1189 
spare bits of the imagery file for any SHARP level 2 generation. In this demonstration the SHARK system is 
used to display the results obtained. These results have been integrated in the 3rd spare bit of the imagery file. 
This bit is set to 1 in Channel 1 if fully cloudy, in Channe l 2 if land, in Channe l 3 if snow, in Channe l 4 if cloud- 
free and in Channel 5 if ice. 
6.2 Atmospheric corrections 
Only the surface brightness temperature retrieval is presented in the software demonstration. The retrieved 
values are surface brightness temperatures for channel 4 and channel 5 where the emissivity has been fixed to 1. 
The emissivity of channel 3 has been set to 0.90 but can be changed by the user. The correction of the infrared 
channels of the AVHRR is performed by creating Look Up Tables (LUT’s) from LOWTRAN 7 with a 8 degrees 
increment for the viewing angles (15 strips for the overall image). Standard atmospheric profiles of the 
LOWTRAN library can be used as input as well as radiosonde data entered by the user. The climatic model used 
has been set following these rules: from -20 to 20 degrees in latitude tropical, from 20 to 60 and -20 to -60 Mid 
latitude and from Latitudes greater than 60 Polar. The seasons obey to the following rules: summer from April to 
September included and winter for the other months in the northern hemisphere, the inverse for the south 
hemisphere. No season change is considered for tropical regions. More than the standard available model within 
LOWTRAN, the users has presently the possibility to input his own radiosondes formatted with the LOWTRAN 
rules. In operation this option will certainly be frozen. 
The visible and near infrared channels of the AVHRR are corrected by inverting an improved version of 5S, 
(Tanre et al„ 1990) where the Rayleigh scattering and the gaseous absorption calculation have been improved. 
Two aerosol models will be available: maritime and continental. Three levels of corrections are planned: 
Rayleigh only. Rayleigh + gaseous absorption by 02, 03, H20, C02, Rayleigh + gaseous absorption + aerosol 
scattering. In operation the software will run using either climatological atmospheric contents for ozone, water 
vapour, and visibility or user defined values. The validation of the implementation of the software using in situ 
measurements over the HAPEX SAHEL site in Niger is in course in CRFE. 
The output format is identical to the input formats and will be called SHARP level 2S with the corrected 
radiometry replacing the TOA measurements and comments of the applied processing in the Radiometric 
Ancillary Record of the Leader File. 255 characters (bytes 893 to 1147) are reserved to c omments the correction 
applied: the climatic model used and the emissivity used for channel 3. 
6.3 LST generation 
Over land, the LST can be obtained using either of the two competitive methods: 
• One uses the 11 micrometer channel by inverting LOWTRAN 7 with the knowledge of the emissivity at this 
wavelength. 
• The other uses the Ottle and Vidal-Madjar, (1992) split window method where the coeffi ci en ts are dependent 
on angle, emissivity and type of atmosphere (polar, temperate, tropical) as previously described in the atmos 
pheric corrections section. The user has actually the possibility to chose its own split window coefficients. 
This possibility will be frozen for operational use. 
The emissivity to be entered is the channel 4 channel 5 average emissivity. To effectively obtain the LST in an 
operational scheme, the average emissivity shall be retrieved from die data set itself. It means that further 
investigation are needed to derive this emissivity, one being to find potential relations between the emissivity in 
these channels and the reflectance in the visible channe ls 
The output format will be put in the SHARP level 2B where, over land, the LST will be put in channel 5 with 
comments on the applied processing in the Radiometric Ancillary Record of the Leader File.