Intercomparison of spaceborne microwave radiometers (ERS1-ERS2-TOPEX-SSMI)

Sid-Ahmed Boukabara and Laurence Eymard

CETP, 10-12 avenue de l'Europe 78140 Velizy France

ABSTRACT

Lot of remote sensing applications use simultaneously data obtained from several 
satellites instruments. Making maps of geophysical products, validation of radiative 
transfer models (by comparing the satellites data to the simulated ones), assimilation 
of the remote sensed data in the meteorological prediction models, are some examples 
where the synergistic use of the different satellites products is necessary and where 
the intercalibration of the measurements is essential (authors like J. Etcheto 1988 
underlined this problem). In order to eliminate the differences (and than the errors) 
due to the instruments calibration differences, we propose to use an inter-calibration 
of these instruments before using their data. The aim is not to change the calibration 
of the instruments but to make homogenous the data for the users who need that for 
their application. We limited this work to the radiometers only (of ERS-1, ERS-2, 
TOPEX-POSEIDON and SSM/I). The work undertaken is based on the cross comparisons 
between the different instruments and the comparison with the ECMWF meteorological 
fields. We use directly the electromagnetic quantities measured by the instruments 
(microwave brightness temperatures MBT) in the different channels (frequency bandwidth: 
18 - 85 GHz). First, the intercalibration of these MBT's is done, with a precision 
of 1 K. Then, an appropriate retrieval algorithm for each instrument is proposed for 
the common retrieved geophysical parameters (integrated water vapour WV, integrated 
water content WC and wet tropospheric correction DH). The algorithms are then 
validated by comparing their results directly to the coincident on-shipment 
measurements. We reach a precision of 1 cm for DH, 0.15 g/cm2 for WV and 2-3 mg/cm2 
for WC.
The geophysical products are retrieved using the same algorithms form (linear 
logarithmic, described by E. G\'erard, 1996). This permits to get homogenous coverage 
when using different remote sensing sources. These algorithms have their own defaults, 
but the geophysical data issued from the different satellites will have the same 
behaviour. All the data used in this paper are issued from the tandem period where the 
four satellites were in orbit (June 1995 to June 1996).

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