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A MATLAB based Distributed Real-time Simulation of Lander-Orbiter-Earth Communication for Lunar Missions

A MATLAB based Distributed Real-time Simulation of Lander-Orbiter-Earth Communication for Lunar Missions
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  See discussions, stats, and author profiles for this publication at: A MATLAB based Distributed Real-timeSimulation of Lander-Orbiter-EarthCommunication for Lunar...  Article  · January 2010 CITATIONS 0 READS 451 8 authors , including:Diptyajit ChoudhuryLund University 1   PUBLICATION   0   CITATIONS   SEE PROFILE Haseeb ZiaÉcole Polytechnique Fédérale de Lausanne 3   PUBLICATIONS   0   CITATIONS   SEE PROFILE Peter LukschUniversity of Rostock 45   PUBLICATIONS   99   CITATIONS   SEE PROFILE Amitava GuptaJadavpur University 64   PUBLICATIONS   620   CITATIONS   SEE PROFILE All content following this page was uploaded by Tiyasa Mitra on 13 January 2017. The user has requested enhancement of the downloaded file.  A MATLAB based Distributed Real-time Simulation of Lander-Orbiter-Earth Communication for Lunar Missions Authors DiptyajitChoudhury  1  , TiyasaMitra 2  , HaseebZiah 3  , AleksandarAngeloski  3  , Hilmar Buchholz 4  , Andr  é Landsmann 4  , Peter Luksch 4  , AmitavaGupta 5 1. Instrumentation and Electronic Engg. Jadavpur University, India {}2. Instrumentation Engineering, Indian Institute of Technology, Kharagpur, India {}3. LRR, Technische Universitaet, Muenchen, Germany {,} 4. VHR, Universitaet Rostock, Germany {,} 5. Power Engg. Department, Jadavpur University, India {} Introduction • Lunar explorations often involve use of a lunar lander , a rover and an orbiter which revolves around the moon with a fixed radius. • Communication in such deep space missions is usually done using a specialized protocol like Proximity-1. • In this paper it is attempted to simulate, in real time, the communication between a tracking station on earth (earth station), a lunar orbiter with a polar lunar orbit and a lunar rover using concepts of Distributed Real-time Simulation(DRTS). • The objective of the simulation is to simulate, in real-time, the time varying communication delays associated with the communicating elements with a facility to integrate specific simulation modules to study different aspects e.g. response due to a specific control command from the earth station to be executed by the rover. Our DRTS application •  We propose to use concepts of Parallel DiscreteEvent Simulation to simulate the interactionbetween the earth-orbiter-lander/rover over amessage passing environment. •  The system comprises 4 tasks and these are:a)Orbiterb)Lander/Roverc) Earth Stationd)Event scheduler • All events are assumed to be time-stamped. The eventscheduler uses a Time-stamp ordering to process events. • Data is sent over a Deep Space Network using a DRTS approach. • This calls for time synchronization which insures that data  sed - receives  lie within specific intervals. • The time synchronization algorithm presented here is adjustedso as to accommodate the scenario discussed above but it iscomposable and hence of emphatic utilization in lunar and otherspace mission benchmark testing. The Individual Processes The Earth Station The earth station islocated on the earthsurface and it is the seatof all data receivedfrom, or control signalssent to, deep space. ADirect-To-Earth (DTE)link is present betweenthe orbiter and theearth station. The Orbiter The orbiter is another stand-aloneprocess which serves as a relay forthe messages from the earthstation to the rover. It is in a polarlunar orbit. This is special becausethe latency for DTEcommunication decreases as itmoves away from the moon. Theorbiter communicates with theearth station as well as the rover. The Rover The lander or the rover is thespacecraft which lands on themoon. The local sensors,instruments on the rover makethe desired data from the  oos surface or atmosphere, availablefor transmission to the orbiter,which in turn forwards the data tothe earth station. The Event Scheduler The fourth process is a virtual oneand has no physical existence.Message form one physicalcomponent to another constitutesan event. The orbiter, earth stationand the rover cannot directly senddata to each other. All events arehandled by the event scheduler.Thus an event representingtransmission from the rover to theorbiter is actually two events viz. atransmission from the rover to theevent scheduler followed by atransmission from the eventscheduler to the orbiter.  State of the system The state of the system is defined by a tuple { d1,d2,f,t  } where , d1  is the distance between orbiter and the earth station; d2  is the distance between rover and the earth station ;  f   is the flag denoting the visibility of the orbiter and rover and t   denotes the time stamp information. The data transmission events • Message from one component to another constitutes an event. • The orbiter, earth station and the rover cannot directly send data to each other -all events are handled by the event scheduler. • Thus an event representing transmission from the rover to the orbiter is actually 2 events viz. a transmission from the rover to the event scheduler followed by a transmission from the event scheduler to the orbiter. Synchronization Event Explained 1.We consider that there are internal clocks in each of the four processes station, lander, orbiter and event scheduler.2.Initially, the clocks of the earth station and the event scheduler are started together, and hence there is inherent time synchronization between them.3.We consider the scenario that the earth station is sending some message to the orbiter. Firstly, it sends the data and its time stamp to the event scheduler.4.The event scheduler uses the value of d1 to calculate the delay (in real time) that will occur as the message is sent over DSN to the orbiter.5.As the simulation is done in real time, the event scheduler waits for that time delay, and then it transmits the message(  trasit -with- delay) .6.As the orbiter is orbiting around the moon, the distance between itself and the earth station changes continuously, giving rise to varying levels of latency and this is appropriatelyaccounted for , by running an independent program which continuously monitors the position of the orbiter and calculates d1.7.The effective idea is that the clock on the orbiter gets synchronized with the one on the earth station as a result of the time scheduler  trasit -with- delay  action.8.Now let us consider the second part of our communication scenario in which the orbiter clock should synchronize with the clock on the rover.9.Here again, the event scheduler uses the value of d2 to calculate the delay (in real time) that will occur as the message is sent over DSN from the orbiter to the rover.10.As the simulation is done in real time, the event scheduler waits for that time delay, and then it performs the  trasit -with- delay  action.11.Now an argument might arise that as the orbiter is orbiting around the moon, varying levels of latency might arise for communication with the rover as well.12.But the proposed model ignores the same, considering the fact that due to the high ratio between d1 and d2 ( 380,000:100) we can safely ignore any such latency for real timecommunication.13.The effective idea is carried forward from the first part of our communication scenario and the rover clock gets synchronized with the one on the orbiter. Hardware used to run the simulation The hardware platform comprises four single board computers operating as stand-alone real time systems (developed by MATLAB xPC target and inter-networked using UDP-IP protocol).The choice of the single board computers over normal PC s is governed by their speed of computation, which is ideal here, as required by the DRTS setup. UDP-IP protocol is chosen overTCP-IP as the primary important criteria for the protocol is time critical transmission rather than the semantics of the message transferred. Software used One of the models developed using the Stateflow tool of the Simulink library is shown here to give a visual aid as to the software interface and programming used in the simulation. Asmentioned, the system is highly composable and any additional components can be added or removed from the models shown below , to cater to the specific benchmarking needs. The OrbitermodelDiscussions and Conclusion • Dynamic (usually mission critical) events like launch, trajectory correction maneuver, spacecraft safing, and orbital insertion generally involve real time monitoring of the spacecraft(orbiter) fromthe earth station through an algorithm containing relevant data and DSN communication considerations. • Prior Mars exploration studies conclude that the deployment of a Mars orbital infrastructure to support the telecommunications and navigation needs of the surface elements is essential to thesuccess of the future Mars exploration missions. • Considering a similar proposition for lunar exploration, the planning and scheduling of the intercommunications between surface elements, orbiters, and Earth subjected to various constraints of deep space communications is a unique and challenging problem, and plays an important role in the efficient utilization of the lunar orbital infrastructure. • But for countries like India, who lack a well developed orbital infrastructure, the proposed DRTS setup serves the same purpose. • Thus, this proposed benchmark testing model will not only propel the space explorations of countries with limited aerospace technologies, but also pave the way for future composable systemswhich can be used by several countries to monitor and successfully carry out their lunar exploration targets. • Further investigation of designing a composable technology base for the international Space station is proposed in conclusion to this paper, which will pave the way for international co-operationin DSN and TT&C constraints, and thus enhancing the realms of human knowledge in the domain of lunar missions in particular, and space exploration in general. 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