NetSat is a satellites constellation of four 3U-Cubesats and flight research mission by the Center for Telematics in Würzburg. Research goals include the application of novel approaches and algorithms, as well as emerging technologies for formation control and inter-satellite coordination. The European Research Council (ERC) funded the construction of the satellites as well as the research and development of formation control algorithms. Zentrum für Telematik was also awarded the Advanced Grant for the NetSat project. In-orbit demonstrations (formation test / ForTe) were funded by the Bavarian Ministry of Economic Affairs, Regional Development and Energy. The mission enables experimental steps towards the "Internet of Space".
Experiments on formation flight of small satellites and inter-satellite communication
The NetSat mission will be the first in the world to demonstrate the cooperation of more than two satellites in a 3-dimensional configuration. For this, the satellites in orbit must form and maintain a formation in 3-dimensional space (formation flight). In this process, the four NetSat satellites fly at close orbital height, less than 1 km (0.62 mi) deviation in altitude. Larger differences cannot be compensated for due to the fuel-saving, low-thrust ion propulsion systems. In order to maintain the close tolerances in orbit, the thrusters must be fired periodically. In contrast to traditional satellite constellations, where individual satellites are monitored by tracking ground stations, the NetSat-mission employs a system whereby the Würzburg ground station supplies only target formation. This is possible because the satellite autonomously decide how to maneuver by adjusting their orbits accordingly.
This process is distributed, not according to the primary/secondary topology. Every satellite processes the information by itself, taking into account distance to its neighbours collected from onboard sensors and coordinates its decisions via inter-satellite radio communications. After initial deployment, the satellites were spaced approximately 50 km (31 mi) apart, which will be reduced to a few meters after about a year. In doing so, they form a tetrahedron that rotates around itself once per revolution of the earth. The tetrahedron’s rotation, describing a helix in several steps like in a DNA molecule, is necessary to evenly distribute the drag caused by friction of the Earth’s atmosphere, as it is still noticeable at this altitude. This flight pattern allows compensation by averaging out the forces on each satellite over time.
Research fundamentals for future sensor networks for 3-dimensional earth observation
Similar to how Synthetic Aperture Radar works, combining picture data from multiple angles allows comprehensive capture of objects on the ground. The required technologies, such as self-ogranised flight in 3-dimensional space are tested experimentally using four satellites. In a future mission currently scheduled for 2021, CloudCT, ten satellites will be launched to adapt the ct-scan technology to space applications. By capturing and analyzing stray sunlight redirected by clouds from multiple angles, spacial distribution of particulates and aerosols may be inferred. This data can be used to aid in climate research. Long-term goals include using a flight formation to allow synthetic aperture telescopy at a single point in time.
Optimization and further development of applications for the “Internet of Space”
In the future, cooperation between smaller fleets and larger satellite, such as those planned by the Chinese Academy of Space Technology between the individual telescopes on satellites, is also conceivable. Key are the opportunities for the “Internet of Space” lie in the self-organizing nature of the satellite formation. In contrast to SpaceX’ Starlink-Project, where inter-satellite communication allows only for exchange of positioning information, the NetSat constellation is capable of autonomously executing collision avoidance maneuvers by design.
The four Cubesats were launched on September 28, 2020 at 11:20 UTC onboard a Soyuz ride-share mission in cooperation with ExoLaunch from Plesetsk Cosmodrome.
The NetSat-constellation tracks a sun-synchronous orbit at an altitude of 575 km (357 mi) and inclination of 97.66°. After detumbling the four satellites are spaced apart slightly their TLEs differ accordingly.
Each NetSat has several battery units to cover the energy demand. The space-grade solar cells mounted to the exterior panels supply additional power and charge the on-board batteries.
Position/Attitude control system
Every Cubesats has an AOCS (Attitude / Orbit Control System) a combination Gyroscope, Magnetorquer and Reaction wheel. This allows the satellites to individually adjust their position relative to the constellation and earth.
The NetSats include several subsystems with (multiple) microprosessors. The main coordination is handled by the OBC, based on an MSP430 16-bit microcontroller from Texas Instruments with a 16 MHz oscillator.
The satellites have a cross dipole antenna to communicate with the ground station in Würzburg during an orbital overflight. The transmission and reception frequency is identical to the 435.600 MHz used by the UWE-4 mission.
The propulsion system is an ion thruster manufactured by the Enpulsion Company of Austria. It provides a maximum thrust of 350 µN, which allows the satellites to adjust their orbits at any time.
- Website for the project ForTe (german)
- Website for the project NetSat (german)
- ERC-Artikel: Small, but cooperative: the future of spacecraft systems (englisch)
- HORIZON-Artikel: Fleets of autonomous satellites to coordinate tasks among themselves (englisch)
- CORDIS-Artikel: Realisierbarkeit selbstorganisierender Kleinsatellitenformationen wird unter Beweis gestellt (deutsch)
- CORDIS-Artikel: Advancing the feasibility of self-organising small satellite formations (englisch)
- EXOLAUNCH-Artikel: Exolaunch to Deliver the NetSat constellation into Orbit for Würzburg Center for Telematics (englisch)
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