The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
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Figure 9: Medusa simulator concept 
Effect of changing focal length due to temp. diff. on direct georeferencing 
Figure 10: Effect of changing focal length on direct 
georeferencing. 
This indicates that a thorough self-calibration approach is 
necessary when metrically correct products need to be derived 
from the Medusa instrument. 
The in-flight calibration will be based on self-calibration by 
means of block bundle adjustment. The DGAP block bundle 
adjustment software (http://www.ifp.uni-stuttgart.de/ 
publications/software/openbundle/index.en.html) will be used 
for this. A predefined flight pattern will be flown over a fixed 
study area with sub centimetre accurately measured GCPs. 
Figure 11 illustrates a minimally required flight pattern 
resulting in maximally overlapping imagery with flight over the 
same area in opposite directions. 
Because of the expected temperature effects, a specific flight 
pattern, oriented relative to the sun’s positions is needed. The 
data will be processed in three independent blocks, illustrated as 
numbered arrows in Figure 11. During the flights within block 
one, the temperature gradient within the instrument will remain 
relatively constant. For flights in block two and three, 
temperature gradients will change direction. The proposed 
flight pattern will allow to examine if the foreseen temperature 
gradients will have a significant effect on the optical 
characteristics of the system. 
Figure 11: Minimal required flight pattern for Medusa 
instrument calibration 
Parameters that will be estimated by block bundle adjustment 
are: 
• Physical models 
o Exterior orientation (6 parameters) 
o Interior orientation 
■ Principal point offset (2 
parameters) 
■ Focal length offset (1 parameter) 
o Radial-symmetric distortions (3 parameters) 
o Radial-asymmetric and tangential 
distortions (2 parameters) 
o Affinity and shear in the image plane (2 
parameters) 
• Mathematical models 
o Ebner (12 parameters) 
o Griin (44 parameters) 
Because of the correlations that may exist between some 
parameters (e.g. Platform altitude versus focal length offset), 
the adjustment will be run piecewise. 
For modem geometric frame cameras, it can be expected that 
the parameters of the mathematical models are not significant: 
these parameters are not correlated with the interior and exterior 
orientation of the imaging system and are used to correct 
distortions in the plane caused by for example deformations of 
analog film products. However, usage of an optical mirror in 
the Medusa instrument might introduce similar deformations 
and will therefore be examined with care. 
5. CONCLUSION AND OUTLOOK 
A specific calibration strategy is proposed for the Medusa 
instrument. Because of the technical constraints of the 
instrument and its specific operation environment, in-flight 
calibration was selected. On the one hand, the instrument 
comprises an optical mirror, which could introduce planar