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    New star tracker for satellites

    08/03/2009

    Small satellites are an exciting option for space projects because their launch costs are low. Before using them for Earth observation or other tasks, however, there are still technical challenges to be met. This is what scientists of the University of Würzburg are currently concerned with.

    Ein Sternenhimmel. Fotos von Sternbildern helfen Satelliten dabei, ihre Lage im Raum zu regeln. Foto: NASA
    Fotos von Sternbildern helfen Satelliten dabei, ihre Lage im Raum zu regeln. Foto: NASA

    Satellites are launched into orbit by means of carrier rockets. The larger and heavier they are, the more expensive the endeavor: “In case of really bulky satellites weighing several tons, the launch costs range from 50 to 150 million euros, depending on the launch rocket,” says Professor Hakan Kayal of the Department of Computer Engineering at the University of Würzburg.

    In contrast, the launch of a miniaturized one-kilogram satellite costs only about 40,000 euros. The Würzburg computer scientists have already gained some experience in working with satellites of this scale. The first specimen of the UWE (University of Würzburg Experimental Satellite) was launched into orbit in 2005; UWE-2 is to follow suit early in September.

    “A particularly promising strategy is to let several miniaturized satellites cooperate as sensor networks,” explains Professor Kayal. This means: The satellites are able to perform simultaneous measurements at different positions in the orbit – gathering much more detailed information than would be possible with only one large satellite.

    Position control of the satellites in space

    Collecting data on planet Earth, localizing orbital space debris or observing remote galaxies: All this and more could be implemented with a fleet of miniaturized satellites. For most of the applications, e.g. for Earth observation, a precisely defined orientation of the satellites is of the highest significance. Without active position control, a satellite would spin in space uncontrollably, for which zero gravity is to blame.

    Star trackers evaluate star images

    How can the position of a satellite be controlled in space? For this task, the satellite needs high-precision sensors to determine its current position. According to Professor Kayal, the most suitable devices for his purpose are so-called star trackers: They evaluate star images that they have previously captured with a camera. After recognizing a certain star pattern, they can autonomously process this information and clearly identify the orientation of the satellite.

    In the next step, the satellite has to be pointed to the desired direction, e.g. through the interaction of small internal wheels. If one of the wheels turns, the satellite moves to the opposite direction under zero gravity conditions. “If you have at least one wheel per axis, you can turn the satellite to any spatial direction,” says Hakan Kayal.

    Objective: star tracker for miniaturized satellites

    Most of the larger satellites already have star trackers in operation. Now, a star tracker for miniaturized satellites to be named STELLA, scheduled to be ready for launch within two years, is planned to be developed and manufactured at the University of Würzburg. Hakan Kayal’s project is funded by the German Aerospace Center (DLR) with just under half a million euros. Two scientific assistants and four student assistants are allocated to take part in the project. Furthermore, some additional students are to be involved within the scope of a master’s or bachelor’s thesis. A new bachelor degree program in aerospace engineering will start at the University of Würzburg in the winter 2009/10.

    “The new star tracker will considerably expand the applicability of pico- and nanosatellites,” predicts the Würzburg professor. The established classification system defines pico- and nanosatellites as satellites with a weight between 0.1 and 1 kg (pico) or between 1 and 10 kg (nano).

    Localization of space debris in orbit

    In future, the Würzburg team wants to use the star tracker for research projects requiring on-board processing of image data. Autonomous destination planning of satellites or the localization of orbital space debris are examples of such projects.

    The latter issue is considered to be urgent: According to Hakan Kayal, the risk of damage to satellites in orbit through space debris has significantly increased in recent years. At the same time, the dependence of people on the infrastructure in orbit for such purposes as communication, navigation or Earth observation has risen.

    So far, there is no European system in existence for monitoring space events. Therefore, the European Space Agency (ESA) has just recently started a new program: It is envisioned to provide Europe with an independent capability to find all objects in orbit and to assess and avoid possible risks to the operation of its satellites. Professor Kayal wants to make his contribution to the implementation of this task.

    Contact

    Prof. Dr. Hakan Kayal, T +49 (0)931 - 31-86649, kayal@informatik.uni-wuerzburg.de

    Appointed in April 2008, Hakan Kayal is Professor for Spacecraft Control and System Design at the Department of Computer Engineering (Robotics and Telematics) of the University of Würzburg.

    Position control of satellites with a star tracker: A camera – of course considerably smaller and more modern than the one shown in the illustration – captures images of star patterns. From this information, the star tracker derives the current position and redirects the satellite to the desired position. The process is illustrated in the picture by means of the x, y und z coordinates. Picture: Hakan Kayal

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