A separate. computer program is run to compute the atmospheric 
corrections. Output of this program is subsequently used in the calibration 
subroutine of the data analysis program. 
Data Input for Atmospheric Correction 
  
Preparatory to running of the atmospheric correction program, the 
distribution of several gaseous attenuators and water vapour must be quantified 
over the atmospheric path length. Water vapour accounts for most of the 
attenuation; it is also the most variable component. Other lesser attenuators 
(carbon dioxide, nitrogen, ozone, etc.) can be assumed to have a relatively 
slowly changing global distribution. Therefore, the distribution of gaseous 
attenuators can be reasonably defined by use of standard model atmospheres, but 
realistic estimates of water vapour distribution through the atmosphere require 
local radiosonde (RAOB) data. 
Pressure, temperature and humidity data from RAOB ascents are input at 
the beginning of the program.  RAOBs from stations nearest to the water body 
and closest in time to the satellite pass are selected. For large water bodies 
PAOBs from several stations may be input; the program merges the data to 
produce an "average" temperature/humidity profile for the atmosphere over the 
survey area. 
Usually the lowest RAOB levels are adjusted prior to input to account 
for: a) average surface air conditions in the survey area at the time of the 
satellite pass, as indicated by hourly observations at stations in the area, 
and b) modification of the air mass by the water body itself. Further 
adjustments are required if the RAOB ascents penetrate cloudy layers, indicated 
by spikes in the humidity profile. Since surface temperatures can be evaluated 
only for cloud-free areas, the cloudy layers are eliminated from the RAOB data 
by smoothing out the humidity spikes. 
Atmospheric Transmittance Model 
  
The program uses the LOWTRAN 3B transmittance model developed by Selby et 
al. (1976). The model is versatile in that atmospheric transmittance can be 
calculated over a range of 0.25 to 28.5 um, for vertical, horizontal and slant 
paths through the atmosphere, and for aircraft as well as satellite heights. 
The model contains transmittance functions for water vapour, uniformly mixed 
gases and ozone, and absorption coefficients for the gaseous elements and 
aerosols. Model atmospheres for water vapour and ozone (six), aerosols (five) 
and haze (two) are also provided. 
Program Functions 
The radiation intensity detected by a sensor through a spectral band-pass 
filter can be expressed as a function of: a) radiation emitted and reflected by 
the target, b) attenuation and emission by the intervening atmosphere, and c) 
transmittance efficiency of the filter/optics system. 
Starting with a given pressure/temperature/humidity profile (from RAOBS) 
and a given distribution of gaseous constituents and aerosols (from LOWTRAN 
model atmospheres), the program uses the radiative transfer equation to 
calculate the upward intensity of radiation transmitted progressively through 
10-millibar layers of the atmosphere, and finally, through the filter/optics 
systems. Contributions from the different terms are evaluated for small wave- 
number intervals and then integrated over the filter band-pass range. 
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