USING EXOTIC GUIDANCE FOR PLEIADES-HR IMAGE QUALITY CALIBRATION 
L.Lebègue, Ph. Kubik, D. Greslou, F. DeLussy, N. Theret 
Centre National d’Etudes Spatiales, 18 av. Edouard Belin 31401 Toulouse cedex, France 
(laurent.lebegue, philippe.kubik, daniel.greslou, francoise.delussy, nicolas.theret)@cnes.fr 
Commission I, WG 1/1 
KEY WORDS: Calibration, Image Quality, Guidance, Star 
ABSTRACT: 
In-flight Image Quality calibration and performance assessment activities depend on specific acquisitions and, for some of them, on 
dedicated guidance of the satellite platform. The operational constraints may be tedious during the commissioning phase. Moreover, 
the length of the requested data collection may be conditioned by uncontrolled parameters such as climatic hazard. The new French 
high resolution earth observing satellite Pleiades-HR will be launched at the beginning of 2010. A specific design and new 
technologies have been embarked to provide great agility. These capabilities offer new methods to perform the image quality 
activities. Two are described in this paper. The first one concerns the so-called AMETHIST method to compute the normalization 
coefficients of the radiometric model. The second one uses the stars to measure the line-of-sight dynamic stability. 
1. INTRODUCTION 
1.1 Pleiades-HR overview 
PLEIADES is the highest resolution civilian earth observing 
system ever developed in Europe. This imagery program is 
conducted by the French National Space Agency, CNES. It is 
the French part of the French-Italian ORFEO program which 
also comprises COSMO-SkyMed, an Italian high-resolution 
radar system. It will operate in 2010-2011 two agile satellites 
designed to provide optical images to civilian and defence users. 
Images will be simultaneously acquired in Panchromatic (PA) 
and multi-spectral (XS) mode, which allows, in nadir 
acquisition condition, to deliver 20 km wide, false or natural 
coloured scenes with a 70 cm ground sampling distance after 
PA+XS fusion. Coverage will be almost world-wide with a 
revisit interval of 24 h for 2 satellites. 
The Image Quality requirements were defined from users 
studies from the different spatial imaging applications, taking 
into account the trade-off between on-board technological 
complexity and ground processing capabilities. The Pleiades- 
HR satellites will benefit from technology improvements in 
various fields which will allow achieving, at an affordable price, 
performances once reserved to ambitious military spacecrafts. 
The major constraints of weight and agility led to the 
development of a highly compact satellite (about 1 ton weight), 
to minimize the moments of inertia. The instrument is partly 
embedded in a hexagonal shaped bus containing all equipment. 
The attitude control system uses 4 fibre-optic gyroscopes and 3 
star trackers to provide restitution accuracy compatible with the 
system location specification of 12m for 90% of the products. 
These attitude sensors are mechanically fixed on the telescope 
support to minimize the thermo-elastic distortions. The satellite 
orientation is ensured by 4 gyroscopic actuators. 
1.2 Agility 
Agility is a characteristic which allows the satellite to acquire 
off-nadir targets rapidly in a large flight envelope, in order to 
sequence numerous images. This agility is imposed by several 
requirements stated by the users. For instance, a 100x100 km2 
zone can be acquired by the satellite from the same orbit thanks 
to a lateral multi-band coverage. As for stereoscopic capacities, 
3 images from the same zone can be acquired in a single pass 
with B/H lying between 0.1 and 0.5. For multi-targets, the time 
between the end of an imaging segment and the start of the next 
segment, including stabilization of the line-of-sight is specified 
less than 10 seconds for an excursion of 10° and less than 26 
seconds for an excursion of 60° from nadir viewing. Guidance 
is mainly performed using roll and/or pitch steering (without 
slow motion), but fine yaw steering has to be used to respect 
the principle of acquisition set by the TDI device in the focal 
plane. 
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©cnes —— 
Figure 1 : The Pleiades-HR satellite 
1.3 Image Quality activities 
The assessment of the Image Quality and the calibration 
operation will be performed by CNES Image Quality team 
during the commissioning phase that will follow the satellite