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Distributed Interactive Simulation forSpace Projects

L. Argüello & J. Miró Modelling and Simulation Section, ESA Directorate of Technical and OperationalSupport, ESTEC, Noordwijk, The Netherlands

What is distributed simulation and whatare its benefits?Distributed Interactive Simulation (DIS) allowsgeographically separated simulators to worktogether, interacting in real-time, to providepredictions just like a single integrated simulator.The technology also allows real entities to beincluded in the simulation loop. Before thisapproach can be applied to space projects,however, it has to be established whether currentsimulation and communications technologycan effectively support the critical requirementsof space scenarios. It also needs to bedemonstrated that this approach results in acost-effective solution compared with theconventional approach of centralised simulators.

The technical challengeThe technical issues involved in implementingthe distributed-simulation paradigm are relatedto interoperability and to the communicationslinks. Interoperability requires the simulator torespect a certain architecture in order to beable to communicate with the outside world. Asfar as the communications links are concerned,the main difficulty is in coping with the time-span data requires to travel from the originatingsimulator to the receiving one (the so-called‘latency’). In a real-time simulation, thedistribution of the simulation models at remotesites introduces an error, because the datarequired by one model from another physicallyremote model needs a finite time to travel overthe network. Assumptions about the currentvalues of the remote model parameterstherefore need to be made on the basis ofearlier values (extrapolation using ‘dead-reckoning algorithms’).

The key question to be answered here iswhether the error introduced by the distributionof the simulation can be kept within pre-defined, acceptable error bounds. This isevaluated by comparing results obtained fromthe distributed system with those obtainedfrom the non-distributed system. This will beparticularly critical for simulation applicationsinvolving closed control loops, such as areencountered in attitude and orbit controlsystems. For simulations with flight softwareand hardware in the loop also, the responsetime expected from the simulator will constitutea critical challenge for this approach. In additionto the latency, the real-time behaviour of thecommunication link will also be a criticalrequirement for real-time simulations, and onenot always possible to meet with conventionalcommunication protocols.

The network requirement is also an importantissue. Setting up a complex communicationsscheme is often difficult and requiresconsiderable effort. The application of modernDIS technology not only simplifies thedistribution of the data and the supporting

Distributed Interactive Simulation is an innovative technology that willdramatically change the way in which simulation is developed andapplied in space projects. It will only be effective, however, if based onwell-accepted standards, such as the IEEE High-Level Architecture(HLA) standard. A number of studies and experiments have beencarried out as part of ESA’s R&D effort to evaluate the benefits ofdistributed simulation for space projects in general, and theInternational Space Station in particular. These have led to the firstever applications of the HLA standard to the space domain. Promisingresults have already been obtained with the simulation of theAutomated Transfer Vehicle’s rendezvous with the Space Station andof satellite payload operations, which can be extrapolated to otherspace projects and scenarios.

distributed interactive simulation

These questions have been addressed in anumber of studies at ESA, including earlyexperiments conducted in the framework of aco-operation between ESTEC and the GagarinCosmonaut Training Centre (GCTC) in Russia.The distributed interactive simulation paradigmcan be implemented in many ways, but in orderto become a useful technology it has to bebased on a standard defining the interfacebetween two interacting simulators/entities, i.e.defining how two or more simulators/entitieshave to talk to each other. The co-operationbetween ESTEC and GCTC has led to the firstever application of this technology in a spacecontext, based on well-accepted standards.

Figure 1. Overview of anHLA-compliant distributed

simulation

network architecture and protocols required,but also reduces the bandwidth needed to aminimum, making the use of affordable ISDNlines and equipment possible.

Applicable standardsThe first standard for interactive distributedsimulation was IEEE 1278.1, also known as theDIS protocol. This standard, whose generationwas sponsored by the US Department ofDefence through the Defence Modelling andSimulation Office (DMSO), was appliedextensively in defence simulations. It wasbased on the use of standard formattedpackets, designed for the data required bythese specific applications. Problems due tothe inflexibility and lack of scalability of thisapproach have eventually led to a completelydifferent approach, the High Level Architecture(HLA), which is in the process of becoming theIEEE 1516 Standard.

The elements of an HLA-compliant distributedsimulation are summarised in Figure 1. Thevarious components of the ‘federation’, the‘federates’, are described using the ObjectModel Template (OMT). During a distributedsimulation, the federates must interact inaccordance with the HLA interface specification.While HLA is an architecture, the Run TimeInfrastructure (RTI) is the software needed tosupport simulation execution.

Practical experienceTo evaluate the distributed simulation approachfor space, a number of practical applications(experiments) have been implemented. The firstever application of this technology to the spacedomain was demonstrated in the framework ofthe co-operation between ESTEC and GCTC.The results obtained both confirmed thefeasibility of the approach and highlighted the

critical issues for its application in the spacedomain. The resulting demonstration systemwas deployed to European industry, andfacilitated the initiation of several related R&Dactivities in the frame of the European Unionprogramme for High Performance ComputerNetworks (HPCN).

Another experiment was performed in parallelusing a satellite simulator to validate the use ofHLA in the context of distributed payload usercentres. The application of distributedsimulation in the context of the InternationalSpace Station, and more precisely forspacecraft proximity operations, was furtherinvestigated in the framework of ESA’sTechnology Research Programme (TRP).

The results of these activities are summarisedin the following paragraphs.

Spacecraft rendezvous A distributed simulation of the rendezvous anddocking (RVD) of the Automated TransferVehicle (ATV) with the International SpaceStation (ISS) has been implemented in order tovalidate the technology in a challenging spacescenario. This scenario is particularly criticaldue to the very tight coupling of the twospacecraft through the ATV trajectory controlloop, and the very small tolerances for thedocking in terms of linear and angulardisplacements. This means that the accuracyrequirements for the position and velocity of theinteracting spacecraft are very high, and theerror introduced by the communication latencyhas to be kept at least one order of magnitudebelow the docking tolerances.

Several simulation experiments were carriedout with simulation nodes at ESTEC, GCTC,ESOC and several industrial sites in Europe.

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Figure 2. Three-dimensionalvisualisation of the ATVapproaching ISS

Figure 3. The ATV viewedthrough the ISS camera

emphasis here is on the early detection ofproblems, which can occur long beforesystem integration is attempted. This shouldalso help shorten the development cycle.

– The operational procedure validation contextassumes the need to define the proceduresand the parameters to be monitored andassociated thresholds for nominal andcontingency scenarios, involving more thanone ISS segment.

– The mission rehearsal and training contextassumes the need for multi-segmentintegrated simulations involving severalcontrol centres and the crew. It also coversremote access to high-fidelity simulations forcrew members.

The RTI software infrastructure required toimplement an HLA federation was madeavailable by the DMSO. The use of ISDN as thebasic communications infrastructure wasselected as the most cost-effective andpractical solution.

Mission scenarioThe simulation scenario was the rendezvousand docking of the ATV to the ISS both inautomatic and in manual mode. 3D visualisationwas used to monitor the manoeuvres and toassist the manual control, activated in case ofcontingencies. Figure 2 shows the finalapproach manoeuvre from above. Figure 3shows a view through an ISS-mountedcamera, used to monitor the final metres of theapproach. The overlay parameters provideinformation on relative position and attitude.

For the purposes of DIS, it was decided toconcentrate only on the ATV manoeuvres to beperformed near the Station, the beginning ofthe final translation being selected as thestarting point for the simulation scenario.

ContextThree different demonstration scenariosrelating to the Rendezvous and Docking (RVD)of the ATV to the ISS were selected, pertinentto different phases in the ATV development life-cycle: – The collaborative engineering context

assumes distributed simulation involvinggeographically distributed industrial partnersresponsible for different parts of the ATV. The

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Figure 4. Geographicalconfiguration of the RVD

distributed simulation

Distributed scenarioThe entities represented in the simulation(‘federation’ in HLA terms) are the ATV vehicle(federate called ATV-F), the ISS (ISS-F) and theMission Control Centre (MCC-F). A federationmanager (FM-F) was defined to implement thesimulation control functions. Switch over fromautomatic to ISS crew control is decided by theMCC, which also prescribes the flight plan andmonitors the manoeuvre.

The geographical allocation of the abovefederates is configurable, but for the experimentthe configuration selected (Fig. 4) was: ISSsimulated at GCTC (Star City, Russia), ATVsimulated at ESTEC (Noordwijk, TheNetherlands), and the Mission Control Centrefederate simulated at ESOC (Darmstadt,Germany). Table 1 shows the functions allocatedto the different simulation nodes.

ResultsThe limits considered allowable based on thesimulation requirements were expressed in theform of misalignments of 0.02 m in position andof 0.3 – 0.5 deg in orientation. However, this isnot sufficient in order to assess some of theintegral performances of a simulation session,e.g. it is conceivable that the differences instate vector components are within theprescribed boundaries, but that the totalamount of fuel consumed differs considerablycompared to the non-distributed simulation.This would render the simulation inadequate,since it could trigger wrong decisions andunnecessary changes in control strategy.

The simulation results show that the accuracycriteria are met even for an acceleration of thesimulation by a factor of 4 with respect to realtime. This is equivalent to increasing the latencyby the same factor. It was therefore proven thatthe delay introduced by the distributedapproach does not affect the overall validity ofthe simulation.

Distributed payload user centresA distributed simulation experiment has beencarried out at ESTEC taking a small technology-demonstration satellite mission, Proba, as abasis. The purpose of this experiment was toevaluate the applicability of HLA for familiarisingand training satellite payload users.

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Table 1. Functions allocated to the different ATV/ISS RVD simulation modes

ATV-F:– GNC subsystem– Chaser orbital mechanics (differential equations of motion)– Computation of Chaser dead-reckoning (DR) parameters– Reconstruction of the Target (ISS) variables using its DR parameters– Computation of variables describing relative (ATV-ISS) motion– Computation of parameters and variables for the GUI– Process inputs from the remote control post– Introduction of failures and contingencies onboard ATV

ISS-F:– Target GNC subsystem– Target orbital mechanics (differential equations of motion)– Computation of Target dead-reckoning (DR) parameters– Reconstruction of the Chaser variables using its DR parameters– Computation of variables describing relative (ATV-ISS) motion– Computation of parameters and variables for the GUI– Remote control post functionality– Computation of the position of Sun– Module to initiate CAM

MCC-F:– Reconstruction of Chaser and Target variables using DR parameters– Remote control post functionality– Computation of variables describing relative Chaser and Target motion in various

coordinate systems (for 3D and 2D graphics, data logger)– Algorithms to form and modify the Mission Plan for Chaser

FM-F:– Federation management (commands like “restart”, “resume”, “pause”)– Changing of the time-scale factor– Introduction of failures and contingencies

Figure 5. Overview of theProba project test-beddistributed simulation

pre-release versions), savings in the time andeffort needed for installation of simulationproducts locally, savings in computer hardware,and access to simulations not otherwiseavailable (e.g. access to high-fidelity simulationsat industrial sites).

In particular, in the case of the ATV study,productivity gains and a reduction of 20% indevelopment time seem achievable throughcollaborative engineering during the developmentphase, depending on the duration and scope ofthe simulation campaign. Early analysis ofcoupled effects is an area where distributedsimulation becomes an enabling technology.

In the case of the ISS, for example, thepotential of distributed simulation to savedevelopment effort and time is significant dueto the distributed nature of the project, involvingnumerous geographically separated partners,and to the large number of simulation facilitiesdistributed throughout the world. The use ofdistributed simulation in support to the multi-segment operations and training involving crewand ground-station personnel within theInternational Space Station programme is in theprocess of evaluation.

The HLA standards are in the process ofbecoming an IEEE standard and showconsiderable potential for being widely appliedin a variety of simulation domains. The real-lifeexperiments carried out by ESA andsummarised here highlight the potential tosupport operations preparation tasks usingaffordable, commercial ISDN lines. Since theuse of this technology only makes sense in aglobal context, its broad adoption by industry

Mission scenarioThe Proba simulation focussed on one of thepayloads, namely an imager. Its users,distributed at different locations, will be able tosend observation requests to the satellite (viathe Control Centre) and will receive directly theimage requested. The mission-simulation partof the Project Test Bed has been re-engineeredto work in a distributed configuration. Using thedistributed approach allows the paralleltransmission of various selections of thetelemetry produced by the simulator to severalremote monitors in parallel, and the receptionof telecommands from a remote user station.

The distributed simulation experiment (Fig. 5)consisted of the mission simulator and theseparate control and monitoring tasks(telecommand, telemetry, event table MMIs,Earth track graphical displays and 3Dvisualisation) running in a distributed manner,both at ESTEC in Noordwijk, representing themission Control Centre, and at Headway (UK)simulating the remote user centre.

The users located at the remote user centresare able to: – send observation requests to the Control

Centre– monitor the outcome of spacecraft autono-

mous operations following user imagerequests

– display spacecraft position, orbital track andground-station visibility zones on a 2D map.

The Control Centre at ESTEC is able, inaddition to the user operations, to:– uplink telecommands and downlink house-

keeping data when the spacecraft is incontact with the ground station

– monitor an on-board event table containing the housekeeping history from the last ground-station contact

– provide visualisation of a realistic model ofthe spacecraft overlaid with spacecraft bodyvectors as well as Sun-, Moon- and Earth-pointing vectors, and real-time visualisationof the pointing manoeuvres required duringthe mission lifetime (i.e. Earth, ground-stationand user-station pointing) on the 3Dvisualisation (Fig. 6).

In particular, the test executed demonstratedthe stability of the distributed system while thedistributed federates join and leave thefederation. Data updates were not affected byfederation management.

Lessons learntThe advantages of distributed simulationversus the conventional approach are: earlieravailability of the simulation (since it can use

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Figure 6. A typical Probatest-bed simulation

and other space agencies is required before itcan be exploited effectively.

Some critical issues associated with thistechnology also need to be pointed out. Thetools needed to build the distributed simulationsystem according to the HLA standards areonly starting now to become available. Thesoftware infrastructure needed to conductdistributed simulations has not yet reached thestandard of a commercial product. Distributedinteractive simulation also conflicts with theimplementation of computer-access securitymeasures in that dedicated systems have to beplaced outside firewalls or access throughfirewalls needs to be granted. Last but notleast, significant expertise is required toconfigure the simulation computers tocommunicate over ISDN lines.

Future workIt is planned to focus the future work on threespecific areas:– Deploying the distributed rendezvous

simulation system implemented in the ESAR&D effort for a transatlantic demonstrationinvolving NASA and ESA: this would allow

NASA to evaluate this technology for ISSoperations and training.

– Establishing a prototype infrastructure atEuropean level to facilitate the use of thistechnology by space industry in support ofcollaborative engineering: the infrastructureshould include a space federation model,guidelines for plug-and-play in this federation,

and the associated software tools.– Extending the demonstration to simulation

systems including flight hardware in the loop,typically the on-board computer.

AcknowledgementsThe authors wish to acknowledge the pioneeringwork carried out by GCTC in the framework ofthe co-operation with ESTEC, and its initiativein promoting the distributed simulationtechnology. The support of DMSO, whichprovided training courses and software to ESAand European industry in order to facilitate theimplementation of the applications presented inthis article, also merits special mention. Finally,the authors want to acknowledge the valuablework carried out by the companies D3 (D), AML(F) and Headway (UK). r

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