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“International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004 
Worldwide coverage and a daily accessibility to any point 
on the globe are requested, and fulfilled by the use of two 
satellites simultaneously in orbit with a 180? phase shift. 
4.2 Satellite architecture 
The main design drivers for the satellite architecture (Fig. 3) 
are the image quality, the agility and the image location 
accuracy. The image quality drives the instrument size. A 
high agility requires a very compact design, with a few stiff 
appendages. As a consequence, the instrument is integrated 
inside the bus. A high image location accuracy is achieved 
by minimizing the interface between the instrument and the 
bus. The star trackers and the gyroscope heads are directly 
supported by the instrument to avoid any thermal distortion 
that could be induced by the bus. 
  
Figure 3 : Satellite In-Flight Configuration 
The bus structure is build on an hexagonal shape, with three 
solar arrays at 120 deg, and three star trackers in a quasi 
tetrahedron — configuration, optimizing the attitude 
determination accuracy. This configuration authorizes an 
easy accommodation of the instrument focal plane radiator 
for maximum heat dissipation (Fig. 4). 
    
Instrument 
Bus 
Figure 4 : Instrument accommodation inside the bus 
An antenna support structure is used to carry the Earth- 
pointing antennas and for the instrument baffle. 
The solar arrays are mounted directly on the bus structure 
without any drive mechanism to ensure a maximum 
stability. Their first flexion mode frequency is increased by 
the use of stiffeners when deployed. 
Moreover the low mass («1000kg) of this very compact 
satellite make it compatible with a large series of low cost 
small launchers (among them Rockot, Soyouz, PSV.) 
4.3 Instrument 
The optical solution chosen for the telescope is a Korsch 
type combination (Fig. 5). The imaging geometry 
optimization induces a primary mirror size of 650 mm 
diameter. 
Carbon-Carbon cylinder 
   
  
  
  
    
  
Carbon-cyanat 
optical bench 
STR 
  
  
    
Highly Integrated 
Detection Unit with its radiator 
FOG Inertial Measurement Unit 
(optical core) 
Pläne mirror 
Figure 5: Instrument Configuration 
The instrument includes a focus function based upon a 
specific finely regulated thermal control of secondary mirror 
structural support. 
The instrument includes also an internal shutter to protect it 
from the sun radiation in non-operational phases such as 
launch, attitude acquisition, or safe modes. This solution 
avoids an external shutter that is generally heavy and 
complex. 
TDI (Time Delay Integration) detectors are used for 
panchromatic detection, with a maximum of 50 integration 
lines. They can be used thanks to an optimized guidance 
strategy of the satellite line of sight including micro- 
vibrations levels minimization, specific geometrical 
accommodation of detector lines in the focal plane to 
minimize optical distortion effects. Five detectors of 6000 
pixels each are used; each pixel having a size of 13 um. A 
lateral anti-blooming structure located in the imaging area of 
the detector array prevents from light spreading along the 
columns. 
The multi-spectral detection is realized with 5 detectors, 
each containing 1500 pixels (13 um size). Each detector set 
consists in a four lines assembly, enabling four colors 
imaging (blue, green, red, near infrared). Interferometric 
filters directly stickled down on the detector glass window 
provide spectral separation of these four channels. 
The focal plane is constituted by two symmetrical 
arrangements of those detectors. To acquire images over a 
field of view of 20 km, each line of sight is composed by 5 
consecutive linear arrays; generating images of 30000 
columns in the Panchromatic channel and 7500 columns in 
the multispectral channel. Among 5 linear arrays of each 
retina, 2 operate by reflection and 3 by transmission across a 
beam splitter mirror device (Divoli) which allows all the 
points in the field of view to be acquired almost