Project: Interplanetary Communications - Action 1: Requirements Analysis

ESTEC Contract No.: 21783/08/NL/HE

Title: Final Report

Doc. No.: IPComm-ASG-FR-001 Issue: 1 Rev. 0 Date: 19.03.2009

Name Date Signature Christian Jentsch Prepared by: Andreas Rathke 19.03.2009 Oswald Wallner

Checked by: Oswald Wallner 19.03.2009

Approved by: Christian Jentsch 19.03.2009

Project Management: Christian Jentsch 19.03.2009

Astrium GmbH 88039 Friedrichshafen - Germany

DL Final Report IPComm

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Quantity Type Name Company / Department 1 PDF M. Wittig ESA/ESTEC 1 PDF A. Ferreol ESA/ESTEC

1 PDF Christian Jentsch Astrium GmbH 1 PDF Andreas Rathke Astrium GmbH 1 PDF Oswald Wallner Astrium GmbH 1 PDF Ulrich Johann Astrium GmbH

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Change Record

Issue Revision Date Sheet Description of Change 1 0 19.03.2009 All First issue

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Table of Content

Distribution List ...... i Change Record ...... iii Table of Content ...... v 1 Introduction ...... 1-1 1.1 Scope of the Document...... 1-1 1.2 Reference Documents ...... 1-1 2 Characteristic Interplanetary Mission Scenarios...... 2-3 2.1 Interplanetary Communication and Navigation...... 2-3 2.2 Characteristic Mission Scenarios ...... 2-4 2.3 User Requirement Classification Criteria...... 2-5 3 Synthesis of Interplanetary Communication Requirements...... 3-7 3.1 User Requirements ...... 3-7 3.2 System Requirements ...... 3-8 4 Synthesis of Interplanetary Navigation Requirements...... 4-11 4.1 User Requirements ...... 4-11 4.2 System Requirements ...... 4-12 5 Synthesis of IPN...... 5-14 6 Summary and Conclusions...... 6-15 Annex A: Abbreviations...... 17

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1 Introduction

1.1 Scope of the Document This document summarises the results of the study "Interplanetary communication - ACTION 1: Requirements Review" primed by Astrium GmbH (Friedrichshafen, Germany). It provides the synthesis of the communication and navigation user requirements for future interplanetary missions as well as of the corresponding system requirements. These have been elaborated in the frame of the study by TU Graz (communication) and Deimos Space S.L. (navigation) for a set of 13 characteristic mission scenarios that serve as a basis for the analysis in the present document.

1.2 Reference Documents [RD 1] ESA NEXT team, "NEXT Mars Orbiter Mission Requirements Document", Issue 1 Rev. 1, 06-11-2007

[RD 2] J. E. Riedel, S. Bhaskaran, S. Desai, D. Han, B. Kennedy, G. W. Null, S. P. Synnott, T. C. Wang, R. A. Werner, E. B. Zamani, T. McElrath, D. Han, M. Ryne, Autonomous Optical Navigation (AutoNav). DS1 Technology Validation Report, JPL undated

[RD 3] Sala, J., Urruela, A., Villares, X., Estalella, R., and Paredes, J. M., Feasibility study for a spacecraft navigation system relying on pulsar timing information, European Space Agency, the Advanced Concepts Team, Ariadna Final Report (03-4202), 2004.

[RD 4] P.S. Ray, K.S. Wood, and B.F. Phlips, Spacecraft Navigation Using X-ray Pulsars, NRL Review, p. 95ff.,2006 [RD 5] Korsmeyer, D., Into the Beyond: A Crewed Mission to a Near-Earth Object, slides of talk at IAC 2007 [RD 6] David E. Smith, et.al., “Two-Way Laser Link over Interplanetary Distance”, Science 6, 53– (2006)

[RD 7] David Smith, Maria Zuber, Mark Torrence, Jan McGarry, Michael Pearlman, Laser Ranging to the Lunar Reconnaissance Orbiter (LRO), 15th International Laser Ranging Workshop, Canberra, Australia, Oct 16-20, 2006 [RD 8] M. Wittig, T. Dreischer, T. Weigl, Optical Data Links for L1 and Mars Missions, IAC-05- B3.6.01, 2005

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2 Characteristic Interplanetary Mission Scenarios

2.1 Interplanetary Communication and Navigation A variety of interplanetary mission scenarios has been established by almost all space agencies playing a major role in space exploration, including ESA and NASA as well as the national agencies of Russia, China (Chang'e programme), India (Chandrayaan programme) and Japan. Typical mission scenarios start with exploration missions to the Moon before proceeding to other destinations as Mars or asteroids. The exploration will in general be based on a first investigation by remote sensing satellites, followed by on surface activities and will eventually lead to human missions. A selection of characteristic mission scenarios, which have been reviewed in the first phase of the study, is depicted in Figure 2-1. It includes missions to Moon, Mars and small bodies, missions at the Lagrange points, in high Earth orbits and in heliocentric orbits as well as deep space missions beyond Mars.

Figure 2-1: A selection of types of planned missions with the associated link architectures.

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ESA has identified the exploration of Mars as its primary exploration target. Therefore, the current European scenario for deep space exploration foresees a step-by-step approach which will increase in complexity over time, finally aiming at a human mission to Mars by the year 2033. In the near term perspective within the Aurora programme, the launch of the ExoMars mission is scheduled for 2013. The spacecraft will consist of a carrier and a descent module. The descent module will land on Mars and deploy a rover for mobile in-situ investigations, while the carrier will separate during approach shortly before entering the atmosphere and burn up. In the medium term perspective the more complex Mars Sample Return Mission (MSR) is planned for launch in 2019. This concept comprises presently five modules for cruise, orbiting, descent, ascent, and Earth re-entry. Reliable communication plays a crucial role in particular for efficient science-data downlink and operations or public outreach for human missions. Currently, the communication of all planned interplanetary missions relies on radio-frequency systems. Any initial plans for optical communications have been limited to technology demonstrations.

2.2 Characteristic Mission Scenarios In the first study phase, the Interplanetary Missions Synthesis Review (IMSR), the systematic review of the worldwide publicly available information has revealed 73 interplanetary missions from various space programs which are already in implementation (at least in Phase B), under detailed investigation (e.g. Phase A), or under analysis by concept studies. Particular emphasis has been put on missions in medium and long-term programmatic planning as these can benefit from today's technology development roadmaps. Based on the findings during the IMSR it turned out that a more general analysis of possible future communication and navigation scenarios would be preferable instead of concentrating on a single reference mission as originally foreseen in the proposal. Hence, a set of 13 characteristic mission scenarios has been identified as basis for the further analysis of the communication and navigation user and system requirements as well as the applicability of the IPN. For this set of missions the following characteristic targets and mission types have been selected:

Moon: (1) Lunar orbiter mission, (2) Lunar impactor network mission, (3) Lunar lander & rover mission, (4) Lunar sample return mission, (5) Lunar human mission.

Mars: (6) Specialised orbiter mission, (7) Robotic lander and rover mission, (8) Lander network mission, (9) Sample return mission, (10) Human Mars mission.

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Lagrange Points: (11) Observatory at L2.

Small Bodies: (12) Asteroid sample return mission.

Giant Planets: (13) Jupiter orbiter & Europa lander mission. The communication and navigation user requirements and system requirements for these mission scenarios were analysed in detail in the study. This document identifies common trends amongst the requirements in order to derive the generic top issues that are relevant for technology development and the planning of space and ground infrastructure.

2.3 User Requirement Classification Criteria The user requirements for communication and navigation have been derived for the set of the 13 characteristic mission scenarios. They specify a performance and functionality envelope which has to be realized by the technical system. For the communication system and the navigation system the user requirements consist of performance requirements and of operational and programmatic requirements. The corresponding system requirements have been derived by mapping the user requirements into system needs. For the communication system the link distance and the payload data rate are the major parameters for the link design. Mainly these parameters determine the subsystem design and the required performance. For the navigation system the link distance and the navigation accuracy are the major parameters for the system design. The navigation accuracy is driven by science requirements or operational requirements and has to be specified for the different mission events.

In order to identify and classify the user requirements which are of highest importance for the system requirements and represent the driving needs a number of classification criteria has been established. All derived communication user requirements have been classified according to the following classification criteria: (1) data volume, (2) distance of furthest link, (3) maximum allowed link interruption time (MALIT), (4) link availability, (5) interoperability with other agencies (mandatory, optional, recommended, not planned). The MALIT and the link availability of the science data channel have been taken into account in addition to the main drivers, data volume and link distance. These operational requirements may affect the system layout significantly. This holds in particular for the requirement of human missions to have nearly 100% of link availability due to safety reasons. For the classification of the navigation user requirements the following criteria have been used: (6) autonomy, (7) relative or absolute navigation, (8) navigation accuracy, (9) user requirements is driven by science requirements or spacecraft operations.

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In the case of navigation it is difficult to distinguish between user and system requirements. Thus, some explanations to the criteria are provided below to permit a common understanding. Autonomy is certainly an important aspect and thus a user requirement for manned missions. Due to safety reasons there will be a back-up navigation system different from the primary system required which shows a high degree of autonomy in these scenarios. However, for other mission scenarios autonomous navigation might be preferred but will rather result from the analysis of possible implementations and is, therefore, a system requirement. It should be classified whether relative or absolute navigation is required because relative navigation is a user requirement if for example the sample canister handling in the MarsSampleReturn mission is considered. More in general, docking or surface operations typically require relative navigation. However, there might be also scenarios where for example a rover on the surface uses relative navigation to a lander base while absolute navigation is required for the rover as well. This would be the case if the rover is supposed to investigate or take samples from a specific surface spot or crater. The navigation accuracy is an important aspect to identify the navigation requirement drivers by classifying the derived accuracies. The navigation accuracy will mainly determine the suitable navigation techniques.

It is important to classify whether a certain navigation accuracy is driven by science requirements or by spacecraft operations. For example radio science experiments will certainly drive the requirements in the case of an asteroid sample return mission for a proper investigation of the gravitational field of the target body rather than the spacecraft operations requirements. As it is important to identify the navigation needs for the specific mission phases the classification has been performed for all applicable mission phases of the characteristic scenarios.

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3 Synthesis of Interplanetary Communication Requirements

3.1 User Requirements Communication user requirements fall into 2 basic classes: • Data volume and data rate requirements. • Availability requirements. For the user data (i.e. mainly science and public outreach data) volume and rate requirements a common trend for all missions could be identified. An exponential growth of the data transmission requirements for interplanetary missions is expected. This growth in demand is simply related to the rapid advance in electronics, in particular detector technology. The detectors of the instruments of future interplanetary mission will have a high spatial and spectral resolution that will enlarge the data volume per image. Hence the same remote sensing task will produce much higher data volumes in future missions as they do today. Since the growth in data rate is connected to advances in detector technology a modified version of Moore's law was assumed to extrapolate the data rates of future missions. In order to take into consideration that advances in electronics for space applications is less dramatic than for terrestrial applications a damped growth is assumed that expects a doubling of data volume every 30 months. This growth factor is assumed to be sufficient to also take into account the increase in the scientific ambition to reach a more complete coverage of the object under consideration or coverage of a broader spectral range. The latter effect will be somewhat limited in impact because it will run into natural limits of scientific interest. For instance for an asteroid mission it seems unlikely that a global coverage of the surface below the 10 cm level can bring any scientific benefit. Similarly also a global coverage of the Moon with 1 m resolution as is planned for the German LEO mission already seems excessive and one can hardly conceive a successor mission that will have an increased resolution on a global scale as a goal (Only on a local scale higher resolution information may be desirable nevertheless).

Figure 3-1 Extrapolation of user data rates for future interplanetary missions

Link availability in general does not seem to be a major driver for future interplanetary missions thanks to the current and expected improvements in satellite autonomy. However for mission-critical situations, efforts are being undertaken to improve availability. For instance the ESA MarsNEXT lander foresees tone-based communications for the approach and entry phase to provide an

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uninterrupted stream of telemetry throughout the critical entry phase [RD 1]. The projected data volume generated per day for the 13 characteristic mission scenarios is shown in Figure 3-2. For the different mission types on Moon and Mars the generated data is in the order of 1014 bit/day for the driving human missions due to the requirement to transmit high resolution videos and medical survey data. The lower values for both targets are required by robotic or the sample return scenarios.

1.0E+15

1.0E+14 Lunar Orbiter Lunar Impactor Network Mission Lunar Lander/Rover Mission 1.0E+13 Lunar Sample Return Mission Lunar Human Mission Observatory at L2 1.0E+12 Mars Specialised Orbiter Mars robotic lander and rover Mars Lander Network 1.0E+11 Mars Sample Return Human Mars Mission Data Amount [bit/day] 1.0E+10 Asteroid Sample Return Jupiter Orbiter & Europa Lander

1.0E+09 1.0E+08 1.0E+09 1.0E+10 1.0E+11 1.0E+12 1.0E+13 Target Distance [m] Moon L2 Asteroid Jupiter Mars

Figure 3-2: Data amount dependent on the target distance for the characteristic missions

3.2 System Requirements The driving system requirements for the communication arise for the user requirements of high data- transmission capabilities and link availability in critical mission phases. The requirements for high data transmission capability can be addressed in a two-fold way, by an increase in the data rate and by an increase in the link duration. The latter option is not straightforward to implement because contact times a mainly constrained by visibility of a suitable ground station from the space element which in turn is determined by the orbital dynamics. Hence an increase in contact time can effectively only be accomplished by considering an additional space element, that has an improved Earth view and the serves as a relay. Both a direct relay and a store and forward relay may be an option. Direct relays where considered in the following scenarios: • Lunar south pole sample return, • Europa lander, • Human Mission to Mars. Store and forward relays were foreseen in the following scenarios:

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• Lunar impactor network, • all robotic Mars missions. Amongst the store and forward relays only the relay for the Lunar impactor network is mandatory because some of the impactors will permanently be out-of-view of the Earth. For the Mars missions also a direct link instead of a relay would have been conceivable and the relay is rather considered to simplify one of the space/surface elements or to enhance the data rate. The principle approach to enhance the data rate is the use of a more capable communication link. For IPcomm both RF links and laser links have been analysed for the characteristic mission scenarios. For robotic missions the comparison did not support a clear cut preference for one of the systems. Instead both options were found feasible, each having a considerable system impact. For the RF systems, transmit powers in the order of 100 W and the use of Ka-band are frequently required. Considering the typical transmitter efficiency of ~50% in Ka-band the RF system and the frequent contacts that are required, the RF system will however be a considerable resource driver. For the optical systems, telescope apertures are of the order of 25 cm and laser powers are around 5 W even if a large ground station of more than 4 m aperture is assumed. The pointing of the onboard telescope will be a considerable challenge due to the long light travel-times to the ground station which makes any scanning procedure that requires an acknowledgement very inefficient.

For both systems the availability of suitable ground stations remains and issue. For the RF links it was considered that a single 35 m ground station can be used for data transmission whenever it is visible from the spacecraft. If several interplanetary missions are to be operated at the same time the two ESA ground stations may soon become insufficient to handle all contacts. For the optical link scenarios ground stations with 4 m to 10 m telescopes are required for several scenarios which would necessitate the institution of large telescopes dedicated to space communications.

For human missions the data rates are assumed to be significantly higher that for robotic mission. The major reason for this is the desire to be able to transmit video streams in realtime at a high quality for public outreach. Also the general telemetry rate will be much higher for human missions because the telemetry will comprise additional types of data such as the health monitoring of the astronauts and the data from the life-support system of the spacecraft. For the ultra-high data-rate streams it is definite that only a laser communication system will be sufficient. In order to relax the pointing requirements in the case of contingencies or orbit manoeuvres without losing the downlink it is furthermore clear that a backup RF system is required. Due to the high telemetry rates also this back- up system will have high resource consumption, in particular in terms of power. For surface elements, mass and power constraints are generally stricter than for orbiters. Consequently the assumption, that a capable RF or optical system can be accommodated, is generally invalid for landers. In order to still accomplish sizeable data rates a relay in orbit around the target planet is desirable. For the surface-element-to-relay communications are trade-off has be made between the achievable data rate and the resource consumption and ruggedness of the communication system. No detailed link analyses have been carried out for these links because the design depends very much on the position of the surface element and the assumed orbit of the relay. Preference is however given to rather low frequency links such as UHF or S-band due to the simplicity of the equipment. For the surface element a steerable antenna of modest gain is realistic. Under these assumptions the short-haul link can provide data rates comparable to the long-haul link between the

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relay and Earth. For surface elements it is desirable to have a back-up link directly to Earth in order to be able to operate the surface element independently of the relay. This is generally foreseen for Mars and Moon landers. Due to the longer distances and resource restrictions it is commonly not considered for landers on the Moons of the giant planets. For this situation it would be desirable to have an established low power telemetry system such as tone-based communications in order to be able to receive a minimal telemetry at Earth via a direct link.

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4 Synthesis of Interplanetary Navigation Requirements

4.1 User Requirements User requirements for navigation arise from various aspects of interplanetary missions: • Guidance of the spacecraft, • Planning of science operations, • Processing of science operations. These three tasks come with different user requirements for the navigation. The navigation for spacecraft operations is required through-out the mission and in a variety of operational situations. The requirement on the timeliness of the navigation fix varies greatly between mission phases. During an interplanetary cruise a weekly navigation solution is sufficient both for coast arcs and for thrust arcs using low-thrust propulsion. For this application the directionality of ground-based navigation measurements along the Earth-spacecraft line of sight does not impair the quality of the solution because previous data points are included in the latest navigation solution. For gravity assist manoeuvres and planetary approaches the navigations fixes need to be obtained more frequently and more quickly and at high 3D accuracy of the order of 10 km. For certain situations the required navigation accuracy may have to be even better. This is mostly the case in the approach to a target that is small. This can either be an asteroid, which can have an overall size below a km. Consequently the relative navigation needs to be accurate a level significantly better. Also landing site for surface elements can be limited to a few 100 m or even less. In planetary orbit, navigation will again be infrequent and a good 3D accuracy can by obtained just by more frequent and long-term observations. As can be seen from the above overview the operational situations where high navigation accuracies and a quick navigation fix are required are mostly concerned with relative navigation with respect to a different celestial body than Earth. This holds for planetary approach, landing, asteroid proximity operations and even for planetary swing-bys. The navigation requirements for the planning of science operations mostly concern the situation in planetary orbit and after landing on the planetary surface. Hence the situation is stationary in the sense that the orbit or position is not changing rapidly over time. For orbiters no particular needs on the timeliness of the navigation fix arise as long as it is available in time for the planning of the mission timeline. While timeliness requirements are relaxed, the requirements on navigation accuracy may be high if the navigation information is needed to point high resolution and small field of view instruments. Also for surface elements the motion is typically slow and hence navigation processing for science operations planning does not need to be timely. For the processing of science data the operational navigation information can be used. This information is however frequently insufficient in accuracy and a more detailed reprocessing or navigation data are needed for the evaluation of the science data. Sometimes it may even be necessary to include additional observables in the processing that are not used for operational navigation. Also for the science data processing the degeneracy of observables due to the directionality of the observables may be a serious challenge. This issue is well known for lunar missions where the navigation accuracy is mainly limited by the lack of navigation on the lunar far side and the orbital uncertainty induced by this cannot be overcome by improved navigation on the near

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side when the Earth is in view.

4.2 System Requirements From the above overview it is clear that advances in navigation for future interplanetary missions will have to address with the following challenges: • Timeliness of navigation fixes for critical operations. • Generation of improved 3D observables. • Means of relative navigation with respect to different celestial bodies than Earth. • Improved navigation accuracy for science operations planning and science data processing. In the following each of these points is addressed separately. Nevertheless some links between this seemingly disjoint topics exist which are also addressed. The time it takes to generate a navigation solution is connected rather with the principle navigation method than with the navigation accuracy. The traditional methods of Doppler and ranging provide information only along the line of sight. Hence the tracking needs to be amended by a dynamical model of the spacecraft motion and several data points along the trajectory are required to provide a navigation solution. This effectively limits the minimal time in which a navigation solution can be obtained.

A combination of ranging and angular measurements such as ΔDOR provides instantaneous 3D information. The ΔDOR measurement requires at least two ground stations (three to obtain an instantaneous measurement of the solid angle). In order to limit the ground infrastructure cost for a mission this method is only used during critical situations where quick information is necessary. Another option to obtain instantaneous navigation information is the use of autonomous navigation methods. Currently the only option demonstrated for deep-space navigation is vision-based navigation using a camera [RD 2]. The navigation fix is obtained by observing asteroids or planets in front of the celestial sphere and carrying out a triangulation with respect to several objects. The current attainable accuracy of this method is in the order of 1000 km. It is unlikely that the accuracy will improve by one order of magnitude in the timeframe until 2025. A similar accuracy in deep space can be attained by making reference to celestial sources only. It has been demonstrated that by observing the extremely regular radio signal from pulsars a triangulation can also be carried out to reach an accuracy of ~1000 km within the solar system [RD 3]. Radio pulsar navigation requires high-sensitivity receivers with antenna sizes of the order of 3 m to 4 m. In order to carry out a triangulation the antenna needs to be pointed towards several pulsars at large angular distance one after each other. Hence radio-pulsar navigation is quite resource-consuming and operationally complicated. This is in contrast to vision- based navigation where several asteroids can be observed with a star tracker-like wide angle camera at the same time and no reorientation of the spacecraft is necessary. Similarly to triangulation using radio pulsars also triangulation using X-ray pulsars has been considered [RD 4]. An X-ray detector has to be operated at low temperature to have a good photon count efficiency. Hence its resource consumption is significantly above that of a camera. In addition, X-ray pulsars are less frequent than radio pulsars and hence re-pointing of the spacecraft or the use of several detectors will be required. The attainable navigation accuracies will be similar to those of vision based navigation and radio pulsar navigation. In consequence vision-based navigation appears to be the most attractive autonomous navigation method due to its lowest resource consumption and its competitive accuracy.

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For close distances to asteroids and for approach to planetary surfaces lidars and radars can be used to obtain altitude information and attitude information with respect to the surface. Also landmark identification in camera images can be used for relative navigation with respect to a planetary surface or an asteroid. For these situations Earth based navigation technologies will not be competitive due to the delay by light travel time and the uncertain distance between the surface/asteroid and the Earth. A cooperative alternative to camera and range-finder navigation could be the use of on-surface beacons that have been deployed by a previous mission. Such scenarios have been considered for human missions to asteroids in NASA's Constellation Program [RD 5]. Due to the infrastructure they require they will generally be limited to human missions. The improved navigation for science operations planning and science data processing is facilitated compared to navigation for spacecraft operations by the fact that longer integration times and a longer processing time on ground are allowed. Consequently, navigation for science tasks is the area which benefits significantly from an improvement in the accuracy of ranging and Doppler, which provide only line-of-sight information. For tracking using radio links, improvements are mainly achievable by going to higher frequencies and by using multiple frequencies at the same time in order to be able to remove disturbances induced by interplanetary plasma, the Earth's ionosphere and troposphere.

A further improvement in navigation accuracy is achievable by the use of laser ranging. Both, in terms of on-board resources and in terms of ground infrastructure, laser-ranging is considerably easier to implement than full-fledged optical communications. Laser ranging is most easily implemented as asynchronous ranging. In this method the received and sent photons are correlated on-board the spacecraft by means of a precise clock and the photon record is transmitted as telemetry via the conventional RF link. This method has been demonstrated using the laser altimeter on Messenger [RD 6] and will also be used on LRO [RD 7]. As a ground station, a lunar laser ranging station is suitable. Hence the development of interplanetary laser ranging could serve as an intermediate step in the development of optical interplanetary communications. In particular the development of suitable on-board telescopes and lasers could be conducted without already having to implement the ground station infrastructure required for interplanetary communications.

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5 Synthesis of IPN

Interplanetary internet (IPN) is an extension of the well known internet protocol suite for space applications. It is particularly suited for the situation of a relay at a distant planet that supports multiple client spacecraft. Such applications are currently handled at Mars via the Proximity-1 protocol. The analysis of the characteristic future mission scenarios has shown that the Proximity-1 protocol will most likely be insufficient for the higher data rates and more complex link topologies of future interplanetary missions. Here, IPN and in particular its implementation of delay tolerant networks could fill the gap. However even for the advanced mission scenarios that have been considered only a limited subset of the features of IPN will be required. This synthesis has been based on a detailed analysis of the applicability and attractiveness of IPN in the 13 characteristic mission scenarios in the course of the study.

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6 Summary and Conclusions

Future interplanetary communications will face steeply increasing data rates. For robotic missions these demands can most likely be fulfilled with either Ka-band radio-frequency communications or laser communications [RD 8]. For human missions it is definite that an optical communication channel will be highly desirable to transmit a wealth of video data. For surface elements the use of more intricate or resource consuming communication systems faces serious resource limitations and challenges from robustness considerations. Hence the communication to surface elements will generally require a relay in orbit. In this way the limited communications equipment on the surface element only needs to bridge a distance of a few thousand km instead of one or more astronomical units. Only for Mars and to the far side of the Moon a dedicated relay is attractive. For the Moon a relay is most sensibly placed in the Earth-Moon L2 where it is too distant from the Moon to be suitable for planetary science tasks. For future navigation multiple user requirements were identified. They lead to an increased demand for autonomous navigation but also to a demand for an improved ground-based ranging accuracy. An attractive approach to the latter task is the use of laser ranging.

Amongst the developments concerning Interplanetary Internet the most relevant for the timeframe considered in the study is the implementation of delay tolerant networks. IPN may in particular become relevant for a Mars relay where several clients are expected to use the same relay.

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Annex A: Abbreviations

ASG Astrium Germany AU Astronomical Unit ΔDOR Delta Differential One-way Range DLR German Aerospace Center DOR Differential One-way Range ESA European Space Agency IPN InterPlanetary Internet ISRO Indian Space Research Organisation JAXA Japan Aerospace Exploration Agency L2 Lagrange point 2 LEO Lunar Exploration Orbiter lidar LIght Detection And Ranging LOLA Lunar reconnaissance Orbiter Laser Altimeter LRO Lunar Reconnaissance Orbiter MALIT Maximum Allowed Link-Interruption Time NASA National Aeronautics and Space Administration radar RAdio Detection And Ranging RF Radio Frequency SELENE SELenological and ENgineering Explorer UHF Ultra-High Frequency

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A

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