This document describes the application of enhanced functional flow block diagrams (eFFBD) on the attitude and orbital control system (AOCS) of an Earth Observation satellite. First requirements and constraints of the satellite and its mission have been identified. Afterwards, these requirements and constraints were used to design the eFFBD of the AOCS

Chaudemar, Jean-Charles

Lizy-Destrez, Stéphanie

Pierl, Christoph

English

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 To cite this document: Pierl, Christoph and Chaudemar, Jean-Charles and Lizy-Destrez, Stéphanie Application of the Systems Engineering methodology to the design of the AOCS of an Earth Observation satellite. In: Complex Systems Design & Management (CSD&M) 2012, 12-14 Dec 2012, Paris, France. (Unpublished) Open Archive Toulouse Archive Ouverte (OATAO)  OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.  This is an author-deposited version published in: http://oatao.univ-toulouse.fr/  Eprints ID: 6802 Any correspondence concerning this service should be sent to the repository administrator: staff-oatao@inp-toulouse.fr  Application of the Systems Engineeringmethodology to the design of the AOCS of anEarth Observation satelliteChristoph Pierl, Jean-Charles Chaudemar and Ste´phanie Lizy-DestrezInstitut Supe´rieur de l’Ae´ronautique et de l’Espace, ToulouseApril 18, 2012AbstractThis document describes the application of enhanced functional flowblock diagrams (eFFBD) on the attitude and orbital control system (AOCS)of an Earth Observation satellite. First requirements and constraints ofthe satellite and its mission have been identified. Afterwards, these re-quirements and constraints were used to design the eFFBD of the AOCS.1 IntroductionSince the first satellites have been placed on orbit around the Earth, it is in-evitable for an optimal use to control its attitude and orbit. It is not sufficientjust to install the satellite on its orbit with the desired orientation because ofthe space environment which disturbs the orientation and the shape of the or-bit of the satellite. Thus a subsystem becomes necessary which measures andcomputes the attitude, computes commands to correct the deviation and appliesthese commands. Such a subsystem is called attitude and orbital control system(AOCS).So as to identify functional design patterns for the attitude and orbital con-trol system for an Earth Observation satellite, we decide to model it with theformalism called enhanced functional flow block diagram (eFFBD). This con-tains beside the modellisation of the AOCS also the modelling of the spaceenvironment and its influences, space debris and the de-orbiting at the end ofthe operational lifetime. Additionally to the de-orbiting modelling, it has to bementioned, that the intended Low Earth Orbit (LEO) of the satellite lies in anenvironment which has been polluted with debris over decades. This fact makesit a dangerous area for space flight and satellite missions and to countersteerthe pollution, the de-orbiting maneuver is considered.The main problematic addressed in this article is complex. For this reason weneed a Systems Engineering methodology. During the last decades it becamemore and more important to have tools, methods and processes available to1handle large projects along the life cycle of the studied system [1]. As thisstudy focus mainly on the architecture process, the notation, used in the frameof this document is eFFBD.Section 2 describes the context of the study. Section 3 gives an overview ofthe applied elements to design the eFFBD models. Section 4 contains the ap-plied eFFBD on the AOCS. Section 5 concludes the document and gives a briefoutlook on future work.2 Satellite and MissionThis section is dedicated to the description of the attitude and orbital controlsubsystem (AOCS) of a satellite. It describes the architecture of the system,the attitude and the orbit of the satellite. The information presented in thissection are used as a basis to apply the eFFBD Systems Engineering approachon the AOCS, which is described in the follwoing sections.2.1 Layered architectureThe introduction of a layered architecture to the AOCS allows to subdividethe satellite into different hierarchical layers representing functionalities. Thearchitecture which is introduced in the paper consists of two functional layers.The opearational layer is the highest which concerns the different operatingmodes of a satellite: operational, safe, recovery and end of mission. The secondlayer is the functional layer which is managed by the operational and consistsof functions to sense, detect and computes the attitude of the satellite andcomputes counter actions. In this layer, we are only concerned with the satellitecontrol functionality[9] [10].2.2 Attitude of the satelliteThe attitude of a satellite is represented with respect to its yaw, roll and pitchaxis of a local reference frame which is determined in the origin of the satellite.The reference frame and the locations of its axis are schematically pictured infig. 1 [7]. In this figure, it is depicted that the yaw-axis is oriented nadir,the roll-axis is tangential to the orbit and the pitch-axis is perpendicular to theorbital plane or rather to the plane spaned by the roll- and yaw-axis. Differencesbetween the local reference frame of the satellite to the local coordinates arerepresented by the angluars: Φ, Θ and Ψ. The AOCS in general has to becapable in order to provide a certain precision around all axis (roll / x, pitch /y, yaw / z) for all angles (Φ, Θ and Ψ) [7].2.3 Orbit of the satelliteThe satellite is considered to be on a Sun-Synchronous polar orbit and it passesthe equator at the same local time on every pass due to the precession of the2Figure 1: Inertial, local coordinate system and angles of the satelliteEarth [5]. The Sun-Synchronous orbit is a prefered orbit for observation satel-lites and makes it easier to detect changes on the Earth’s surface due to aconstant angle of incidence of the sun light. It is necessary to make adjustmentregularly to maintain the orbit, which is done by on-board actuators of thesatellite.3 Overview on eFFBDThis section gives an overview of the elements, structure and properties ofeFFBD which are used for the design of the attitude and orbital control subsys-tem of the satellite. A set of elements connected to a structure are presented,whose properties and structural behaviour are explained. The connected ele-ments represent of what must happen during the execution of the diagram.3.1 Elements of eFFBDThis section comprehends the employed eFFBD elements to design the struc-ture of the AOCS. A description, illustration of elements and its properties arepresented. Furthermore symbols to create branches and loops are presented anddescribed [2].3.1.1 FunctionA function is a central element in an eFFBD. It is represented by a rectanglewhich is labeled [3]. Here, we mainly use verbs in infinite form for the functionlabel. This label describes an implied function, which is performed when theblock has been enabled. In fig. 2 a function block is pictured [2]. Besidea basic function, which contains actions to be executed there exist so calleddecomposed functions which are additionally labeled with a black rectangular.A decomposed function is determined by its subfunctions. Such a block can be3Figure 2: eFFBD function blockbrought down to a simplifying and compact way of designing a model. Such ablock is pictured in fig. 3. [2]Figure 3: eFFBD decomposed function block3.1.2 Execution of functionsFor the execution of a function, triggering data is necessary. This is effected bymeans of the control flow, items, trigger and ressources. In our model, mainlyitems are used to model the data flow. Items can be exchanged at a time andcan only be consumed in integer quantities, which have to be ≥ 1. Items aredepicted as rectangulars with rounded down edges in eFFBD [3] and [4].• Item: Represent data stores (e.g. output of a sensor)• Trigger: Orders which are issued by a controller• Ressource: It can have different natures, such as: energy, CPU memory,etc.3.1.3 Multiple branch and repetition structuresFor the modelling of the dataflow, certain elements are necessary to createparallel structures and probabilistic choices of functions, to conduct iterationsand loops. To model parallel structures an AND or ”&” is applied at thebeginning, where the parallel structure starts and at the end of the parallelstructure. The subsequent function of the closing element will not be executedbefore the last function of the parallel structure is completed. It is possible toimplement several subsequent functions, loops, iterations and OR structures [4].Such a parallel structure is schematically shown in fig. 4. In the same way asthe AND structures, the OR structures are modelled. On the contrary to ANDelements, the OR elements are choices of probabilistic nature and can also bemodelled as a parallel structure. Only one function at once can be executed[4]. Such a choice structure is schematically shown in fig. 5. For this case,the probability that Function 1 will be chosen is: p = 1 − x and for Function2 the probability is: p = x. For the modelling of the system it is inevitable to4Figure 4: Parallel structure with AND-ElementFigure 5: Choice structure with OR-Elementapply loops (LP). Loops are often used systems. In eFFBD, the loop will be runthrough infinitely and it can be left with the loop exit element LE, which causesthat either a subsequent function will be executed or the data flow correspondinghigher system level will be continued. It has also to be mentioned that a loopexit element, embedded in a diagram is framed of an OR structure. [4] In fig.6 is a loop structure schematically pictured.Figure 6: Loop structure4 eFFBD applied on an AOCSThe attitude and orbital control subsystem of a satellite contains the devices tostabilize the satellite and orients it in desired directions during the mission. [8]To determine and control the attitude and orbit of the satellite physical devicesare used: sensors, computers and actuators. The outputs of the sensors can alsobe transmitted via the telemetry channel to a groundstation. It is possible toreceive set-values via the telecommand channel and also to operate the actuatorsdue to several reasons such as avoiding collisions with space debris.54.1 eFFBD of operational modeThis section is dedicated to the operational layer of the AOCS, which is shownin Fig. 7. The function Detect condition is used to describe the differentoperational modes: Operational mode, Recovery mode, Safe mode and End ofoperation mode. The choice of the exit branch is either done inside of thefunction which is not decomposed or in a subscenario which is decomposed[2]. This layer of the system affects the functional layer which is described inSec. 4.2. At the end of the operation, the functional construct is left by anLE-element.Figure 7: eFFBD of the operational layer4.2 eFFBD of attitude control subsystemThis section is dedicated to the functional layer of the eFFBD of the attitudecontrol system of the AOCS. Fig. 8 shows the eFFBD with its inputs, outputsand dataflows. The eFFBD shows that the process involves generally [7]:• Measurement of attitude and errors with reespect to external references• Determination of the attitude with respect to the local coordinate system• Computation of commands to correct the error• Execution of correction• Evolution of the attitude in accordance with the satellite dynamics6The listing above has been translated into a construct of functions whose labelsdescribe processes inside of the functions.The functional layer consists of an AND consturct with two parallel branches.The first AND branch describes the control functions whereas the second onerepresents the communication function. The subsequent branch of the Transmitdata to Telemetry Subsystem function enables the data to transmit, which is la-beled with Data to transmit. The functional series, which starts with the Detectfunction represents the steps for the attitude determination, computation ofcounter actions and application of these counter actions. First the environmentis detected by sensors. Then the signal is being processed to compute the atti-tude of the satellite. In the following step the computed attitude is comparedto a nominal value and the result is used to compute commands which will besend to the actuators where they will be applied.Figure 8: eFFBD of the functional layer of attitude control system74.2.1 Decomposition of ComputeThe function Compute is decomposed in the next lower system layer and pic-tured in fig. 9. It describes the functional series of steps to determine theposition. These steps are based on applying techniques of measurement tech-nology and signal processing. The input to this system layer is the output of theDetect function from the top-layer and are detected angles (Φ,Θ,Ψ)S . Thesesignals are supposed to be discrete and getting amplified and filtered to removethe noise in the signal. After processing the signal, it can be used to determinethe position in orbit, which is applied in the function Determine the position.Figure 9: eFFBD of decomposed function Compute4.2.2 Decomposition of Compute counter actionsThe function Compute counter actions computes the angular accelerations ofthe satellite due to external impacts and calculates counter actions in terms offorces, revolutions, etc are based on the angular accelerations (see: fig. 10).8Figure 10: eFFBD of AOCS49Additionally to the external influences, like air drag, gravity gradient, Earth’smagnetic field or solar pressure, the inertia of the satellite has to be regarded.First, the resulting disturbing momentums due to the space environment arecalculated in parallel branches by using AND elements. The functions are listedright below:• Calculate air drag torque• Calculate gravity gradient torque• Calculate magnetic torque• Calculate solar pressure torqueThe output results are added in the function Sum disturbances, whose outputis the momentum vector ~MDT . This momentum vector is used to calculatethe angular acceleration, which is the output result of the subsequent function.The angular acceleration is the input of the function to calculate the actuatorcommand to counter the disturbances and to move the satellite back to itsdesired position.4.2.3 Layered architecture of the AOCSThis chapter is dedicated to the merging of the operational with the functionallayer. The resulting eFFBD diagram is shown in fig. 11. In this eFFBD ithas been regarded, that the determination of the attitude of the satellite is aniterative process, which has the structure of a loop. The condition to leave theloop is when the end of operational lifetime has been reached. The structureof the loop and loop exit are describe by two LP and one LE element. Forthe arrangement of the operational and functional layer the hierarchy was takeninto account so that the operational order is above the functional layer.5 ConclusionThe eFFBD systems engineering approach was chosen due to the fact that itdescribes functions of the system and its order in which they are executed. Hencedata dependencies and decision flows between functions can be represented. Anoverview and description of the eFFBD structural elements are given which areapplied to design the AOCS.It could be shown that it is possible to design the AOCS of a scientific EarthObservation satellite with the eFFBD approach. Processes of keeping the orbit,attitude and the demanded deorbiting at the end of lifetime could be translatedinto a sequence of functions succesfully. It can be seen as a spin doctor toapply this approach to other subsystems of a satellite and even to their parentsystem. The data flow can be traced at any time in the diagram as well as inits decomposed functions. The future works are concerning the specifications ofthe links between each operational mode and some functions in the functionallayer.10Figure 11: eFFBD of the layered architecture of the AOCSReferences[1] National Aeronautics and Space Administration: Systems EngineeringHandbook, NASA/SP-2007-6105 Rev. 1, 2007[2] Charlotte Seidner: Ve´rification des EFFBDs: Model checking en Ingn´ierieSystm`e, Doctoral Thesis, Nantes, 2007[3] Ste´phanie Lizy-Destrez: Functional Design: eFFBD modelling, IS548-SDA/eFFBD, Lecture Script, ISAE-SUPAERO, Toulouse[4] Charlotte Seidner and Olivier H. Roux: On the Formal Verification ofEFFBD Models Using a Structural Translation to Time Petri Nets, Tech-nical Report RI20073 3695, Nantes,Revised, 2008[5] Holli Riebeck: Catalogue of Earth Satellite Orbits, last modi-fied: 04.09.2009, Internet, last access: 08.02.2012, 11:55am CET.http://earthobservatory.nasa.gov/Features/OrbitsCatalog/[6] Domoinique Luzeaux, Jean-Rene´ Ruault and Jean-Luc Wippler: Maitrisede l’inge´nerie des syste`mes complexes et des systmes des syste`mes,Lavoisier, Paris, 2011[7] Ge´rard Maral and Michel Bousquet: Satellite Communications Systems,John Wiley & Sons Ltd., Chichester, 5th Edition, 2007[8] Wiley J. Larson: and James R. Wertz Space Mission Analysis and Design,Microcosm, Inc., Torrance, California, 2nd Edition, 199211[9] Lemai, S., Charmeau, M.-C. and Olive, X. Inte´grer des planificateurs dansle logiciel de vol d’un satellite autonome, JFPDA’06 - Journe´es Franco-phones Planification, De´cision, Apprentissage pour la conduite de syste`me,Toulouse, FR, 2006[10] Jean-Charles Chaudemar and Eric Bensana and Christel Seguin Modelbased safety analysis for an Unmanned Aerial System, 7th IARP Workshopon Technical Challenges for Dependable Robots in Human Environments(DRHE), Toulouse, FR, June, 2010A Table of AcronymsAOCS - Attitude and orbital control systemeFFBD - enhenced functional flow block diagramLEO - Low Earth OrbitOBC - On-board computerB Table of SymbolsΦ - Yaw-angleΘ - Roll-angleΨ - Pitch-angle12

Catalogue of Earth Satellite Orbits, last modiﬁed: 04.09.2009, Internet, last access: 08.02.2012, 11:55am CET.

Jean-Ren´ e Ruault and Jean-Luc Wippler: Maitrise de l’ing´ enerie des syst` emes complexes et des systmes des syst` emes, Lavoisier,

Lizy-Destrez: Functional Design: eFFBD modelling,

Model based safety analysis for an Unmanned Aerial System, 7th IARP Workshop on Technical Challenges for Dependable Robots in Human Environments (DRHE),

National Aeronautics and Space Administration: Systems Engineering Handbook,

On the Formal Veriﬁcation of EFFBD Models Using a Structural Translation to Time Petri Nets,

Satellite Communications Systems,

V´ eriﬁcation des EFFBDs: Model checking en Ing´ nierie Syst` me, Doctoral Thesis,

Wertz Space Mission Analysis and Design,

Application of the Systems Engineering methodology to the design of the AOCS of an Earth Observation satellite

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Application of the Systems Engineering methodology to the design of the AOCS of an Earth Observation satellite

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