542 
which can provide simultaneous measurements of the columnar water vapour content In this paper, we present 
how we have fitted an algorithm to retrieve water vapour content from ATSR/IR split-window channels and its 
validation on a set of 35 ATSR images. The methodology is presented and the first results are discussed. 
2 - DIFFERENTIAL ABSORPTION TECHNIQUE 
This technique consists in viewing a scene at two wavelengths for which the absorption of the atmospheric 
water vapour is different. This method has been developed in the atmospheric window in the 11 pm region 
which can be separated in two bands, one band being more sensitive to water vapour than the other. 
If we consider the case of a cloudless region for which the atmosphere is supposed to be homogeneous 
and if the radiative transfer equation is written for two pixels whose surface temperature is different, Kleespies 
and McMillin(1990) have shown that the ratio of the brightness temperature difference at the two wavelengths 
is equal to the ratio of the transmittances : 
(AT//)ii _ (T, - Tj)\\ _ en Tu 
(A7ÿ)l2 (Ti - Tj) 12 £12 Tl2 
The subscripts 11 and 12 refer to the split window channels and i and j refer to two pixels for which the surface 
temperature changes measurably and which have the same surface emissivity. These conditions may be found 
over sea regions where Sea Surface Temperature fields occasionnaly exhibit greater spatial variability than 
atmospheric fields (Harris and Mason, 1992). 
As shown by Kleespies and McMillin(1990), Jediovec(1990) and Harris and Mason(1992), the ratio of the 
transmittances in these two channels (R 11,12) is related to the water vapour (Wv) in the atmospheric column 
by an inverse relationship. 
Rn.n = K\Wv + K 2 (2) 
where Ki and K2 are constants related to the absorption coefficients of the atmospheric constituents. 
These authors have determined this relationship by regression between rawinsonde measurements and 
computed transmittances ratio with radiative transfer models. Their results do not show a good agreement 
since the differences on the estimated water vapour may be greater than 50% . 
In order to see whether these differences are related to the accuracy of the radiative transfer model 
used or if they depend on the set of atmospheres chosen, we have calculated this relationship for a great 
number of ATSR images corresponding to various atmospheric situations . 
3 - DESCRIPTION OF THE ATSR-IR AND ATSR-MW RADIOMETERS 
3.1. ATSR-IR 
The Along-Track Scanning Radiometer (ATSR) is a four channel passive infrared radiometer designed 
specifically to measure Sea-Surface Temperatures (SST) and also cloud top temperatures. The ATSR observes 
the Earth's surface along two curved swaths : the nadir swath ( a nearly vertical path through the atmosphere ) 
and the forward swath ( an inclined path forward the sub-satellite point ). These swaths are 500 km in width 
and the two views are separated by approximately 900 km in along-track distance and separated in time by 
about 2mn. Figure 1 shows the viewing geometry of the instrument. The spatial resolution is about lxlkm 2 at 
nadir and a little larger at forward equal to 1.5x2 km 2 . Its four channels are fully co-registered and provide 
measurements at 1.6, 3.7, 10.8 and 12 pm. More details on this instrument may be found in the ERS-1 
reference manual ( 1989).