National Academies Press: OpenBook
« Previous: 7 Communications and Navigation Architecture
Suggested Citation:"8 Technology." National Research Council. 2007. Review of the Space Communications Program of NASA's Space Operations Mission Directorate. Washington, DC: The National Academies Press. doi: 10.17226/11718.
×
Page 56
Suggested Citation:"8 Technology." National Research Council. 2007. Review of the Space Communications Program of NASA's Space Operations Mission Directorate. Washington, DC: The National Academies Press. doi: 10.17226/11718.
×
Page 57
Suggested Citation:"8 Technology." National Research Council. 2007. Review of the Space Communications Program of NASA's Space Operations Mission Directorate. Washington, DC: The National Academies Press. doi: 10.17226/11718.
×
Page 58
Suggested Citation:"8 Technology." National Research Council. 2007. Review of the Space Communications Program of NASA's Space Operations Mission Directorate. Washington, DC: The National Academies Press. doi: 10.17226/11718.
×
Page 59
Suggested Citation:"8 Technology." National Research Council. 2007. Review of the Space Communications Program of NASA's Space Operations Mission Directorate. Washington, DC: The National Academies Press. doi: 10.17226/11718.
×
Page 60
Suggested Citation:"8 Technology." National Research Council. 2007. Review of the Space Communications Program of NASA's Space Operations Mission Directorate. Washington, DC: The National Academies Press. doi: 10.17226/11718.
×
Page 61

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

8 Technology INTRODUCTION and navigation architecture is a service-based infrastructure providing command, telemetry, data return and forwarding, It is the mission of the technology program element of emergency services, and astronaut communications between the Space Communications Office (SCO) to constantly look each other and to mission control. These activities are per- to the future in order to find and develop new technologies formed during all phases of a space mission, including launch that will enhance NASA space communications and naviga- and transit, as well as for all possible final destinations, in- tion capabilities, or enable new capabilities that will improve cluding Earth orbit, the Moon, Mars, and anywhere else in service to NASA exploration and science mission users. The the solar system and beyond its boundaries. Table 8.2 shows Technology element is funded at $17 million per year and is how these various technology areas relate to these mission managed by NASA headquarters. This budget supports a phases and destinations.4 civil service workforce of 32 full-time equivalents and 15 For each of these technology areas, NASA identified a work-year equivalents of contractor support. The Technol- key capability that was selected to meet evolving NASA ogy element has contracts with Spectrolab Inc., Intevac Inc., mission needs. The capability is based on assumed data rates, OEC Inc., Princeton Lightwave Inc., and General Dynam- link availability, and quality of service expected. NASA also ics.1 Owing to proprietary concerns, details on these con- identified the current state of practice for each capability as tracts were not provided to the committee, which thus did well as the estimated development time needed to achieve not have further insight into the size of each of these con- the capability. Each of the key capabilities with its associ- tracts, the work being done, the schedule and tasks planned, ated data is shown in Table 8.3.5 the contract length, or how the contractors were selected. NASA uses the output of the Space Communications ASSESSMENT Architecture Working Group (SCAWG) and its Technology Assessment Team, described in Chapter 7, to select the tech- Space communications is a critical service that enables nologies on which it will focus its resources. Technologies NASA to perform its missions; therefore it follows that tech- that serve all of NASA’s mission directorates are included in nology developed for space communications also should be the SCO’s technology element portfolio. of critical importance. This report’s focus was limited to the The unifying challenge in space communications is the Space Operations Mission Directorate’s (SOMD’s) space need to transport data with higher quality, efficiency, flex- communications program. However, this program provides ibility, and interoperability than is currently possible. This only a portion of the overall NASA space communications need creates architectural challenges that vary depending on work, and also, only a portion of the spending for communi- where a NASA mission is going, when it is going, and what cations technology development. Since space communica- it will be doing when it gets there. Table 8.1 shows notional tions is in fact a critical function for any space mission, data rates for various communications services.2 NASA’s investment is further only a portion of the overall NASA has chosen to divide the Technology element into technology investment in this area, with the Department of six areas: optical communications, uplink arraying, space- Defense (DOD) and commercial entities also investing in craft radio frequency technology, programmable communi- space communication technologies. Where possible, the cations systems (software defined radio), navigation, and committee’s review of the space communications program’s plug-and-play interoperability.3 NASA’s communications technology development is placed in this larger context. 56

57 TECHNOLOGY TABLE 8.1 Notional Data Requirements User Channel Content Latency No. of Channels Channel Rate Total Rate Operational Base Speech NRT 2 10 kbps 20 kbps Engineering NRT 1 100 kbps 100 kbps Astronauts Speech NRT 4 10 kbps 40 kbps Helmet camera NRT 4 100 kbps 400 kbps Engineering NRT 4 20 kbps 80 kbps Human transports Video NRT 2 1.5 Mbps 3 Mbps Engineering NRT 2 20 kbps 40 kbps Robotic rovers Video NRT 8 1.5 Mbps 12 Mbps Engineering NRT 8 20 kbps 160 kbps Science orbiters Quick look NRT 4 1 Mbps 4 Mbps Engineering NRT 4 20 kbps 80 kbps High Rate Base HDTV 1 day 1 20 Mbps 20 Mbps Human transports HDTV (medical and PIO) NRT 2 20 Mbps 40 Mbps Hyperspectral imaging 1 day 1 150 Mbps 150 Mbps Robotic rovers Surface radar 1 day 1 100 Mbps 100 Mbps Hyperspectral imaging 1 day 1 150 Mbps 150 Mbps Science orbiters Orbiting radar 1 day 2 100 Mbps 200 Mbps Hyperspectral imaging 1 day 2 150 Mbps 300 Mbps SOURCE: John Rush and Dan Williams, NASA, NASA Communication and Navigation Technology Capability Portfolio, August 19, 2005. TABLE 8.2 Technology Area Relationship to Destinations Technology Areas Spacecraft Programmable Capability Optical Uplink Radio Communications Plug-and-Play Support Areas Communications Arraying Frequency Systems Navigation Interoperability Launch X X X Earth orbit X X X X Transit X X X X X X Lunar X X X X X Mars X X X X X X Solar system and beyond X X X X X X SOURCE: John Rush and Dan Williams, NASA, NASA Communication and Navigation Technology Capability Portfolio, August 19, 2005. However, understanding this larger context and how it af- Recommendation: As stated in the NASA Space Communi- fects the technology portfolio has proven challenging for cations and Navigation Architecture presentation to the Stra- NASA, as was confirmed by the Space Communications and tegic Management Council on March 17, 2006, a strategic Navigation Architecture presentation to the Strategic Man- communication technology program should be initiated to agement Council (SMC) on March 17, 2006, that called for improve coordinated technology investment in this critical the initiation of an integrated strategic communication tech- mission function. nology program.6 In that briefing, the need for a multicenter campaign that would involve other government agency par- To review the SCO technology element, the committee ticipation was identified. requested several presentations and supporting documents. Personnel involved with this review included experts previ-

58 REVIEW OF THE SPACE COMMUNICATIONS PROGRAM TABLE 8.3 Status of Key Capabilities Capability/Subcapability Mission or Roadmap Enabled Current State of Practice Minimum Estimated Development Time High-data-rate optical technology High data rate from Mars, solar None 4 years (demo 1 Mbps) 16 years (1 Gbps from Mars maximum system, and beyond; lower mass, (operational 1 Gbps) distance) power, volume for lunar mission spacecraft 2012 demo lunar capability Uplink antenna array—initial Deep space, Mars, and transit Single-dish antennas 5-8 years 12-m antenna array and extended to both High-data-rate radio frequency High data rate from Mars, solar Example: Mars Global Surveyor 10 years technology (1 Gbps from Mars system, and beyond 33 kbps, Mars Odyssey 14 kbps maximum distance) Programmable communications All missions Starlight, Electra, and LPT 15 years (25 Mbps landers, 500 Mbps systems (software-defined radio) orbiters, full autonomous independent platform software) Navigation All missions Radiometric techniques 5 years (x-ray pulsar navigation) Plug-and-play interoperability All missions Limited protocols for large delays Delay-tolerant protocols demonstrated on simulation and emulation testbed Downlink antenna array—initial Decommissioning of large Single-dish antennas 3 years 12-m antenna array and extended Deep Space Network antennas SOURCE: John Rush and Dan Williams, NASA, NASA Communication and Navigation Technology Capability Portfolio, August 19, 2005. ously tapped for the NRC review of the communications and cess. The technology program element is executed in a col- navigation roadmap conducted in March 2005,7 therefore legial fashion with many efforts receiving funds indepen- providing some continuity with that effort. Even after the dently from the Space Operations Mission Directorate as cancellation of that NRC effort, within NASA an effort con- well as from the Science Mission Directorate. Presumably, tinued to create the NASA Communication and Navigation if things were allowed to continue in this manner, the Explo- Technology Capability Portfolio report,8 which was an im- ration Systems Mission Directorate would become a third portant source of information in this assessment. uncoordinated funding source. The Space Communications Coordination and Integra- tion Board (SCCIB) technology working group that spans Formulation of the Program Plan directorates is officially supposed to coordinate technology NASA presented to the committee a top-level overview efforts, but unofficially NASA stated that the process is fairly of the process for determining technology needs in space ad hoc and informal. Since the SCCIB lacks “control,” it communications. That process begins with the exploration cannot prevent the mission directorates from acting only in and science missions (with their associated roadmaps), iden- their own best interests, and there is the risk that an inte- tifying needed capabilities and the time period in which they grated, efficient approach may not result. This includes the will be needed. That information is incorporated into the risk of duplicating and misdirecting efforts. For example, design of the communications and navigation architecture NASA cited the fact that optical communications technol- by the SCAWG as discussed in Chapter 7. The SCAWG in ogy efforts have focused on multiple wavelengths rather than turn determines the technologies needed to support the ar- concentrating efforts on a single one. NASA remarked that chitecture. This process ensures that the goals and objectives the technology program was more focused when there was a consolidated organization.10 As indicated above in connec- of the technology program are consistent with the NASA strategic plan and lower-level plans of the Science Mission tion with the March 2006 presentation to the Strategic Man- and Exploration Systems Mission Directorates. However, as agement Council, NASA has acknowledged the need for its NASA itself has acknowledged,9 the technology element space communications technology development to become managed by the SCO has little insight into the overall NASA a coordinated multicenter effort that spans all of NASA. funding of communications technology efforts, creating a A specific program plan document has not been written disconnect in this technology portfolio determination pro- for the technology element. As a surrogate, the NASA Com-

59 TECHNOLOGY munication and Navigation Technology Capability Portfo- It was difficult to assess from the data provided whether lio report contains some of what one would expect in a pro- the technology plans can be accomplished, and whether the gram plan. It discusses the portfolio and the process for de- planning is adequate and has sufficient decision points, down termining it without including specific information on selects, customer agreements, and/or unallocated out-year allocation of resources and schedule. It also lacks lower-level funding. Quarterly reviews with each of the centers sup- plans for each of the technology areas. Examples of lower- ported by the SCO technology program are identified, but level plans separate from this portfolio document were given NASA acknowledged that maintaining the quarterly sched- to the committee regarding how different specific technol- ule has been challenging and that the reviews were not al- ways consistent.12 Again without detailed data about each ogy projects within the technology areas are executed, but there is not a complete set of plans for all of the technology of the technology areas and the projects supported under projects, and there was little evidence of a uniform process those technology areas (examples were provided, but not a to plan and assess these efforts. This lack of detail made a complete set), it was difficult for the committee to assess complete evaluation of the technology element’s goals and specifics regarding deliverables, progress, off-ramps, and objectives difficult, including whether appropriate time ho- sunsets. Risks and risk management were not discussed for rizons are identified for technology advancement, how risk the various technologies, and this is a deficiency that should is managed, and the availability of critical personnel and fa- be addressed. A lack of information made it difficult for the cilities. committee to completely assess the adequacy of the plan- To address questions regarding the adequacy of facili- ning and the process used to complete this planning. ties and personnel, NASA did supply a document11 that In general NASA’s technology assessment process is stated the following list of facilities and personnel issues described as consisting of four steps: (1) identify system- needed to support NASA’s communications and navigations level issues, (2) identify performance requirements, (3) de- capabilities: termine technology and possible performance, and (4) iden- tify transformational technologies and track performance. Out of this process is to emerge the recommendations that Facilities and Assets determine the technology portfolio composition, schedules, • Deep Space Network ground stations at Canberra, and resource allocation. In determining this portfolio of in- Goldstone, and Madrid vestments, options are selected by NASA as a function of • Ground stations including White Sands Complex, potential “return on investment,” stated more specifically as MILA, KSC, WFF, GRGT an identification of the potential benefit of a technology in • Research and test facilities at JPL, GSFC, and GRC terms of reduction of user burden. NASA also tries to avoid • Tracking and Data Relay Satellite System (TDRSS) duplication of investments made by other U.S. government Critical Workforce Competencies agencies through dialogue within the large national space communications community as well as by looking for op- • RF and optical communications technologists portunities for partnerships with other agencies and indus- • NASA: GSFC, JPL, GRC, JSC, KSC and associated try. This portfolio is also integrated with NASA Small Busi- contractors ness Innovation Research (SBIR) program investments, • Laboratories: MIT Lincoln Labs, JHU Applied Phys- ics Lab, Naval Research Lab, Sandia National Lab, Air Force which appears to be one of NASA’s primary methods to Research Lab obtain industry involvement.13 • Universities NASA measures progress primarily by using technol- ogy readiness levels (TRLs) with each plan and providing a Human Capital Considerations technology maturation plan with TRL milestones aligned • Critical competencies must be maintained with cost estimates for achievement. Technology program • Improved workforce competency in new and emerg- performance is measured as a function of planned versus ing technology areas such as optical communications and actual TRL advancement.14 Examples of technology plans programmable communication systems were provided to the committee, and it appears that the ap- proach is sound if applied uniformly. Although the level of information provided did not al- low the committee to assess whether the facilities and per- Finding: Examples of specific technology development plans sonnel to support the technology element are adequate or if provided by NASA to the committee exhibited the character- and how the personnel issues are being addressed, the com- istics of a sound technology planning process; however, mittee agrees that this is a comprehensive top-level list. How- there was evidence suggesting that such a process was not ever, it is difficult to see how the leadership of the SCO can applied uniformly to all of the projects, with the most obvi- influence this large list beyond its organizational boundaries ous being the inability of NASA to provide this data for all of without more formal interagency relationships and increased the projects in the SCO technology element portfolio. resources to meet the need for all of these critical workforce competencies.

60 REVIEW OF THE SPACE COMMUNICATIONS PROGRAM Connections to the Broader Community Methodology The space communications community is quite vast, The lack of a complete technology element plan and the spanning not only NASA’s needs, but also those of the DOD challenge of providing the committee with requested infor- and other agencies. It is also a segment of the space market mation made assessment difficult. Those examples seen by with extensive commercial success and assets that NASA the committee appeared well crafted, but integration seemed can take advantage of to leverage its efforts. Without details to be lacking. The examples shown to the committee of how on all of the technology areas it was difficult to assess the SCAWG performed system-level assessments appeared whether the technology element utilized appropriate tech- to indicate a sound process (more completely described in nology work already done by the DOD, the U.S. commercial Chapter 7). Again, whether system-level assessments were space industry, and others. Knowledge of work done outside done for all of the technologies considered and how this in- the SCO program within NASA as well as in the larger com- fluenced the selection of the complete portfolio was unclear. munity is gathered informally. Without further detailed re- view it is difficult to assess the quality of the SCO technol- Finding: The connection between the top-down mission- ogy element relative to leaders in the field. A past review by driven technology needs of the NASA missions and NASA’s the National Research Council ranked several of NASA’s bottom-up technology planning must be tighter and must be space communications technology projects as world-class applied uniformly. The process is in place and simply needs efforts;15 however, a review of this depth was not performed to be completely executed. for this report. NASA does have unique technological re- quirements that need to be addressed, and its track record in In an ideal technology planning process, plans (includ- communications supports its position as a leader in address- ing tasks, priorities, schedules, and resources) are created ing these unique challenges. and accepted by all stakeholders. Periodic reviews are used Also, it was difficult to assess, from the details avail- to assess progress and make project adjustments based on able, whether the strategy for out-of-house work (competi- this progress. There is likely no single right answer for port- tions, partnerships, and so on) was well chosen and well folio composition, and the optimal composition will certainly managed. There was evidence that there is out-of-house com- change over time, but it is important to try to maintain some petition, with the SBIR process appearing to be the primary stability so that adjustments are minor and done mainly to mechanism. Examples were given of partnerships with other improve the technology portfolio as a whole. The impor- agencies, but a complete overview was not provided. Again, tance of stability and continuity in technology development because of the lack of complete information, the benefits should not be underestimated. (and costs) of increasing interoperability with military space Systems analysis is a crucial part of technology portfo- systems, commercial space systems, and the systems of for- lio management, enabling competitive task selection and eign space agencies were not assessed. ongoing refinement and redirection as technical progress is It was difficult to assess the role of external peer review made. Systems analysis also leads to an awareness of the in the SCO technology element, as information on how in- system-level impact of individual technologies under devel- ternal and external projects were selected was available only opment, allowing for a more holistic judgment. The commit- for isolated examples. The committee suggests that the SCO tee observed gaps in system-level analysis in the technology institutionalize a process for external peer review of all of its element. It suggests that, for every one of the projects within technology projects, both internal and external. External peer the technology element, some form of systems analysis ca- review should serve a role in task selection, ongoing reviews pability be applied. The methods can range from low-fidel- of progress, and a final assessment of results. It is important ity, back-of-the-envelope approaches to methods of in- for this process to be credible, and so a number of non-advo- creased fidelity, including parametric analysis and specific cate reviewers should be included. External peer review has point designs. Encouraging this system-level awareness proven beneficial in other government technology programs down to the lowest levels of individual technology projects within NASA as well as in other government agencies. If will serve as a mechanism to ensure that research goals re- executed properly it can provide a relatively unbiased re- tain their relevance. The fidelity of the method can be appro- view that creates defensible results to justify selections. The priate to the level of the project, but even performing a low- following recommendation is not new to NASA, having been fidelity analysis for the lowest-level project is important as suggested by the NRC in a previous report.16 opposed to conducting no analysis at all. A recommendation for improving NASA’s systems analysis capability as a tool Recommendation: The Space Communications Office should for technology portfolio management is not new, having been offered before.17 establish a formal external peer review process that would assess all aspects of the technology program element, in- cluding task selection, progress toward goals, and assess- Recommendation: To support technology investment deci- ment of final results. This process should be applied to exter- sions, systems analysis should be strengthened and made nal and internal technology projects.

61 TECHNOLOGY more uniform across the SCO technology element as a cru- proved stability and continuity in space communications cial part of the portfolio management and project selection technology development, the importance of which should process. The outlined general process of linking technology not be underestimated. As has been recommended by the NRC to NASA previously,18 performing systems analysis at decisions back up to architectures that are designed to meet mission requirements—which in turn are determined by the all levels of the technology portfolio, uniformly executing missions selected as a part of NASA’s strategic plan—is a the strategic management process outlined, and effectively good approach, but it needs to be applied uniformly so that using external peer review can all be methods to ensure suc- all technology projects have this top-to-bottom linkage. This cessful technology development. linkage will allow the lowest-level projects to retain their relevance. However, the process must be flexible enough to NOTES accommodate changing needs and new technology discov- 1. Rush, John, “NASA Communications Technology,” brief- eries. ing to the NRC Committee to Review NASA’s Space Communica- tions Program, Washington, D.C., January 26-27, 2006. CONCLUDING COMMENTS 2. Rush, John, and Dan Williams, NASA Communication and Navigation Technology Capability Portfolio, August 19, 2005. Through its excellence in mission execution, NASA has 3. Rush, John, and Dan Williams, NASA Communication and demonstrated that many of its efforts in space communica- Navigation Technology Capability Portfolio, August 19, 2005. tions technology are world-class and enabling for the sci- 4. Rush, John, and Dan Williams, NASA Communication and ence discoveries it has made. Space communications will Navigation Technology Capability Portfolio, August 19, 2005. continue to be an essential aspect of mission success and 5. Rush, John, and Dan Williams, NASA Communication and will always pose critical challenges that have to be met to Navigation Technology Capability Portfolio, August 19, 2005. enhance missions of the future. 6. Rush, John, “Space Communication and Navigation Ar- chitecture,” briefing to the NASA Strategic Management Council, To achieve this success has required critical workforce March 17, 2006. capabilities and unique facilities. To continue to achieve 7. The final stage of this study was cancelled as a result of mission success in the future, NASA must maintain and en- administrative changes at NASA. hance its workforce and facilities to keep pace. Insufficient 8. Rush, John, and Dan Williams, NASA Communication and detail was available to enable the committee to assess the Navigation Technology Capability Portfolio, August 19, 2005. current state of the workforce and facilities supporting the 9. Rush, John, and Dan Williams, personal communication, technology element or to assess whether plans will suffi- March 27, 2006. ciently support this critical NASA capability in the future. 10. Spearing, Robert, comment during briefing to NRC Com- Further review is merited to ensure that the capability to cre- mittee to Review NASA’s Space Communication Program, Wash- ate world-class technologies to support NASA’s critical ington, D.C., March 14, 2006. space communications function is maintained and enhanced. 11. Spearing, Robert, and M. Regan, NASA Communication and Navigation Capability Roadmap, May 2005. If the recommendation to the NASA Strategic Management 12. Rush, John, and Dan Williams, personal communication, Council to create an integrated NASA communications tech- March 27, 2006. nology program across all of NASA is executed, then a re- 13. Rush, John, “NASA Communications Technology,” brief- view of the integrated program will be merited to see if goals ing to the NRC Committee to Review NASA’s Space Communica- are being met and the recommendations provided here have tions Program, Washington, D.C., January 26-27, 2006. been incorporated. With sufficient information and time for 14. Rush, John, “NASA Communications Technology,” brief- analysis, such a review could explore more deeply the devel- ing to the NRC Committee to Review NASA’s Space Communica- opment of the technologies on an individual project level so tions Program, Washington, D.C., January 26-27, 2006. that the overall NASA space communications technology 15. National Research Council (NRC). 2003. R eview of portfolio can be properly weighed. Unfortunately, the sched- NASA’s Aerospace Technology Enterprise: An Assessment of ule for and the data available during this review were not NASA’s Pioneering Revolutionary Technology Program. The Na- tional Academies Press, Washington, D.C. adequate for exploring NASA’s space communications tech- 16. NRC. 2003. Review of NASA’s Aerospace Technology En- nology development to the depth that it deserves. terprise: An Assessment of NASA’s Pioneering Revolutionary Tech- If NASA creates an integrated technology development nology Program. The National Academies Press, Washington, program, this integration of efforts should go a long way D.C. toward addressing shortfalls of the technology element, 17. NRC. 2003. Review of NASA’s Aerospace Technology En- which appear to involve primarily a lack of coordination. terprise: An Assessment of NASA’s Pioneering Revolutionary Tech- Processes are in place at NASA that, if applied uniformly, nology Program. The National Academies Press, Washington, could result in a technology program that strives for the ideal D.C. technology planning process, whereby plans (including 18. NRC. 2003. Review of NASA’s Aerospace Technology En- tasks, priorities, schedules, and resources) are created and terprise: An Assessment of NASA’s Pioneering Revolutionary Tech- accepted by all stakeholders. If so, the result could be im- nology Program. The National Academies Press, Washington, D.C.

Next: 9 Operations Integration Program Element »
Review of the Space Communications Program of NASA's Space Operations Mission Directorate Get This Book
×
Buy Paperback | $29.00 Buy Ebook | $23.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

The Space Communications Office (SCO) at NASA has two primary roles. The first is to manage two of the communications networks that enable spaceflight operations and research, and the second is to integrate agency-wide telecommunications issues. In 2005, NASA asked the NRC to review the effectiveness of the SCO in carrying out its responsibilities by assessing the overall quality of the space communications program. This report presents a review of each of the program elements within the SCO—the space network, NASA’s integrated space network (NISN), spectrum management, standards management, search and rescue, communications and navigation architecture, technology, and operations integration. The review focuses on formulation of plans for each element, plan development methodology, connections with the broader community, and overall capabilities. Recommendations for improving SCO operations and organization are provided.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!