IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring", Hyderabad, India,2002 
  
and could be compared with the satellite measured radiation Ls' 
given by the following expression : 
Ly = C ( Lux = Linin ) / Conax + Luin (4) 
In the above equation, C is the count as recorded by the sensor 
for the target. Cmax is the maximum count corresponding to 
maximum radiance Lmax ANd Lmin iS the minimum radiance as 
determined from the Light transfer characteristics measured for 
the sensor in the laboratory before satellite launch. 
Transmittance of the atmosphere T is dependent on the 
attenuation produced by intervening molecules and particulates 
present in the atmosphere : 
Ts exp(-1) (5) 
The total optical depth t is comprised of three components 
given by : 
= Tm+ Tat Ty (6) 
where, Tm, T, and T, denote optical thicknesses due to 
molecular scattering, molecular absorption and aerosol (Mie) 
scattering respectively. Though the absorption by ozone and 
water molecules are very less yet they have been accounted in 
present work. Total optical depth of the atmosphere T is a 
measurable quantity and is determined by Langley plot 
technique. In this technique, a series of measurements of solar 
irradiance in narrow FOV and falling on the earth’s surface is 
taken against different solar zenith angle. Once t4, t, and 1 are 
known, aerosol optical depth t, can be computed by Equation- 
6. Since aerosol optical depth is spectrally varying, spectral 
response of the sensor is used in the determination of this 
parameter. Computation of path radiance L, and total down- 
welling irradiance — E, , as mentioned in Equation-3, is 
essentially based on the simplified theory of radiative transfer 
given by Turner and Spencer(1972) which involves single 
scattering phase functions and single scattering albedo. 
Based on the algorithm as discussed above, a software package 
named ‘SATCOR’ has been designed and developed in SAC in 
collaboration with SPL/VSSC. The software handles IRS data 
as input along with other parameters and displays corrected 
radiance and reflectance image on SGI display system. IRS 
LISS/PANCHROMATIC data is downloaded through 8mm 
DAT/CD in a band sequential form on the display system to 
extract the 512x512 image which is used as input file in the 
correction of gray count for atmospheric effects. Synchronous 
data of target reflectance, total optical depth, aerosol optical 
depth, ozone content and water content are given as input in 
the command procedure along with other information for data 
correction. Extraterrestrial solar irradiance values for each 
spectral band is computed based on the band width and center 
wavelength. Window size in terms of number of scan line/pixel 
surrounding the calibration target is also given as input to 
  
compute background reflectance. Based on the input provided, 
the program computes optical depths, path radiance total down- 
welling irradiance, computed target reflectance, target radiance 
at the top of atmosphere, sensor calibration coefficients etc. and 
generates corrected 512x512 radiance and reflectance image 
file for each band automatically for display and false color 
image generation. SATCOR is working in an integrated mode 
and efforts have been initiated to develop it into a GUI based 
user friendly package. 
5. DEVELOPMENT OF CHHARODI CALIBRATION 
SITE 
The reflectance based approach requires calibration targets 
which are large and uniform in size so that sufficient number 
are pixels are available for reflectance measurement and 
radiance computation. Various requirements were considered 
before the selection of calibration site near SAC campus. Based 
on the requirements, following criteria were adopted for the 
selection of calibration site : 
Large and homogeneous targets of different reflectance 
Stability of reflectance of targets with time 
Knowledge of accurate dimension of targets 
Flat terrain and cloud free region during large periods 
Accurate identification of pixels in the satellite image 
Accessible for conducting synchronous experiments as and 
when required with satellite pass 
A large number of survey was conducted in and around 
Ahmedabad to locate areas satisfying above mentioned criteria. 
Out of many areas selected during survey, NorthCot cattle farm 
located about 30 kms. from SAC campus in the west was found 
satisfying most of the conditions as mentioned above and was 
found very convenient for carrying out controlled synchronous 
experiments with satellite pass. This farm is located in the 
Gujarat Agricultural University campus at CHHARODI village, 
Sanand in Ahmedabad district free from pollution and remains 
cloud free for about 7096 of the time in a year. It was very 
difficult to find natural targets of different reflectance in the 
campus which were static in nature. Two years satellite data 
analysis revealed large spatial as well as temporal variations for 
the features existing in the campus which is contrary to the 
requirements of the reflectance based calibration approach. For 
this approach, calibration targets must be static in nature and 
must not have spatial variations more than 2 to 3%. Further, it 
is preferable to have targets which have uniform reflectance 
over complete spectral range in the visible and NIR region of 
e.m. spectrum. Hence, this exercise led to the development of a 
controlled artificial calibration site named as ‘CHHARODI’ 
of 300m x 300m size for multi point calibration of high 
resolution remote sensing sensors.  Figure-l shows design 
concept of the artificial calibration site. 
    
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