Challenges of the ExoMars Rover Control Luc Joudrier and A. Elfving et al.

ESA, Noordwijk, The Netherlands, 2200

F. Ravera, G. Battistoni, P. Francescetti et al. Thales-Alenia Space Italy, Torino, Italy

and

Nuno Silva and Andrew Davies et al. ASTRIUM Ltd, Stevenage, UK

This paper describes the control aspects of the Martian rover that has been studied in the frame of the phase B2 of the ExoMars project. This challenging European mission is aiming to search for extinct or extant life on Mars by sending a rover fitted with a drill able to collect subsurface samples down to a depth preserved from radiation effects. The rover is carrying a suite of scientific instruments, the Pasteur payload, aiming to analyze the crushed samples. The operations are foreseen over six months, on a daily basis, imposing a certain level of autonomy & automation as discussed in the paper.

I. Introduction he ExoMars mission’s scientific objectives, in order of priority, are: • •

T To search for signs of past and present life on Mars; To characterize the water/geochemical environment as a function of depth in the shallow subsurface;

• To study the surface environment and identify hazards to future human missions; • To investigate the planet’s subsurface and deep interior to better understand the evolution and habitability of Mars.

The search for life and sub-surface sampling (drill) represent capabilities not previously flown on other Mars Exploration Missions. NASA’s two Mars Exploration Rovers (MER) were devoted to geological investigations. The Mars Science Laboratory (MSL), planned to be launched in 2011, will have the goal to identify habitable environments. ExoMars (launch 2016) will be the first mission, following the Viking (1976) landers, directly addressing the search for signs of life on Mars. ExoMars will combine mobility and access to subsurface locations, where organic molecules may be well-preserved; thus allowing, for the first time, to investigate Mars’s third dimension: depth.

ExoMars is the first European planetary rover mission. The rover control capabilities required to achieve the mission objectives presented in this paper are only one aspect of this challenging mission. A comprehensive overview showing the phase B1 design can be found in [1].

The rover Guidance, Navigation and Control (GNC) architecture will be first briefly presented with the intended supporting sensors. Then, the three major functions will be described:

• Navigation to perform path planning on the terrain; • Localization to estimate rover position and attitude; • Control to execute the trajectory remaining in a safe corridor.

In addition, the intended control of the arm implementing single command placement using vision closed loop will be presented.

Finally, rover autonomy aspects are discussed and put into the context of the Rover Operation Control Center process that will command the rover in a sol by sol scheme.

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AIAA Infotech@Aerospace Conference <br>and <br>AIAA Unmanned...Unlimited Conference 6 - 9 April 2009, Seattle, Washington

AIAA 2009-1807

Copyright © 2009 by ESA, ASU, TAS-I. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

Figure 1: ExoMars Rover Design at the end of phase B2

II. The rover locomotion subsystem design

The locomotion subsystem is shown in Figure 1 above as designed at the end of phase B2. It includes passive suspension as well as active mechanisms used to deploy the rover and to enhance traversability capabilities.

The locomotion subsystem (terminology that is preferred to rover chassis) is the results of extensive studies based on work performed by the Russian company RCL for ESA [8] that has been enhanced by the ExoMars locomotion subsystem contractor [6] and [7].

The locomotion formula is 6*6*6+6W, meaning, 6 wheels, 6 driving wheels, 6 steering wheels and 6 walking wheels. The suspension has been reduced to the minimum of 3 bogies directly attached to the rover body. In order to avoid averaging mechanisms necessary for rocker-bogie-like locomotion concepts, a transversal bogie has been put at the rover’s rear. This has some impact of the traversability capabilities of the rover but the savings in mass, complexity and AIV (Assembly, Integration and Verification) are significant.

The 6-wheel steering is also an asset for locomotion and navigation, as it will allow decoupling the rover heading and position to facilitate placement of the arm and drill tool to the selected location. Double Ackerman steering and point turn capabilities are however the main capabilities required to follow a trajectory.

The 6 walking wheels provide the capability to self-deploy from the stowed configuration in the descent module. They also provide additional capabilities using wheel-walking coordinated movements, also called peristaltic movements that had been demonstrated by Russian Marsokhod rovers in the past. This additional feature, combined with slippage estimation will provide the capabilities to extract the rover from difficult situations similar to those which the MER rovers have experienced in being stuck into loose soil dunes.

An additional peculiar characteristic of the locomotion subsystem are the flexible wheels that allow keeping the wheel diameter and width to a reasonable size for easing stowage and deployment while providing reasonable traction [9]. The effect of this flexibility during specific operations like drilling will be tested soon on the ExoMars breadboard that will be fitted with a drill functional breadboard.

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The other mechanisms fitted on the rover are: • The deployable mast and associated Pan&Tilt mechanism holding the navigation cameras, the

PanCam scientific instrument (2 Wide Angle Cameras and 1 High Resolution Camera), and the infrared spectrometer MIMA.

• The drill deployment mechanism (1 rotation and 1 translation) including a jettisoning mechanism should the drill be stuck in a hole, the Drill tool itself, including a number of mechanisms for rotation, advance, and connection/deconnection of the drill rods.

• The 5DoF Instrument Arm and its Rock Abrasion Tool holding contact instruments. • Solar Array mechanisms to deploy and trim the orientation toward the sun. • The Analytical Laboratory Drawer (ALD) is which the analytical instruments of Pasteur payload

are fitted with a number of mechanisms to transport, crush and expose the sample fines to the instruments.

The design and control of Drill and ALD are very complex taking into account the Planetary Protection

requirements to avoid the samples being contaminated by Earth. Their description is out of scope of this paper.

III. Rover GNC Requirements and Architecture The driving requirement for the rover GNC are:

• 100m traverse per sol on a reference terrain with the slopes of 18° (3σ) over 10m and 10° (3σ) over 100m with rock distribution according to Viking Landing 1 abundance [10].

• Any longitude with latitude in the range 5° South to 45° North • Autonomously travel 100m per sol, to within 1m in position and 2º in heading at arrival (an

additional 1m error is currently allocated to the target definition error by the ground) In addition to these testing requirements, additional requirements coming from the operations elevate the rover control challenge:

• Single placement command of the instrument arm from 3m away of the target • Accurate knowledge of the trajectory actually executed during the ground penetrating radar mapping • Accurate deployment of the drill to the ground specified drilling location

The combination of Localisation, Navigation and Locomotion control will allow the rover to safely reach targets

assigned by the Ground. The Figure 2 below describes the intended GNC architecture further described in [2]. As Localization and Navigation are based on image processing, a dedicated co-processor and an FPGA will help to reach the mean velocity requirements (about 40m/h). It is intended to overcome the processing limitations encountered by MER rovers, described by Mark Maimone in [4], thanks to the now common availability of FPGA technology for space.

Main processor

IMU

SS

LocCam

IMU processing

SS processing

Cyclic TM

Acyclic TM

Localisation Function

Trajectory Control

Function(<=5Hz)

LSS High-Level

Control Function

(5Hz)LSS

hardwareRover

DynamicsLSS TM

processingCyclicCAN TM (10Hz)

LSS forward kinematics

(10Hz)

Cyclic TM LocCam processing

Acyclic TM NavCam processing

Navigation Function

Localisation Visual Odometry

LSS TC processing

Cyclic CAN TC (5Hz)

Co-processor (while rover is moving)

Co-processor (path planning, while stationary)

Rover real continuous state

NavCam

Figure 2: ExoMars Rover GNC Architecture

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The navigation will use the stereo cameras (Navcam) placed on the mast at 1.7m above the ground. Alternatively the localization cameras fixed at the front of the rover at 0.8m will be used to perform the visual odometry. LocCam will have the same stereo baseline with a wider field of view as the Navcam. They will also be used as redundancy for the NavCam, accepting that navigation operations would be degraded depending on the terrain. The rear LocCam are used for redundancy of the front LocCam, in addition to provide better view of the rear landscape. Note that the functions and names of the various stereo cameras are different from the MER terminology, though ultimately, most of the capabilities described in [4] are also intended to be implemented on ExoMars.

IV. Local Autonomous Navigation Local Autonomous Navigation will reuse the algorithms developed by CNES over the past ten years [3],

performing path planning from DEMs computed with stereo cameras. The process involves stopping every few meters to plan the path that avoids non traversable areas and then execute the trajectory within an allowed corridor. It is not foreseen to have active hazard detection during the rover movements and the robustness of the concept relies on the localisation function that will be described below.

Starting from the algorithms provided and supported by CNES [3] as contribution to the ExoMars project, ASU has experienced the process with breadboard stereo cameras:

• Perception: creation of a local terrain Digital

Elevation Model from stereo image processing of the navigation camera placed on the mast.

• Navigation Map creation: classification of DEM

cells. It includes “binary” classification as non-navigable and navigable, but also an optimised weight distribution through the navigable cells - see Figure 3. The Navigation safety is achieved by checking if the rover can be safely placed at every point of the DEM and turn on spot. The drawback is the processing time, thus in order to have it deemed acceptable, checks are performed with an idealised simple rover model described in Figure 4.

• Path planning: based on the Navigation Map,

computation of a path that allows the Rover to remain safe and to maximize its travelled distance/speed. Initial primitive path is enhanced with a dynamic based planner to take the rover capabilities into account and optimize the planning, see [2] for more details.

Figure 3: Schematic of navigation maps and path

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6 wheeled Rover Model

Simplistic Rover Model

Digitalized Simplistic

Rover Model

Wheel width

Wheel width

Circle diameter = 2*parameter Each square

corresponds to the DEM resolution

6 wheeled Rover Model

Simplistic Rover Model

Digitalized Simplistic

Rover Model

Wheel width

Wheel width

Circle diameter = 2*parameter Each square

corresponds to the DEM resolution

Digital Elevation MapDigital Elevation Map

Figure 4: process of Digitalized Simplistic Rover Model to be computed in the DEM

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The centre of the Digitalized Simplistic Rover Model is placed in each point of the DEM for different orientations (turn on spot).

The navigation criteria are then checked: • Discontinuity: The difference between the lowest and highest altitude under the wheels shall be lower

than the maximum discontinuities parameter. • Slope: The inclination of the model when the wheels are “touching” the highest altitude shall be lower

than the maximum slope parameter. Test campaigns have been performed in order to verify the DEM creation process both with real images and with

virtual images produced from a simulator - see figure 5 below.

(a) (b) (c)

Figure 5: Left (a) and right (b) stereo images leading to DEM (c)

To account for perception, localization and execution errors the obstacles are dilated in the navigation maps. The navigation process as described above occurs about every 2 meters. The navigation maps are merged and a traveled area is maintained in memory should the rover need to go back.

The intention is to have the regional planning being performed by the ground operators based on orbital images. The operator will then upload intermediate targets and forbidden areas. The rover will provide a concatenation of the navigation maps in order to help the operator to understand the actual difficulties of the terrain ahead of the rover by analogy with previous similar terrains.

The rover stops and computes the path before execution of the trajectory. The current computation estimations lead to the need to stop for about 50seconds every few meters (typically 2m, but the length is dependant of the terrain difficulty). This duration accounts for the mast oscillation damping.

V. Localisation The localisation function, a key GNC rover functionality, will use advanced visual odometry techniques to

compute relative position and attitude variations. The Localisation function will allow estimating rover slippage and therefore, will represent an essential element of the rover’s safety, complementing the FDIS (Fault Detection Isolation & “Safing”) monitoring.

As the Technology Readiness Level (TRL) of the visual odometry was deemed very low, in addition to the need of being able to compare various selected techniques on a comparable and realistic environment, ASU organized in June 2008, a test campaign in the CNES Mars Yard in Toulouse called SEROM. This 50m by 80m terrain covered with basaltic grained soil and rocks of various sizes is equipped with a laser tracker allowing registration of targets fitted on a rover and thus providing a ground truth to compare with the rover localization estimation. The ASU Bridget rover was used for the test (see figure 6), while the two ExoMars locomotion subsystems breadboards were being manufactured.

Figure 6: ASU Bridget rover fitted with 5 different localization sensors and the laser tracker targets (square plate)

Five different techniques have been tested and the Structure-from-Motion developed by Roke Manor Research Limited, based on Harris points tracking, has been baselined while the CNES visual odometry and NPAL system [10] developed for planetary landing, have been kept as back-up solutions.

The achieved performance over 55m traverse was 0.4m and 2 degrees error in heading without use of IMU or sun sensor information. This performance was reached with operational conditions far from perfect, giving confidence that the ExoMars localization requirements can be met. See figure 7 below for the path computation. This test has been very useful to the rover GNC stakeholders. Lessons learned at this occasion will clearly enhance the future tests with the ExoMars rover breadboards.

Figure 7: Post-processed trajectory with various image rates

Post-processing of the stereo images at different rates (see figure 7 above) also allows a trade-off comparison between performance and processing load.

One important challenge is the implementation of the Structure-from-Motion algorithms within the available co-processor, without reducing the locomotion mean velocity. Current estimations predict processing the images within 10seconds. ASU is currently working to confirm the feasibility to achieve these figures.

Under this assumption, about every 10cm (mean rover velocity of 1cm/s), the localization is updated. However, this update arrives 10s after the rover has passed the update location. Between the updates, the rover relies on internal sensors and more classical odometry in order to keep the knowledge of the rover position within 10cm at all time (target is 5cm).

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The rover also has need of computing the absolute attitude and especially the absolute heading. ASU has tested a number of algorithms to achieve this (see [2]). Sun sensors are currently the baseline in which two are present in order to cope with mast shadows in addition to provide redundancy. The Navcam could also be used to compute the absolute heading from sun elevation with the drawback that this information could not be used while driving. The dust is not seen as an issue due to the limited operational lifetime of 6 months falling outside the dust storm season where there is sufficient transmission of light through fine dust layer.

Further investigations are on-going to understand which sensors are the best to compute absolute heading and relative heading (IMU and/or sun sensor and/or Navcam) with minimal mass impact.

VI. Locomotion Control The rover locomotion control aims at executing the trajectory computed by the path planner. It is responsible for

keeping the rover in a safe corridor using the localization information. The design of the dynamics-based path/trajectory control system is broken down into the following sub modes:

• Nominal Trajectory Following Mode (NTFM) • Point Turn Mode (PTM) • Large Disturbance Recovery Mode (LDRM) The margins concept to trigger the various control modes are illustrated in Figure 8 below:

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The Nominal Trajectory Following Mode (NTFM) is designed to drive the Rover along a refined trajectory using only Ackermann turns. There is a lateral displacement controller, a heading controller and a speed controller. It is envisaged that nominally the speed controller will feed the speed demand from the speed profile directly into the locomotion system. However the speed controller may decide to back off the speed from the value held in the speed profile if a situation occurs that was not predicted and that would benefit from a reduction in speed.

Point Turn Mode (PTM) is only entered when the rover is at the start of a path segment and the rover’s heading

is not within tolerance of the initial trajectory tangent of the path segment. The maximum acceptable offset will be defined by NTFM’s ability to handle misalignments. It is thus envisaged that the maximum offset will be in the order of 10°. PTM will command the rover to make a point turn to align the rover’s heading to within the permitted tolerances. Once this is achieved, PTM exits and NTFM is entered to drive the rover along the path segment. The rover controller cyclically determines the corrective maneuvers that need to be performed. This is sent to the locomotion subsystem control which transforms the vehicle commands (velocity, mode, center of rotation) into motor commands. Large Disturbance Recovery Mode (LDRM) is triggered when the rover is found outside the NTFM corridor. It mainly uses Point Turn Mode (PTM) to return the NTFM corridor. Should the rover go further away from this boundary, it would plan a new path.

VII. Instrument Arm Control The instrument arm (figure 9 & 10) control is

composed of the classical robotic manipulator control capabilities: joint space motion, Cartesian space motion, self-collision and obstacle collision detection.

As the instrument arm will be deployed on an area that will have been imaged by the PanCam stereo cameras, the operators on ground will have sufficient information to define the path toward the defined target before upload. It is therefore not necessary to implement a complex on-board collision avoidance system. Margins into the planning will be set accordingly.

However, the requirement for a single command to place the arm from 3 metres means that the rover must move toward the target and then deploy the arm to place it on the targeted rock to within a few cm accuracy.

In order to reliably achieve this accuracy on a rough terrain, a vision closed loop is envisaged,

reusing the capabilities of the Structure-From-Motion that will be developed for the visual localization.

Figure 9: 5 Dofs Instrument Arm Design

Figure 10: Instrument arm control architecture.

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VIII. Rover Operation Control Centre & Autonomy The Rover Operation Control Centre (ROCC) will be located in ALTEC, Turin, Italy. It will communicate with

the Mission Operation Control Center (MOC) in ESOC where the cruise and entry, descent and landing will be controlled. The MOC will be the interface toward the relay orbiter available at the time of the surface operations.

Due to the launch delay, some uncertainty exists on which orbiter will be available and its suitability for the ExoMars mission. The NASA selected mission MAVEN due to launch end of 2013 does not seem very practical as relay orbiter for ExoMars and other options are being studied, also in the frame of international cooperation with NASA.

During the surface operations that are intended to last 6 months, the rover will perform deployment, egress from the landing platform and commissioning within the first ten sols. Upon completion of these initial activities, experiment cycles have been defined and are being used as reference for the rest of the mission design.

The experiment cycle (Figure 11) consists of traveling to a location defined by the scientists on ground (typically a few hundred meters), while performing subsurface scanning every 20m with the ground penetrating radar called WISDOM. Based on PanCam panoramas acquired at the end of the traveling sols, scientists select target features (ideally outcrops) that the rover will be monitoring more precisely with its High Resolution Camera (HRC) and infra-red spectrometer MIMA. Scientists on ground can then decide where to place contact instruments fitted on the arm.

Figure 11: Experiment cycle with use of the various Pasteur payload instruments. In red, the ground decision points

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Once the geological interest of the site has been confirmed on ground, the rover is commanded to scan an area of

typically 5m by 5m with the WISDOM ground penetrating radar. During this operation, the rover stops every 10cm to perform a WISDOM full measurement lasting about 30seconds. All the collected data are transmitted to the scientists to determine the best location to acquire a sample with the drill.

Once the drilling location is determined and commanded, the rover drives to the target location, deploys the drill tool and drills to the specified depth. In the mean time, the instrument arm is deployed to acquire data about the drill fines. The drill tool is retracted and the sample provided to the analytical laboratory inside the rover where it is crushed into fine powder and distributed to the analytical instruments. For greater drill depths (>1 m), the drilling operation can take several sols. The baseline is to retract the drill at the end of each day such as to mitigate the risk of getting stuck with the drill tool in the frozen soil during the very cold night.

ExoMars rover surface reference mission is intended to perform seven of these experiment cycles in addition to two vertical surveys where the samples are acquired and analyzed on the same location but at different depths.

Notes : - I/F with external users not shown - Not all interfaces are shown

Rover Planning

Communication

I/F

TM Acquisition And Processing

Rover Plan Validation and

Command Generation

Rover HK Data Assessment and

Planning

Activity

Planning

Science Data

Assessment and Planning

Rover

Operational Simulator

TM

Science TM

Vehicle TM

Simulated data

Vehicle Status / constraints

TC uplink

Data Archive and Retrieve

Module (interface with all the other modules)

Science Planning

= External Tools / Systems

Comm Pass Allocation / Other

planning data

On Board Software

Management

SW Upload

Rover HW in the loop

(MTS)

Test data

Science Post Mission Data

Product generation

Robotic

Visualization and Planning

Vehicle Status / constraints

P/L Status /Constraints

Mission DB

(copy)

Plan

TC

Processing

=Core MCS Functions

ROCS System Control

and Administration

TC

ROCS Mission

Configuration Contral

Test TC

TC Sequence For Uplink

Uplink Verification

Data Dump

Comm. data

Preliminary Engineering Plan

Activity

Preparation and Validation

New Activity Template

Figure 12: ROCC Functional Architecture

The whole mission is ultimately aiming at analyzing subsurface samples at locations that are selected with the

process described above. It is a rather sequential process – no need of drilling before the ground penetrating radar has scanned an area that shall be selected based on confirmed geological interest. In every steps of the experiment

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cycle, the scientists on the ground shall analyze the scientific data retrieved from Mars surface before proceeding with the next step.

A functional and a task analysis have been performed in order to define the proper level of autonomy actually required by the mission operations. The need of commanding the rover with high level commands has been acknowledged. However, additional on-board deliberative capabilities will be reduced to a bare minimum, due to the sol by sol operations with ground scientists in the loop and the minimum operational alternatives possible in such sequential envisaged scenario. In addition, the rover will not implement any automatic recovery requiring switching redundancy, unless time critical hazards would endanger the rover. Ultimately, the level of autonomy is dependent on the capabilities of the ground to properly predict resources and major events for the next sols activities. This is also closely linked to the communication capabilities and thus to the orbiter characteristics. Therefore, definition of the rover autonomy is still an on-going process. It must be clear that autonomous scientist type of robot is out scope for the ExoMars mission.

High-level commands will correspond to rover behaviors that will be verified extensively on ground and will constitute the various blocks for the planning of daily activities.

In the previous phase of the study, an X-band antenna was foreseen allowing Direct from Earth (DFE) and some limited Direct To Earth (DTE) communication. This capability of uploading a new plan just at the start of the rover activities has been descoped for mass and cost reasons. The operations will therefore rely solely on orbiter capabilities with the current assumption of two communication windows per sol, one at the end of the afternoon and one during the late night. This is a limiting factor that will slow down the operations.

The Rover Operation Control Centre (ROCC) has been designed in order to cope with sol by sol planning activities. Accounting for the various latencies, the control centre is expected to have about eight hours (under the MRO orbiter capabilities considered so far) to complete its process from reception of the telemetry until upload of validated commands. This scheme may evolve once the orbiter characteristics are frozen.

Strategic planning is run on a weekly basis whereas tactical planning will serve a daily basis to upload the next two sols activities. The second sol activity should in principle be overridden every sol but would allow to cope with a missed communication opportunity. I.e. multi-sol traveling could be performed but any sol requiring scientist decision or inputs would be lost and a default activity may be run in order not to fully lose the sol.

The ROCC functional architecture, provided in figure 12, includes the ground control process of telemetry analysis, planning and telecommands verification before upload. More detailed ROCC description can be found in [5].

Housekeeping data are analyzed and rover resources predicted as input to a parallel process of assessment planning of scientific activities and engineering planning. The planning requests are merged to constitute a unique plan that can be verified before upload. The process of data assessment and planning will be supported by the rover operational simulator.

A visualization environment will allow co-registration of images and scientific data. It will also serve as a front end to define the specific robotic tasks e.g. placement of the arm on target, verification of collision, definition of targets for the navigation, evaluation of the control parameter of the locomotion.

Capabilities to compute the absolute position of the rover on Mars will be integrated in the ROCC. The current assumption is that ROCC operators will benefit from rather precise orbital images mainly coming from the HIRISE and CTX instruments fitted on MRO. It is not intended to count on images of the rover at the time of actual surface operations, as MRO will be over its designed lifetime. However, the ExoMars landing site selection has already been initiated. Agreement with NASA to progressively map a few selected landing site ellipses has been made. Absolute localization of the rover on the surface of Mars will be possible though a variety of possible techniques already experienced with the MER mission, like panorama features registration or Doppler ranging using the orbiter telecommunication capabilities. Even if operators and scientists are able to use absolute coordinates on Mars to define targets for navigation, the rover will work with relative coordinates from the stopping point where the ground defined the target’s coordinates.

Ground will benefit from the rover odometry discussed previously. A Mars terrain simulator (MTS) will be available to run a real rover model on an approximately 14m by 20m sand bin, the main objective is to perform operator’s training. Since ALTEC premises in Turin are next to Thales Alenia Space – ExoMars project prime contractor – the MTS is also intended for end-to-end functional verification. It may be also used during operations for troubleshooting, should it be necessary.

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The science data will be post-processed at ROCC and archived. Once the mission is completed, the data will be transferred to ESA-ESAC facility in Spain for long term archiving and dissemination.

IX. Conclusion An overview of control aspectsThis paper has presented an of the control functions of the ExoMars mission as

studied and experienced so far by the ExoMars industrial team. Since the ESA ministerial conference in November 2008, the ExoMars project has entered a redefinition phase in order to fit the international cooperation and budgetary constraints. The result may have significant impact on the rover control – mainly toward simplification – that is under discussion at the time of writing. Nevertheless, the essential GNC features presented in this paper will for sure form the basis for the ExoMars Rover during its surface operation in 2018.

Acknowledgments This paper represents the work of ExoMars rover teams from both ESA and Industry - TAS-I, Astrium UK,

ALTEC and their subcontractors- with contribution of CNES to the autonomous navigation developments.

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