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7 Communications and Navigation Architecture INTRODUCTION modulation formats and other details of the physical layer (layer 1 of the OSI model) for communication, including The communications and navigation architecture pro- the need for accurate and synchronized time sources. For gram element is responsible for defining the space commu- example, a fraction of a spacecraft’s transmitter power can nications and navigation architecture to support NASA’s be dedicated to an unmodulated radio frequency beacon that science and exploration missions through 2030. This archi- is included strictly for the purpose of computing the range tecture must evolve through 2030 and beyond to keep pace and range rate from Earth to the spacecraft. If a user space- with the needs of future science and exploration users and, craft needs to find its position by comparing beacons from potentially, non-NASA users. The communications and different sources, as in the Global Positioning System, those navigation architecture necessarily encompasses compo- sources must synchronize their transmissions. Navigation nents such as the Deep Space Network and the Ground Net- requirements can influence the selection of a particular con- work that are managed outside the Space Operations Mis- stellation of communication relay satellites orbiting a planet, sion Directorate (SOMD). As per its charge, in this chapter but these requirements are not expected to significantly in- the committee examines NASA’s approach to developing fluence the choice of whether or not to link specific nodes the architecture, and the resources and capabilities that will in the space network. Therefore, it is not unreasonable support that development. that navigation is mentioned only occasionally in SCAWG The communications and navigation architecture pro- documents. The remainder of this chapter focuses on gram element accomplishes its task through NASA’s communications. agency-wide Space Communications Architecture Working Group (SCAWG), whose purpose is to develop a future ASSESSMENT space communications architecture and identify associated technology investments necessary to support all future NASA exploration, science, and human-tended missions.1 Formulation of the Program Plan SCAWG’S scope is shown in Figure 7.1. Project Objectives The purpose of NASA’s space communications archi- tecture is “to concurrently architect the Space Communica- NASA’s SCAWG encompasses members that carry out tions Network to enable NASA’s changing mission of Ex- both technical and programmatic/authoritative functions, and ploration . ” 2 To do this, NASA is developing 5-year that represent each NASA mission directorate, the Strategic “snapshots” of the space communications architecture that Investment Division of the chief financial officer, and com- must evolve from the present Deep Space Network, Space munication networks and the user community, including Network, and Ground Network in order to provide the nec- NASA centers. Evidence of the breadth of SCAWG’s mem- essary communication capabilities to support NASA explo- bership was confirmed by the committee through its review ration and science programs. Figure 7.2 shows the elements of documentation3 and in presentations to the committee by that will be associated with NASA’s space communications the SCAWG’s chair. Structurally, SCAWG’s membership architecture by approximately 2030. allows for inviting subject-matter experts from government, Regarding the navigation portion of the communica- academia, and industry to participate in specific studies on tions and navigation architecture, NASA believes that navi- an as-needed basis. gation requirements will continue to heavily influence 51

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52 REVIEW OF THE SPACE COMMUNICATIONS PROGRAM FIGURE 7.1 Scope of NASA’s Space Communications Architecture Working Group, as indicated by the continuous thin blue line encom- passing capabilities such as Earth, lunar, and Mars communication and navigation relays. SOURCE: John Rush, NASA, “NASA Navigation and Communications Architecture,” briefing to the NRC Committee to Review NASA’s Space Communications Program, Washington, D.C., January 26-27, 2006, p. 7. Because all interests are represented in the SCAWG, date on the capabilities that lunar relay is expected to pro- the recommendations are likely to flow both to and from vide. Eventually, lunar relay will be one more existing infra- organizations represented in the SCAWG and to flow both structure capability forming a basis for future space commu- from work in process and any formal reports by the SCAWG. nications architectures. Given this high degree of continuing interaction, the com- As it was explained to the committee, the 2006 SCAWG mittee believes that the objective of involving mission direc- activity is a one-time effort to defragment NASA’s vision torates in planning and review is adequately addressed. for space communications infrastructure. Now that the SCAWG has established its vision for a 2030 communica- Finding: NASA’s Space Communications Architecture tions architecture and has conducted a lunar relay architec- Working Group appears to have all the necessary qualifica- ture study, its level of activity is expected to decrease within tions, capabilities, and facilities to perform its work, and its the next 2 years. The nature of that activity could also change output is of high quality. as today’s proposed architectures become tomorrow’s projects. For example, the SCAWG is responsible for keep- ing track of GSFC’s preliminary design work on a lunar re- Project Deliverables lay project and any other near-term design efforts. One example of SCAWG activities is a recent lunar re- lay architecture study4 that produced a preliminary evalua- Recommendation: NASA’s Space Communications Architec- tion of the options shown in Figure 7.3. Further refinement ture Working Group should continue as planned to carefully of the elliptical orbit option is now beginning at GSFC, as evaluate near-term and intermediate-term architecture op- lunar relay becomes a project rather than a concept. The tions while promoting development of components such as SCAWG will have one or more members from GSFC’s lu- relay satellites and ground stations consistent with the long- nar relay project team, keeping the architecture team up to term communications architecture.

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53 COMMUNICATIONS AND NAVIGATION ARCHITECTURE FIGURE 7.2 Top-level view of NASA’s space communications architecture circa 2030. SOURCE: John Rush, NASA, “NASA Navigation and Communications Architecture,” briefing to the NRC Committee to Review NASA’s Space Communications Program, Washington, D.C., January 26-27, 2006, p. 10. Expected Services ellite, but the features of this satellite are unspecified. NASA needs to have a working concept (or concepts) of the next- The development path to the SCAWG’s long-term com- generation Earth relay satellite, if only to allow for an or- munications architecture vision will be determined by pro- derly transition of service from the current TDRSS and its grammatic decisions yet to be made, namely the definition planned replenishment. and scheduling of specific missions. As long as the interme- The committee does not presume to define an evolu- diate steps provide proper software layering and allow for tionary path from today’s TDRSS forward, but only to point some software upgradability, the path to the long-term vi- out that some baseline definition, however imperfect, should sion is quite flexible. This is one of the greatest technical be undertaken before NASA issues requests for proposals strengths of the SCAWG’s recommended communications for future Earth relay satellites. This suggestion is not incon- architecture. sistent with NASA’s present plan to include acquisition in The SCAWG will produce occasional updates on the the FY 2008 budget of “clone” TDRSS satellite replacements long-term architecture plus individual studies of how to sat- for launch in 2015. isfy the communication and navigation needs of specific missions. These updates should enable NASA to assess Finding: A critical near-term task for NASA’s Space Com- progress, should prompt feedback from internal customers munications Architecture Working Group (SCAWG) will be (the designers for each mission), and should ultimately make to define one or more potential evolutionary paths from the case for reallocating funding as needed. Adequacy of today’s TDRSS to an Earth relay system consistent with the staffing to support this level of activity depends on active SCAWG’s vision for a long-term communications architec- support from other mission directorates. ture. The SCAWG has not yet attempted to define whether or how the existing near-Earth network (Tracking and Data Technologies that might be worth further study as com- Relay Satellite System; TDRSS) could evolve to support the ponents of some of the evolutionary paths include those for long-term communications architecture. One of the elements augmentation of the ground network (more sites, fiber inter- of that long-term vision is a next-generation Earth relay sat- connection) to provide better coverage in low Earth orbit;

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54 REVIEW OF THE SPACE COMMUNICATIONS PROGRAM Malapert Station: Polar Circular Orbit: Communications base at Varying numbers of orbital planes and Malapert Mountain, elevation spacecraft provide differing levels of 5 km. Near-continuous redundancy and availability. Circular coverage between Earth and orbits are stable and the proper the Moon. 89% full Sun and phasing of spacecraft will guarantee 4% partial Sun, total darkness continuous coverage of the polar up to 7 days, 5 times/year. region. Inclined Circular Orbit: Elliptical Orbit: Inclination aids in a more even Apoapsis beneath the distribution of coverage over the South Pole increases full lunar surface. dwell time above that region. L2 L1 & L2 Halo Orbits: Hybrid Constellation: Continuous direct Example = combination of communications with Earth. Lagrange point orbits and a L1 and L2 are unstable polar orbit. points, and the orbits require L1 station-keeping maneuvers. FIGURE 7.3 Lunar relay constellation options considered by the SCAWG. SOURCE: John Rush, “NASA Navigation and Communications Architecture,” briefing to the NRC Committee to Review NASA’s Space Communications Program, Washington, D.C., January 26-27, 2006, p. 14. technologies for improved performance of the multiple ac- The committee is aware of the differences of opinion cess service (higher transmitted signal levels and greater re- regarding space communications protocols. (See “The Inter- ceiving sensitivity) to allow some current S-band single-ac- planetary Internet,” IEEE Spectrum magazine, August 2005.) cess users to move to multiple access; near-Earth relay However, that conflict is not primarily a technical one: ev- crosslinks into the DOD’s transformational satellite (TSAT) eryone agrees that IP (Internet Protocol, the basic packet data network (in order to share TSAT’s communications back- communication standard) will work fine at short ranges and bone); technologies for packet switching (with or without a that UDP (User Datagram Protocol for one-way transport of router); optical links to user satellites; and combined DOD/ data packets) will be the foundation for longer-range com- NASA satellite payloads. munication. The SCAWG should promote the development In addition to working on the evolution of Earth relay of long-term protocol stack solutions consistent with the capabilities, the SCAWG will need to work closely with ongoing use of IP in networks on the Moon and Mars. This Goddard’s Exploration, Operations, Communications, and should include consideration of any applicable delay-han- dling and QoS5 techniques currently being pioneering in Navigation Systems organization to maximize the long-term utility of the technologies and components of the lunar relay DOD’s TSAT effort. project. As these protocol solutions progress from concept to The SCAWG did not present to the committee any re- implementation, they will move under the control of the Data sults on protocol stack software for space networks, but the Standards program element. This process is analogous to the requirements for this software are largely independent of the movement of the architecture concepts from the SCAWG to constellation design. This problem can therefore be ad- individual programs as the elements (such as lunar relay) dressed separately. JPL is currently taking the most promi- become real projects. nent role in design of protocol software for space networks.

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55 COMMUNICATIONS AND NAVIGATION ARCHITECTURE Finding: Protocol stack solutions for space communications DOD. Although the committee is aware of the occasional networks beyond Earth orbit are an important foundational conflicts over resource allocation between DOD and NASA element for the long-term communications system architec- users today, the potential overall benefit from combining ture. networks is too attractive to ignore. Additionally, some portion of NASA’s communications Recommendation: NASA’s Space Communications Architec- backbone could be provided by DOD systems, including ture Working Group should promote cooperation between TSAT and the Global Information Grid, which could relieve the Jet Propulsion Laboratory and other groups, both within some of NASA’s budget pressure while still providing vital NASA and in DOD and academia, that are doing significant communications capability. work to design the protocol stack software necessary to op- erate packet networks in deep space. Other Space Agencies In his presentations to the committee the SCAWG’s Long-Term Project Goals and Objectives chair did not mention cooperation with foreign space agen- The primary challenge to implementing the architecture cies. Cooperation on specific missions for common relay developed by the SCAWG will be ensuring support and com- capabilities and sharing of ground network resources is prob- pliance by the component projects and user entities. The ably best handled by the project managers for the specific agency-wide representation in the SCAWG is most appro- missions. Nevertheless, it would be useful for the SCAWG priate for the current effort, and an extension of that approach to establish a mechanism for periodic identification of op- that includes all users should be considered as the communi- portunities for international cooperation. cations and navigation architecture moves along the path to fruition. NOTES 1. National Aeronautics and Space Administration (NASA), Recommendation: NASA’s top management should imple- SCAWG Charter, date unknown. ment a management structure that involves the affected sci- 2. NASA, Space Communications Architecture Vision, avail- ence and mission programs and other users and ensures able at https://www.spacecomm.nasa.gov/spacecomm/programs/ support for, and compliance with, the long-term communi- architecture.cfm. cations and navigation architecture. 3. NASA, SCAWG Charter, date unknown. 4. NASA, NASA Space Communications and Navigation Ar- chitecture Recommendations for 2005-2030, available at https:// Connections to the Broader Community www.spacecomm.nasa.gov/spacecomm/programs/architecture. cfm. Department of Defense 5. QoS (quality of service) refers to a system by which some An advantage of having the SCAWG take the lead in packets receive improved handling (greater precedence, for ex- ample) according to the requirements of the application. defining the path to the next-generation Earth relay satellite is the need to fully explore possible future cooperation with