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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part BI. Istanbul 2004
Figure 4: Camera electronics
Storage: Thanks to two adding up Solid State Data Recorder
(SSDR) based on Power-PC processor and totalising 1Gbytes
data and one redundant unit of 128Mbytes based on
StrongARM (SA 1100) and acting as a backup unit.
Each bank is linked to the three storage units via a synchronous
serial input streams at nominal 20Mbit/s allowing a flexible
satellite operation.
3. QUALIFICATION TESTS
For Alsat-1 mission, the storage units and the camera represent
new modules (except SA1100 which was flown onboard a
SNAP nanosatellite). Therefore, they follow all, several
qualification tests (vibration, environmental test, thermal
vacuum test, electromagnetic compatibility, etc.).
3.1 Imager Alignment tests
Because of their relative complexity with its six independents
channels, the imager on Alsat-1 was the critical subsystem. It
has a high demand on the optical and opto-mechanical
alignment accuracies. Hence, two types of alignment were
applied:
Interchannel alignment: The central channel of the bank (red
filter) was selected as reference then two by two alignments
carried out with the two other channels. The goal was to have
the three lines sensor parallels within an acceptable margin. A
complex mechanical adjustment was done.
Interbank Alignment: the same central channel was selected
for both bank then an interbank alignment was achieved so the
two same channels (i.e.: red) will have their sensor line aligned
in continuity to achieve a double line sensor (within an
acceptable margin).
4. ALSAT-1 FIRST RESULTS
We began the commissioning phase just after the launch. The
camera and the SSDR showed their characteristics close to the
nominal values.
330
The first successful image of the Algiers bay was obtained a
week after the beginning of the imaging tests because of the bad
meteorological conditions (it was winter in Algeria).
A first commissioning review one month later showed that the
equipments were reliable (no electric breakdown until now) but
the orbit required a correction (due to a slight injection error of
the satellite). This was achieved throughout six weeks (over
March & April 2003) to pass from the elliptic orbit [684-745
km] to a circular one with a 686km altitude. It did not affect in a
visible way the images quality.
The examination of the 300 images taken during the first year
showed qualities close to those of Landsat. The spectral
characteristics are almost identical on the other hand the images
of Alsat-1 are geometrically less fine (in urban zones) as shown
on the photo 5.
vs
A
taken on 15 Mar. 1993
© LANDSAT data
Alsat-1 image on region of Oran taken on 28 May 2003
© CNTS 2003
Figure 5: Alsat-1 vs. Landsat images
The images also showed the other distortions more classic like:
The visible “vignettage” effects (unequal optical transmission
factor between the centre and the edge of the optics) due to the
exceptional wide swath width. This effect identified during the
pre-flight tests was easily corrected.
The visible "stripping" effects on both bank (more visible on
bank0). This effect is created by the CCD sensor. It would result
from bad "Reset" for pixels pairs (some electrons remains in the
output stage increasing by a constant value the grey levels of