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CALIBRATION STRATEGY FOR THE MEDUSA CAMERA SYSTEM 
K. Nackaerts *, J. Everaerts, R. Choi, B. Delauré, T. Van Achteren, J. Biesemans, 
Vito/TAP, Flemish Institute for Technological Research/Remote Sensing Research Unit, Boeretang 200, BE-2400 
Belgium - (kris.nackaerts;jurgen.everaerts;bavo.delaure;tanja.vanachteren;jan.biesemans)@vito.be 
Commission I, WG 1/4 
KEY WORDS: Optical, Sensor, Calibration, Camera, Bundle, High Resolution 
ABSTRACT: 
The MEDUSA camera system is a high resolution earth observation instrument designed to operate from a long endurance UAV 
flying at stratospheric altitudes. Due to the technical constraints imposed on the instrument regarding mass, power and dimensions, 
the MEDUSA instrument shall experience large temperature variations induced by its varying environment in which it is operated. 
The induced physical changes of the optical system (lenses and opto-mechanics) imply that the parameters of the geometric camera 
model of the MEDUSA instrument cannot be assumed to be constant. Generating geometric “correct” images under those 
circumstances requires a calibration procedure which is able to respond to this dynamic behaviour. 
This paper presents the calibration strategy for the MEDUSA instrument which is based on a full in-flight self-calibration with block 
bundle adjustment. A theoretical estimate of the geometrical accuracy of MEDUSA images has been explored. Based on this 
approach a first sensitivity check to certain temperature variations within the optical system has been addressed. A more detailed 
study has started making use of a refined image simulator based on the collinearity equations, a camera model and image distortion 
models. First preliminary results of this approach show the geometric accuracy over the complete image within different temperature 
windows for a camera model with constant parameters. 
1. INTRODUCTION 
The PEGASUS (Policy support for European Government by 
Acquisition of information from Satellite and UAV-bome 
Sensors) project aims to fill in the gaps that conventional Earth 
observation cannot provide in terms of spatial and temporal 
aspects (Everaerts, 2004). It uses a High Altitude Long 
Endurance (HALE) Unmanned Aerial Vehicle (UAV), called 
Mercator-1, which will ultimately fly at stratospheric altitudes 
persistently for weeks and even months. To do this, the platform 
is powered by solar energy during daytime and high capacity 
batteries during the night. The airplane design is largely driven 
by mass minimization where the use of lightweight composite 
material allowed to realize a total mass of about 30 kg. In order 
to accomplish disaster monitoring and large-scale mapping as 
its first phase target applications for the Pegasus project, a high 
resolution camera system, MEDUSA (Monitoring Equipment 
and Devices for Unmanned Systems at high Altitude), is being 
developed by a consortium led by VITO (Delauré 2007, Van 
Achteren 2006, 2007). 
The MEDUSA camera system is designed to deliver imagery in 
the visual spectrum with a spatial resolution of 30 cm from an 
altitude of 18km and covers a swath of 3 km. 
The technical constraints which the MEDUSA instrument is 
facing, are severe due to the restricted payload capacity of 
Mercator-1: no more than 2.5 kg total system mass, fitting 
within a horizontally oriented cylinder of 12 cm diameter and 1 
m in length, consuming less than 50 W of electrical power. 
Apart from those limitations the environmental conditions of 
the stratosphere are an important factor to be taken into account 
in the instrument development process. Since the variation of 
the environmental parameters induce physical changes of the 
optical system, the interior parameters (e.g. focal length, 
principal point coordinates, etc) of the optical system are 
expected to vary accordingly. Calibration of MEDUSA needs to 
take the consequences of these constraints into account. In this 
paper, we discuss the geometric calibration strategy. 
2. MEDUSA INSTRUMENT 
2.1 Top-level system requirements 
The MEDUSA instrument is designed for large scale mapping 
and disaster monitoring. The top-level system requirements of 
the MEDUSA camera are summarized in Table 2-1. 
Ground resolution 
30 cm (@ 18 km ) or less 
Spectral range 
400 - 650 nm (RGB) 
Swath width 
3000 m (>= 10 000 pixels) 
SNR 
100 @ 8:00 am equinox 
Sensor type 
Frame 
Shutter 
Electronic 
Forward overlap 
60% max 
RF downlink range 
150 km from the ground 
station 
Table 2-1 MEDUSA system requirements 
* Corresponding author.