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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