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The system is highly sensitive to errors due to miscalibration. Three
measurements are made every time a pair altitude/GP is obtained:
- An image is acquired.
- An angle is obtained through a digital inclinometer.
- Greenwich Mean Time is stored.
Measument of time is precise and can be easily calibrated. We are
using a digital inclinometer with simple re-calibration procedure, but limited
to 
degrees of precision. Currently the precision of our system
is limited by the inclinometer precision.
Finally, camera calibration is necessary for minimizing errors in the
detection of Sun altitude.
We calibrated our camera/lens apparatus using a calibration method proposed
by Robert [10]. The method requires a series of measurements
of a target object. The dimensions of the target object must be precisely
known. The user starts the calibration process by indicating correspondences
in the calibration image and the real target object. The method generates
the perspective projection matrix [4], which contains all the
relevant information about the imaging system.
Since we work with huge zoom lenses, our images are essentially
ruled by orthographic projection. In fact, our perspective projection
matrix is (up to a constant factor):
Numeric determination of intrinsic parameters using a perspective model
is highly unstable due to ill-conditioning of this matrix. Instead we
set the first three elements of the last column to zero; by so doing
we obtained an orthographic projection matrix. Obtaining the image center and
the magnification factor is immediate. In our case (units in pixels):
