In this paper, a constellation of four low earth orbit nanosatellites for disaster monitoring is presented; their small size, low cost, and short time development give them the opportunity to be widely selected. The space segment is based on existing subsystems that are assembled on the bottom part of a developed structure and present acceptable performance for the mission. The payload is a multispectral camera which fulfills the mission's requirements (remote sensing). The ground segment is based on an existing modular ground station, used for the ALSAT-1 DMC, which will be adapted to fit with the specifications of the mission.
The Algerian Space Agency has been active in the field of microsatellite engineering for more than 15 years and has successfully developed microsatellites under several know-how transfer technology programs, six to date. This paper presents the flight results and lessons learned from the attitude determination and control system (ADCS) flown on the ALSAT-2B satellite, an Earth observation microsatellite, by analysing the behaviour of the satellite from the initial attitude acquisition through the coarse pointing mode then the nominal mode, where the payload is first tested, and finally the orbit control mode. The spacecraft was launched on 26 September 2016 and placed into a 670km Sun-synchronous orbit with a solar local time at an ascending node of 22:15. The ADCS performance presented here mainly focuses on the launch and early operation results. ALSAT-2B includes four reaction wheels in a pyramidal configuration, three gyros, three Sun sensors, three magneto-torquers, one magnetometer, and one star tracker for agile and accurate attitude control. In addition, a propulsion system based on four 1N hydrazine thrusters is also used on board the microsatellite. The main new development in this platform compared with previous ones of the same type is the fusion of the star tracker and measurements by the three gyroscopes into one gyrostellar estimator that was implemented for the first time on ALSAT-2B, and the pyramidal configuration of the wheels, aiming to increase the angular momentum. The results obtained from the early launch operations for different ADCS modes are very encouraging and fulfil all the requirements set during design and testing. Currently, the satellite has accomplished its fourth year in orbit and is still operational and producing high-quality images.
- In this paper we present the in orbit performance during the initial attitude acquisition, immediately after separation from the final stage of the launcher, until the satellite converges toward a sun pointing. On 12th july 2010 ALSAT-2A microsatellite was launched into a 670 km sun synchronous orbit, with a solar local time at an ascending node of 22h15. In the initial acquisition mode or even in the safe mode, only sun sensors and magnetometer are used for attitude determination. Knowing that, the satellite once is separated from the launcher it starts tumbling. So, as to detumble the satellite, a strategy consisting of three phases is set. The first phase consists on reducing the velocity using only the magnetorquers, until reaching the threshold angular momentum of 0.05 Nms, once this is done, the second phase is automatically enabled in such a way, the four reaction wheels are ON and speed up to get an angular momentum of -0.15 Nms along the satellite x-axis. At the end of this phase the satellite is completely detumbled and a third phase is carried out, in which the satellite minus X axis is pointed toward sun with a small rotation.
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