Institut für Informatik Lehrstuhl für Robotik und Telematik Prof. Dr. K. Schilling Würzburger Forschungsberichte in Robotik und Telematik Uni Wuerzburg Research Notes in Robotics and Telematics Marco Schmidt Ground Station Networks for Efficient Operation of Distributed Small Satellite Systems Band 6 Die Schriftenreihe wird vom Lehrstuhl für Informatik VII: Robotik und Telematik der Universität Würzburg herausgegeben und präsentiert innovative Forschung aus den Bereichen der Robotik und der Telematik. Die Kombination fortgeschrittener Informationsverarbeitungsmethoden mit Verfahren der Regelungstechnik eröffnet hier interessante Forschungs- und Anwendungsperspektiven. Es werden dabei folgende interdisziplinäre Aufgaben- schwerpunkte bearbeitet: Ÿ Robotik und Mechatronik: Kombination von Informatik, Elektronik, Mechanik, Sensorik, Regelungs- und Steuerungstechnik, um Roboter adaptiv und flexibel ihrer Arbeitsumgebung anzupassen. Ÿ Telematik: Integration von Telekommunikation, Infor-matik und Steuerungs- technik, um Dienstleistungen an entfernten Standorten zu erbringen. Anwendungsschwerpunkte sind u.a. mobile Roboter, Tele-Robotik, Raumfahrts- ysteme und Medizin-Robotik. Lehrstuhl Informatik VII Robotik und Telematik Am Hubland D-97074 Wuerzburg Tel.: +49 (0) 931 - 31 - 86678 Fax: +49 (0) 931 - 31 - 86679 [email protected] http://www7.informatik.uni-wuerzburg.de Dieses Dokument wird bereitgestellt durch den Online- Publikationsserver der Universität Würzburg. Universitätsbibliothek Würzburg Am Hubland D-97074 Würzburg Tel.: +49 (0) 931 - 31 - 85917 Fax: +49 (0) 931 - 31 - 85970 [email protected] http://opus.bibliothek.uni-wuerzburg.de ISSN (Internet): 1868-7474 ISSN (Print): 1868-7466 eISBN: 978-3-923959-77-8 Zitation dieser Publikation SCHMIDT, M. (2011). Ground station networks for efficient operation of distributed small satellite systems. Schriftenreihe Würzburger Forschungsberichte in Robotik und Telematik, Band 6. Würzburg: Universität Würzburg. Ground Station Networks for Efficient Operation of Distributed Small Satellite Systems Dissertation zur Erlangung des naturwissenschaftlichen Doktorgrades der Julius-Maximilians-Universität Würzburg vorgelegt von Marco Schmidt aus Würzburg Würzburg 2011 Eingereicht am: 13.5.2011 bei der Fakultät für Mathematik und Informatik 1. Gutachter: Prof. Dr. Klaus Schilling 2. Gutachter: Prof. Dr. Shinichi Nakasuka 3. Gutachter: Prof. Dr. Hakan Kayal Tag der mündlichen Prüfung: 29.7.2011 Contents 1 Introduction1 1.1 Motivation.................................1 1.2 Contributions...............................2 1.3 Outline...................................4 2 Background7 2.1 Small satellites in education and research................7 2.1.1 The small satellite concept....................7 2.1.2 Evolution of the pico and nano-satellite approach....... 10 2.1.3 The UWE satellite series..................... 12 2.2 A new concept of ground station networks............... 17 2.2.1 Classic and academic ground station networks......... 18 2.2.2 Projects establishing academic ground station networks.... 23 2.3 Distributed / multi satellite systems................... 26 2.3.1 Distributed satellite systems as a new paradigm for satellite missions.............................. 29 2.3.2 Application scenarios for distributed satellite systems..... 30 2.3.3 Example mission......................... 32 2.4 Challenges and technologies for distributed space missions...... 32 2.4.1 Challenges for highly distributed satellite missions....... 33 2.4.2 Technologies for distributed space missions........... 35 3 Redundant scheduling for ground station networks 39 3.1 State of the art.............................. 41 3.1.1 Satellite Range Scheduling.................... 41 3.1.2 Earth Observation Scheduling (EOS).............. 45 iii 3.1.3 ESA tracking stations - ESTRACK............... 47 3.1.4 Summary............................. 48 3.2 Scheduling for low cost ground station networks............ 48 3.2.1 Scheduling requirements of academic ground station networks 51 3.2.2 Problem description....................... 52 3.2.3 Scheduling objective....................... 55 3.2.4 Classification........................... 56 3.3 Scheduling approach........................... 57 3.3.1 System overview......................... 58 3.3.2 Scheduling objective function.................. 59 3.3.3 Behavior of the penalty function for two arbitrary requests.. 62 3.3.4 Redundancy distribution for more than two arbitrary requests 64 3.3.5 Search algorithms......................... 69 3.3.6 Implementation.......................... 71 3.4 Performance evaluation.......................... 74 3.4.1 Evaluation criteria........................ 75 3.4.2 Experiments............................ 76 3.4.3 Results............................... 79 3.5 Conclusion and discussion........................ 87 4 Data management for information recovery in ground networks 91 4.1 Problem definition and state of the art................. 93 4.1.1 Problem description....................... 94 4.1.2 Data and network management in computer networks..... 98 4.1.3 Data management for information recovery........... 99 4.1.4 Time synchronization....................... 102 4.2 Synchronization in academic ground station networks......... 103 4.2.1 Time synchronization between ground stations......... 106 4.2.2 Data synchronization on frame level............... 114 4.3 Data combination in academic ground station networks........ 118 4.3.1 Ground station majority voting approach............ 118 4.3.2 Brute force method for data recovery.............. 122 4.3.3 Single bit error correction in AX.25............... 123 4.4 Performance analysis........................... 127 4.4.1 Performance of the data combination algorithm........ 127 iv 4.5 Hardware tests and results........................ 133 4.5.1 Ground station network simulation............... 133 4.5.2 Local ground station network with radio link.......... 142 4.6 Conclusion and future work....................... 149 5 Conclusion 151 A Propagation delay in Low Earth Orbits (LEO) 155 B OSI layer model for satellite communication 159 C Satellite orbit data for evaluation of CUSS 163 v Chapter 1 Introduction 1.1 Motivation In the last 10 years emerged a new approach in the field of satellite engineering. The idea of sending extremely small satellites into space originated from educa- tional institutions and then adopted in diverse application fields. This paradigm shift involved building satellites from commercial of-the-shelf components, designed for restricted lifetime, but at affordable prices. Many of these pico and nano-satellites were already brought into orbit. The University of Würzburg contributed in that scope with the University of Wuerzburg Experimental satellites (UWE) and demon- strated successfully how extremely small satellites can be used to perform innovative space research. A benefit of designing lightweight satellites is, that they can be launched ’piggyback’ as secondary payloads, and hence at moderate costs. This makes it possible to in- stall networks of satellites, carried from a single launch vehicle into space. Many researchers look at satellite networks (especially formations) as the next necessary step to transfer successful terrestrial distributed system technology to the context of space exploration, for example at utilizing virtual instruments or very long base- line interferometry. The concept of networked satellites promises progress in diverse application fields. Specifically from a computer scientists point of view, the network aspect is very interesting, although it is known that handling and controlling dis- tributed systems is very challenging. To better illustrate such issues, let us assume a satellite cluster of 10 small satellites is placed in similar low Earth orbits, acting together to achieve a common mission 1 2 goal. On the receiving side, 30 ground stations are available, geographically dis- tributed over Europe. The starting point for this work was a simple question: How can data from vehicles in space be transferred efficiently to an operator on Earth? One could think of this as a simple deterministic problem, taking into account the orbit geometry and the transmission rate of each satellite. However, a closer look reveals many sophisticated theoretical and technical problems: For example, which specific ground station should track a satellite when several are visible at the same time? How can the restricted link capacity of a single satellite be compensated, when a superior number of ground stations are available? In which way can ground stations be efficiently synchronized with each other? Starting from this point, a detailed investigation of current ground station networks was performed with focus on established infrastructure at academic institutions. These satellite receiving stations are built from low-cost devices and components, the architecture being similar all over the world as they are primarily used for oper- ation of small satellites. The actual transmission rate of these satellites are limited to utilization of commercial of-the-shelf components and low frequency bands for communication. Nevertheless, the lack of performance can be compensated with in- telligent utilization of network resources and redundant communication links. The central topic of this contribution are different strategies to improve the operation of small satellites with ground station networks. In the following chapters are different concepts that optimize the