International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004 
  
simultaneously (Fig. 6). This great proximity in time makes 
the reconstruction of the continuous image line insensitive to 
the parallax effects introduced by the relief, and by temporal 
attitude variations between the 2 acquisitions of the same 
point on the ground (by 2 adjacent linear arrays). 
Thanks to a separation mirror, the XS and PAN viewing 
planes are separated only by 1.5 mrad in the field, which 
makes PAN and Multispectral channels registration possible 
by a rather simple ground processing (re-sampling). 
  
Theoritical Panchro and Multispectral chanels focal plane 
       
    
  
     
Multispe ectors illuminated 
p 
: PAN detectors illuminated 
inreflection [numbers 2 and 4) 
A in transmission (numbers 1.3 and 5) 
  
  
  
  
  
Mutispectral divoli mirror 
  
! 
Panchro detectors illum inat ed 
in transmission (numbers 1.3 and 5) 
FAN divoli mirror 
Multispectra! detectors iliuminated 
intrans mission (numbers 1, 3 and 5) 
  
  
  
    
   
   
   
   
  
   
    
   
  
     
     
    
   
   
   
     
   
Figure 6 : PAN and Multispectral bands linking and 
separation principle 
4.4 Payload Data Handling and Transmission 
The video data are output from the instrument at 4.5 Gbits/s 
total output rate, are compressed in the Payload Data 
Compression Unit. A wavelets transform algorithm is used, 
that enables the compression ratio to go up to 7, while in 
standard operation the ratio is 5. 
The compressed data are then memorized in the Solid State 
Mass Memory (SSMM). This memory has a storage 
capacity of 600 Gbits. The maximum image data input rate 
is 1.5 Gbits/s. The output rate is nominally of 465 Mbits/s, 
on three individual channels of 155 Mbits/s each. 
The data are then coded following a trellis-coded scheme in 
8-PSK type modulators coupled to Traveling Waves Tube 
power Amplifiers (TWTA). They are multiplexed and down- 
linked with an omni-directional 64? aperture horn antenna. 
Pleiades restored image (D=0.65m) 
These high storage capacity and high transmission rate allow 
high reactivity of the Pléiades system with few ground image 
receiving stations. Coupled with high agility, a great amount 
of user's requests can be satisfied within one day due to 24 
hours reprogramming capabilities offered by centralized data 
collection. 
4.5 Attitude and Orbit Determination 
In order to reach very high level of ground location 
accuracy, i.e. 10 m for 90 % probability ground circular 
error without ground control points (GCP), new very high 
precision technological developments have been taken into 
account for Pléiades HR satellite attitude restitution. 
The autonomous orbit determination is performed by a Doris 
receiver, which allows reaching an accuracy of about 1m (on 
the three axes). 
The attitude determination is performed by a gyro-stellar 
system. Very accurate solid state gyroscopes are used to 
ensure high accuracy attitude determination while 
maneuvering. Fiber Optic Gyros (FOG) allow high 
performances, such as a scale factor stability of a few ppm, a 
random drift of 0.002 deg/h, and an angular random walk of 
0.0002 deg/root-hour. Both star trackers and inertial 
measurement unit have separated optical heads and 
electronic units. The optical heads are placed onto the 
instrument structure to minimize the thermal distortion with 
respect to the instrument line of sight 
4.6 Ground Image Processing 
Radiometric quality of the images results from the 
combination of the instrument performances with optimized 
image processing which can remove many defects (noise, 
blurring, ..) 
Ground image quality enhancement relies on restoration 
process. Due to the medium performances of the instrument, 
raw images down-linked from board are relatively “blurred” 
at Nyquist sampling frequency (fe/2), that is for 0.7 m 
resolution (ground sampling frequency at nadir). The 
simulated pictures (Fig 7) show how image restoration can 
improve the raw image quality: 
To make it sharper, restoration process raises the high 
frequencies in the image. In the Fourier domain, this 
corresponds to multiplying the image spectrum by the inverse 
of the MTF, to aim for a frequency response of 1 over the 
interval [-fe/2, fe/2]. In fact, a more sophisticated 
  
   
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