. The 
iteration method is compared to a statistical coupling method, where the geophysical 
parameters are expressed as a function of all the active and passive measurements 
together. An important database of active  and passive data (scattering coefficients 
and brightness temperatures) is simulated using the double model. Then, a multilinear 
regression fit is applied  to this database in order to retrieve the 
surface/atmosphere parameters that served for the direct simulation. This leads to a 
statistical algorithm that we compare the performances to the iteration method.
        The geophysical products so obtained are compared to the standard geophysical 
products, derived separately. An other comparison is done with meteorological ECMWF 
coincident measurements ( 2hours in time and  0.5 in latitude and longitude). They are 
also compared to in-situ and radiosounding measurements and this  constitutes the 
final validation.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry