IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India,2002 
  
considered as options along with asbestos sheets. The 
requirement of modular design of targets was an important 
aspect as it was envisaged to re-deploy and use the same 
material for different design requirements. Handling in high 
wind conditions and maintenance of the targets were other 
problems envisaged on the site. Due to this reason, cloth and 
tarpaulene based targets were rejected and asbestos sheets of 8ft 
x 4ft dimension and 6 mm thickness were finalized as basic unit 
in the construction of large size targets of 50m x50m. This 
basic size of asbestos sheet was found very convenient from the 
point of arranging them in matrix pattern on the calibration site. 
Another very important use of the asbestos material was its all 
weather proof quality and easy application on the surface to 
realize different reflectance. As far as realizing targets of 
different reflectance was concerned, different choices were 
considered for painting the top surfaces of asbestos sheets after 
deployment at their identified locations as shown in Figure-1. 
Cement and oil based paints of different types were attempted 
and large number of samples of asbestos sheets coated with 
various shades ranging from deep black to white with matte 
finish were prepared and spectral reflectance measurements 
were carried out using Field SPEC Pro spectro-radiometer. 
Exercises were also carried out to test the durability and 
adhesion of these paints as these sheets are planned to be kept 
on the calibration site all throughout for almost six months. 
Based on the durability and other operational considerations, 
  
Figure 2. High resolution image of CHHARODI calibration site 
6. MEASURING INSTRUMENTS FOR VICARIOUS 
CALIBRATION 
Reflectance based vicarious calibration approach for absolute 
calibration of spacecraft sensors requires measurement of 
reflectance of artificial targets/bare soil and atmospheric 
parameters over calibration site during satellite pass. These 
parameters are given as inputs in SATCOR software to 
compute top-of-atmosphere corrected radiance and calibration 
coefficients of sensors along with path radiance, total down- 
welling irradiance etc. as intermediate outputs. The atmospheric 
parameters are also used to compute intrinsic target reflectance 
for comparison with measured reflectance of targets. Field 
Spec-Pro Spectro-radiometer has been used for reflectance and 
total down-welling irradiance measurements. It is a field proven 
spectro-radiometer procured from M/s Analytical Spectral 
Devices, USA. It measures the in-coming radiation in the 
350nm to 1050nm spectral range with a 1.4nm spectral 
  
special oil based paints of three types were finalized for 
application on the site. Deployment of these sheets was a very 
difficult task and prior to laying them on the site, theodolite 
measurements were taken with laser ranging to finalize the 
location thereby deciding relative distances between targets and 
their orientations. The reflectance finalized for three types of 
painted asbestos targets are 4.7%, 38% and 45%. Bare soil of 
17% reflectance as shown at the identified location in figure-1 
has been used as fourth target. Figure-1 shows four square 
patches also on the four corners of the calibration site. These 
are RCC constructed patches of 6mx6m dimension and are 
painted to give 88% reflectance. These four patches are 
accurately constructed reference points serving as Ground 
control points(GCPs) and have been very useful in finding out 
geometric accuracy of the satellite image. The modular concept 
of artificial targets evolved during the development of this 
methodology has been very useful and convenient to perceive 
and design any type of target for sensor parameter evaluation 
by simply rearranging them as per design requirements. This 
has been demonstrated recently by constructing targets of 
different types using 4ftx8ft asbestos sheet material for 
evaluating 1 mtr. PANCHROMATIC sensor image as shown in 
Figure-2. These evaluation targets have been designed for 
MTF, SWR, ERE parameter determination. Figure-3 shows 
some of the artificial targets in actual deployed condition on 
CHHARODI calibration site. 
  
  
Figure 3. Artificial targets deployed on the site 
resolution. The data is collected through fibre optic cable on 
note book PC. Field Spec Pro spectro-radiometer has also been 
used successfully to derive total optical depth of the atmosphere 
by Langley plot technique. Prior to plot, since ASD data is in 
1.4nm resolution, the data is processed suitably to take into 
account filter response of IRS sensors. 
Microtops Sun photometer is a factory calibrated instrument to 
record aerosol optical depth of the atmosphere in five narrow 
spectral bands through 2.5 degree FOV. The instrument has 
five optical filters of 10nm bandwidth with center wavelength 
at 380,440,500,675 and 870nm. Instantaneous aerosol optical 
depth is displayed on LCD within 10 seconds after pointing to 
the sun. Externally supplied parameters to the instruments are : 
lattitude/longitude of place of observation, altitude, atmospheric 
pressure. Based on the response of the IRS spectral bands, 
AOD values are interpolated through measured data. Ozono 
    
    
   
    
  
  
     
  
   
   
   
    
   
  
  
  
  
  
    
   
   
   
   
  
£50 'O £g ed MO OL 709 (€ CTO 14) Tt ot nM 
Sel 
se 
co 
Af 
foi 
gth 
Nc 
the 
tha