Robust Satellite Engineering in Educational Cubesat Missions at the Example of the UWE-3 Project

19th IFAC Symposium on Automatic Control in Aerospace September 2-6, 2013. Würzburg, Germany

Robust Satellite Engineering in Educational Cubesat Missions at the Example of the UWE-3 Project Stephan Busch ∗ , Klaus Schilling ∗ , Philip Bangert ∗∗ , Florian Reichel ∗∗ ∗

Julius Maximilians Universit¨ at, W¨ urzburg, Germany (e-mail: {busch, schi}@informatik.uni-wuerzburg.de) ∗∗ Zentrum f¨ ur Telematik, W¨ urzburg, Germany (e-mail: {philip.bangert, florian.reichel}@telematik-zentrum.de) Abstract: With the introduction of the cubesat standard hands-on satellite engineering received a significant boost at educational university programs. However, not all missions could demonstrate successful operation in orbit. Besides technical challenges due to the extremely limited mass and volume imposed by the standard, cubesat projects at university level typically inhere challenges due to high student fluctuation together with significant launch delays and limited financial budgets. In order to ensure feasibility and continuity over long time, special care in the engineering process, especially design, production, integration and testing, has to be taken. This contribution gives an overview about tools and procedures developed in the context of the University of Wuerzburgs UWE project, which support robust satellite engineering despite mentioned challenges. Keywords: picosatellite; engineering; education.. 1. INTRODUCTION With the introduction of the cubesat standard in 2002 hands-on satellite engineering received a significant boost at educational university programs. In the last decade more than 80 educational institutions from all around the world have been attracted by the flexible and cost efficient access to space as these projects not only take forward research in science and technology related to small satellites, but also provide a unique training platform for the next generation of scientists and engineers (Chin et al., 2008) (Woellert et al., 2011). However, not all missions could demonstrate successful operation in orbit (Bouwmeester and Guo, 2010). Besides technical challenges due to the extremely limited mass and volume imposed by the standard, cubesat projects at university level typically inhere challenges due to high student fluctuation together with significant launch delays and limited financial budgets. While a typical cubesat project lasts a few years, the active contribution of a student team member is in the order of a few months only. The risk connected to a generation change in the project team is not only the loss of expertise related to a particular component developed, but also the loss of a responsible person ensuring functionality of the inherited developments until actual launch date. The complex nature of a satellite system together with the limited access to a complete set of test facilities already during development phase further complicates robust development for a single university. In order to ensure feasibility and continuity over long time, special care in the engineering process, especially design, production, integration and testing, has to be taken. 978-3-902823-46-5/2013 © IFAC

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The engineering philosophy of the educational UWE project focuses on three key elements: access to hardware, access to software, and access to test facilities which are further described in the following sections. 2. ACCESS TO HARDWARE Whenever highly integrated miniaturized electrical systems are target of close inspection, the developer faces the problem that many parts or signals can not be properly accessed for debugging. For this reason quite often a so-called flat-sat model is introduced during satetellite development phase. Unfortunately, some problems, mostly related to EMC, would only show up in a compact integrated configuration. For this reason, with UWE-3 a flexible and modular satellite bus was introduced, intended to support robust and rapid development, integration and testing of the satellite throughout the entire development cycle (Busch and Schilling, 2012). One of the most important design rules of the UWE architecture constitutes the support for simple, standardized, and continuous testing at satellite bus architecture, as well as for the subsystems. For the bus architecture this supports the requirement for simple and quick assembly and disassembly of the entire setup. Although it is not intended to disassemble the flight model for late modifications, nevertheless it would be possible, if needed. The prototype can thus be easily tested and debugged in a flight model configuration and disassembled again for further modifications with a minimum of time and cost overhead. 10.3182/20130902-5-DE-2040.00127

2013 IFAC ACA September 2-6, 2013. Würzburg, Germany

Further objectives for the conception of the UWE bus beside reduction of mass, compactness and efficiency was modularity, with special focus on support for future extension. The careful definition of simple and clear mechanical, electrical and logical interfaces significantly decrease complexity, reduce dependencies and ensure compatibility in future. Hence, in contrast to many other CubeSat Missions, the design philosophy for the UWE next generation is to preserve the authority over the subsystem interface, relying on third party products only on component level. This way individual components can be much easier replaced or upgraded at subsystem level, while maintaining compatibility to the satellite bus.

could recently be demonstrated when the complete UWE-3 flight model was assembled from its individual subsystems and tested for functionality within a few hours only (see Fig. 2).

Fig. 2. Completely integrated UWE-3 flight model running various embedded software unit self tests via its umbilical line. 3. ACCESS TO SOFTWARE

Fig. 1. Checkout test on the UWE-3 flight model with detached side panels (top) for comfortable access to the subsystem stack. In order to ensure comfortable access to all subsystems in a functional configuration and to protect the interior from thermal hazards the mechanical structure of UWE-3 is decoupled from the electronics (inner) structure (see Fig. 1). The set of subsystems, each implemented on an individual printed circuit boards (PCB), includes all standard satellite subsystems like power, onboard data handling, communication and attitude determination and control, optimized with respect to size and mass. The subsystem PCBs are mechanically fixed in position by four sets of titan screws, which provide a good thermal resistant and constitute (except for electrical connections) the only physical connection of the inner structure to the outer structure. The individual subsystems are entirely interconnected with the other subsystems by a backplane with standardized connectors so that no wiring is necessary. The bus interface combines multiple digital interfaces like SPI or I2 C along with various power interfaces for different voltages. The pico-satellites side panels contribute to achieve a miniaturized design, as they combine structural stability, thermal balance, radiation protection, antenna ground plane, and electronics in an integrated compact and lightweight design. Being desiged as a double sided PCB with alumnium core, they already include the entire wiring necessary and can just be plugged into the backplane to be detected automatically by the OBDH core module. Advancements in robust and rapid satellite development, integration and testing in the context of the UWE project 237

From software developers perspective, the most crucial access to embedded devices allowing robust software development is given by a set of appropriate debug interfaces. For embedded hardware in the UWE-3 project, at minimum the following three interfaces are usually implemented: A simple digital IO line with controllable LED for debugging of timing issues and to be used as simple heart beat indicator, an in-system-debugging interface for life memory inspection and brakepoint debugging, and further a bidirectional communication interface for logging, event triggering and data exchange. Especially the communication interface is crucial when it comes to automated hadware-in-the-loop tests. In order to encourage the student developers to utilize this opportunity frequently from beginning on, a set of software tools have been developed which allow the students to define and perform automated tests of the embedded software with minimal effort. 3.1 Leightweight and Flexible Debug Protocol The UWE-3 debug protocol is a leightweight and flexible transport protocol for simple structured data exchange. It was introduced for two purposes. First, to provide a framework with a set of handy functions in order to reduce the initial workload whenever test interaction with an embedded component might be helpful. Second, to enable independant, maintainable and hence sustainable software development by many developers working on the same hardware but on different tasks. In order to be deployed on various embedded architectures the protocol is implemented in ANSI C with a minimal footprint. Operating on a stream-based serial data link, the protocol API allows the developer to send and receive data packets via a set of virtual data channels. As long as the developer uses a unique channel for his specific application, collisions with other services, available or implemented in

2013 IFAC ACA September 2-6, 2013. Würzburg, Germany

future, are avoided. The protocol also identifies events of packet loss and data corruption. Further, it reliably handles packet synchronization after corrupted packets have been received for any reason. As the average protocol overhead is only about four bytes, the protocol can be used effectively with low transmission rates. A virtual channel might transport a defined set of structured data frames from a PC to the embedded device and vice versa. These data frames are typically defined as nested C-struct types including arbitrary primitive data types. On PC side, two protocol implementations exist: A Java based implementation and a pure MATLABTM implementation. In both implementations a set of convenience functions takes care of automatic serialization from and de-serialization to the corresponding high level data types in Java or MATLABTM . Thus, arbitrary subprotocols can be implemented within seconds. While the MATLABTM frontend is usually used for rapid implementation of automated test and data logging scripts, the Java frontend can be used to implement proven protocols for data exchange with a solid platform independant user interface. For this reason, a generic and extensible user interface application is provided to the students. Basing on the Eclipse RCP 1 framework, the software combines a handy user interface framework with a powerful plugin functionality. This way, any extension implemented can be encapsuled as a modular plugin which perfectly accompanies with the protocol philosophy of promoting sustainable software in the context of educational projects. Despite various device specific protocol definitions, a few services implemented are more generic and useful in most embedded applications to support robust software development. Two of them are described in more detail in the following sections. 3.2 Embedded Unit Testing One of the most frequently used service implemented for the debug protocol is a framework for embedded unit testing. It brings along several C macros for rapid test definition and execution of assertions. An example test case implementation can be seen in Fig. 3. Once declared anywere in the source code a specifically developed preprocessor explores all available test definitions and links them into a test database structure during compile time. At runtime, a list of available test cases and their hieracical relations can be requested via the debug protocol. A specific view in the user interface presents the test cases in a tree view and lets the developer select a subset of tests for execution. This will trigger the corresponding source code to execute the implemented assertions. Any performed assertion will update the user interface which displays a colored status bar indicating progress and success (see Fig. 5). A second view displays all messages logged by a set of convenience macros for more detailed analysis. The unit testing framework has shown great acceptance among the developers due to its simple and ditributed way of defining test cases which further supports parallel development without the need for maintaining a central 1

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EUNIT_DEFINE ( mytest , " descripton " ,2 , $PARENT ) { ... // do some stuff EASSERT ( " assert condition " , condition == x , " error : % i " , condition ) ... }

Fig. 3. Simple distributed definition of embedded unit test case. database of tests. This way, any code developed any time during the development cycle for testing any specific part of the hardware or software, remains accesible in the system to be executed any time in future. 3.3 Embedded Variable Synchronization A similar and not less powerful framework implemented on the debug protocol is the so-called variable synchronization. Instead of the normal declaration of a static variable a specific macro can be used to publish the variable also via the debug protocol. The variable itself can still be used in the same way as before, but it can now further be read and written via the PC user interface. Optional hooks for the registration of external set and get events can be registered in terms of function pointers. This way an externally modified or requested value can be further synchronized with hardware periphery such as connected I2 C devices. // static int16_t m y v a r i a b l e ; VARSY NC_DEFI NE ( myvariable , int16_t , " descripton " , & setFunction , & getFunction , $PARENT )

Fig. 4. Definition of synchronized variable simply replaces conventional static variable definition. The user interface diplays the entire variable tree and enables to poll recent variable values as well as to overwrite a specific value at any time. All values received can be plotted over time and are further loggeded to a file for later analysis. The variable synchronization protocol constitutes a powerful tool allowing the software developer to get access to the running software by simply adding a line of code. Using the protocol with the MATLABTM frontend renders the framwework as a valuable tool for script based hardware-in-the-loop tests. In the scope of the UWE-3 project it has been frequently used for data logging during long term tests, for quick identification of adequate parameters and for verification of formula implemplementations by simply comparing the embedded calculations with MATLABTM reference implementations. 4. ACCESS TO TEST FACILITIES The complex nature of a satellite system underlines the importance of testing. High student fluctuations in typical educational projects, together with the limited responsibility of student developers, make continous testing with minimum effort even more important in order to ensure functionality of the inherited developments until actual launch date. Unfortunately, not all universities have available all necessary test facilities or can provide frequent, cost-efficient, and unbureaucratic access for stundet developers througout the entire development phase.

2013 IFAC ACA September 2-6, 2013. Würzburg, Germany

Fig. 6. CAD Drawing of UWE-3 Space Environment Simulator Components. Fig. 5. Screenshot of the Java debug protocol frontend showing views for embedded unit test and embedded variable synchronization. Despite the challenges imposed by the extremely limited mass and volume of the Cubesat standard, exacly these properties are very advantageous when it comes to testing. So can the low weight or low power consumption of a Cubesat enable to design test setups which are much simpler, more compact, more portable and especially much more cost-efficient than typical test environments of larger missions. The design philosophy of the UWE-3 project forsees not only to design a satellite, but also to design simple test setups and procedures which support continous automated tests whenever possible. Located right at the students workplace and being designed to gernerate reproducable test results with little work overhead the student developers should be animated to test and verify not only their own hardware but also inherrited dependant components as often as possible during all project phases. In the following sections a selection of such test facilities developed during the UWE-3 project is presented.

inspection, several feedthroughs are available to connect power, data and the liquid nitrogen pressure tubes to the interior. Thermal Space background is provided by a black coated shroud surrounding the test specimen which is cooled down to −196 ◦ C by liquid nitrogen guided through a helical pipe. The usage of materials with high thermal capacitance in a solid design ensures thermal stability of the shroud. Solar irradiance simulation is realized by a combination of halogen and high pressure metal vapor discharge lamps, properly aligned to ensure a homogenous illumination at a realistic solar equivalent flux of about 1367 W/m2 . The superposition of the different light characteristics allows the approximation of the Solar spectrum to provide realistic thermal absorption. Thus, it further allows the operation of solar cells under realistic conditions.

4.1 Cost-Efficient Long Term Space Environment Tests One of the most important but also most expensive tests for Spacecraft are themal vacuum tests. Fortunately, the limited size of Cubesats allow the utilization of costefficient designs and procedures compared to larger missions. The UWE-3 Space Environment Simulator, itself developed in the context of a student project (Kerstner et al., 2010), is optimized for typical Cubesat requirememts and enables student developers to perform long term thermal vacuum tests on system and subsystem level throughout the whole satellite design phase and for later verification. The compact test setup is usally located right at the student developers workplace which reduces administrative and financial overhead, and thus, encurages the students to frequently use this tool to qualify their designs whenever it is indicated. The test setup consists of a pressure chamber, a thermal shroud for cool space background simulation, solar irradiation simulation with realistic light spectrum, and a precise measuerment and control unit for data logging and satellite access (see Fig. 6). High-vacuum inside the pressure chamber suppresses convection and gaseous conduction to create a Space equivalent heat transfer environment. Besides a window for visual 239

Fig. 7. Thermal Balance Tests with UWE-3 in the Space Environment Simulator In the course of the UWE-3 project several tests such as thermal balanve tests or antenna deployment test have been successfully conducted with the described setup (see Fig. 7). The system has been proven to be a valuable and cost effective tool to enhance robust satellite engineering in educational projects. 4.2 Attitude Determination Calibration and Verification Attitude determination is a key capability in modern day spacecrafts. Testing such a system is usually a complex task requiring sophisticated setups for environment and attitude simulation. Attitude determination testing facilities for large spacecraft are too complex and expensive to be used for small satellite projects which is the reason why many picosatellite attitude determination systems have only been simulated before flight.

2013 IFAC ACA September 2-6, 2013. Würzburg, Germany

In order to follow the philosophy of a robust satellite system, a simple small-scale attitude determination testbed has been developed to enable verification tests at any stage of development. Moreover, this testbed is used for precise calibration of the satellites sensors at the same time (Kiefel et al., 2011). The testbed features a leveled rotational platform combined with a high precision optical encoder (See Fig. 8). While the present Earth magnetic field can be used as reference as is, a narrow spot-light is employed to mimic the Sun such that experiments can be conducted inside the laboratory. In order to guarantee a consistent environment regarding the Earth magnetic field and the artificial Sun with respect to the internal reference models, an innovative approach modifying the experiment time instead of mechanically tracking the simulated Sun constellation has been implemented.

Fig. 9. The aircushion testbed with the UWE-3 engineering model mounted on top. The testbed can be used to simulate frictionless satellite dynamics and to perform attitude control experiments. With the help of the aircushion testbed the integration of a miniature reaction wheel in the UWE-3 system could sucessfully be demonstrated by performing slew maneuvers of the picosatellite with the wheel actuator. Detumbling experiments with an initially spinning satellite have sucessfully been performed and stabilized UWE-3, showing the effective interaction of magnetic torquers and corresponding control algorithms. 4.4 Power Budget Verification

Fig. 8. Attitude determination testbed at the University of W¨ urzburg: 1) UWE-3, 2) optical incremental encoder, 3) electric motor, 4) computer with live visualization, and 5) spotlight. This setup has successfully been used and extended by students during all stages of the ADS development. It served as a reliable testbed from the integration of the various sensor types up to the verification of the complete system where an attitude determination accuracy of better than 5 deg could be demonstrated. 4.3 Attitude Control Verification One of the main mission goals of UWE-3 is the in orbit demonstration of a miniature attitude control system (ACS) based on magnetic torquers and one reaction wheel. In order to enhance the robustness of a reliable ACS, a special attitude control test setup for picosatellites has been developed, shown in Fig. 9. This testbed, which is based on a spherical aircushion bearing, can simulate frictionless satellite dynamics on ground and therefore allows for attitude control experiments already during the development phase. With this hardware-in-the-loop approach, students can test newly developed algorithms and verify their performance, as well as perform hardware functionality tests at their workplace. 240

Power budgets of very small satellites are usually constrained by the satellite’s dimension as they do not possess large solar panels and active attitude control. In order to efficiently utilize the limited available energy, sophisticated power subsystems have to be developed. Efficiencies of peak power trackers (PPT) might degrade with higher spinning rates of the satellite while efficiencies of power converters and high power paths might vary significantly with higher currents as bus voltages are relatively small compared to big satellites. Thus, testing the power budget in a hardware-in-the-loop configuration can help to ensure the power strategy despite mentioned uncertainties. For the evaluation of PPT efficiency and total power management strategy, realistic conditions regarding varying temperature and irradiance of each solar panel have to be emulated physically. Fortunately, the low power application of picosatellites allow to use compact and cost-efficient hardware setups. The UWE Solar Cell Emulator has been developed to precisely mimic the characteristic behavior of small photovoltaic (PV) arrays under realistic time varying conditions of typical picosatellite operation scenarios. The hardware design of the emulator is pretty much straight forward as it can be seen in Fig. 10. The setup is designed around a linear power operational amplifier meeting the requirements for the output signal ranges. The power op-amp is used in a simple non-inverting amplifier configuration. A low pass filter characteristic with a designed cut-off frequency of about 1 KHz stabilizes the output signal by suppressing high frequency noise. The input signal of the amplifier is provided by a precise digitalto-analog converter hooked up to a stable temperature

2013 IFAC ACA September 2-6, 2013. Würzburg, Germany

compensated reference voltage source. The same voltage reference is used by the fast analog-to-digital converters which sample the output voltage and a current proportional reference signal generated by the power op-amp.

Fig. 10. Block diagram of the photovoltaic emulator. The test setup has been used frequently throughout the development of UWE-3 in various automated tests in order to verify the PPT operation and to determine and verify the effective powerbudget on the satellite. For the latter tests a set of three emulators have been controlled by a MATLABTM script to emulate a typical long term orbit scenario of a spinning picosatellite with multiple solar panels. Further, a programmable load has been attached to the individual power buses on the satellite to simulate extra power consumption on the bus. During several long term tests, each lasting several days, the MATLABTM script could automatically determine the maximum extra load on the individual buses such that the total power budget remains positive. As these values naturally include all uncertain efficiency parameters including PPT, power path, batteries, converters etc., they provide a robust estimation of safety margins in the power budget.

automated tests have been implemented during the development phase whereas a large fraction of them became part of the automated final checkout tests. In order to enable access to test facilities, several compact and cost efficient test setups have been designed and frequently used by multiple student generations in the course of the UWE-3 project. Thereby it was achieved, that all developments could be verified continuously in house throughout the entire development and integration phase. This way, the student projects did not end with a working simulation only, but instead lead further to a verified embedded hardware component operating in a realistic environment. Within the UWE-3 project, more than a dozen directly related final degree theses have successfully been completed in the last 4 years. Besides robustness, the applied engineering philosophy demonstrated advancements in knowledge transfer and preservation, which constitutes an important factor in the context of an university project with a high fluctuation of students. Based on the described infrastructure, the future vision of the UWE project is to enable more advanced space and even science missions and to make them available to students. 6. ACKNOWLEDGEMENTS The authors appreciated the support for UWE-3 by the German national space agency DLR (Raumfahrt-Agentur des Deutschen Zentrums f¨ ur Luft- und Raumfahrt e.V.) by funding from the Federal Ministry of Economics and Technology by approval from German Parliament with reference 50 RU 0901. REFERENCES

5. CONCLUSION In order to ensure feasibility and continuity in educational satellite projects over long time, the engineering philosophy of the UWE project focuses on three key elements: access to hardware, access to software, and access to test facilities. Access to hardware is achived by the introduction of the UWE-3 satellite bus architecture, supporting easy maintenance, extension and replacement of subsystems in any configuration, even after final integration. In the course of the UWE-3 project the ease of rapidly changing the hardware configuration encouraged the students of concurrently running thesis projects to test their subsystem developments frequently also in the shared engineering model in a compact flight-like configuration. This way, various bugs due to incompatibilities of the different subsystem could be solved immediately such that the final flight model assembly could be conducted within less then two hours without any further surprises. Access to software refers to a set of software tools allowing the students to define and perform automated tests of the embedded software with minimal effort. The implemented tools showed great acceptance among the student engineers as they accelerated the training phase and significantly shortened the time until first results could be generated with the embedded hardware. Hundreds of 241

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