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
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