Swarm satellites Design, methods and applications

1. Swarm satellitesDesign, methods and applications PhD Candidate: Steven Engelen Department: SpE Section: Space Systems Engineering Supervisor: C.J.M. Verhoeven Promoter: E.K.A. Gill Start date: 1-4-2010 Funding: STW Cooperations: UTwente, ASTRON This causes a great deal of confusion as to what a satellite swarm is, and one of my research goals is therefore to formally distinguish satellite swarms from other forms of distributed architectures, such as constellations and formation flights. The current consensus as to what a satellite swarm is, is defined as: “A satellite swarm is ideally defined as a distributed space system consisting of many identical, low-cost spacecraft, autonomously cooperating to achieve a common global goal”. What defines a swarm satellite? Using the above definition, distinguishing features of a swarm satellite can be identified. This allows defining the properties of the individual satellites through a novel “hybrid” systems engineering appoach, which uses a top-down requirements definition process, yet matches it with a bottom-up element design process. This was required due to an effect called “emergent behaviour”, which is (unexpected) behaviour which only emerges due to interactions between elements, and is therefore very difficult to design for. A bottom-up approach can allow for simulations to identifiy the interactions which cause emergent behaviour, which can then be tuned to suit the application. In general, a swarm satellite should be as simple as possible. An overview of the preliminary element design for an OLFAR spacecraft can be seen on the right. Fivemostly independent functionality groups can be identified: The power system Attitude control and locomotion Communication, intelligence and control The payload An end-of-life device These can function almost independently, and should be present in one form or other in any swarm satellite. The end-of-life device is a special addition, to comply with space-junk regulations, as satellite swarms will contain many potentially unreliable elements, which would need disposal. How to design the most basic swarm satellite (for the OLFAR swarm) This is the crucial question. A novel systems engineering approach has been proposed, and it is being applied to the OLFAR element requirements as defined by the OLFAR team. Monte-carlo simulations of the lifetime estimates have been performed for OLFAR-like swarms and swarm satellites, rendering a tool to define the system- and element (useful) lifetime, taking reduced operational states into account, as well as the element reliability. One such result is shown on the right. The satellites in this siumation had an expected internal component lifetime of around 3 years, which results in a useful lifetime of less than 0,4 years, and an extended lifetime up to 0,45 year. This lead to the conclusion the swarm satellite design used in this model is fundamentally flawed, and an improved version is currently being investigated. • Background • Radio astronomers, interested in low frequency signals are troubled by Earth’s ionosphere, as it distorts and even blocks signals below 20 MHz. This realisation lead to studies to attempt to construct radio telescopes in space. • Two precursor missions were also launched (RAE1 and RAE-2) in the early 70’s. They discovered Earth’s ionosphere is also a very strong emittor or Radio Frequency Interference. Also man-made signals were detectable from space, causing RAE-2 to be launched into lunar orbit. It detected that behind the Moon (relative to Earth), the RFI was significantly reduced, to below the detection threshold of the satellite. Launching a large satellite into lunar orbit or beyond was deemed un-feasible back then. • Recently, with the advent of distributed Earth-based radio telescopes such as the WSRT, SKA and LOFAR, we have seen a revival in studies of low frequency space-based radio telescopes. The latest in this series is a proposal called OLFAR (Orbiting Low Frequency Antennas for Radio Astronomy), which proposes a swarm of nano-satellites to form a virtual, distributed radio telescope in Lunar orbit or beyond. The main difference in the OLFAR studies, and its competitors is that it attempts to use (beyond) state-of-the-art technologies, rather than proven space-qualified technology, which would drive up the cost of the mission. • This in turn renders access to modern, high performance processing and fast inter-satellite links, which allows for an increase in the number of nodes in the array. The reduced form factor is therefore less of a nuissance than it would initially seem. • Progress and Objectives • Currently, a workable definition of a satellite swarm is defined, and the impications are being investigated. This would, in time, allow for an unambiguous distinction between the different types of distributed satellite systems. • A novel systems engineering approach, tailored to satellite swarms, has been proposed. It will still have to prove its merits , yet initial results are promising. • A throughough invesitgation into the reliability aspects of using low-cost (nano-) satellites in swarms has been performed, and is submitted to a Journal for peer-review. The tools developed will allow tailoring the element design to the lifetime and relibaility requirements of the overall swarm. • The OLFAR satellites are currently undergoing their preliminary design phase. Crucial elements, such as the payload, are being developed. Those will be tested in pathfinder missions, which will also determine the RFI environment in the various candidate orbits. • Using a satellite swarm for OLFAR is extremely promising, but a lot of additional research and design has to be performed before the mission can be launched. Precursor missions are therefore already being outlined. Preliminary element design of an OLFAR element Aerospace Engineering Swarm lifetime simulation result for a swarm of 100 satellites Artist impression of OLFAR in Lunar orbit Swarm satellites Satellite swarms are a novelty. My PhD research focusses on design methods for satellite swarm elements (i.e. the individual spacecraft which, when combined, form the swarm). In order to structure this, several research questions are defined. They can be categorised as follows: What is a satellite swarm? Unlike the mass-based definitions of monolithic satellites, which have grown over the years to become an accepted standard, distributed space systems are still in their infancy, and terms such as swarms, clusters and formations are used rather arbitrarily. • Select publications • S. Engelen, E. Gill, C. Verhoeven, “On the reliability of spacecraft swarms”, Proceedings of the 4S Conference, Portoroz, Slovenia, 2012 • S. Engelen, E.Gill, C. Verhoeven, “On the Reliability, Availability and Throughput of Satellite Swarms”, IEEE Transactions on Aerospace and Electronic Systems (March 2013, Paper submitted) • C.J.M. Verhoeven, M.J. Bentum, J. Rotteveel, B. Monna and J. Guo, “On the Origin of satellite Swarms,” ActaAstronautica, 68 (7-8). pp 1392-1395, 2011. ISSN 0094-5765 • E. Dekens, S. Engelen, R. Noomen, “A Satellite Swarm for Radio Astronomy”, IWSCFF 7th International Workshop on Satellite Constellations and Formation Flying, March 13-15, Lisbon, Portugal • S. Engelen, C.J.M. Verhoeven and E.K.A. Gill, “Systems Engineering Challenges for Satellite Swarms”, IEEE Aerospace Conference, March 5-12, 2011, Big Sky, Montana, US • R.T. Rajan, S. Engelen, M.J.Bentum and C.J.M. Verhoeven, “Orbiting Low Frequency Array for radio astronomy”, IEEE Aerospace Conference, March 5-12, 2011, Big Sky, Montana, US