The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
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2.2 Environmental conditions 
The lower stratosphere’s environment is characterized by low 
air pressure (down to 60 mbar) and low relative humidity. The 
ambient temperature is low and varies around -55°C with a 3 
sigma deviation going up to 15°C (based on averaged 
temperature-data acquired above Belgium over a 20 year period. 
Source: the Royal Metrological Institute of Belgium). Apart 
from the fluctuating air temperature, the temperature within the 
instrument is dominated by its two main heat sources: sun 
(external) and electronic power dissipation (internal). As the 
MEDUSA camera system is not screened by the air plane it 
experiences a strong temperature variation induced by the 
relative orientation of the sun with respect to the instrument. 
This is shown in Figure 1. After a sharp rise in temperature at 
start-up in the morning, the electronics modules of the 
instrument produce a constant heat input. 
Figure 1: temperature variation of the MEDUSA system during 
one day (21 st June above Belgium) at various positions within 
the MEDUSA instrument. 
The following effects occur: 
• Temperature variation during day-time 
• Temperature variation depending on day in the operational 
season 
• Thermal gradient along the axis (nose is coldest, back side 
warmest due to heat dissipation of the electronics) 
• Thermal gradient top-down due to the different heat input 
(sun versus earth) 
2.3 MEDUSA Camera System Design 
The MEDUSA instrument is plugged onto the front side of the 
fuselage of the Mercator-1 UAV. To comply with the frontal 
area constraints of Mercator, the MEDUSA instrument is 
housed in a horizontally oriented cylindrical volume with a 
diameter of 12 cm and a length of about 1 m. The MEDUSA 
instrument, of which more details are described in (Delaure 
2007, Van Achteren 2006, 2007), consists of an optics and an 
electronics compartment Figure 2 
Figure 2 shows the layout of the MEDUSA instrument. The 
former is located at the front of the instrument. It houses the 
optical system which consists of a folding mirror (to cope with 
the horizontal orientation of the instrument), a set of lenses and 
the focal plane assembly containing two CMOS sensors (one 
panchromatic and one colour) of 10000x1200 pixels each. The 
focussing elements of the optical system are fixed together in a 
metal structure (4-lens tube) which is mounted in the Carbon 
Fibre housing. 
The electronics compartment is located at the backside of the 
instrument. It houses the command and data handling unit 
(responsible for on-board data processing and housekeeping 
within the instrument), and a lightweight IMU and dual 
frequency GPS-system. Via an S-band transmitter and antenna 
JPEG2000 compressed image data is being transferred at a rate 
of 20 Mbps to the ground control station. The instrument is 
designed to generate a forward overlap of 70% taking into 
account the forward speed and attitude variations of the aircraft. 
This is realized at a frame rate of 0.7 fps. 
Due to the large temperature variations to which the MEDUSA 
instrument is subjected and the temperature sensitivity of the 
optical system (focal shift of 20pm/°C), a passive thermal 
compensation system is implemented. This compensator rod, 
shown in Figure 2, adjusts the distance between the 4-lens tube 
and sensor plane, driven by the temperature within the optics 
compartment. This way the optical system is kept in focus 
continuously. 
Apart from the temperature sensitivity the optical system is also 
pressure dependent. A focal shift of about 300pm takes place 
when reducing the pressure from ground-level atmospheric 
conditions to stratospheric pressure (60 mbar). For this reason a 
mechanical adjustment needs to be performed on-ground 
depending on the expected operational in-flight pressure regime. 
Mirror Entrance window Passive compensator 
4 lens tube 
baffles 
Figure 2: Layout of the Medusa instrument 
3. GEOMETRIC CALIBRATION CONSIDERATIONS 
Due to the specific design and operational environment of the 
MEDUSA system, the geometric calibration of the MEDUSA 
instrument is not straightforward. 
Since the MEDUSA instrument faces strong variations of its 
environment during its operational cycle, the optical system 
undergoes physical changes. For this reason the camera 
parameters describing the MEDUSA imaging system cannot be 
considered to be constant during the operational window. First 
analyses of the current design indicate the following 
dependencies: 
• Focal length variation (due to change of refractive 
index and lens surfaces) 20 pm/°C 
• Due to the fact that the fixation of the 4-lens tube to 
the carbon fibre structure is not athermal, the 
principal point can shift with about 22 pm over a 
temperature range of 50°C 
First simulations show no significant temperature dependence 
on the radial and tangential lens distortions.