ınbul 2004
jl. 36, No.
directional
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G., 1908.
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pp. 176 —
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ata, /EEE
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119 pp. |
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w. [EEE
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ns T.L,
pact High
ng of SPIE
2003, San
insberg, I.,
ations and
$ MN-160,
Alsat-1: First Results of Multispectral Imager
A. Rachedi * *, N. Hadj-Sahraoui *, A.Brewer"
* Centre National des Techniques Spatiales, BP13, Arzew 31200, Algeria - (rachedia, hadjsahraouin)@cnts.dz
? Surrey Satellite Technology Limited, Guildford, Surrey GU2 7XH, United Kingdom, - a.brewer@sstl.co.uk
ThS 3: Small Satellite for Earth Observation
KEY WORDS: Small/micro satellites, Observations, Camera, Imagery, Sensor, Pushbroom
ABSTRACT:
In November 2002, the 28", at 06:07am GMT, was launched the 1* Algerian satellite from the cosmodrome of Plesetsk in Russia
into a 700 km sun synchronous orbit. Alsat-1 is an enhanced earth observation microsatellite (<100kg), stabilised in 3 axes for image
acquisition mode. It was designed for disaster monitoring and is part of the international constellation dedicated to Disaster
Monitoring (DMC),
Alsat-1 payload is a multispectral imager, with two banks of 3 channels. Each bank works in a push broom mode and covers half of
the whole image size with 5% overlap. The total swath width is 600 km and images are taken at 32m GSD, on three spectral bands
(Green, Red, NIR), identical to those of Landsat-TM. A windowing mode was implemented to add more flexibility to the system and
to prevent the saturation of the three storage units: Two adding up Solid State Data Recorder (SSDR) totalising 1Gbytes data and
one redundant unit of 128MBytes. The image data are sent to the ground station thanks to the high rate transmitters working in S-
band at 8Mbps.
Since Alsat-1 launch, the imager has taken more than 300 images. Their first evaluation showed a comparable quality to those from
the similar satellites (Landsat-TM). The paper presents an overview of the Alsat-1 characteristics and shows the first results of the
multispectral imager.
1. ALSAT-1 MISSION
Alsat-1 is an earth observation satellite which evolves in a sun-
synchronous retrograde circular orbit. Designed to be part of a
constellation for daily disaster monitoring, Alsat-1 is equipped
with two banks of cameras giving a total of 600km field of view
at 32 meters ground sampling distance in three spectral bands:
Red, Green and Near Infra-Red. This field of view allows the
constellation to cover the whole earth within 24 hours. In
absence of disaster, Alsat-1 is dedicated for Algerian purposes:
mainly for remote sensing applications
2. SATELLITE DESCRIPTION
Alsat-1 has a three axis stabilization in imaging mode and
evolves in a BBQ mode out of imaging time. The attitude
determination and control subsystem gives a good attitude
pitch/roll/yaw stability during imaging (=5 m®/s) and the Orbit
filter provides a maximum track error of the scene position of
5km (GPS On during one orbit a day).
The imaging system allows windowing and it is supported by a
total storage capacity of two 0.5Gbytes of data which could be
downloaded to a ground station at 8 Mbps (within
10min/SSDR). The downlink and the uplink, both operate in S
band at 8 Mbps in normal operation and 38.4 / 9.6 kbps during
commissioning for the downlink and uplink respectively.
* Corresponding author.
Most of the Alsat-1 subsystems were designed with no single
point failure to be highly resilient to non-nominal situation.
Therefore, most of the critical subsystems for the mission are
redundant, such as:
— 2 x receivers/transmitters to communicate with the
spacecraft.
— 2 x Power Supply Modules (PCM) and one Power
Distribution Module (PDM) providing two redundant
power lines from each single switch.
— 2 x On Board Computers (OBC) manage the satellites tasks
(OBC-386 as primary & OBC-186 as secondary).
— 3x Image Data Storage Memory (2x0.5 Gbytes SSDRs and
one 128 Mbytes SA1100).
— 2 x ADCS modules. Each controls one magnetometer, two
sun-sensors, and three coils of the three magnetorquers.
— 2 x boom controllers to insure the pyro-firing for boom
deployment.
Alsat-1 as part of the constellation is equipped with a
propulsion system with 50mN thrust and two tanks with a
capacity of 2.5 litres each. The propulsion system is needed to
assure:
— The circularization of the constellation satellites is
maintained so the sun-synchronicity of the mission too.
— The station acquisition which allows the satellites to equally
separate from each other so the daily global coverage is
respected.
— The station keeping of the satellites to avoid them to deviate
from their original orbit under drag effect and keep the daily
global coverage.
