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Review of the Space Communications Program of Nasas Space Operations Mission Directorate
7
Communications and Navigation Architecture
INTRODUCTION
The communications and navigation architecture program element is responsible for defining the space communications and navigation architecture to support NASA’s science and exploration missions through 2030. This architecture must evolve through 2030 and beyond to keep pace with the needs of future science and exploration users and, potentially, non-NASA users. The communications and navigation architecture necessarily encompasses components such as the Deep Space Network and the Ground Network that are managed outside the Space Operations Mission Directorate (SOMD). As per its charge, in this chapter the committee examines NASA’s approach to developing the architecture, and the resources and capabilities that will support that development.
The communications and navigation architecture program element accomplishes its task through NASA’s agency-wide Space Communications Architecture Working Group (SCAWG), whose purpose is to develop a future space communications architecture and identify associated technology investments necessary to support all future NASA exploration, science, and human-tended missions.1 SCAWG’S scope is shown in Figure 7.1.
The purpose of NASA’s space communications architecture is “to concurrently architect the Space Communications Network to enable NASA’s changing mission of Exploration.”2 To do this, NASA is developing 5-year “snapshots” of the space communications architecture that must evolve from the present Deep Space Network, Space Network, and Ground Network in order to provide the necessary communication capabilities to support NASA exploration and science programs. Figure 7.2 shows the elements that will be associated with NASA’s space communications architecture by approximately 2030.
Regarding the navigation portion of the communications and navigation architecture, NASA believes that navigation requirements will continue to heavily influence modulation formats and other details of the physical layer (layer 1 of the OSI model) for communication, including the need for accurate and synchronized time sources. For example, a fraction of a spacecraft’s transmitter power can be dedicated to an unmodulated radio frequency beacon that is included strictly for the purpose of computing the range and range rate from Earth to the spacecraft. If a user spacecraft needs to find its position by comparing beacons from different sources, as in the Global Positioning System, those sources must synchronize their transmissions. Navigation requirements can influence the selection of a particular constellation of communication relay satellites orbiting a planet, but these requirements are not expected to significantly influence the choice of whether or not to link specific nodes in the space network. Therefore, it is not unreasonable that navigation is mentioned only occasionally in SCAWG documents. The remainder of this chapter focuses on communications.
ASSESSMENT
Formulation of the Program Plan
Project Objectives
NASA’s SCAWG encompasses members that carry out both technical and programmatic/authoritative functions, and that represent each NASA mission directorate, the Strategic Investment Division of the chief financial officer, and communication networks and the user community, including NASA centers. Evidence of the breadth of SCAWG’s membership was confirmed by the committee through its review of documentation3 and in presentations to the committee by the SCAWG’s chair. Structurally, SCAWG’s membership allows for inviting subject-matter experts from government, academia, and industry to participate in specific studies on an as-needed basis.
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Review of the Space Communications Program of Nasas Space Operations Mission Directorate
FIGURE 7.1 Scope of NASA’s Space Communications Architecture Working Group, as indicated by the continuous thin blue line encompassing 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, the recommendations are likely to flow both to and from organizations represented in the SCAWG and to flow both from work in process and any formal reports by the SCAWG. Given this high degree of continuing interaction, the committee believes that the objective of involving mission directorates in planning and review is adequately addressed.
Finding: NASA’s Space Communications Architecture Working Group appears to have all the necessary qualifications, capabilities, and facilities to perform its work, and its output is of high quality.
Project Deliverables
One example of SCAWG activities is a recent lunar relay architecture study4 that produced a preliminary evaluation of the options shown in Figure 7.3. Further refinement of the elliptical orbit option is now beginning at GSFC, as lunar relay becomes a project rather than a concept. The SCAWG will have one or more members from GSFC’s lunar relay project team, keeping the architecture team up to date on the capabilities that lunar relay is expected to provide. Eventually, lunar relay will be one more existing infrastructure capability forming a basis for future space communications architectures.
As it was explained to the committee, the 2006 SCAWG activity is a one-time effort to defragment NASA’s vision for space communications infrastructure. Now that the SCAWG has established its vision for a 2030 communications architecture and has conducted a lunar relay architecture study, its level of activity is expected to decrease within the next 2 years. The nature of that activity could also change as today’s proposed architectures become tomorrow’s projects. For example, the SCAWG is responsible for keeping track of GSFC’s preliminary design work on a lunar relay project and any other near-term design efforts.
Recommendation: NASA’s Space Communications Architecture Working Group should continue as planned to carefully evaluate near-term and intermediate-term architecture options while promoting development of components such as relay satellites and ground stations consistent with the long-term communications architecture.
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Review of the Space Communications Program of Nasas Space Operations Mission Directorate
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
The development path to the SCAWG’s long-term communications architecture vision will be determined by programmatic decisions yet to be made, namely the definition and scheduling of specific missions. As long as the intermediate steps provide proper software layering and allow for some software upgradability, the path to the long-term vision is quite flexible. This is one of the greatest technical strengths of the SCAWG’s recommended communications architecture.
The SCAWG will produce occasional updates on the long-term architecture plus individual studies of how to satisfy the communication and navigation needs of specific missions. These updates should enable NASA to assess progress, should prompt feedback from internal customers (the designers for each mission), and should ultimately make the case for reallocating funding as needed. Adequacy of staffing to support this level of activity depends on active support from other mission directorates.
The SCAWG has not yet attempted to define whether or how the existing near-Earth network (Tracking and Data Relay Satellite System; TDRSS) could evolve to support the long-term communications architecture. One of the elements of that long-term vision is a next-generation Earth relay satellite, but the features of this satellite are unspecified. NASA needs to have a working concept (or concepts) of the next-generation Earth relay satellite, if only to allow for an orderly transition of service from the current TDRSS and its planned replenishment.
The committee does not presume to define an evolutionary path from today’s TDRSS forward, but only to point out that some baseline definition, however imperfect, should be undertaken before NASA issues requests for proposals for future Earth relay satellites. This suggestion is not inconsistent with NASA’s present plan to include acquisition in the FY 2008 budget of “clone” TDRSS satellite replacements for launch in 2015.
Finding: A critical near-term task for NASA’s Space Communications Architecture Working Group (SCAWG) will be to define one or more potential evolutionary paths from today’s TDRSS to an Earth relay system consistent with the SCAWG’s vision for a long-term communications architecture.
Technologies that might be worth further study as components of some of the evolutionary paths include those for augmentation of the ground network (more sites, fiber interconnection) to provide better coverage in low Earth orbit;
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Review of the Space Communications Program of Nasas Space Operations Mission Directorate
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 access service (higher transmitted signal levels and greater receiving sensitivity) to allow some current S-band single-access users to move to multiple access; near-Earth relay crosslinks into the DOD’s transformational satellite (TSAT) network (in order to share TSAT’s communications backbone); technologies for packet switching (with or without a router); optical links to user satellites; and combined DOD/ NASA satellite payloads.
In addition to working on the evolution of Earth relay capabilities, the SCAWG will need to work closely with Goddard’s Exploration, Operations, Communications, and Navigation Systems organization to maximize the long-term utility of the technologies and components of the lunar relay project.
The SCAWG did not present to the committee any results on protocol stack software for space networks, but the requirements for this software are largely independent of the constellation design. This problem can therefore be addressed separately. JPL is currently taking the most prominent role in design of protocol software for space networks.
The committee is aware of the differences of opinion regarding space communications protocols. (See “The Inter-planetary Internet,” IEEE Spectrum magazine, August 2005.) However, that conflict is not primarily a technical one: everyone agrees that IP (Internet Protocol, the basic packet data communication standard) will work fine at short ranges and that UDP (User Datagram Protocol for one-way transport of data packets) will be the foundation for longer-range communication. The SCAWG should promote the development of long-term protocol stack solutions consistent with the ongoing use of IP in networks on the Moon and Mars. This should include consideration of any applicable delay-handling and QoS5 techniques currently being pioneering in DOD’s TSAT effort.
As these protocol solutions progress from concept to implementation, they will move under the control of the Data Standards program element. This process is analogous to the movement of the architecture concepts from the SCAWG to individual programs as the elements (such as lunar relay) become real projects.
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Review of the Space Communications Program of Nasas Space Operations Mission Directorate
Finding: Protocol stack solutions for space communications networks beyond Earth orbit are an important foundational element for the long-term communications system architecture.
Recommendation: NASA’s Space Communications Architecture Working Group should promote cooperation between the Jet Propulsion Laboratory and other groups, both within NASA and in DOD and academia, that are doing significant work to design the protocol stack software necessary to operate packet networks in deep space.
Long-Term Project Goals and Objectives
The primary challenge to implementing the architecture developed by the SCAWG will be ensuring support and compliance by the component projects and user entities. The agency-wide representation in the SCAWG is most appropriate for the current effort, and an extension of that approach that includes all users should be considered as the communications and navigation architecture moves along the path to fruition.
Recommendation: NASA’s top management should implement a management structure that involves the affected science and mission programs and other users and ensures support for, and compliance with, the long-term communications and navigation architecture.
Connections to the Broader Community
Department of Defense
An advantage of having the SCAWG take the lead in defining the path to the next-generation Earth relay satellite is the need to fully explore possible future cooperation with DOD. Although the committee is aware of the occasional conflicts over resource allocation between DOD and NASA users today, the potential overall benefit from combining networks is too attractive to ignore.
Additionally, some portion of NASA’s communications backbone could be provided by DOD systems, including TSAT and the Global Information Grid, which could relieve some of NASA’s budget pressure while still providing vital communications capability.
Other Space Agencies
In his presentations to the committee the SCAWG’s chair did not mention cooperation with foreign space agencies. Cooperation on specific missions for common relay capabilities and sharing of ground network resources is probably best handled by the project managers for the specific missions. Nevertheless, it would be useful for the SCAWG to establish a mechanism for periodic identification of opportunities for international cooperation.
NOTES
1. National Aeronautics and Space Administration (NASA), SCAWG Charter, date unknown.
2. NASA, Space Communications Architecture Vision, available at https://www.spacecomm.nasa.gov/spacecomm/programs/architecture.cfm.
3. NASA, SCAWG Charter, date unknown.
4. NASA, NASA Space Communications and Navigation Architecture Recommendations for 2005-2030, available at https://www.spacecomm.nasa.gov/spacecomm/programs/architecture.cfm.
5. QoS (quality of service) refers to a system by which some packets receive improved handling (greater precedence, for example) according to the requirements of the application.
Representative terms from entire chapter:
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