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A PSEUDO-5S CODE FOR THE ATMOSPHERIC CORRECTION OF 
ATSR-2 VISIBLE AND NEAR-INFRARED LAND-SURFACE DATA 
G. MACKAY, M.D. STEVEN, J.A. CLARK* 
Department of Geography, University of Nottingham, University Park, Nottingham, 
NG7 2RD, UK. 
*Department of Physiology and Environmental Science, University of Nottingham, 
Sutton Bonington Campus, Sutton Bonington, Loughborough LE12 5RD, UK. 
ABSTRACT 
A pseudo-5S code is described. The code has been developed for use as one component of an 
operational atmospheric correction procedure for the visible and near-infrared land-surface 
data from the ATSR-2 sensor. The code has been rationalized from the original 5S code, which 
computes the apparent reflectance assuming uniform Lambertian surface reflectance, and 
inverted to retrieve the surface reflectance from the apparent reflectance. For operational 
purposes, the aim has been to reduce computation time without compromising the accuracy of 
the result. Extending the code to include cases of heterogeneous surface reflectance, as provided 
in the 5S code using an environmental weighting function, has not been considered as an option 
in the pseudo-code. Spectral integration using the detector spectral response functions for the 
ATSR-2 channels provides pre-computed bandpass integrated parameters specific to standard 
atmospheric models and to characteristic aerosol types used within the code. Comparison of 
the pseudo-code and 5S indicates that the run time using the pseudo-code is reduced by a 
factor in excess of 200 from 5S with less than l.0% average difference in the derived values 
of apparent reflectance; the operational code is approximately 100 lines in length. 
KEYWORDS : ATSR-2, Atmospheric correction code, 5S 
1 INTRODUCTION 
The ERS-2 remote sensing satellite, scheduled for launch in late 1994, is intended to succeed 
ERS-1, ESA’s first remote sensing satellite. The main improvement is that the ATSR-2 carries 
three additional channels in the visible and near-infrared: VI (0.555pm), V2 (0.659pm) and V3 
(0.865 pm), which complement the existing ATSR-1 lb channel (1.61pm) operating in the solar 
reflecting spectrum. The intention of these additional channels on the ATSR-2 instrument is 
to provide a sensor system for global vegetation monitoring which can combine the glo 
bal-coverage of an AVHRR-type instrument with the spectral coverage of a Thematic 
Mapper-type instrument. The ATSR-2 radiometer will provide better spatial resolution than 
AVHRR, with a pixel resolution at nadir of lkm, and narrower spectral bands than the AVHRR 
or the Thematic Mapper: VI (0.545-0.565 pm), V2 (0.649-0.669 pm), V3 (0.855-0.875 pm) and lb 
(1.58-1.64 pm). 
An additional feature of the ATSR is that the conical scan of the sensor system provides two 
view angles; any position that is scanned on the forward view at the leading edge of the ’detector 
cone’ is scanned again on the nadir view at the trailing edge of the cone, providing an along 
track swath with view zenith in the range 52.7° to 55.1°, in addition to a nadir swath with view 
zenith in the range 0° to 21.8°, of approximately 500km width. This unique capability to 
monitor the radiances in the along-track view as well as at nadir can be exploited for the 
atmospheric correction, as well as providing information on vegetation reflectance at an additional 
angle; if these two effects can be separated.