Brian D. Haynes

Agile Defense LLC, Minneapolis, Minnesota

Technology Candidates for Air-to-Air

and Air-to-Ground Data Exchange

NASA/CR—2015-218843

September 2015
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Brian D. Haynes

Agile Defense LLC, Minneapolis, Minnesota

Technology Candidates for Air-to-Air

and Air-to-Ground Data Exchange

NASA/CR—2015-218843

September 2015

National Aeronautics and

Space Administration

Glenn Research Center

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Table of Contents 

1 EXECUTIVE SUMMARY ..................................................................................................................................5

2 INTRODUCTION ...............................................................................................................................................7

3 RESEARCH PLAN .............................................................................................................................................9

3.1 RESEARCH PROCESS.................................................................................................................................................... 9

3.2 WORK BREAKDOWN.................................................................................................................................................12

3.3 RESEARCH TEAM.......................................................................................................................................................13

4 LITERATURE REVIEW ................................................................................................................................ 17

5 FUTURE DATA EXCHANGE REQUIREMENTS ...................................................................................... 18

5.1 FUTURE NAS SCENARIOS.........................................................................................................................................18

5.2 INFORMATION EXCHANGE BREAKDOWN ..............................................................................................................23

5.3 DATA COMMUNICATION FUNCTIONS .....................................................................................................................25

6 CANDIDATE TECHNOLOGIES................................................................................................................... 30

6.1 ENABLING TECHNOLOGIES ......................................................................................................................................30

6.2 LINK TECHNOLOGY CANDIDATES ...........................................................................................................................40

7 CANDIDATE TECHNOLOGY COMPARISON .......................................................................................... 57

7.1 CANDIDATE SUITABILITY RATINGS ........................................................................................................................57

7.2 CANDIDATE COMPARISON RESULTS.......................................................................................................................69

7.3 TECHNICAL ANALYSIS CONCLUSIONS.....................................................................................................................73

7.4 TECHNICAL GAP ANALYSIS ......................................................................................................................................74

8 BUSINESS CASE ANALYSIS ........................................................................................................................ 76

8.1 BUSINESS CASE ANALYSIS APPROACH...................................................................................................................76

8.2 UNDERLYING ASSUMPTIONS....................................................................................................................................78

8.3 REFERENCE ARCHITECTURES..................................................................................................................................81

8.4 BUSINESS CASE FINDINGS........................................................................................................................................92

8.5 BUSINESS CASE GAP ANALYSIS ............................................................................................................................102

9 MOST PROMISING TECHNOLOGIES .....................................................................................................103

9.1 BBSDR .....................................................................................................................................................................103

9.2 DELIVERY MANAGER .............................................................................................................................................104

9.3 AEROWAN, AEROMACS, AND MERGED WIFI................................................................................................. 104

9.4 ADS-B NEXT...........................................................................................................................................................104

9.5 VDL-NEXT...............................................................................................................................................................105

9.6 COMMERCIAL CELLULAR DATA SYSTEMS ..........................................................................................................105

10 CONCLUSIONS AND RECOMMENDATIONS......................................................................................106

10.1 OVERALL CONCLUSIONS .....................................................................................................................................106

10.2 RECOMMENDATIONS ...........................................................................................................................................108

10.3 FOLLOW-ON RESEARCH RECOMMENDATIONS...............................................................................................108

10.4 SUMMARY..............................................................................................................................................................109

NASA/CR—2015-218843 1
11 APPENDICES .............................................................................................................................................111

11.1 FUTURE DATALINK TECHNOLOGY CANDIDATE MATRIX ..............................................................................112

11.2 APPENDIX 2 - LITERATURE REVIEW BIBLIOGRAPHY AND NOTES ..............................................................118

11.3 APPENDIX 3 – AIR TRANSPORT – DOMESTIC BUSINESS CASE ANALYSIS..................................................127

11.4 APPENDIX 4 – AIR TRANSPORT – OCEANIC BUSINESS CASE ANALYSIS.....................................................129

11.5 APPENDIX 5 – GENERAL AVIATION BUSINESS CASE ANALYSIS...................................................................131

11.6 APPENDIX 6 – CANDIDATE COMPARISON DETAIL .........................................................................................133

11.7 APPENDIX 7 – SDR AND DM CONCEPTUAL ARCHITECTURE.......................................................................144

11.8 APPENDIX 8 – ACRONYMS AND TERMS ...........................................................................................................146

LIST OF TABLES 

TABLE 3-1: ORGANIZATIONAL ROLES AND RESPONSIBILITIES..................................................................................................................14

TABLE 3-2: KEY PERSONNEL...........................................................................................................................................................................14

TABLE 8-1: AIR TRANSPORT DOMESTIC, CURRENT AND FUTURE ARCHITECTURE ...............................................................................83

TABLE 8-2: AIR TRANSPORT OCEANIC, CURRENT AND FUTURE ARCHITECTURE..................................................................................87

TABLE 8-3: GENERAL AVIATION, CURRENT AND FUTURE ARCHITECTURE.............................................................................................91

TABLE 8-4: AIR TRANSPORT – DOMESTIC BUSINESS CASE RESULTS.......................................................................................................95

TABLE 8-5: AIR TRANSPORT – OCEANIC BUSINESS CASE RESULTS .........................................................................................................97

TABLE 8-6: GENERAL AVIATION BUSINESS CASE RESULTS .......................................................................................................................98

TABLE 11-1: AIRBORNE AIRCRAFT, INBOUND COMMUNICATIONS........................................................................................................112

TABLE 11-2: AIRBORNE AIRCRAFT, INBOUND COMMUNICATIONS (CONT’D) .....................................................................................113

TABLE 11-4: AIRCRAFT ON GROUND, INBOUND COMMUNICATIONS.....................................................................................................115

TABLE 11-5: AIRCRAFT ON GROUND, INBOUND COMMUNICATIONS (CONT’D) ..................................................................................116

TABLE 11-6: AIRCRAFT ON GROUND, OUTBOUND COMMUNICATIONS ................................................................................................. 117

TABLE 11-7: LITERATURE REVIEW BIBLIOGRAPHY AND NOTES...........................................................................................................118

TABLE 11-8: AIR TRANSPORT – DOMESTIC BUSINESS CASE ANALYSIS ...............................................................................................127

TABLE 11-9: AIR TRANSPORT – DOMESTIC BUSINESS CASE ANALYSIS (CONT.)...............................................................................128

TABLE 11-11: AIR TRANSPORT – OCEANIC BUSINESS CASE ANALYSIS (CONT.)...............................................................................130

TABLE 11-12: – GENERAL AVIATION BUSINESS CASE ANALYSIS ..........................................................................................................131

TABLE 11-13: – GENERAL AVIATION BUSINESS CASE ANALYSIS (CONT.)..........................................................................................132

TABLE 11-14: ACRONYMS AND TERMS ......................................................................................................................................................146

LIST OF FIGURES 

FIGURE 3-1 RESEARCH FLOW CHART ............................................................................................................................................................11

FIGURE 6-1: BBSDR ARCHITECTURE............................................................................................................................................................32

FIGURE 6-2: FUTURE AIRCRAFT COMMUNICATION ARCHITECTURE WITH DM AND BBSDR .............................................................36

FIGURE 6-3: FUTURE TERRESTRIAL COMMUNICATION DISTRIBUTED DM CLIENT/SERVER ARCHITECTURE ................................. 37

FIGURE 7-1: LINK CANDIDATE SUITABILITY RATINGS................................................................................................................................68

FIGURE 7-2: AGGREGATE WEIGHTED SCORE WITH OBSOLESCENCE........................................................................................................70

NASA/CR—2015-218843 2
FIGURE 7-3: AVERAGE WEIGHTED SCORE WITH OBSOLESCENCE.............................................................................................................71

FIGURE 7-4: AGGREGATE WEIGHTED SCORING SHOWING MERGED AEROWAN / AEROMACS SCORE...........................................72

FIGURE 11-1: AGGREGATE LINK SCORES - BASELINE..............................................................................................................................133

FIGURE 11-2: AVERAGE LINK SCORES - BASELINE...................................................................................................................................134

FIGURE 11-3: FUNCTION PRIORITY WEIGHTED AGGREGATE SCORES ..................................................................................................135

FIGURE 11-4: FUNCTION PRIORITY WEIGHTED AVERAGE SCORES.......................................................................................................136

FIGURE 11-5: AGGREGATE WEIGHTED SCORE WITH OBSOLESCENCE ..................................................................................................140

FIGURE 11-6: AVERAGE WEIGHTED SCORE WITH OBSOLESCENCE .......................................................................................................141

FIGURE 11-7: AGGREGATE WEIGHTED SCORING SHOWING MERGED AEROWAN / AEROMACS SCORE.....................................142

FIGURE 11-8: CONCEPTUAL ARCHITECTURE OF DELIVERY MANAGER AND BBSDR IN AIRCRAFT SYSTEMS................................144

FIGURE 11-9: CONCEPTUAL ARCHITECTURE OF DELIVERY MANAGER AND BBSDR IN AIRCRAFT SYSTEMS (CONT.)................145

NASA/CR—2015-218843 3

1 Executive Summary 

Technology Candidates for Air-to-Air and Air-to-Ground Data Exchange is a two-year research 

effort to visualize the U. S. aviation industry at a point 50 years in the future, and to define 

potential communication solutions to meet those future data exchange needs. The research 

team, led by XCELAR, was tasked with identifying future National Airspace System (NAS) 

scenarios, determining requirements and functions (including gaps), investigating technical and 

business issues for air, ground, & air-to-ground interactions, and reporting on the results. The 

project was conducted under technical direction from NASA and in collaboration with XCELAR’s 

partner, National Institute of Aerospace, and NASA technical representatives. 

Parallel efforts were initiated to define the information exchange functional needs of the 

future NAS, and specific communication link technologies to potentially serve those needs. 

Those efforts converged with the mapping of each identified future NAS function to potential 

enabling communication solutions; those solutions were then compared with, and ranked 

relative to, each other on a technical basis in a structured analysis process. The technical 

solutions emerging from that process were then assessed from a business case perspective to 

determine their viability from a real-world adoption and deployment standpoint. The results 

of that analysis produced a proposed set of future solutions and most promising candidate 

technologies. Gap analyses were conducted at two points in the process, the first examining 

technical factors, and the second as part of the business case analysis. In each case, no gaps or 

unmet needs were identified in applying the solutions evaluated to the requirements 

identified. 

The future communication solutions identified in the research comprise both specific link 

technologies and two enabling technologies that apply to most or all specific links. As a result, 

the research resulted in a new analysis approach, viewing the underlying architecture of 

ground-air and air-air communications as a whole, rather than as simple “link to function” 

paired solutions. For the business case analysis, a number of “reference architectures” were 

developed for both the future technologies and the current systems, based on three typical 

configurations of current aircraft. Current and future costs were assigned, and various 

comparisons made between the current and future architectures. 

In general, it was assumed that if a future architecture offers lower cost than the current 

typical architecture, while delivering equivalent or better performance, it is likely that the 

future solution will gain industry acceptance. Conversely, future architectures presenting 

higher costs than their current counterparts must present a compelling benefit case in other 

areas or risk a lack of industry acceptance. The business case analysis consistently indicated 

lower costs for the proposed future architectures, and in most cases, significantly so. The 

proposed future solutions were found to offer significantly greater functionality, flexibility, and 

Technology Candidates for Air-to-Air 

and Air-to-Ground Data Exchange 

Brian D. Haynes 

Agile Defense LLC 

Minneapolis, Minnesota 55450 

NASA/CR—2015-218843 5
growth potential over time, at lower cost, than current systems. This was true for overall, 

fleet-wide equipage for domestic and oceanic air carriers, as well as for single, General 

Aviation (GA) aircraft. 

The overall research results indicate that all identified requirements can be met by the 

proposed solutions with significant capacity for future growth. Results also illustrate that the 

majority of the future communication needs can be met using currently allocated aviation RF 

spectrum, if used in more effective ways than it is today. A combination of such optimized 

aviation–specific links and commercial communication systems meets all identified needs for 

the 50-year future and beyond, with the caveat that a new, overall function will be needed to 

manage all information exchange, individual links, security, cost, and other factors. This 

function was labeled “Delivery Manager” (DM) within this research. DM employs a distributed 

client/server architecture, for both airborne and ground communications architectures. 

Final research results included identifying the most promising candidate technologies for the 

future system, conclusions and recommendations, and identifying areas where further 

research should be considered. 

NASA/CR—2015-218843 6
2 Introduction 

The effort to define, characterize and assess the relative merits of Technology Candidates for 

Air-to-Air and Air-to-Ground Data Exchange is designed to visualize and analyze the future 

aviation communication needs of the National Airspace System (NAS), and ultimately to 

recommend the optimum solutions based on the overall research results. Agile Defense, DBA 

XCELAR (XCELAR) performed the research summarized herein under contract to NASA, to 

perform a study of future air-to-air and air-to-ground datalink technologies as part of NASA’s 

Airspace Systems Program. The study focused on identifying technologies and potential 

solutions to address datalink needs of the air transportation system fifty years into the future, 

nominally from 2013, when research began, through 2063. XCELAR’s approach incorporated 

both technical and business considerations, and considered the needs of General Aviation and 

Unmanned Aerial Systems in addition to those of air carriers and other jet operators. The 

perspective of the aircraft operator was an integral element of the evaluation process. 

A key consideration was to correctly define the underlying assumptions for the air 

transportation system in the year 2063, particularly in terms of regulatory considerations, 

system user expectations and sensitivities, and their rate of evolution over the 50-year period. 

One useful input to that process was to look back over a similar period, and assess the rate of 

change over the past 50 years in the same industry. 

Examples of significant technical events approximately 50 years ago include: 

 1963 - B727 first flight 

 1963 – Cessna 336 enters service; 1965 – Cessna 337 in certification 

 1963 – Lear 23 first flight 

 1963 – First Laser Ring Gyro demonstrated 

 SELCAL entered service approximately 50 years ago 

More recent events help to put the timeline into better perspective: these notable events took 

place approximately 30 years ago: 

 1983 – First cellular telephone received FCC approval (Motorola DynaTac) 

 1983 – First PDA (Psion 1); 1993 – Apple Newton 

 1983 – GPS made available for civil use 

 1981 – First Laser Ring Gyro certified 

 ACARS Entered service 

NASA/CR—2015-218843 7
This illustrates that the entire history of aviation datalink services to date is only approximately 

30 years, making the 50-year future projected herein nearly 40% longer than its whole service 

history. Thus, it is reasonable to postulate that the next 50 years could produce a greater 

scope of innovation, technology introduction, performance gains, and overall progress than 

aviation data link has seen since its inception. 

The XCELAR research team includes industry professionals whose experience spans the history 

of aviation data link to the present time, with an impressive track record in developing, 

implementing, operating, and improving all types of aviation data links, in all segments of the 

industry. The team believes that the research conducted, and the results presented herein, 

are both achievable and practical, and present a viable approach to meeting the aviation 

communication needs of the future. 

NASA/CR—2015-218843 8
3 Research Plan 

The research was conducted in accordance with a structured plan that included parallel work 

flows to define the future datalink requirements, and to identify and characterize technical 

candidates. Those two paths converged in comparison and gap analysis steps, with provisions 

for a “feedback loop” process to revisit previous steps as needed to fill gaps identified. 

Technical analysis results were then examined from a business perspective, and another gap 

analysis/feedback stage planned prior to final identification and characterization of the most 

favorable candidates. 

A regular reporting process was used, employing a combination of monthly and other 

scheduled reports, and periodic results reports at key stages of the research process, among 

the program deliverables. 

3.1 Research Process 

The anticipated functions and requirements of the NAS of the future were defined using 

input from NASA, industry, academia, and the study team, using a structure based on 

phase of flight and vehicle/operator types. These scenarios were then used to derive the 

associated information flow and data communication requirements. In parallel, initial 

analysis of candidate link technologies was initiated, starting with projected capabilities, 

loading, and improvements of currently available technologies through the study period. 

An initial analysis of currently known link technologies was conducted to assess their 

current and probable future capabilities compared to projected loads and other factors. 

This “look forward” analysis provided a starting point in the identification and analysis of 

future solutions. It quantified potential unmet needs in current systems, and thus 

identified that may require new solutions. An example is the current use of 1030/1090 

MHz for ATCRBS, ADS-B, TCAS, and other proposed functions such as air-to-air linking of 

wake turbulence data. The research team’s analysis indicates that the current system will 

reach its capacity limit well before the 50-year future point, and alternate solutions must 

be considered as part of the study. 

The research then considered various candidate technologies for alternative future links, 

characterizing those that showed the most promise from a technical standpoint, and 

incrementally maturing the most viable ones. Architectural and infrastructure issues were 

considered, along with compatibility with other current and future systems. Performance 

modeling was conducted to allow technical comparisons between candidates, and a 

structured candidate comparison was developed between the twelve most viable 

candidates from a technical perspective. 

NASA/CR—2015-218843 9
Finally, those candidate solutions that were deemed technically viable were subjected to 

business case analysis, which examined the costs of proposed future solutions relative to 

current industry practice. The ultimate goal of the research was to identify a group of most 

promising technologies for the aviation datalink 50 years in the future. 

Fig. 1 depicts the overall flow of the research from beginning to end, with reports at 

various key points in the process. The parallel paths of defining future NAS functions and 

derived communication requirements, and of identifying potential candidate link 

technologies, are shown in the upper left area. Those two efforts converge with the 

evaluation of candidates against technical requirements, with those deemed to be worthy 

of further research documented in the Technology Candidate Descriptions report. An 

analysis of various system considerations was then initiated, with the results documented 

in the Infrastructure and Architectural Needs report. 

At this point in the research, any identified gaps or unmet needs would have initiated a 

parallel effort to identify additional or alternative candidates to potentially fill the gap. This 

part of the process is depicted in the upper right portion of the flow chart. Remaining 

candidates were subjected to a structured comparison process to quantify the relative 

merits of all viable candidates for each identified function, from a technical perspective. 

Results were documented in the Candidate Comparison report. One of the conclusions of 

the research to that point was that there were in fact no gaps or unmet needs, and a group 

of nine candidate link technologies, and two underlying or enabling technologies, became 

the focus of the business case analysis. 

The business case analysis, shown near the bottom center of the flow chart, examined the 

potential costs of the 50-year future solution proposed as a result of the technical research, 

relative to current industry solutions and their costs. Another gap analysis was performed 

based on the totality of the research and its results, with another potential “feedback loop” 

to examine potential ways to fill identified gaps. Again, no gaps were identified, and the 

research proceeded to the final planned step, identifying the most promising candidate 

solutions. Both the business case analysis and the most promising solutions were 

documented in the Most Promising Technology Alternatives report. The research was 

divided into two segments, designated the Base Year and Option Year one. Base Year 

activities were initiated in October 2012, and Option Year 1 concluded in January 2015. 

NASA/CR—2015-218843 10
Figure 3-1 Research Flow Chart 

NASA/CR—2015-218843 11
3.2 Work Breakdown 

The research was conducted in accordance with a structured Work Plan, submitted as the first 

Deliverable. Major tasks included the following, structured in accordance with the research 

process shown in Fig. 1. 

 KOM and Work Plan 

o Work Plan document 

 Literature Review 

 Define Future NAS Scenarios 

o Host and Organize a NAS Communications Workshop 

o “Summary of Results of the Future NAS Communications Workshop” report 

o “Future NAS Scenarios” report 

 Define Data Exchange Functions 

 Define Data Communications Functional Requirements for Each Scenario 

o Identify Candidate Link Technologies 

o Derive Functional Requirements 

o Evaluate the Suitability of the Various Candidate Link Technologies 

o Prepare a Functional Requirements Matrix 

o Include value-weightings (must-have, very desirable, nice-to-have) 

 Define Potential Datalink Technologies 

o Evaluate the suitability of the identified datalink technologies 

o Identify existing, updated and new datalink systems 

o “Technology Candidate Descriptions” report 

 Infrastructure and Architecture Needs 

o Analyze the System Level considerations of each Candidate datalink technology 

o Interoperability with other systems 

o Applicability across multiple user groups, scalability over time and changing 

industry needs 

NASA/CR—2015-218843 12
o Failure modes, effects, and backups needed to achieve the functional 

requirements 

o “Infrastructure and Architectural Needs” report 

 Conference Presentation 

o ICNS 2013 

 Base Year Report 

 Characterize and Compare Candidates 

o Comparison matrix correlating functional requirements against both existing 

and emerging candidate systems 

o Comparing candidates for the same functions with each other 

o Identify functional or operational gaps for each candidate system 

o “Candidate Comparison” report

o Prepare a business case for each candidate technology 

o Consider current conditions, projection of future influences like advances in 

component design and increased production volumes 

o “Most Promising Technology Alternatives” report 

 Gap Analysis and Alternative Technologies 

o “Alternative Technologies” report 

 Conference Presentation 

o ICNS 2014 

 Final Review Presentation and Final Comprehensive Report 

o “Final Comprehensive Report”

3.3 Research Team 

The research team was conducted by prime contractor XCELAR and its subcontractor, National 

Institute of Aerospace (NIA). Organizational roles and responsibilities are summarized in Table 

1, and Table 2 provides information about key personnel contributing to the research. 

NASA/CR—2015-218843 13
Table 3-1: Organizational Roles and Responsibilities 

Organization Roles and Responsibility

AGILE DEFENSE / XCELAR

Principal Investigator

Science CO-I

CO-I

Program Management

National Institute of Aerospace

(NIA)

SME / Academic Coordination

Project Management

Table 3-2: Key Personnel 

Brian Haynes / XCELAR • Overall program leadership 

• Manage conduct of research, and overall technical 

direction 

• Subject Matter Expert (SME) – multiple areas, including: 

airline and General Aviation (GA) operations; ground-air 

communication systems; weather data systems; aircraft 

integration; business case analysis; avionics; small UAS 

design & avionics considerations 

• Schedule, logistics, progress reports, budgetary 

management 

• Contact PI 

Captain Robert 

“Rocky” Stone

• Scientific direction of work

• SME – multiple areas, including: airline operations; flight 

deck systems; ADS-B; air traffic management; turbulence 

mitigation; flight deck weather applications; NextGen; 

business case analysis; system engineering

NASA/CR—2015-218843 14
• Requirements definition & suitability analysis

• Airline industry outreach / input solicitation

Richard Haendel / 

XCELAR

• Communication Systems Analysis

• SME – multiple areas, including: avionics components, 

systems and design; avionics manufacturing, certification, 

and cost projection; RF propagation and performance 

analysis; antenna design and integration; satellite 

communication systems and technologies; ground-air & 

air-air communications; network design, analysis, and 

fault tolerance assessment; Airline, Business, GA, large 

UAS, and military avionics systems

• Requirements definition & suitability analysis

• Avionics industry outreach / input solicitation

Dan Johnson / XCELAR • Market Impact Analysis

• SME – multiple areas, including: GA manufacturing, cost 

projection, market analysis, and operations; aircraft 

sensors and air data systems; GA datalink and weather 

systems; small UAS applications & market analysis

• Program logistic and administrative support

• GA industry outreach and input solicitation

Fred Brooks / NIA  Academic community coordination

 GA and VLJ systems analysis

 SME – multiple areas, including: GA ADS-B systems and 

applications; VLJ design, market analysis, and operations; 

NextGen; FAA, NASA, and academic research in aeronautics 

 Program logistics support

 NASA outreach and input solicitation

 Academic community outreach and input solicitation

NASA/CR—2015-218843 15
Hank Jarrett  Literature Review Lead

 SME – multiple areas, including: systems engineering; satellite 

communication systems design; safety engineering; GA ADSB applications; NASA AGATE and SATS programs, and 

subsequent related research and literature; business case 

analysis

 Coordination and harmonization of NAS scenarios with other 

related research

 NASA outreach and input solicitation

NASA/CR—2015-218843 16
4 Literature Review 

A Literature Review was conducted as an initial step to familiarize the team with previous 

efforts that might affect the conduct of the research, such as avoiding duplication of efforts, or 

suggesting areas where additional or targeted study is needed. The scope of the review 

included future NAS operations concepts, future datalink technologies, and future information 

exchange needs. A detailed summary of literature reviewed can be found in Appendix 2. 

The goal of the literature review was to identify and document past or existing efforts and 

documentation regarding the datalink needs of the future National Airspace System (NAS) Air 

Transportation Systems for the 2013-2063 time period. Objectives were to identify relevant 

analysis, studies, and methodology during the literature review, to help bring the current state 

of knowledge in synch with the team’s hypotheses, including the potential identification of:

 Applicable “Future NAS Scenario” inputs

 Other research that has investigated closely related issues 

 Extant models applied to similar problems, and their assumptions 

 Alternate analysis methods that could apply to this project 

 Existing questions that can be applied to this work effort 

 Harmonize study elements with other related efforts (example: harmonize Future NAS 

Scenarios with other Future NAS studies and outputs) 

 A roadmap of future research to be conducted in the remainder of the effort in this 

contract. 

Each Lead Reviewer, and other contributors as needed, captured notes from each document 

that were deemed to have possible relevance to the conduct of this research. These notes 

were then reviewed and evaluated by the overall team, and used to produce the following 

findings: 

 There has been very limited applicable study work in the past 10 – 15 years 

 The majority of previous work pertains to already-identified link technologies and/or 

NAS concepts, not future concepts 

 There is a tendency to focus on specific link technologies and their capabilities rather 

than future functions/unmet needs 

 There is very limited consideration of link-independent, result-based data delivery 

concepts rather than specific links for each purpose 

 The current research effort’s planned approach of defining the future NAS, then 

functions/unmet needs, then delivery solutions continues to look like the correct 

methodology 

 “Form follows function” thinking is needed, rather than the reverse

NASA/CR—2015-218843 17
5 Future Data Exchange Requirements 

A workshop was held to develop the underlying assumptions to be used in defining future 

information exchange needs, and to develop consensus on the working definition of the “50-

year Future”, and related scope issues, for NAS scenario development and for the remainder of 

the study. The research team considered the overall question of expected characteristics of 

the future NAS, along with more in-depth discussions of specific NAS capabilities, vehicle 

capabilities, and potential “mid-point” scenarios to understand how transitions from current to 

future states might be accomplished. Due to limitations of workshop time, efforts were 

focused primarily on air transport users; in some cases additional sub-scenarios were 

developed later in the research where the defining NAS characteristics were expected to differ 

for other user groups (i.e., General Aviation, charter, Business Aviation). In some cases more 

detail was added to the future NAS concepts as the analysis of various link candidates raised 

additional requirements questions. The results of the workshop and future NAS definitions 

were documented in the Future NAS Scenarios report. The following sections summarize the 

future NAS scenarios, derived information exchange needs, and future communication 

functions. 

Scenarios were developed based on the needs of the future NAS for safety, efficiency, and 

capacity. Scenarios were structured by phase of flight, and included departure, enroute, 

arrival, and ground operations. Four baseline scenarios were developed during the workshop 

to illustrate the considerations to be used throughout the research effort. 

5.1 Future NAS Scenarios 

The underlying concept for future National Airspace System (NAS) operations was based on a 

network-based Air Traffic Management (ATM) approach, in which the aerial vehicle (including 

commercial aircraft, corporate and private jets, General Aviation (GA), and a wide range of 

Unmanned Aerial Systems (UAS)) has a high degree of autonomy. Universal ADS-B deployment 

for all IFR operations, and most VFR operations, was assumed. Overall traffic flow would be 

optimized across the system based on four types of interactions between the aircraft and the 

system: 

 Establish/negotiate expected flight conduct; 

 Monitor compliance; annunciate actual/expected deviations; 

 Adapt network to deviations; 

 Negotiate / establish new expectations as needed. 

NASA/CR—2015-218843 18
This can either be centrally managed (i.e., a ground function) or distributed. The basic 

communication elements are essentially the same in either case under the study concept, but 

the distribution of traffic changes. For example, in a centrally-managed model more of the 

conduct-of-flight communications would involve ground-air-ground communications, whereas 

in a distributed model more air-air communications would be required. For initial Phase 1 

evaluation a ground-based ATM function was assumed for operations over land masses; 

oceanic operations were assumed to involve a greater degree of distributed management and 

separation, facilitated by ground ATM via space-based ADS-B. The study was structured to 

accommodate straightforward adaptation to varying levels of distributed ATM in future 

iterations. 

The future NAS scenarios defined in the workshop and subsequent updates are summarized 

below. Four basic scenarios were developed, to characterize information exchange needs 

during four phases of flight: Departures, Enroute/Cruise, Arrivals, and Ground Operations. 

5.1.1 Scenario 1: Terminal Operations - Departures 

 User Group – Air Transport 

 Top-Level Goals: 

 Increased Capacity over Current Levels 

o Maintain Capacity Over Wide Range of Operating Conditions 

 Maximize Efficiency for All Operations 

 Primary Measure: Aggregate Cost 

 Includes fuel, crew cost/time, asset utilization 

 Equivalent or Greater Level of Safety 

5.1.2 Expected Capabilities & Needs 

 Departure throughput is limited by runway occupancy and wake vortex avoidance 

criteria 

o Departure throughput requires ground operations to deliver aircraft to the 

runway with minimum queueing and delay 

 Optimum routing on departure with collision avoidance 

o Eliminate the climb corridor; begin wind-optimized Great Circle Route as 

soon as possible after lift-off 

o Minimized tunneling or standard departure routing, allow for User Preferred 

Routing (UPR) shortly after take-off with real time de-confliction as 

necessary from arrivals, en-route traffic 

 Optimum profile climb on departure 

o Uninterrupted climb / high-speed 

o Prioritized over the Optimum Profile Descent 

NASA/CR—2015-218843 19
 Wake Vortex avoidance 

 Hazardous weather avoidance 

o Convection, icing, turbulence 

 Terrain clearance 

5.1.3 Scenario 2: Enroute Operations 

 Defined Enroute to begin at Top of Climb (TOC) and end at Pre-Arrival Sequencing 

point) 

 Primary User Group – Air Transport 

 Top-Level Goals: 

o Increased Capacity over Current Levels enabled by new separation standards 

and better surveillance tools 

o Two separation standards, tailored to avoid the specific hazard 

 Collision avoidance separation standard 

 Wake vortex avoidance separation standard 

o Maximize Efficiency for All Operations 

o Primary Measure: Aggregate Cost 

 Includes fuel, crew cost/time, asset utilization 

o Equivalent or Greater Level of Safety 

o Maintain Capacity Over Wide Range of Operating Conditions 

5.1.3.1 Expected Capabilities & Needs 

 Flow control / pre-arrival conditioning (course granularity) 

o Pre-arrival sequencing nominally begins at 150-500 miles 

 Increased traffic density / decreased spacing 

 Higher cruise altitudes 

 Some sub-orbital operations 

 Collision avoidance 

o Optimized conflict detection and resolution 

 Pair-wise Trajectory Management (PTM) 

o Deviations from the optimum only to resolve actual conflicts 

o Multiple layers of conflict detection and collision avoidance 

o Predictive behavior of adjacent aircraft 

 Altitude optimization for wind and aircraft performance or ride quality 

o Flights not required to be at cardinal cruise altitude, all aircraft performing 

constant cruise/climb 

 Route optimization for fuel burn and time 

 Hazardous weather avoidance 

NASA/CR—2015-218843 20
o Convection, turbulence, volcanic ash 

 Wake vortex avoidance 

5.1.4 Scenario 3: Terminal Operations - Arrivals 

 User Group – Air Transport 

 Top-Level Goals: 

o Increased Capacity over Current Levels 

o Maximize Efficiency for All Operations 

o Primary Measure: Aggregate Cost 

o Includes fuel, crew cost/time, asset utilization 

o Equivalent or Greater Level of Safety 

o Maintain Capacity Over Wide Range of Operating Conditions 

5.1.4.1 Expected Capabilities & Needs 

 Traffic sequencing to minimize arrival constraints 

o Assumed that runway utilization is the limiting factor for increasing arrival 

throughput 

 Any delays for sequencing and runway availability absorbed while the aircraft is 

more efficiently operating at cruise altitude 

o I.e., rather than in an “trombone downwind” 

o Process (communications) begins: 150-500 nm “upstream”

 Optimum User Preferred Routing (UPR) from the en route environment to the 

runway 

 Optimum Profile Descent (OPD) along optimum route 

 Collision avoidance throughout arrival 

 Terrain clearance assured throughout arrival 

 Wake vortex avoidance throughout the arrival 

o Wake vortex efforts are significant (now) and we assume that the 

phenomenon will be much less of an impact to the NAS – thus, the 

“Communication” needs of the NAS will be impacted by an increase in 

airspace density (congested terminal area) 

 Assumption: closer spacing than current (<1 mile) – for “worst Case” datalink 

modeling 

 Assumption: wake alleviation technologies 

 Hazardous weather avoided throughout arrival 

 Convection, Icing, Turbulence 

 Stabilized approach 

NASA/CR—2015-218843 21
 Glide path, final approach speed, and landing configuration by 3 miles on final 

approach 

5.1.5 Scenario 4: Ground Operations 

 User Group – Air Transport 

 Assumed no significant increase in surface landing facilities (Concrete) 

 Top-Level Goals: 

o Increased Capacity over Current Levels 

o Maximize Efficiency for All Operations 

o Primary Measure: Aggregate Cost 

o Includes fuel, crew cost/time, asset utilization 

o Equivalent or Greater Level of Safety 

o Maintain Capacity Over Wide Range of Operating Conditions 

5.1.5.1 Expected Capabilities & Needs 

 Gate departure metering & arrival scheduling to achieve minimized queuing at the 

runways, taxiways, and gates 

o Accurate communication of “anticipated” departure time to ATM

o Coordinated pushback/flow management/departure clearance timing 

 Autonomous Taxiing 

o Guided taxiing 

o Now: green lights depict “your” path to the gate from your runway exit

o Cockpit will receive optimized ground routing, speed, etc. to and from runway 

o Includes: Runway incursion protection, metering, collision avoidance, optimized 

routing 

 Low visibility flow management & collision avoidance; synthetic vision navigation, 

ground collision avoidance system 

 Deice routing/queue management; weather/holdover time monitoring 

o Pass-through de-icing

o Aircraft takes care of de-icing / anti-icing themselves 

 Safety separation for small aircraft behind large aircraft / jet blast 

 Security / Police / SWAT / Secret Service / TSA 

 Fire / Emergency Ops / Medical 

 Customs / Immigration 

 Weather information for all ground operations 

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5.2 Information Exchange Breakdown 

Using the scenarios as a guide, a breakdown of overall information exchange needs was 

developed. Information exchange needs were categorized into general groups that were not 

limited to a particular type or class of aircraft. As a reminder both to the research team and 

future reader that information exchange needs apply to all types of airspace users, the term 

“air vehicle” was used, to include not only traditional aircraft, but other types of current and 

future vehicles including lighter than air craft, space vehicles, UAS, hypersonic vehicles, and 

others. Information exchange needs were organized by the destination and direction of 

exchange: 

 Information from the air vehicle 

 Information to the air vehicle 

 Information from ground-based Air Traffic Management (ATM) 

 Information to ATM 

 Information from ground-based non-ATM (i.e., dispatch, flight following) 

 Information to ground-based non-ATM 

Functional categories of information exchange were defined for each group, as the basis for 

developing more detailed functional requirements. The following section summarizes the top 

level information exchange needs identified in conjunction with the NAS scenarios. 

Information from air vehicle 

 Vehicle status 

o Health 

o System status 

o Departure from normal conditions 

o Location 

o State Vector / Aerodynamic information about vehicle 

o Intentions 

 Local environment atmospherics 

 Acknowledgement of select input data 

 Special requests 

 Stream of data from occupants / cargo on board 

Information to air vehicle 

 Other proximate vehicle status / information 

o Location 

o State Vector / Aerodynamic information about vehicle 

o Intentions 

o Atmospherics 

NASA/CR—2015-218843 23
 Airspace system information 

o Enroute 

o Destination 

o Atmospherics 

 ATM Guidance & relevant information 

 Acknowledgement of select output data 

o e.g. Conflict resolution negotiation 

 Replies to special requests 

 Stream of data to occupants / cargo on board

Information from ground-based ATM 

 Airspace system information 

o ATM Guidance & relevant information 

o Processed Atmospherics 

 Replies to special requests 

 Non-cooperative airborne objects (aircraft, birds, balloons, etc.) 

o Location / State Vector / Aerodynamic information about objects 

Information to ground-based ATM 

 All vehicle status / information 

o Location 

o Health / departure from normal 

o State Vector / Aerodynamic information about vehicle 

o Intentions 

 Atmospherics 

 Inputs from/about non-cooperating vehicles 

 Acknowledgements & Replies to special requests 

 Monitoring of conflict resolution, routing changes 

Information from ground-based non-ATM 

 “Company” communication

o AOC, AAC 

o Mission pertinent 

 “Personal” communication (to passengers)

o Voice, data 

 Security 

 Command & Control for autonomous operations 

 Atmospheric 

NASA/CR—2015-218843 24
Information to ground-based non-ATM 

 “Company” communication

o AOC, AAC 

o Mission pertinent 

 “Personal” communication

o Voice, data 

o Telemetry [Data] 

 Video / Graphics 

 Security 

o e.g. FAMS air-to-ground, cabin surveillance, emergency control of air vehicle 

(dialog with ground-based emergency control) 

 Command & Control acknowledgements to ground 

 Atmospheric information 

These functional information exchange needs formed the basis for deriving specific data 

communication functions and associated requirements to be used to assess various candidate 

solution technologies. Candidate link technologies were then vetted against those 

requirements to measure how effectively the various candidates fulfilled the relevant 

requirements and their associated functional objectives, as summarized in the following 

sections. 

5.3 Data Communication Functions 

This section summarizes the user functions defined for use within the research effort. Future 

NAS participating aircraft considered in identifying user functions included Air Transport, 

General Aviation, UAS, and Hypersonic aircraft. Operations ranged from commercial airlines, 

to corporate jets, to private pilots, to large and small UASs, to space operations. Hypersonic 

and space operations analysis was limited to Mach 8 and below, and specialized 

communication considerations such as space vehicle reentry ionization blackouts were not 

considered to be within the study scope. 

Each function was given a Function Code for ease and consistency of reference, categorized by 

the general type of operation involved. Four types of functions and codes were used: 

 AAI-x refers to Airborne Aircraft, Inbound Communications; 

 AAO-x refers to Airborne Aircraft, Outbound Communications; 

 AGI-x refers to Aircraft on the Ground, Inbound Communications; and 

NASA/CR—2015-218843 25
 AGO-x refers to Aircraft on the Ground, Outbound Communications 

For example, AAI-1 denotes the first function defined under Airborne Aircraft, Inbound 

Communication, AAI-2 the next, and so on. Function codes for each function can be found in 

the leftmost column of the tables in Appendix 1. 

5.3.1 Airborne Aircraft, Inbound Communications (See also Appendix 1A) 

This category includes communications received by aircraft in flight, both from airborne 

and ground sources. Appendix 1A provides more detail on aircraft types: Unmanned 

Aircraft (“U”), General Aviation (“G”), Air Transport (“A”), or Hypersonic (“H”), defined 

for this study as a maximum if Mach 8. 

 AAI-1 Information from other proximate aircraft used by own ship for flight 

path de-confliction, collision avoidance, and wake vortex avoidance. Own ship 

must be able to see aircraft far enough away to allow for all three applications. 

80 mile range suggested from RTCA DO-289, Minimum Aviation System 

Performance Standards (MASPS) for Aircraft Surveillance Applications (ASA). 

 AAI-2 Other information used by own ship for general situational awareness 

about proximate traffic. 

 AAI-3 Other information used by own ship for the prediction of a “wake free” 

flight path. From RTCA DO-339. 

 AAI-4 Digital information used by own ship systems for the avoidance of 

significant hazardous weather or the mitigation of significant hazardous 

weather encounters. Information used for pilot situational awareness. From 

RTCA DO-340. 

 AAI-5 Digital information used by own ship systems for the avoidance of 

hazardous weather or the mitigation of hazardous weather encounters. 

Information used for pilot situational awareness. From RTCA DO-340. 

 AAI-6 Digital information used by own ship for the avoidance of immediately 

hazardous weather, as defined by RTCA DO-308. 

 AAI-7 Information to allow for the efficient management of own ship with 

regard to Time Based Flow Management (TBFM) constraints. 

 AAI-8 Additional general pertinent information required for arrival at an 

airport.

 AAI-9 Graphical depictions of weather information used by general aviation 

flight crews for the avoidance of hazardous weather or the mitigation of 

hazardous weather encounters. Information used for pilot situational 

awareness. From RTCA DO-340. 

 AAI-10 Graphical depictions of weather information used by air transport flight 

crews for the avoidance of hazardous weather or the mitigation of hazardous 

NASA/CR—2015-218843 26
weather encounters. Information used for pilot situational awareness. From 

RTCA DO-340. 

 AAI-11 Textual weather information used by air transport flight crews for the 

avoidance of hazardous weather or the mitigation of hazardous weather 

encounters. Information used for pilot situational awareness. From RTCA DO340. 

 AAI-12 Textual weather information used by general aviation flight crews for 

the avoidance of hazardous weather or the mitigation of hazardous weather 

encounters. Information used for pilot situational awareness. From RTCA DO340.

 AAI-13 Numerical weather information used by UAV operators or UAV onboard 

systems for the avoidance of hazardous weather or the mitigation of hazardous 

weather encounters. Information used for pilot situational awareness. 

 AAI-14 Miscellaneous Air Traffic Management information primarily used for 

long term planning. 

 AAI-15 Airline Operational Control (AOC) messages. 

 AAI-16 Airline messages for passenger convenience. 

5.3.2 Airborne Aircraft, Outbound Communications (See also Appendix 1B) 

This category includes communications transmitted from aircraft in flight, both to other 

aircraft and to ground destinations. 

 AAO-1 Information for ATM and other proximate aircraft to be used for flight path deconfliction, collision avoidance, and wake vortex avoidance. Own ship must be able to 

see aircraft far enough away to allow for all three applications. 80 mile range 

suggested from RTCA DO-289, Minimum Aviation System Performance Standards 

(MASPS) for Aircraft Surveillance Applications (ASA). 

 AAO-2 Other information used by ATM and proximate aircraft for general situational 

awareness about transmitting traffic. 

 AAO-3 Other information used by ATM and proximate aircraft for the prediction of a 

“wake free” flight path. From RTCA DO-339. 

 AAO-4 Special category of transmission of location and state information primarily 

from small UAVs for the use of ATM and proximate aircraft. 

 AAO-5 Vehicle health information from manned vehicles. 

 AAO-6 Vehicle health information from UAVs. 

NASA/CR—2015-218843 27
 AAO-7 General atmospheric information used to initiate and validate numerical 

weather models. 

 AAO-8 General atmospheric information used to initiate and validate numerical 

weather models, at higher data collection rates for better resolution in the terminal 

area.

 AAO-9 Special requests from aircraft. 

5.3.3 Aircraft on Ground, Inbound Communications (See also Appendix 1C) 

Communication with aircraft that are not in flight was considered separately, due both to the 

availability of different link options and to the potential inefficiency of using ground-air or airair link for ground-ground communications. This section summarizes communication functions 

inbound to the aircraft while it is on the ground. 

 AGI-1 Non-instructional situational awareness information such as ATIS, NOTAMS, 

Traffic Flow Management restrictions, active runways, etc. 

 AGI-2 Graphical weather products for general situational awareness. 

 AGI-3 Graphical weather products for specific hazards, such as Terminal Doppler 

Weather Radar (TDWR) for convection, gust fronts, etc., WSDM for icing information, 

etc. 

 AGI-4 Textual weather products for air transport pilot situational awareness. 

 AGI-5 Textual weather products for general aviation pilot situational awareness. 

 AGI-6 Numerical weather products necessary for UAV flight operations. 

 AGI-7 Taxi out instructions from ramp and ATC. 

 AGI-8 Departure clearance, and any other ATC instructions. 

 AGI-9 Airline operational control messages such as destination gate assignment, 

maintenance information, weight and balance information. 

 AGI-10 Airline administrative information, such as passenger connecting gate 

information. 

 AGI-11 Information from proximate vehicles on their location and intentions. 

 AGI-12 Information about proximate vehicle characteristics. 

 AGI-13 Information from proximate vehicles needed to determine any wake vortex 

constraints on departure. 

 AGI-14 Hazardous weather reports from other proximate aircraft in the terminal area. 

NASA/CR—2015-218843 28
5.3.4 Aircraft on Ground, Outbound Communications (See also Appendix 1D) 

These functions are similar to AGI, but focus on communications outbound from the aircraft 

and are accordingly designated as “AGO-x”.

 AGO-1 Vehicle position and velocity information, including an indication that the 

vehicle is “on the ground”.

 AGO-2 Vehicle position and velocity information from a UAV, including an indication 

that the vehicle is “on the ground”.

 AGO-3 Vehicle status and health information. 

 AGO-4 Vehicle status and health information from a UAV. 

 AGO-5 Transmission of the value of the initial circulation strength of the wake vortex of 

the aircraft once it becomes airborne. This information can be used to plan departure 

queues and for wake vortex mitigation upon departure. 

 AGO-6 Local weather observations. Not all information may be available until the 

aircraft is in the air. 

 AGO-7 Special Requests. A general category to include various specialized needs such 

as gate assignment, passenger connection information, and medical information for 

passenger emergencies. 

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30

6 Candidate Technologies 

This section summarizes the candidate technologies that were included in the candidate 

comparison process, including both specific links and the two enabling technologies that are 

associated with most or all of the proposed link implementations. Each candidate technology 

is summarized from a technical perspective, including its associated architecture 

considerations such as interoperability, applicability to multiple user groups, scalability over 

time and changing industry needs, and the potential impacts of failures on the overall system. 

Architectures are described functionally, in keeping with the conceptual nature of the study at 

the current stage. The candidate technologies described are: 

 Broad-Band Software-Defined Radio (SDR) 

 Delivery Manager / Overall System Architecture 

 VHF Datalink, including VDL Mode 2 (VDL-2)

 ADS-B and ADS-B Next 

 AeroMACS 

 AeroWAN 

 SDARS

 Commercial Cellular Links and “Cloud Communications”

 Ku-/Ka-Band satellite 

 Iridium/Next and similar L-Band LEO links 

6.1 Enabling Technologies 

As previously discussed, the research team identified two enabling technologies considered to 

be important to the successful implementation of various link-specific technologies in the 

future architecture: Broad Band software Defined Radio (BBSDR) technology, and the Delivery 

Manager (DM) function. These two technologies serve as a platform on which various linkspecific solutions are integrated in a system-based approach. As such, while neither functions 

directly as a communication link, they are common to the overall success of all. 

6.1.1 Broadband Software-Defined Radio (BBSDR) Technology 

BBSDR technology allows a single radio device to operate across an entire frequency band 

simultaneously, operating multiple links of various types using Digital Signal Processing (DSP) 

techniques. The BBSDR Architecture is shown in Fig. 2. The Software Defined Radio (SDR) 

NASA/CR—2015-218843 30
component allows the device to adapt to new types of modulation and other link 

characteristics by changing its software, rather than altering or replacing its hardware as in 

conventional radio systems. By 2063 this technology will enable a single receiver to monitor 

the entire L-Band, or VHF Band, large segments of the cellular communications spectrum, or 

even large portions of the Ku- or Ka-Band, demodulating and outputting link content to the DM 

from multiple links over a single high-speed interface. It will also provide the DM with 

detailed information on link quality and other characteristics for the DM to use in managing 

overall communication effectively. The same is true for transmission, with slightly different 

architecture. 

This technology allows the operation of many links in parallel, using a small number of Line 

Replaceable Units (LRUs) for maximum capability at minimum cost and complexity. 

Redundancy is greatly simplified; for example, all L-Band links could be made fully redundant 

using two identical L-Band BBSDR units. 

A BBSDR-based architecture can enable substantially increased longevity for avionics, and 

reduced susceptibility to obsolescence, through its ability to use software-only updates to 

many formerly hardware-based link characteristics. Modulation and demodulation, data 

encoding and decoding, tuning and filtering, bandwidth and channelization, error correction, 

and data rates are all examples of link characteristics that can be changed in a wide variety of 

ways via software load, or in many cases, even simpler field configuration data updates on an 

SDR-based device. This allows a single avionics unit to be upgraded and adapted to evolving 

system capabilities, spectrum availability, and other technologies to a much greater degree 

than current equipment. 

6.1.1.1 BBSDR Architecture 

A single BBSDR would be able to access all links in a given frequency band. Two or more 

identical units would be used to provide redundant backup where required. Initially this would 

include one BBSDR unit (plus backups) each for the VHF aviation band and the entire L-Band. 

L-Band links could include the current DME, TACAN, ADS-B, TCAS, UAT, GNSS, and Iridium 

communication systems, along with the proposed 1030 MHz ADS-B Next, and other services. 

Each BBSDR will be managed by the DM. 

Receive (RX) and Transmit (TX) functions for each BBSDR have somewhat different 

architectures. TX and RX antennas should be physically separated to minimize TX interference 

with RX operations. A single BBSDR may also have more than one TX subsystem depending on 

performance requirements for simultaneous transmissions at high power on different links. A 

single TX subsystem can be configured to support multiple modulation and data formats but 

cannot simultaneously support multiple simultaneous transmissions. Configuration of 

adaptive filtering and other mitigation methods will be coordinated dynamically between the 

NASA/CR—2015-218843 31
TX and RX functions to further minimize interference between simultaneous TX and RX 

operations. In general this will apply more to commercial and/or larger aircraft; in GA 

applications it is more likely that transmissions at different frequencies and/or modulations 

can be temporarily interleaved to allow use of a single modulation, power amplification, and 

dynamic filtering resource to be used to multiple purposes at the same time. A conceptual 

SDR architecture is shown in Appendix 7 for reference. 

Figure 6-1: BBSDR Architecture 

NASA/CR—2015-218843 32
6.1.1.2 BBSDR Interoperability and Applicability 

The proposed BBSDR technology is fully interoperable with current proposed future systems. 

The SDR capability allows adaptation to a wide range of related systems. Spectrum 

interoperability with legacy and other systems, such as in the VHF and L-Bands, is addressed in 

other sections. Broadband Software-Defined Radio technology is applicable to all user groups. 

6.1.1.3 BBSDR Scalability over Time and Changing Industry Needs 

BBSDR is highly scalable: the software-defined operation allows a wide range of upgrades and 

adaptations to new link types, deployment of additional ground stations, adaptive modulation 

based on link quality, and other technical developments. The ability to meet more and more 

challenging price points is also promising, through the ability to have a single device serve 

multiple functions that currently require separate, purpose-built equipment, and through the 

switch from primarily analog to primarily digital components, and resulting increases in 

production volume and other economies of scale. Significant portions of the BBSDR can also 

be common between models for use in different bands; the DM interface, DSP and SDR 

infrastructure for both RX and TX, and much of the software can be used with different RF 

components to operate in different bands. 

6.1.1.4 BBSDR Failure Modes, Effects, and Backup Options 

The application of a single SDR to multiple functions inherently increases the severity of failure 

effects; a single SDR failure can cause the loss of multiple functions, links, or capabilities. 

However, the solution is also inherent, in that backup equipment only requires a single type of 

Line Replaceable Unit (LRU) – two or three identical BBSDRs back up all SDR-enabled functions, 

instead of requiring separate backups for each type of communication, navigation, and/or 

surveillance radio device. In higher-end aircraft two or three identical SDR units (for each 

frequency band, i.e., VHF, L-Band, Ku-Band) may be required. For GA aircraft 2 identical units 

for each band used are expected to be acceptable. 

6.1.1.5 BBSDR Relationship to Delivery Manager 

The capability of the BBSDR is not limited to the reception and output of the various 

communication payloads within its operating frequency range. Its processor will also 

dynamically derive a wealth of useful information about the performance of each link. Signal 

quality, link margin, improving and degrading channels as the aircraft moves (AeroWAN for 

example), sources and frequencies of interference, and other parameters will be available for 

use in making optimal use of available communications. 

The Delivery Manager, described in the following section, will have access to this BBSDRderived information as one of its many inputs in optimizing overall communications. Unlike a 

simple router, this access to link-level status and trend information gives the Delivery Manager 

NASA/CR—2015-218843 33
more powerful tools to deliver optimum communication performance using whatever 

individual links are available at any given time. 

6.1.2 Delivery Manager (DM) 

The Delivery Manager enables multiple individual links or link technologies to be harnessed as 

a group, routing information dynamically across the most favorable link at any given time 

based on required availability, integrity, capacity, cost and other criteria. This could also allow 

for the use of non-aviation protected spectrum for certain functions, while still utilizing 

aviation protected spectrum where criteria require it. 

Aircraft today are typically equipped with multiple separate communication links, with little or 

no interaction. Communication Management Units (CMUs) in larger aircraft provide some 

rudimentary coordination of communication payloads and individual links, but are limited in 

their capabilities. Optimized communications using future link technologies can be greatly 

improved by adding a more comprehensive overall management capability across all available 

links. In principle, this function can be described as an extrapolation of today’s router, but 

with access to a large variety of links, access to status information from multiple layers of the 

communication process (link layer included), inputs and outputs in many different formats, 

and responsibility for delivery and integrity of payload information. The research team 

designated references to this larger set of delivery / integrity capabilities as the Delivery 

Manager (DM). 

The Delivery Manager is the communications hub of the aircraft. It connects to the various 

information systems on the aircraft that use or generate information, and to the various 

communication links that the aircraft is capable of accessing. This includes both aviationspecific links and commercial or non-aviation specific links. It is the Delivery Manager’s role to 

assure delivery of communications at or above the required levels of timeliness, reliability, and 

integrity, at the lowest practicable cost. Where high-speed, commercial links are available, the 

DM may use them to route larger data messages, using encryption or other means to assure 

appropriate integrity. When high-speed links are not available, the DM relies more on lowerspeed, aviation-specific links to assure delivery, even if at reduced speeds. Information 

deemed to be integrity- or latency-critical may always be routed via aviation-specific links, with 

multiple such links available to assure delivery. 

For example, CPDLC messages could “default” to the VDL-Mode 2 link, but if for any reason 

that link is temporarily unavailable, messages could be routed via AeroWAN; on the ground, 

CPDLC traffic could be routed via AeroMACS to reduce VDL-Mode 2 system loading. The DM 

has access to link-layer information such as specific stations currently in range, signal strength 

and noise level, and performance trends for each ground station; for example, which stations 

NASA/CR—2015-218843 34
are fading and which are improving as the aircraft moves. This allows the DM to route via the 

most viable link at all times, and to strategically switch from one ground station to another to 

maintain best communication capability at all times. In addition, for some links the DM 

participates in link-acceleration decisions under optimum conditions; for example, AeroWAN 

can switch to a higher-speed method of transmission when link margins are high, minimizing 

delivery times for large payloads, and back to more conservative methods as the link 

deteriorates with aircraft movement. 

Delivery decisions are made dynamically and without crew interaction. Crews can access link 

status and current routing information, similar to a pilot accessing the GPS constellation status 

using today’s GPS receivers, but under normal conditions the DM manages communications 

autonomously and transparently to the pilot. It should be noted also that some ground 

stakeholders may also use a subset of its capabilities; for example, an airline dispatch 

operation may use similar capabilities to assure delivery of AOC or AAC data to an aircraft via 

the various links available at a given time. 

The viability of non-aviation specific candidate technologies for conduct of flight 

communications is greatly enhanced by the Delivery Manager function, because the DM is key 

to their applicability and its scope. For example, cellular technologies may be used for a wider 

range of functions with the presence of the DM and its ability to encrypt, and to switch to 

alternate links automatically to maintain the required link availability.

6.1.2.1 DM Architecture 

The DM coordinates the use of the various specific links based on availability, timeliness, 

capacity, performance relative to requirements, cost, and other factors. The DM also 

addresses issues such as encryption and security, receipt verification, and probability of 

delivery across each link utilized from an approval/certification standpoint. A combination of 

aviation-specific links and commercial links are available to the DM, including but not limited 

to those described in this report. The DM is also capable of initiating relay communications 

using nearby aircraft (and their DMs), such as in the case of high-priority messages in primary 

link failure conditions. Polar route position reports, for example, could be relayed from aircraft 

to aircraft until access to an air-ground link is obtained. 

The DM presents some overall architecture considerations. To be fully effective, the DM must 

have access to most, if not all, available ground-air and air-air links. In the aircraft, the DM 

functions as the central manager for communications, interconnected with all source and user 

systems on one side, and each individual link on the other. Multiple links may be provided by a 

single BBSDR package, as described previously. 

NASA/CR—2015-218843 35
Figure 6-2: Future Aircraft Communication Architecture with DM and BBSDR 

The conceptual communications architecture for a 2063-era aircraft is shown in Fig. 3. A 

Communications Bus connects the major components of the system, connecting primary and 

backup DMs with multiple link-specific devices. DMs have access to both communications 

payload data from each link, and detailed link status information for use in managing link 

availability as conditions change. To the extent that commercial link providers require 

separate, link-specific LRUs for their services, each such LRU would interface with the DM via 

the Communications Bus. The addition of multiple redundant LRUs, such as two or three LBand BBSDRs, is architecturally simple, as additional nodes on the Communications Bus. 

NASA/CR—2015-218843 36
Figure 6-3: Future Terrestrial Communication Distributed DM Client/Server Architecture 

On the ground, the DM function must have access to various communication links and 

networks, including those with widely distributed architecture such as networks of ground 

stations. Multiple ground users must have means to access the DM function to submit data for 

transmission, and to receive transmissions addressed to them. Ground user access to the 

ground-based DM constitutes another area of Cloud Communications, where different users 

may access the DM function via various means ranging from dedicated, secure links to a 

dynamic, web-based IP connection. 

A distributed architecture is envisioned for the ground DM, where end users (i.e., an airline 

dispatch office) are equipped with a “DM Client” which accesses a “DM Server” to establish a 

secure, authorized user link to authorized DM functions, and perform link selection and 

routing, message prioritization, security, and other DM functions collaboratively. Different 

NASA/CR—2015-218843 37
ground users have different authorization profiles controlling which links may be accessed, 

information types which may be sent, encryption and security requirements, and other 

parameters. The DM client associated with each user negotiates via the cloud with one or 

more DM servers, through which communication access is established and maintained. A 

number of DM Controllers manage overall operation of the system, including client and server 

configuration management, security, traffic flow management and balancing, and other 

optimizations. 

6.1.2.2 DM Interoperability 

Aircraft: 

Proposed aircraft communication systems are interoperable with all relevant legacy and new 

systems. Existing CMUs and DMUs and other current purpose-specific link equipment are 

expected to have been replaced with DM-compatible equipment before 2063. Interim systems 

may have a stand-alone architecture with a DM interface that allows upgrade to DM 

architecture, possibly with reduced DM effectiveness due to limitations in the scope of DM 

access to detailed link information. 

Ground: 

A high degree of interoperability is required for the ground segment, due to the number and 

diversity of users, user locations, information types, links available, and geographically 

dispersed ground stations and other infrastructure. The ground system is expected to be fully 

interoperable with all relevant systems. 

6.1.2.3 DM Applicability Across Multiple User Groups 

The architecture differs slightly from commercial aircraft to GA, but will be applicable to all 

user groups. Some specific links will only be practicable for larger aircraft, and a higher level of 

redundancy and integration is expected for higher-end aircraft. However, the overall system 

architecture, and the DM functionality, will apply to all. 

Commercial aircraft typically will be equipped with two or more redundant DM units, crosslinked to maintain current status data for all, with dynamic fail-over from one to another. GA 

aircraft will typically have two redundant DM units, with a choice of manual or automatic 

switch to backup based on user needs and price sensitivity. 

6.1.2.4 DM Scalability Over Time and Changing Industry Needs 

Aircraft: 

The DM will be highly scalable, primarily through software upgrades to the DM itself and to the 

BBSDRs to add/upgrade capabilities of specific links. Backup DM units can also be used to 

manage some workload in a distributed processing environment as system loading increases 

over the life of the system. 

NASA/CR—2015-218843 38
Ground: 

Also highly scalable, with upgrades of both software and hardware being more practicable due 

to easier access, the limited number of ground stations, and the ability to apply cloud-based 

communication and routing capabilities to the ground-to-ground user access functions. 

6.1.2.5 DM Failure Modes, Effects, and Backup Options 

Aircraft: 

 In the event of a DM failure, at least one backup DM unit will be required. Higher-end aircraft 

would have three redundant units with auto fail-over. Loss of any individual link is mitigated 

automatically by the DM function and its ability to switch to alternate links as needed. 

Ground: 

Three major types of failure are possible: 

 Ground Station Failure; 

 Ground Interconnect Failure; 

 Ground DM Function Failure. 

A failure of any specific ground station is mitigated first by in situ backup equipment and 

automatic fail-over. Complete failure of a ground station location is mitigated primarily by DM 

function, which immediately attempts to access an alternate link. Options include a more 

distant station of the same link type or use of an entirely separate link. 

Failure of the interconnection between any one ground station and the network results in a 

similar situation to that of a complete ground station location failure. Failure of the entire 

ground interconnect system, between the DM and all link access points, is prevented by the 

distributed nature of the ground DM function, and the use of “cloud communication” to use 

diverse links to the various interconnect points. 

Failure of the ground DM function is also precluded by the distributed nature of the DM 

function; there is no single DM location, but rather multiple DM entities operating in a 

collaborative fashion. Link capabilities are distributed across a number of DM servers in 

diverse spatial and connectivity locations, such that if one DM server does not respond to a 

request for a DM client, others do. 

NASA/CR—2015-218843 39
6.1.3 Influence of BBSDR and DM on Link Favorability 

Both BBSDR and DM were assumed to be available to all links at the 50-year reference point, 

with the exception of current ADS-B and UAT, and current VDL-2. Both are integral parts of the 

future datalink architecture envisioned by the research team, and are essentially integral with 

each other as well. In that context, it could be argued that their presence benefits all 

candidates equally. 

In fact, there are some additional factors to be considered. Some links may derive more 

benefit from BBSDR and DM technology than others, such as in relation to steerable-antenna 

satellite links and links offering more different service types in the same contiguous 

bandwidth. Some commercial systems may be a good fit with BBSDR architecture for technical 

reasons, but precluded by business considerations such as subscription control and fee 

management, or protection of proprietary technology. 

By the same token, such commercial candidates may derive significant benefit from the DM 

technology, a core purpose of which is to provide a link-independent, performance-based 

method for providing and maintaining information “delivery” regardless of specific link or 

provider. This link-independent “pedigree” can provide the means to obtain approval for using 

some commercial links for functions that otherwise would not be compatible with noncertified equipment, providers, and spectrum. 

Taken as an overall system, it is likely that aviation-specific candidates such as AeroWAN and 

VDL Next will derive greater benefit from BBSDR than commercial links such as cellular and 

SDARS. However, the commercial links may also derive greater benefit from the Delivery 

Manager than some aviation-specific links. In total, the BBSDR and DM technologies together 

help provide a more flexible and capable system, with each complementing the other in 

multiple ways. 

6.2 Link Technology Candidates 

The following section summarizes each candidate link, its proposed architecture, its 

anticipated interoperability with other systems and applicability to multiple user groups, and 

its scalability over time and changing industry needs. Related failures modes, effects, and back 

up options are also identified. 

The team considered a combination of aviation-specific options and commercial candidates to 

provide the broadest overall capabilities and lowest overall cost, while always assuring the 

availability of aviation protected spectrum operations for critical functions. A wide range of 

solutions were considered, and the proposed candidate technologies fall into three categories: 

extensions and enhancements to current, existing aviation links, re-architecture of current 

NASA/CR—2015-218843 40
aviation systems and/or RF spectrum, and the application of new, primarily commercial link 

technologies not currently associated with aviation applications, and not located in aviation 

protected RF spectrum. The first two are defined herein as “Aviation-Specific Candidates” due 

to their operation in aviation specific RF spectrum. The third is defined as “Commercial Link 

Candidates”, being designed primarily for use by non-aviation user populations, typically 

operating outside aviation spectrum, and on a fee-for-use basis. 

6.2.1 Aviation-Specific Link Candidates 

Aviation-specific candidate technologies include: 

 VDL Mode 2 (VDL-2) and aggregated VDL2, herein referred to as VDL-Next 

 ADS-B based on the current 1090 MHz architecture with a new low power option (1 

watt or less transmit power for ground operations);

 A restructured ADS-B link system re-using 1030 MHz spectrum, referred to herein as 

“ADS-B Next”;

 Space-based ADS-B;

 AeroMACS; and 

 AeroWAN, a new wireless Aeronautical Wide-Area Network re-using portions of the 

current DME/TACAN frequency band. 

6.2.2 Commercial Link Candidates 

As noted previously, the DM increases the overall viability of using some non-aviation-specific 

candidate technologies, through its ability to manage their use according to availability, 

integrity, and cost. These DM capabilities allow expanded use of non-aviation specific links 

such as cellular technologies, commercial SATCOM, Iridium/Next and similar L-Band LEO 

systems, and SDARS, while maintaining required performance and integrity levels. For 

example, if required weather information is successfully received via SDARS or other broadcast 

means in a timely way, no further action is required; if not, the DM may request it via alternate 

means such as AeroWAN, cellular technology, VDL-2 or Iridium. Inventory lists of expected 

data can be sent via AeroWAN or VDL-2 and, if received successfully via commercial broadcast, 

no further communication loading (or cost) is required. AOC / AAC information may be 

transmitted via a commercial link (i.e., cabin internet) with appropriate encryption and 

segregation when those links are available. When they are not, the DM switches to alternate 

options such as VDL-2, Iridium, or AeroWAN. 

Commercial candidate link technologies include: 

 4G/LTE and future generation technologies; 

 Iridium/Next and similar L-Band LEO Satellite; 

 Ku- and Ka-Band satellite systems. 

NASA/CR—2015-218843 41
6.2.2.1 VHF Data Link (VDL) 

Candidate links include three types of VHF Data Link: the currently operational VDL Mode 2, a 

modification of current VDL-2 allowing data broadcast to multiple aircraft, and a re-architected 

VHD data link system not limited to operations within the current 25 KHz channel structure of 

the VHF aviation band. 

VDL Mode 2: 

This category includes the current VDL Mode 2 VHF datalink (VDL-2), and the legacy ACARS 

links to the extent that they are still relevant in 2063. VDL-2 and the future VHF datalinks are 

expected to be available to all user segments, including GA, at market-compatible price. 

VDL-2B (Broadcast) 

Same as VDL-2 except with the additional capability of broadcast messages from ground-to-air; 

abbreviated in some tables herein as VDL-2B. 

VDL-Next 

Present capacity of single channel VDL-2 is limited, and recent saturation problems 

encountered in Europe with current VDL-2 highlight the potential problems in the current VDL2 architecture. Aggregation of a number of predefined RF frequencies with a new connection 

protocol would greatly increase VDL-2 capability. For this study, an aggregated VDL structure 

based on the use of a total of 38 channels in the 136-137 MHz band, having 36 data channels 

and two control channels at 25 KHz intervals, was used. One or more channels would also be 

allocated to broadcast data, reducing loading on interactive channels from repetitive 

dissemination of data common to multiple aircraft. A BBSDR architecture allows simultaneous 

reception of control channels, broadcast channel, and assigned interactive channel. This type 

of aggregated-bandwidth VHF Data Link is referred to as “VDL Next”.

6.2.2.2 VDL Architecture 

A single BBSDR unit (TX and RX) can be used to accomplish all communication in the aviation 

VHF band, with one or more backups as needed. Current VDL-2 communication methodology 

would be accommodated along with future upgrades such as VDL-Next, and the advent of 

higher throughput link technologies. It is assumed that by 2063 the current analog voice 

operations in the VHF band will have been replaced by digital voice, CPDLC, and other 

alternatives. The DM will manage the information. 

6.2.2.3 VDL Interoperability and Applicability 

BBSDR is interoperable with current VDL-2 and future proposed systems. BBSDR is also 

capable of supporting current VHF analog voice communications to the extent that it is still in 

use. BBSDR is applicable to all user groups. 

NASA/CR—2015-218843 42
6.2.2.4 VDL Scalability over Time and Changing Industry Needs 

All current and future VHF data links are fully compatible with Broadband software Defined 

Radio technology, and BBSDR is highly scalable in terms of the adaptability of the BBSDR to 

expanded bandwidth operations, parallel channel operations, and future modulation 

upgrades. Scalability is somewhat limited by the inherent bandwidth limitations of VHF 

operations and the size of the aviation VHF band itself. 

6.2.2.5 Failure Modes, Effects, and Backups 

The application of a single BBSDR to multiple functions inherently increases the severity of 

failure effects; a single BBSDR failure can cause the loss of multiple functions, links, or 

capabilities. However, the solution is also inherent, in that backup equipment only requires a 

single type of LRU; two or three identical BBSDRs back up all BBSDR-enabled functions, instead 

of requiring separate backups for each. In higher-end aircraft two or more identical VHF 

BBSDR units are expected. For GA aircraft one or more units, similar to current VHF 

communication radios, are expected to be acceptable. 

6.2.3 ADS-B 

ADS-B in this context includes both current ADS-B systems (1030/1090 MHz, and UAT), and the 

proposed restructured ADS-B NEXT system centered initially at 1030 MHz. 

6.2.3.1 Current ADS-B 1090/1030 MHz System 

By 2063 it is expected that the current 1090 MHz-based ADS-B system will have reached 

“legacy” status, but may still be in service in parallel with the new 1030 MHz system (see 

following sections) during a potentially lengthy transition period. This legacy system will be 

compatible with all proposed new systems, with no mutual interference expected. After the 

legacy 1090 system has reached its sunset, this spectrum can be re-allocated to provide 

additional bandwidth for the new 1030 MHz ADS-B Next system long into the future. 

6.2.3.2 ADS-B Next 

A new ADS-B system is proposed that reuses RF spectrum centered around 1030 MHZ in a 

robust, efficient way to replace all current 1090 MHZ and UAT ADS-B functions. The 

Technology Candidate Descriptions report includes an overview of the strategy for 

decommissioning all current functions that occupy the 1030 MHZ spectrum, including active 

interrogations by ATCRBS, TCAS, and multilateration systems. A key enabler of this strategy is 

the premise that by 2063 ADS-B will be universally deployed on all aircraft operating under 

Instrument Flight Rules (IFR), and a majority of Visual Flight Rules (VFR) aircraft as well. Some 

realignment of DME channel assignments may also be needed to aggregate a single broadband 

channel centered around 1030 MHZ. 

NASA/CR—2015-218843 43
6.2.3.3 ADS-B Next Architecture 

Aircraft: 

Reception of the legacy 1090 ADS-B and UAT signals, and the new 1030 MHz ADS-B Next 

signals, will be among the multiple functions of the L-Band BBSDR. The DM will support 

delivery of legacy system data to the appropriate legacy systems as needed, as well as data 

from the new ADS-B Next system. Transmission of new ADS-B Next squitters will also be a 

function of the BBSDR; transmissions of legacy 1090 and UAT signals, during transition, can be 

done by legacy equipment or the L-Band BBSDR. 

Both the legacy and new ADS-B systems will be applicable to both air-to-air and air-to-ground 

applications. 

Ground: 

New ADS-B Next ground receivers can be collocated with legacy equipment during transition. 

ADS-B Next stations will connect to users via the DM function. This will allow both Air 

Navigation Service Provider (ANSP) users and others (i.e., airline dispatch operations) to 

become “clients” of ADS-B ground station output data via their DM client and associated 

authorization profile if desired. 

6.2.3.4 ADS-B Next Interoperability and Applicability 

The proposed system is fully interoperable with current 1090 MHZ ADS-B, and proposed future 

1030 MHZ ADS-B Next systems. It is not interoperable with other current 1030 MHZ functions 

including active interrogations by ATCRBS radar systems, TCAS, and multilateration systems. It 

is interoperable with DME/TACAN systems with the exception of the possible need to relocate 

some DME channel assignments adjacent to 1030 MHZ to allow the aggregation of a single, 

broadband channel centered at 1030 MHz. This new system will be applicable to, and 

available to, all user segments at market-compatible price, including current users of the UAT 

system for ADS-B (primarily GA). Space-Based ADS-B systems would eventually need to be 

upgraded for compatibility with ADS-B Next transmissions at 1030 MHz, but due to the ability 

of the L-Band BBSDR to generate legacy ADS-B transmissions this would not be a requirement 

until the end of the transition period. 

6.2.3.5 ADS-B Next Scalability over Time and Changing industry Needs 

ADS-B Next is highly scalable due to inherently higher bandwidth than current systems, 

providing significant room for future growth in the number of users and in the addition of 

applications. It is also scalable due to the inherent scalability of BBSDR technology, as 

discussed in other sections. In addition, after the current 1090 MHz and UAT ADS-B systems 

are retired their spectrum can be reused to provide additional growth capacity for ADS-B and 

other functions in parallel with the new 1030 MHz system. 

NASA/CR—2015-218843 44
6.2.3.6 ADS-B Next Failure Modes, Effects, and Backups 

Aircraft: 

Three primary types of failure modes need to be considered: RX failure, TX failure, and 

uncommanded transmissions (e.g., “stuck mic”). RX and TX failures are mitigated by the 

presence of backup BBSDRs and DMs and the inherent “fail-operational” architecture of the 

system. In addition, if the DM recognizes a multiple failure condition, it can attempt to 

establish position reporting via alternate links, at least to the ATM client for ATM-assisted 

separation. The ATM can also attempt to initiate similar connectivity to the aircraft if its DM 

recognizes the failure. 

The uncommanded transmission condition is also unlikely, as system design will include 

multiple fail-safe provisions to prevent a single aircraft from “jamming” all nearby ADS-B 

signals. If this condition were to occur, ADS-B operations would be disrupted in the vicinity of 

the offending aircraft. All associated DMs would attempt to access an alternate link, and ATM 

instructions via other links would not be affected – for example, CPDLC messages via VHF 

datalink would continue to operate. It should be noted that this theoretical condition exists 

today with UAT ADS-B, and with intentional jamming of 1090 MHz ADS-B signals. ATM 

procedures will need to include provisions for this condition. 

Ground: 

Ground ADS-B system failures are similarly mitigated by redundant receivers and DM 

capabilities discussed in other parts of this report. In the event of widespread interruption, it 

should first be noted that air-to-air ADS-B would continue to operate, allowing self-separation. 

ATM DM clients would immediately seek alternate links, and could use VHF datalink, 

AeroWAN, AeroMACS (for surface movements), SATCOM, and available cellular links to restore 

capability. ATM procedures will need to include provisions for this condition, as they do now. 

6.2.4 AeroMACS 

AeroMACS has been the subject of extensive study in other forums, and this report does not 

attempt to summarize that information. AeroMACS is a ground-aircraft system designed for 

communications while aircraft are on the ground. The system uses the IEEE 802.16 “Mobile 

WiFi” standard which provides for internet like connectivity and uses Internet Protocol (IP). It 

operates in a portion of the 5 GHz band designated for aircraft use, and is scalable for both 

large and small airports, with a range up to 3 Km. Data rates are comparable to broadband 

WiFi up to 54MB/sec. Applications include AOC messaging, ground traffic control, CPDLC 

messaging, weather information, and many other applications. 

In the 2063 system AeroMACS is expected to have been deployed and matured significantly, 

and to be an important element of the future communication system, used by all segments. 

AeroMACS is primarily applicable to ground operations on and near airports. It provides 

NASA/CR—2015-218843 45
wireless networking connectivity for surface clients, including vehicles of various types, ATM, 

airport operations, airport operators, and others. 

6.2.4.1 AeroMACS Architecture 

Strategically located ground stations and antennas provide wireless signal coverage 

throughout the airport environment. This network of stations connects via the ground DM 

with various clients including ATM, airlines, aircraft operators, weather information providers, 

and other authorized users.

AeroMACS operates in the 5 GHz band currently allocated to MLS, and will require one or more 

AeroMACS transceivers to access the system, under the management of the aircraft’s DM. 

Ground-Mobile Vehicles, with little or no need for alternate links, access AeroMACS directly 

from a user terminal device to their onboard AeroMACS transceiver. 

6.2.4.2 AeroMACS Interoperability and Applicability 

The future AeroMACS will be interoperable with current and proposed systems, with the 

caveat that as AeroMACS bandwidth needs grow over time, increased allocation of RF 

spectrum to AeroMACS will be required. It is expected that adequate spectrum will become 

available as legacy systems are decommissioned. AeroMACS is applicable to all user groups, 

including not only aircraft but also mobile ground vehicles and other ground users in the 

airport surface environment. 

6.2.4.3 AeroMACS Scalability over Time and Changing Industry Needs 

AeroMACS is highly scalable through assignment of additional spectrum and other channel 

management strategies, upgrades of BBSDR-based access equipment to improved throughput 

technologies, and deployment of additional ground stations at additional airports. 

6.2.4.4 AeroMACS Failure Modes, Effects, and Backups 

Three primary AeroMACS failure modes have been identified: Aircraft Equipment Failure, 

Ground Equipment Failure, and Interference. 

6.2.4.4.1 Aircraft Equipment Failure 

Aircraft AeroMACS equipment failure affects an individual aircraft which, in most cases, 

will be engaged in surface operations. In general, the inherent aircraft system 

architecture will provide backup via the redundant DM and link-specific equipment, 

and the DM’s ability to switch to alternate links dynamically. Dynamic backup options 

include using ADS-B Next, VHF datalink, cellular, AeroWAN, or SATCOM for 

communication. In cases of multiple failures affecting not only AeroMACS but the DM 

and alternate links, removal of the aircraft from service for repair would be required. 

ATM procedures will need to include monitoring of the communication status of all 

participating aircraft, and practical, safe methods of recognizing and annunciating 

NASA/CR—2015-218843 46
communication failures, and of removing a communication-impaired aircraft from 

operations. These procedures will need to accommodate manned aircraft, reducedcrew operations, and unmanned aircraft operations. 

6.2.4.4.2 Ground Equipment Failure 

Ground equipment failure presents a relatively straightforward mitigation in most 

cases, through redundant systems and automated fail-operational methods. The 

exception to this is a major outage of some type, such as damage to both primary and 

backup equipment, antennas or cabling, and other infrastructure. In such an event 

airport surface operations could be significantly disrupted, affecting a wide range of 

preflight planning, traffic flow management, and situational awareness functions 

including ground ADS-B, AOC/AAC, and airport operations coordination. In visual 

conditions, the loss of ADS-B capability could be mitigated by visual separation; during 

poor visibility, use of the ADS-B Next airborne ADS-B system is a viable reversion mode. 

For other functions the DM would switch automatically to other available links, 

including VHF datalink, cellular, and even SATCOM to continue operations, albeit at 

increased cost and possibly reduced pace. It should also be noted that events of this 

magnitude may cause interruptions of other airport operations as well for reasons 

unrelated to AeroMACS. 

6.2.4.4.3 Interference 

Interference, whether malicious or unintentional, has the potential to disrupt 

AeroMACS-enabled airport operations to a significant degree. As with other failure 

modes, the first fallback is the inherent capability of the DM, both ground and aircraft, 

to dynamically switch to alternate links when needed. The availability of spectrally 

diverse link options at VHF, L-band and cellular frequencies, as well as SATCOM for 

larger aircraft, offers a range of inherent mitigations. In some cases, such as SATCOM, 

increased link latency must be taken into account, and would potentially impact the 

overall pace of operations. 

In the event of a major, broad-spectrum RF attack, it is possible that airport operations 

could be seriously impacted. In such an event much more than AeroMACS would be 

affected, and contingency planning is outside the scope of this study. The necessity for 

ATM procedures to include loss-of-communication recognition and mitigation, 

discussed in the previous section, applies here as well. For example, in the event of a 

major, broad loss of communication connectivity, an “everyone stop” process may be 

the first step in resolving ground operations issues; air operations obviously present a 

more complex problem. 

NASA/CR—2015-218843 47
6.2.5 AeroWAN 

The proposed Aeronautical Wide-Area Network employs wireless network technology to 

provide broadband bidirectional communications between aircraft and ground access points in 

repurposed aviation L-band spectrum formerly used for DME and TACAN. AeroWAN is based 

on the same mobile WiFi technology used in AeroMACS, but adapted for use with aircraft in 

flight. The system is accessible to all user segments, and provides air-to-ground 

communications for multiple users and purposes, as well as air-to-air connectivity via routing 

similar to existing WiFi systems. 

6.2.5.1 AeroWAN Architecture 

A comprehensive network of ground stations spaced to provide access to all users at 

appropriate altitudes provides wireless network access to client aircraft in flight, 

complementing AeroMACS coverage for on-airport operations. Subnetworks of “high” and 

“low” ground stations may be required, to allocate bandwidth, power, and coverage to highaltitude aircraft with higher bandwidth needs, versus lower altitude and/or lower bandwidth 

aircraft such as small GA aircraft and UASs. 

Ground stations are managed and interconnected by the ground DM function. Ground users 

access the network as clients via their DM client function depicted in Fig. 4. 

Aircraft access AeroWAN via the BBSDR avionics, managed by the DM. A wide range of 

information types can be sent and received by AeroWAN, ranging from AOC/AAC and weather 

information to use as a backup link for key functions such as CPDLC and ADS-B. 

6.2.5.2 AeroWAN Interoperability and Applicability 

AeroWAN is interoperable with all proposed future systems, and with current/legacy systems 

including TACAN, DME, and ADS-B with the caveat that allocation of spectrum within the 

current DME/TACAN band will be required during transition. It is expected that by 2063 all 

TACAN operations, and a majority of DME stations, will have been decommissioned. The 

future concept of ground-based Pseudolites, which provide alternate position, navigation, and 

time transmissions to supplement and back up GPS and other SATNAV systems is also 

interoperable with AeroWAN through simple coordination of channel/spectrum assignments. 

More likely, the proposed AeroWAN and Peudolite functions would be merged into a single, 

multi-purpose system with ample bandwidth for both functions. AeroWAN is applicable to all 

user groups. 

6.2.5.3 AeroWAN Scalability over Time and Changing Industry Needs 

AeroWAN provides significant scalability, through allocation of additional L-band spectrum 

bandwidth and /or deployment of additional access point ground stations. As TACAN channels 

become de-activated, each frequency available can be converted into a new channel providing 

NASA/CR—2015-218843 48
another 4 MB/sec. of capacity, operating in aviation-allocated protected frequency spectrum. 

This can be continued until there are dozens of channels available within the TACAN band. 

Using BBSDR radio technology, each aircraft can receive multiple ground-to-air transmissions 

simultaneously, and avionics can adapt to the addition and reassignment of channels 

dynamically without equipment changes. Higher bandwidth users can also be upgraded to 

adaptive modulation, dynamically switching to higher bandwidth link methodologies during 

favorable link conditions. The software-defined L-Band radio avionics facilitate this, coupled 

with the likelihood that large commercial aircraft, serving passenger connectivity needs, will be 

the most likely higher bandwidth users, and typically operate at higher altitudes where 

adaptive modulation can be used more effectively. 

6.2.5.4 AeroWAN Failure Modes, Effects, and Backups 

Failure modes are divided into four types: Ground Station Failure, Ground System Failure, 

Aircraft System Failure, and Interference. 

In the event of a single ground station failure, higher-altitude aircraft will frequently have 

connectivity with more than one ground station, and the DM simply connects with a more 

distant station. When this is not possible, the DM accesses alternate links including VHF 

datalink, cellular, and SATCOM. In the event of complete loss of AeroWAN connectivity some 

support information services may be lost; however, conduct of flight services such as ADS-B, 

CPDLC and SATNAV can still be maintained using other links. 

Aircraft system failures are mitigated by the redundant DM and L-Band BBSDR architectures. 

In the event of a complete loss of AeroWAN connectivity some support information services 

may be lost; however, conduct of flight services such as ADS-B, CPDLC and SATNAV can still be 

maintained using other links. 

Interference, particularly in the form of intentional jamming, is expected to be localized in 

nature, and produces a similar effect to the loss of a single ground station. 

6.2.6 SDARS 

Satellite Digital Audio Radio Systems (SDARS) provide broadcast of audio programming and 

data services to mobile and other users from geosynchronous and Molniya orbiting satellites. 

SDARS is used today to deliver weather and other support information to aircraft, primarily for 

GA users. It is expected that this system or a comparable follow-on will continue to be 

available for aviation use, and provides a potentially valuable additional link option for GA 

aircraft, as well as other user segments. 

NASA/CR—2015-218843 49
6.2.6.1 SDARS Architecture, Interoperability, and Applicability 

SDARS uses orbiting satellites for broadcast transmission of predictable data sets such as 

weather information. Reception by aircraft is accomplished via the L-band BBSDR or a service 

provider supplied receiver (which may be necessary for subscription control), supported by the 

DM. Data to be transmitted is delivered by the supplier to the SDARS provider’s gateway for 

uplink. 

SDARS is interoperable with all current and proposed systems. SDARS is applicable to all user 

groups, with the possible exception of UAS, where it is technically available but less clear 

whether information applicable to UAS operations will be transmitted via this type of service. 

6.2.6.2 SDARS Scalability, Failure Modes, and Backups 

SDARS scalability is limited primarily by the business case considerations of the commercial 

service provider. Substantial additional bandwidth is available in the overall system, but will 

only be allocated to aviation information broadcast if it promises to generate more revenue 

per unit bandwidth than audio programming. 

Failure of the SDARS aircraft reception equipment, the terrestrial data delivery to the uplink, or 

the entire system result in the loss of some support information to the flight crew. The DM 

automatically switches to other, less efficient and/or more expensive links to obtain 

information from alternate sources as needed. 

6.2.7 Cellular Technologies 

Cellular technologies can be expected to continue their rapid development and capability 

expansion over the study term. It should be noted that looking back 50 years, cellular 

technology was not yet available; the first cellular phone became available in the United States 

in 1983, just over 30 years ago. Thus the 50-year future research term will be nearly 40% 

longer than the entire lifetime of cellular communications to date. Given the rapid pace of its 

historical development, predicting the state of this technology nearly twice as far in the future 

as its total existence to date is a challenge. It is safe to assume that the cellular technology of 

2063 will be far advanced from its current state. 

Current cellular communication systems are purposely structured to limit signal coverage as 

closely as possible to ground users, for technical, regulatory, and commercial reasons. Cellular 

spectrum is re-used from cell to cell to maximize system bandwidth, facilitated by the limited 

range of a ground-based user device. An aircraft-based device has much longer range due to 

its antenna elevation, and would “blanket” a large number of cells in the current architecture, 

seriously impacting overall system available bandwidth. For current commercial cellular 

systems to have practical use for aviation, these problems need to be resolved. The research 

team identified straightforward technical and regulatory steps that could enable practical use 

NASA/CR—2015-218843 50
of cellular systems for aviation applications. This opens up potential access to significant 

communication bandwidth and related technical capabilities outside the limited aviation 

spectrum. Properly managed by the DM function, this offers an intriguing potential source of 

additional communication connectivity. In addition to technical and regulatory considerations, 

however, commercial services are also influenced by business case considerations. Section 8, 

Business Case Analysis, includes additional detail on some of the key business considerations 

related to the use of commercial, non-aviation services for aviation use. 

For future applications in the mid-term study period and beyond, it is reasonable to assume 

that cellular providers may be induced to implement changes in the current architecture as 

described, in order to capture additional business, particularly in the non-urban areas where 

passengers are able to use cellular devices onboard aircraft. By that time 4G/LTE can be 

expected to have been supplanted by “5G/LTE”, or 6G, or additional generations. The key will 

be to stimulate the necessary regulatory and architectural changes to facilitate technical 

compatibility. 

Another option is to make a portion of current aviation spectrum available for one or more 

providers to replicate their commercial infrastructure for aviation use. The research team has 

not studied this in detail, but conceptually it is feasible, and might help offset the commercial 

issues of reserving valuable mass-market bandwidth for a smaller aviation market segment. 

6.2.7.1 General Aviation Cellular Options 

Cellular communications using 4G/LTE are, as a practical matter, already in use to some extent 

in GA. Pilot-owned cellular phones are operable to some extent at low altitudes, and are 

occasionally used by GA pilots in flight to access weather information and other conduct of 

flight functions. This is an application where a relatively minor change in ground antenna 

orientation could create significantly improved performance for GA users at low altitudes. 

Additional research in this area would be valuable. 

6.2.7.2 Aviation Cellular Architecture 

Cellular systems use large numbers of base stations to communicate with mobile devices in 

close proximity. To adapt this architecture to aviation use, a small subset of these stations 

would be modified to include antennas oriented to provide signal to aircraft, coupled with 

modifications to the system’s algorithms for controlling cell size from the channels allocated to 

the aviation antennas, and for dynamically controlling channel assignments and bandwidth 

allocations. Modifying rural stations is a logical starting point, as they typically have fewer 

terrestrial users competing for the same bandwidth, cover larger areas already, and have 

fewer obstructions to antenna coverage for distant aircraft users. 

NASA/CR—2015-218843 51
Base stations are connected by the cellular provider’s infrastructure. A bridge between that 

infrastructure and the ground DM function provides access to aviation users. 

6.2.7.3 Aviation Cellular Interoperability and Applicability 

Within the caveats discussed previously, the proposed cellular capability is interoperable with 

other relevant systems, most notably the ground users of the same system. It is also 

interoperable with all current and proposed aviation systems. The cellular system is applicable 

to all user groups, with the exception of oceanic aviation operations where no commercial 

cellular infrastructure exists. In particular, the GA user would be well served on a technical 

basis by the small size and cost of equipment and antennas, and excellent low-altitude 

coverage across a wide range of topography. Airline passenger users could also be well served 

by the inherent compatibility between existing personal communication devices and the 

proposed architecture, and by the potentially large bandwidth available at demonstrably 

acceptable commercial terms. 

6.2.7.4 Aviation Cellular Scalability, Failure Modes, Effects, and Backups 

With the cellular system, scalability may ultimately depend more on commercial 

considerations than technical ones. Technical scalability is high, being based on similar 

strategies of cell size and spectrum re-use to those applied to ground user services. 

By definition, commercial cellular communication is considered an alternative to aviationspecific links for any type of conduct-of-flight functions. On that basis its failure effects are 

relatively benign, primarily impacting passenger access, except in cases of compound failures 

where it fails coincident with multiple aviation links. The likelihood of such coincident failure is 

small, but would need to be included in implementation planning. 

One possible exception to the assumption that cellular links are only alternatives for conduct 

of flight information is GA. Cellular systems may be used more heavily by GA for flight support 

information such as in-flight updates of weather, NOTAMS, TFRs and similar data due to its 

relatively low, usage-related cost. In that case the GA user would be compelled to depend on 

other links and services, dynamically accessed by the DM, and/or possibly degraded visual 

displays for some flight support information. Cellular communications may also offer 

particular advantages to sUAS operations, an area that merits further study. 

6.2.8 Ku/Ka Band Links 

Ku/Ka-band links typically use geosynchronous (GEO) satellites, which can offer relatively high 

bandwidth communication to larger aircraft, including the oceanic regions. Geosynchronous 

satellite coverage is latitude limited, and is not accessible by aircraft operating in the polar 

regions. One notable exception is the Iridium/NEXT Low Earth Orbit (LEO) Ka-Band payload, 

which will be capable of providing Ka-Band connectivity globally, including the polar regions. 

NASA/CR—2015-218843 52
The research team believes that Iridium/NEXT will be the first of multiple LEO Ka- and possibly 

Ku-Band satellite systems, opening a new category of satellite links compatible with all aircraft 

sizes via small patch-type antennas. It should be noted, however, that Ku-band propagation is 

inherently weather-susceptible, and Ka-Band is even more so. For that reason, even with the 

higher link margins made possible by LEO-based architectures, use at low altitudes, such as by 

GA aircraft and sUAS, will have significant reliability considerations. 

Ku/Ka-Band GEO systems use electronically steerable aircraft antennas, to connect aircraft 

systems via the satellite to a terrestrial earth station which functions as a communication 

gateway. In general, Ku/Ka GEO system steered beam antennas are relatively large, limiting 

their use to larger aircraft. Ka-band communications is a viable system for many aircraft 

applications; satellites will provide increased data rates over Ku-band satellites and will use 

smaller antennas for a given data rate. User costs should be lower than Ku-band. For two way 

messaging via satellite, round trip messaging delays need to be taken into account for each 

proposed application. In many cases the ground network is actually the largest contributer to 

the overall link delay. For broadcast applications including NOTAMS, graphical weather, and 

AAC, network delays should not be a factor. For transporting time critical messages, however, 

the inherent latency of Ku- or Ka-band systems is a significant concern. 

A number of airlines have justified Ku/Ka-Band satellite installations by capitalizing on 

customer demands. Inflight Entertainment (IFE) and airline passengers have driven the push –

and the business case - for faster data rates, leading to Ku- and Ka-Band equipage. Although 

the majority of airlines charge customers to utilize WiFi, and more customers continue to 

demand connectivity, very few are willing to actually pay for the service. Currently, less than 

10 percent of passengers purchase connectivity. If every passenger did “log onto” these 

services, performance with currently available data rates would be severely degraded. 

Airlines are beginning to leverage this passenger demand for connectivity to improve flight 

deck and inflight needs. The traditional means of obtaining such information forces flight 

crews to rely on antiquated systems such as ACARS. Slow bandwidth coupled with the high 

work load of flying the aircraft in critical phases of flight often do not allow this information to 

be disseminated. As cabin wireless becomes available, airlines are using dedicated onboard 

WiFi SSID’s to equip pilots with access to real-time weather on the flight deck, to improve 

operational decision making and reduce block times. Further, flight attendants can check gate 

connections and passengers can even rebook themselves if necessary. 

6.2.8.1 Ku/Ka-Band Aviation Architecture 

In the proposed architecture the terrestrial satellite communications gateway would in turn 

connect with the ground DM function to accomplish link selection from available options, 

message routing, and the required security, reliability, and other requirements for conduct of 

NASA/CR—2015-218843 53
flight information. Aircraft link equipment would also be managed by the aircraft DM, and 

information routed via the DM to various onboard users including passengers, cabin crew, and 

flight crew. For larger aircraft, either GEO or LEO systems will be viable; smaller aircraft will be 

compatible with future LEO systems only, subject to the weather interruption considerations 

discussed previously. 

6.2.8.2 Ku/Ka-Band Interoperability and Applicability 

The future Ku/Ka-Band systems are expected to be interoperable with other proposed 

systems, including the DM, VHF and L-Band links, and legacy systems. Future Ku/Ka-Band 

SATCOM systems will be applicable primarily to larger aircraft that operate at cruise altitudes 

above the troposphere (and potential weather interruptions), such as airline and business jet 

aircraft and larger UASs. GA aircraft and smaller UASs will have very limited ability to support 

the antennas required for GEO systems, and will find equipment size and cost to be challenges 

as well. Aircraft whose primary operations take place within the troposphere will have limited 

applicability due to weather-induced service disruptions. Personal jets or VLJs may benefit 

from Ku/Ka-Band systems depending on their size and operating altitudes. 

6.2.8.3 Ku/Ka-Band Scalability, Failures Modes, Effects and Backups 

For the compatible users, scalability of these systems is relatively high. Significant bandwidth 

is available, particularly at Ka-Band, and satellite architectures will become capable of 

delivering improved performance during the study period. By 2063 it is likely possible that 

antenna and filtering advances will allow closer spacing of geosynchronous satellites, with a 

direct increase in bandwidth as a result. LEO configurations also open a new area of scalability, 

allowing spectrum re-use across relatively small geographic areas within the footprints of the 

low orbiting satellites. 

Three primary failure modes are involved: failure of the aircraft equipment, of the satellite 

itself, or of the earth station/ground connection segment. Due to their limited reliability at 

lower altitudes, the primary application of satellite systems will not be conduct of flight 

services, minimizing their failure effects in most cases. 

Failure of aircraft equipment is primarily mitigated by the use of onboard backup systems. 

Specific backup architectures will be based on the criticality of the system to safe operations; if 

only passenger connectivity is affected, more limited backup capabilities may be needed. The 

DM function also dynamically attempts to find and access alternate links, again depending on 

criticality and end-user cost/priority criteria. 

Satellite and ground segment failures may be mitigated by the DM and aircraft link equipment 

attempting to access an alternate satellite, particularly if satellite spacing has decreased by 

2063. If an alternate satellite is unavailable, the DM will attempt to access alternate links – for 

NASA/CR—2015-218843 54
example, Iridium or other L-Band SATCOM or, over land, the various VHF, L-Band and cellular 

links available. In the case of LEO systems, an inoperative satellite quickly moves along its 

orbital track, limiting interruptions from individual spacecraft to a few minutes duration in 

most cases. 

6.2.9 Iridium/Next (L band) 

The Iridium system of L-Band SATCOM is used today for air-to-ground communications, and its 

successor system Iridium/NEXT is expected to see increased aviation use. Iridium/NEXT is 

scheduled to become operational in 2017-2018, and was used as the baseline for evaluation of 

offerings of that type. Iridium/Next is also designed to provide a capability for true “Push-ToTalk” (PTT) voice communications in real time, anywhere in the world. 

Iridium/Next will include not only communication but also a space-based ADS-B payload that 

will provide direct air-to-ground relay of aircraft ADS-B transmissions in the oceanic regions. 

This capability will allow radar-like air traffic surveillance in those regions, allowing tactical 

rather than procedural traffic separation techniques to be used, fundamentally changing air 

traffic practices worldwide, and increasing capacity significantly in many areas. This capability 

could also change the link loading and other aspects of the current ADS-B links (Mode S and 

UAT) in future applications in the oceanic regions where it will be available. Due to 

competition for capacity from ground/mobile users in populated areas, Iridium space-based 

ADS-B will not be offered in populated land areas. It should be noted that space-based ADS-B 

capabilities have also been proposed based on both Globalstar and INMARSAT platforms; for 

various technical reasons the XCELAR team expects the Iridium-based offering, called AIREON, 

to be the most viable of these. 

Iridium/NEXT also includes global Ka-Band capability (see previous section). By 2063, 

Iridium/NEXT will have reached its design life and is expected to have been replaced with a 

new generation system offering further capability enhancements. 

User costs on Iridium/Next are expected to be relatively high. Pre-paid phone use is currently 

hovering around $1.00/minute and SMS messaging is $ 0.60 per message. Generic data rates 

are $1.80 per 1000 bytes. As an illustration, a typical NEXRAD weather radar image in 

compressed format is about 100 Kbytes. If the present addressing scheme is used, this graphic 

would cost $180.00. Iridium/Next is an addressed system, and does not appear to be well 

suited to broadcast operations. 

Iridium deployment and use on aircraft will continue to increase, but is expected to be used 

primarily for specialized needs (such as polar communications) and backup capability due to its 

relatively high cost. 

NASA/CR—2015-218843 55
6.2.9.1 Iridium/Next Architecture 

The architecture for Iridium/Next is a Low Earth Orbit (LEO) satellite constellation with aircraft 

link equipment connecting to one or more earth station gateways. The DM function on aircraft 

and ground manage the link and information transported. 

6.2.9.2 Iridium/Next Interoperability and Applicability 

Iridium communication will not interfere with any future or existing systems, and will be 

interoperable with all proposed future links. Specific capabilities and backward compatibility 

between the Iridium/NEXT follow-on and its predecessor is beyond the scope of this study. 

Iridium-based communication will be applicable to all user groups, including commercial, 

business, GA, personal jets, and large and small UASs. 

6.2.9.3 Iridium/Next Scalability, Failure Modes, Effects, and Backups 

The Iridium/Next L-Band system itself may be subject to growth limits due to its current 

spectrum allocation. However, its successful application to space-based ADS-B and growing 

role in aviation communication may facilitate either additional spectrum allocations, or a new 

generation of similar competing commercial services. For air transport users, its primary LBand use may be as a backup to Ku/Ka-Band SATCOM at high latitudes and/or low altitudes, 

and as a low-bandwidth link for ATM-related messaging. In these applications its scalability is 

moderate or better. Iridium/NEXT’s Ka-Band capability could provide much higher bandwidth 

connectivity globally. 

The Iridium constellation itself is relatively failure tolerant due to its large number of 

spacecraft and their rapid motion – for any given user, any single spacecraft failure quickly 

propagates to other locations. Aircraft equipment failure could result in loss of ATM 

communications in certain conditions, such as during polar operations, and onboard backup 

equipment architectures should be determined accordingly. Multiple, geographically diverse 

ground gateways provide inherent backup, particularly in conjunction with the Iridium system 

architecture and its intersatellite link capability. 

A major failure of the Iridium system, such as from an extreme space weather event, could 

cause significant disruptions in polar and oceanic operations as its use becomes more central 

to those operations. Self-separation of aircraft via air-to-air ADS-B provides one mitigation 

element; in addition, ATM communication fallback procedures will need to be developed. 

NASA/CR—2015-218843 56
7 Candidate Technology Comparison 

The parallel efforts to identify and characterize future NAS communication functions, and 

candidate link technologies, converged in the candidate comparison process. Each function 

defined in Section 5 was used to rank each relevant link in terms of its suitability, on a scale 

from 1 to 10. Those rankings were then analyzed in a series of comparative steps to identify 

the best-suited candidates for each function, under the most relevant conditions. This section 

summarizes the results of the comparison process.

7.1 Candidate Suitability Ratings 

This section summarizes the relationship between each function, one or more candidate 

communication links, and a relative ranking of overall merit for each candidate from a 

technical standpoint. A numerical suitability rating is given for each candidate, relative to its 

suitability for that specific function, on a ten-point scale based on its capabilities and 

characteristics relative to the requirements of each function. Only links with a suitability 

rating of 5 or higher are shown for brevity. In cases where several links had ratings of 5 or 

more, only the top 3 or 4 are shown, again for conciseness. Candidates are listed in order of 

relative suitability, and a brief summary of underlying rationale for ratings and other relevant 

ranking factors is included where applicable. The same link candidate may receive different 

ratings for its suitability for different functions. It should be remembered that this assessment 

is based on technical considerations only; business case considerations were addressed in the 

next phase of the program, and are summarized in Section 8, Business Case Analysis. 

Each function is referenced by its Function Code, as defined in Section 5 and summarized in the 

Future Datalink Technology Candidate Matrix in Appendix 1. 

7.1.1 Airborne Aircraft, Inbound Communications 

The recommended data links below are listed in priority order based on our assessment of 

their ability to most effectively and economically accomplish the mission. 

AAI-1: Location/State Vector: 

Three attributes are drivers: latency, system capacity, and communications range. 

Candidates Include: 

1. ADS-B Next Suitability 9

2. AeroWAN Suitability 9

3. ADS-B present implementation Suitability 7 

(With phased modulation and low-power on-ground mode) 

4. UAT Suitability 5

(Air-to-air, supplemented by TIS-B ground-to-air) 

NASA/CR—2015-218843 57
Rationale: Multiple data links carrying location/state vector information may be 

necessary in the future to guarantee the continuity of service to achieve airborne 

self-separation. ADS-B Next offers the best throughput and long-term capacity. 

ADS-B Next has the capacity to support 4 times as much traffic as the present ADSB, or 6,400 squitters per second within a service volume, supporting increased 

future traffic density. 

Current ADS-B, when enhanced via phased modulation and low-power ground 

operation mode, is capable of meeting the requirement, with acceptable 

throughput and potentially fewer transition issues than ADS-B Next, but with 

much less capability for future expansion. UAT, when supplemented by TIS-B 

ground-to-air, provides a multi-use link, but lower overall performance than either 

ADS-B or ADS-B Next.

It is worthy of note that current ADS-B has the capacity to accommodate 

additional traffic without becoming overloaded for many years, if transmissions 

are limited to collision avoidance messages at 1 or 2 per second. It is not 

necessary to send 6 pulse groups per second per aircraft as in current practice. 

Increased message rates provide no additional position/state vector information. 

One feature that should be added is anti-spoofing (coded) preambles into the 

message to avoid detection of false messages from hackers. 

AeroWAN provides an alternate method of broadcasting state vector messages. It 

has sufficient range and enormous capacity. It will be able to support additional 

messages and larger messages than present ADS-B. AeroWAN operates in similar 

aviation protected spectrum to that of ADS-B Next, but uses multiple frequencies 

in a mobile Wide-Area Network (WAN) architecture for greater flexibility and 

bandwidth capability. 

AAI-2, AAI-3: Same as AAI-1, same rationale. 

AAI-4, AAI-5, AAI-6: Vehicle (in) Hazardous messages, AMDAR reports are expected to be 

broadcast to aircraft. 

Candidates Include: 

1. ADS-B Next Suitability 9

2. UAT FIS-B with revisions Suitability 8 

3. AeroWAN Suitability 8

NASA/CR—2015-218843 58
Rationale: Messages are expected to be larger than 112 bits, therefore current 

ADS-B would not be a good choice. UAT FIS-B message lengths are a 

maximum of 422 bytes which can support a variety of broadcast messages. If 

AMDAR and Hazmet Weather reports were limited to less than 422 bytes or 

multiple 422 byte messages were combined, then UAT would be applicable –

but UAT is currently limited to lower altitude use. 

At present UAT FIS-B data is limited to a maximum reporting altitude of 24,000 

ft. This is suitable for GA and some turboprops at these cruise altitudes and 

may also be useful for air transport aircraft during climb or descent. Weather 

data for air transport aircraft will require support from other data links or 

revision of the UAT data messages to include higher altitudes. 

For these reasons, ADS-B Next is recommended, as it can serve both high and 

low-altitude aircraft, and offers superior bandwidth and flexibility. In addition, 

the ADS-B Next 1030 MHz data link could easily handle broadcast message 

sizes of 10K bytes and larger. Larger data files transmitted by UAT must be 

partitioned into smaller blocks and transferred slowly or be truncated. 

AeroWAN can support transfer of larger text messages without truncation of 

graphical hazardous area files. Its WAN-based architecture could introduce 

some additional latency in routing messages from a source aircraft to all other 

aircraft logged onto the network, but this is expected to be minimal, especially 

with SDR-enabled multi-channel reception of the proposed broadcast channel. 

VDL-2 is not recommended as a single channel data link, as it is expected to 

quickly become congested. 

AAI-7: Traffic Flow Management 

Candidates Include: 

1. AeroWAN Suitability 9

2. VDL-2 Next Suitability 7

3. UAT Suitability 5

Rationale: Data is not time critical but acknowledgement is required; coverage 

area is localized to a few hundred miles and perhaps less. AeroWAN is well 

suited to this type of data, offers robust bandwidth and growth capability, and 

operates in the aviation spectrum. VDL-2 Next can meet the requirements, but 

NASA/CR—2015-218843 59
is more bandwidth and growth constrained. UAT’s inherent spectrum 

limitations make future saturation a serious consideration. 

AAI-8: NOTAMS Broadcast 

Candidates Include: 

1. AeroWAN Suitability 9

2. SDARS Suitability 7

3. VDL-2 Next Broadcast Suitability 6

4. UAT FIS-B Suitability 5

Rationale: AeroWAN offers high bandwidth and flexibility, and is compatible 

with all altitudes of operation. SDARS offers wide-area broadcast coverage, but 

may have bandwidth limitations; also, use of a wide-area satellite system to 

distribute regional data has efficiency limitations. Both UAT FIS-B and VDL-2B 

are capable of providing the required information; however, UAT has altitude 

limitations, and VDL-2B may be capacity limited. 

AAI-9 through AAI-13: Weather information (various types) 

Required range of information is up to 1000 miles; same information is useable by many 

aircraft. 

Candidates Include: 

1. AeroWAN Suitability 9

2. SDARS Suitability 8

3. VDL-2 Next Suitability 6

4. Ka band broadcast Suitability 6

Rationale: AeroWAN offers high bandwidth, flexibility, and potential 

expandability as traffic grows over time; range of any single ground station is 

limited to 100 miles, so coverage may not be available in certain geographical 

areas at low altitudes. SDARS has the largest area coverage and high data 

capacity, allowing more efficient distribution overall. Ka band broadcast has 

the highest data capacity, and large area coverage, but reception is highly 

dependent upon antenna size, limiting compatibility with smaller aircraft in 

particular. Ka band can also be subject to weather attenuation, which also 

affects smaller aircraft users. VDL-2 and UAT are not expected to have the 

NASA/CR—2015-218843 60
remaining capacity in 2063 to provide high resolution graphics over a large 

area. 

AAI-14 through AAI-16: AOC, AAC, Optimization data 

File sizes are indeterminate, latency is modest. ACK/NAC is required. 

Candidates Include: 

1. AeroWAN Suitability 9

2. Iridium / Iridium/Next Suitability 8

3. VDL Next Suitability 6

4. Cellular Suitability 6

5. Ka band ACK/NAC. Suitability 6

Rationale: Routine traffic, data latency is not critical. Any link that can support 

routine traffic across CONUS coverage with modest queueing delay is suitable. 

AeroWAN provides high bandwidth, the flexibility to accommodate traffic from 

both ATM and AOC/AAC sources, and compatibility with all aircraft types. 

Iridium may be more bandwidth limited, but offers global coverage. VDL Next 

has bandwidth limitations, but is compatible with all aircraft types, whereas 

interactive Ka band has high bandwidth but antenna size issues for smaller 

aircraft. Cellular can address the AOC/AAC applications, given changes to the 

current operating altitude limitations. 

7.1.2 Airborne Aircraft, Outbound Communications 

AAO-1: Location/State vector: 

Candidates Include: 

1. ADS-B Next Suitability 9

2. ADS-B Suitability 7

3. UAT Suitability 7

4. AeroWAN Suitability 5

Rationale: The three key attributes are data latency, system capacity and 

communication range. ADS-B Next has higher capacity then present ADS-B, 

and the inherent capability to expand capacity through additional spectrum 

allocation which current ADS-B does not. UAT has additional data latency if 

traffic is derived from surveillance radars. Otherwise UAT air-to-air and current 

ADS-B air-to-air are equal. AeroWAN offers an interesting option as a backup 

NASA/CR—2015-218843 61
or fallback link in case of primary link failure; its latency would be comparable 

to UAT when traffic is derived from surveillance radars. It operates in the 

aviation spectrum, has high bandwidth, and compatibility with all aircraft 

types. 

AAO-2, AAO-3, AAO-4: 

Same as AAO-1. Note that aircraft on ground should use ADS-B at low power (1 watt) 

to reduce airborne spectrum congestion, while allowing for more aircraft to aircraft 

information transfer using short messages. 

AAO-5, AAO-6: 

Message sizes are expected to be larger than 112 bits. ACK/NAC is required. Data 

latency is not critical 

 

Candidates Include: 

1. AeroWan Suitability 8

2. Iridium/Next Suitability 8

3. Ka band ACK/NAC. Suitability 7

4. Cellular Suitability 6

Rationale: AeroWan has high capacity to download significant engine and 

other maintenance data. Iridium or Iridium/Next have similar capabilities but 

data rate on L band Iridium will be lower. Interactive Ka band offers very high 

capacity, but has limited compatibility with smaller aircraft and UASs. Cellular 

links would meet the requirements, and serve all aircraft types, if current 

altitude operation limits were changed. 

AAO-7, AAO-8: 

Candidates Include: 

1. VDL-2 / VDL-2 Next Suitability 8

2. AeroWAN Suitability 8

3. ADS-B Next Suitability 7

4. UAT downlink Suitability 7

Rationale: Wind and temperature data represent a relatively small traffic 

segment; data latency is not critical. Multiple links are capable of meeting the 

requirements. VLD-2 and AeroWAN are compatible with all aircraft types, and 

offer routing capability to both ATM and non-ATM data users. ADS-B Next and 

UAT also meet the requirements, but offer more limited routing options to 

non-ATM users. 

NASA/CR—2015-218843 62
AAO-9: Special Requests. 

A general category to include various specialized needs such as medical information for 

passenger emergencies. 

Candidates Include: 

1. AeroWAN Suitability 9

2. Iridium/Next Suitability 8

3. Cellular Suitability 6

Rationale: AeroWAN is the best candidate as it can support increasing file 

sizes and high network capacity, compatibility with all aircraft types, and 

robust routing to non-ATM ground users. Range is more than adequate for 

reporting. ACK/NAC is available. 

Iridium/Next can support large file sizes with good transfer rate. Network 

delays of a few hundred milliseconds are not an issue for these types of 

messages. ACK/NAC protocol is built in to the two way messaging. Alternately 

Iridium/Next Push to Talk (PTT) could be used as a common channel for a 

number of aircraft as long as the transmit percentage of each aircraft is low. A 

continued issue for Iridium is network availability as satellite connectivity is 

limited by design although once connected, data transfer rates are more than 

sufficient. 

Cellular links would meet the requirements, and serve all aircraft types, if 

current altitude operation limits were changed. 

7.1.3 Aircraft on Ground, Inbound Communications 

AGI-1 through AGI-6: Airspace System information and Atmospherics 

Requirements include support of large number of aircraft at close range, and variable message 

sizes including graphics. Latency requirement is modest, and ACK/NAC is not needed. 

Candidates Include: 

1. AeroMACS Suitability 9

2. Cellular (4GLTE) Suitability 8 

3. VDL-2 Next Suitability 6

4. SDARS Suitability 5

NASA/CR—2015-218843 63
Rationale: AeroMACS is well suited for this application. Link has high capacity 

and spectrum is reserved for aircraft applications. Many aircraft can use this 

system at the same time. AeroMACS information will be localized for the 

specific airport. It should be noted that there may be airports which are not 

equipped with AeroMACS, but may be within ground range of an AeroWAN 

station, which could provide similar capability. 

Cellular links can provide the same information as AeroMACS but spectrum will 

be shared by consumers and data transfer rates may lag due to network 

delays. SDARS also has capacity for graphic files but localized information may 

not be available and data rate will be slower with greater information latency. 

AGI-7 through AGI-10: Taxi instructions, departure clearance, gate assignments 

Candidates Include: 

1. AeroMACS with ACK/NAC Suitability 9

2. VDL-2 Next Suitability 8

3. VDL-2 Suitability 7

4. Iridium Suitability 5

Rationale: Small file sizes similar to ACARS information, ACK/NAC protocol is 

required. There may be several messages sent to the vehicle in a period of a 

few minutes. VDL-2 and AeroMACS are packet switched, and within the 

capacity limits of both; AeroMACS can service more aircraft. 

Iridium is circuit switched, and keeping a channel open during all of ground 

movement for each aircraft may saturate the satellite’s capacity. Iridium Push 

to Talk networked users on a single frequency could greatly increase capacity. 

Message initiation may take many seconds to minutes. 

Cellular links can meet requirements for AOC and AAC in AGI-9 and -10, but 

are less likely to be deemed acceptable for AGI-7 and -8 messages. 

AGI-11, AGI-12: Information from proximate vehicles on their location and intentions 

Low latency, to include other ground vehicles, high capacity on airport. 

Candidates Include: 

1. ADS-B Next Suitability 9

NASA/CR—2015-218843 64
2. AeroMACS Suitability 7

3. ADS-B Low Power Suitability 5

4. UAT Suitability 5

 Rationale: ADS-B messaging is vehicle to vehicle. AeroMACS information 

needs to go through a router. Latency here is very important; a delay of just a 

few seconds may give flight crews and ground personnel misleading 

information. UAT could theoretically meet the needs of GA-only airports, but 

latency induced by relay of messages between 1090- and UAT-equipped 

vehicles compromises its performance where there is mixed equipage. 

Capacity of all systems is adequate. 

AGI-13, AGI-14: Wake vortex reports; AMDAR reports 

Candidates Include: 

1. AeroMACS Suitability 9

2. ADS-B Next Suitability 8

3. VDL-2 Suitability 8

4. Cellular Suitability 7

Rationale: Reports are broadcast and file sizes are small. Communication range 

is small. Latency is not a significant consideration. AeroMACS is well suited to 

this application. ADS-B Next and VDL-2 B are also well suited, but may require 

routing capability of data to ground ATM and non-ATM users. AeroMACS and 

VDL-2B can continue connectivity until out of range of ground stations. 

For cellular links, pilots will need to stay connected to the network prior to and 

during taxi maneuvers to takeoff; under current FCC regulations, 4GLTE 

communications must be discontinued when aircraft is off the ground. Local 

network coverage could affect reliability. 

7.1.4 Aircraft on Ground, Outbound Communications 

AGO-1, AGO-2: Vehicle position and velocity information 

Low latency, other ground vehicles should be capable of receiving A/C squitter and sending 

their own position reports. High total capacity on airport is required. 

Candidates Include: 

1. ADS-B Next, Low Power Suitability 9

NASA/CR—2015-218843 65
2. AeroMACS Suitability 7

3. ADS-B Suitability 5

4. UAT Suitability 5

Rationale: ADS-B Next offers the best combination of low latency, capacity, and 

compatibility with all aircraft types. AeroMACS offers similar advantages, but 

is dependent on the presence of a ground station on or near the airport, 

whereas ADS-B Next works directly from vehicle to vehicle. 

Both current ADS-B and UAT can meet the performance requirements, but 

share the drawback of interoperability versus latency in a mixed-equipage 

environment. Resolution of the interoperability problem requires a local 

ground station, and introduces latency. 

AGO-3, AGO-4: Maintenance data 

Candidates Include: 

1. AeroMACS; Suitability 9

2. Cellular, with ACK/NAC protocol. Suitability 8 

3. VDL-2 Next Suitability 7

4. VDL-2 Suitability 5

Rationale: Data latency is not critical, a few seconds delay is acceptable. 

Message sizes may be highly variable. Cellular links can support data transfer 

with ACK/NAC. VDL-2 Next is expected to have the capacity. Maintenance 

data file sizes may become too large for VDL-2 to manage without significant 

delays. 

AGO-5, AGO-6: Local wake turbulence, winds 

Note: This is broadcast data for local conditions. 

Candidates Include: 

1. AeroMACS Suitability 9

2. Cellular Suitability 8

3. VDL-2 Next Suitability 8

4. VDL-2 Suitability 6

Rationale: Small file size, local data favor the use of local links versus satellite; 

capacity limits of VDL-2 are a consideration. 

NASA/CR—2015-218843 66
AGO-7: Special requests, medical information 

Candidates Include: 

1. AeroMACS Suitability 9

2. Cellular(Using ACK/NAC protocol) Suitability 8

3. VDL-2 Next Suitability 6

Rationale: Data file sizes are indeterminate. Either AeroMACS or Cellular can 

handle variable file sizes. VDL-2 Next may have delays with large free text 

messages or graphics. 

7.1.5 Suitability Rating Summary 

Fig. 5 summarizes the suitability ratings assigned to each applicable link for each function. This 

rating data was used to perform a series of comparisons between candidates, the results of 

which are summarized in the Section 7.2. Additional detail on functions, applicable links, and 

suitability ratings can also be found in Appendix 1, Future Datalink Technology Comparison 

Matrix. 

NASA/CR—2015-218843 67
Figure 7-1: Link Candidate Suitability Ratings 

ADS-B 

Next

ADS-B/ 

Lo Pwr

Aero

MACS

Aero

WAN UAT

VDL-2 

Next VDL-2 SDARS Cellular

Iridium

/Next Ka Bcast

Ka 

Inter

Avg/ 

Function

AAI-1 9 7 9 5 30 7.5

AAI-2 9 7 9 5 30 7.5

AAI-3 9 7 9 5 30 7.5

AAI-4 9 8 8 25 8.3

AAI-5 9 8 8 25 8.3

AAI-6 9 8 8 25 8.3

AAI-7 9 5 7 21 7.0

AAI-8 9 5 6 7 27 6.8

AAI-9 9 8 6 23 7.7

AAI-10 9 8 6 23 7.7

AAI-11 9 8 6 23 7.7

AAI-12 9 8 6 23 7.7

AAI-13 9 8 6 23 7.7

AAI-14 9 6 6 8 6 35 7.0

AAI-15 9 6 6 8 6 35 7.0

AAI-16 9 6 6 8 6 35 7.0

54 21 141 49 31 47 18 24 30 18 433 120.6

Avg/Link 9.0 7.0 8.8 6.1 6.2 0.0 7.8 6.0 8.0 6.0 6.0 7.5

ADS-B 

Next

ADS-B/ 

Lo Pwr

Aero

MACS

Aero

WAN UAT

VDL-2 

Next VDL-2 SDARS Cellular

Iridium

/Next Ka Bcast

Ka 

Inter

Avg/ 

Function

AAO-1 9 7 5 7 28 7.0

AAO-2 9 7 5 7 28 7.0

AAO-3 9 7 5 7 28 7.0

AAO-4 9 7 5 7 28 7.0

AAO-5 8 6 8 7 29 7.3

AAO-6 8 6 8 7 29 7.3

AAO-7 7 8 7 8 30 7.5

AAO-8 7 8 7 8 30 7.5

AAO-9 9 6 8 23 7.7

50.0 28.0 61.0 42.0 16.0 18.0 24.0 14.0 253.0 65.2

Avg/Link 8.3 7.0 0.0 6.8 7.0 0.0 8.0 0.0 6.0 8.0 0.0 7.0 7.2

ADS-B 

Next

ADS-B/ 

Lo Pwr

Aero

MACS

Aero

WAN UAT

VDL-2 

Next VDL-2 SDARS Cellular

Iridium

/Next Ka Bcast

Ka 

Inter

Avg/ 

Function

AGI-1 9 5 8 22 7.3

AGI-2 9 5 8 22 7.3

AGI-3 9 5 8 22 7.3

AGI-4 9 5 8 22 7.3

AGI-5 9 5 8 22 7.3

AGI-6 9 5 8 22 7.3

AGI-7 9 7 5 5 26 6.5

AGI-8 9 7 5 5 26 6.5

AGI-9 9 7 5 5 26 6.5

AGI-10 9 7 5 5 26 6.5

AGI-11 9 7 5 21 7.0

AGI-12 9 7 5 21 7.0

AGI-13 8 9 8 7 32 8.0

AGI-14 0.0

8.0 18.0 113.0 18.0 28.0 30.0 75.0 20.0 310.0 92.0

Avg/Link 8.0 9.0 8.7 0.0 6.0 0.0 7.0 5.0 6.8 5.0 0.0 0.0 6.6

ADS-B 

Next

ADS-B/ 

Lo Pwr

Aero

MACS

Aero

WAN UAT

VDL-2 

Next VDL-2 SDARS Cellular

Iridium

/Next Ka Bcast

Ka 

Inter

Avg/ 

Function

AGO-1 9 5 7 5 26 6.5

AGO-2 9 5 7 5 26 6.5

AGO-3 9 7 5 8 29 7.3

AGO-4 9 7 5 8 29 7.3

AGO-5 9 6 8 23 7.7

AGO-6 9 6 8 23 7.7

AGO-7 9 6 8 23 7.7

18 10 59 10 20 22 40 179 50.5

Avg/Link 9.0 5.0 8.4 0.0 5.0 6.7 5.5 0.0 8.0 0.0 0.0 0.0 7.2

Aggregate/link 130.0 77.0 172.0 202.0 119.0 51.0 66.0 77.0 151.0 68.0 30.0 32.0

Function

Function

Airborne Aircraft, Inbound Communications

Airborne Aircraft, Inbound Communications

Aircraft on Ground, Inbound Communications

Aircraft on Ground, Outbound Communications

Function

Function

NASA/CR—2015-218843 68
7.2 Candidate Comparison Results 

The candidate ranking data was subjected to a number of analysis steps in order to understand 

how the candidates compare to each other in various ways. Three levels of result analysis 

were used, each building on its predecessors: 

 Baseline Analysis 

 Function Priority Weighted Analysis 

 Weighted Score with Obsolescence 

For each level of analysis, two comparison summaries were developed. One was based on the 

aggregate score for each link across all functions for which it was deemed applicable, and the 

other was an average score for each link based only on those functions for which it was 

deemed applicable. This provides two perspectives: 

 Aggregate Scores illustrate how well each link serves a wide range of functions; for 

example, a link that provides excellent service for only one of the identified functions 

would score lower than a link that provides suitable service for many functions; 

 Average Scores highlight how well each link serves those functions for which it is used, 

without considering how broad-based its applications may be. 

The Baseline Analysis was done based on the suitability scores for each function summarized in 

Fig. 5, and Aggregate and Average scores determined for each candidate link. Then two types 

of weighting factors were applied. In the Function Priority Weighted Analysis, each candidate’s 

score was weighted based on each function’s priority level (i.e., Must-have, Highly Desirable, 

or Nice-to-have). This produces a higher score when a link enables a must-have function than 

a nice-to-have one, based on the relative importance of each function to the conduct of flight. 

A 5-point weighting scale was used, with Must-have weighted at 5 points, Highly Desirable at 3 

points, and Nice-to-have weighted at 1 point. 

In the Weighted Score with Obsolescence, a second factor was then applied to the Function 

Priority Weighted Analysis scores based on the team’s assessment of each link’s susceptibility 

to obsolescence, again using a 5 point scale. This produces higher scores for those links 

deemed to be less susceptible to obsolescence, in the context of looking forward from the 50-

year future research reference point. Those links offering the lowest susceptibility to 

obsolescence were scored higher than those deemed more susceptible to becoming obsolete 

in the 50+ year time period. 

The results of the various analyses were compared and evaluated in terms of the overall 

research goals, and the research team determined that the final step, Weighted Score with 

Obsolescence, provided the best overall guidance toward the optimum future solutions. A 

NASA/CR—2015-218843 69
summary of all the analysis steps and their results is included in Appendix 6 for reference; 

notes regarding obsolescence susceptibility can be found there as well. 

7.2.1 Weighted Scores with Obsolescence 

Fig. 6 shows the aggregate scores for all candidates with obsolescence taken into account. This 

may provide the clearest “investment case” perspective of which candidates merit the most 

investment in research and development to realize the maximum future benefit. It can be 

seen that the highest ranking candidates, in order, are AeroWAN, VDL-2 Next, and ADS-B Next. 

ADS-B Next and AeroMACS have nearly identical rankings; AeroMACS is already the subject of 

significant research, the merit of which is reinforced by this analysis. 

Figure 7-2: Aggregate Weighted Score with Obsolescence 

The average scores with obsolescence considered, shown below in Fig. 7, show a somewhat 

different order of the same four top candidates, with ADS-B Next scoring highest in is 

performance within its more limited areas of application, followed by AeroMACS, AeroWAN, 

and VDL-2 Next. Note that both cellular and interactive Ka-band also score well. 

NASA/CR—2015-218843 70
ADS-B Next, AeroMACS, and interactive Ka-Band all score well in part because they are 

purposely designed to address specific needs in specific areas rather than broad-based links for 

many applications. AeroMACS only serves ground-based users in close proximity to airports, 

Ka-Band serves large aircraft at cruise altitudes in oceanic areas, and ADS-B Next is purposely 

limited to proximate aircraft awareness functions. Cellular candidates also score well for 

similar reasons, being primarily applicable to ground-based uses not unlike AeroMACS. 

Figure 7-3: Average Weighted Score with Obsolescence 

7.2.2 Merged AeroWAN and AeroMACS Rankings 

One special case was also analyzed by the team. AeroWAN and AeroMACS, as currently 

defined, are based on similar technology and operate in similar ways, but serve two disparate 

operational contexts using two relatively divergent frequency bands. As a result, each scores 

very well in its own area: AeroWAN in airborne functions only, and AeroMACS in ground 

functions only. If these two systems could be merged over the 50 year research period, a 

number of advantages would be gained, including commonality of both ground and airborne 

NASA/CR—2015-218843 71
equipment, potentially one less avionics device needed, and increased coverage by using 

airport systems to also serve as AeroWAN ground stations and vice versa. 

The team opted to model a merged system in the AeroWAN L-Band spectrum primarily due to 

the broader applications of an L-Band SDR; however, more detailed technical analysis would be 

needed to formulate a considered recommendation. The aggregate scores under this scenario 

are shown in Fig. 8 below. It can be seen that the “Merged WiFi” system outscores all other 

candidates by a significant margin, due to its broad-based range of functions served coupled 

with its high bandwidth and other strong performance rankings. This suggests that, of all the 

candidates analyzed, such a merged AeroWAN/AeroMACS system would serve the most users 

and functions, across all user segments, of any single link studied. 

Figure 7-4: Aggregate Weighted Scoring Showing Merged AeroWAN / AeroMACS Score 

The Gap Analysis input downselect step identified the top three or more candidates for each 

function; in each case there were at least 2 candidates with scores of 7 or higher. This 

indicates that suitable performance can be obtained for each function from two or more 

candidates. As a result, while the Average rankings provide an interesting look at optimum 

options, which may be useful in choosing between two qualified candidates, in general the 

research team concluded that the Aggregate Rankings provide a more compelling measure of 

NASA/CR—2015-218843 72
overall candidate merit, particularly in the context of defining development priorities going 

forward. A link candidate that provides suitable service for many functions may well support a 

better business case for development investment than one that provides somewhat better 

service for a small number of functions. Using the Aggregate Weighted Score with 

Obsolescence analysis results, as shown in Fig. 6, the highest-ranking candidates are AeroWAN, 

VDL-2 Next, ADS-B Next, and AeroMACS. 

7.2.3 Minimum Candidate Set to Enable All Functions 

Analysis of the Baseline scoring data shows that, technically, all identified functions could be 

served at some level by AeroMACS and AeroWAN. However, there are two fallacies in this. 

First, the defined functions do not break out oceanic operations separately from others, and 

neither AeroMACS nor AeroWAN is well suited to oceanic operations. Second, this would 

require serving some Priority 1 functions with a suitability level of 5 (AeroWAN for ADS-B 

related functions). AeroWAN is better suited to serve as a backup for this high-priority service 

rather than as the primary link. 

As a result, a realistic minimum candidate set would need to include at least four links for 

adequate service. Based on the analysis, the optimum minimum set would consist of ADS-B 

Next, AeroWAN, AeroMACs, and either Iridium or Ka-Band satellite for oceanic operations. If 

AeroWAN and AeroMACS were merged, only three links would be required. 

7.3 Technical Analysis Conclusions 

A total of twelve candidate link technologies were compared against the future NAS functions 

identified previously in the study, five systems currently in service or being deployed within the 

next three years, and seven new or significantly enhanced candidates. Using all twelve 

candidates, all functions were deemed to be successfully enabled. 

The comparison also shows the important potential role of some future technologies studied in 

a successful future NAS. This can be seen in the top four candidates, with the highest 

aggregate ratings, all of which are new candidate technologies. The four highest aggregate 

scores (considering obsolescence) were, in order: 

 AeroWAN 

 VDL-2 Next 

 AeroMACS 

 ADS-B Next 

Another important point is the impact of two elements that do not appear in the comparison 

scores, as they are not link technologies per se, but are important enabling technologies for 

NASA/CR—2015-218843 73
both technical and business case viability of the overall system: BBSDR, and the Delivery 

Manager. These two capabilities allow much broader application of multipurpose hardware 

than current architectures, and the seamless use of multiple links. They also are expected to 

extend the service life of communication avionics, due to their ability to implement 

enhancements via software rather than replacement of hardware onboard the aircraft. 

Both BBSDR and DM were assumed to be available to all links at the 50-year reference point, 

with the exception of current ADS-B and UAT, and current VDL-2, as discussed previously. 

Both are integral parts of the future datalink architecture envisioned by the team, and are 

essentially integral with each other as well. In that context, it could be argued that their 

presence benefits all candidates equally. 

7.4 Technical Gap Analysis 

The preceding comparison data allows examination of possible gaps, or functions not 

adequately enabled by the candidate technologies. It can be seen that in the context of 

current technologies only, there are functions that are expected to be required in the 2063 

aviation industry that are unlikely to be adequately served by today’s technology without 

enhancements. Analysis of the comparison data also indicates that through the application of 

future link technologies identified in the research, all identified future functions could be 

enabled successfully. 

The research team did not address any hypersonic vehicles at speeds beyond Mach 8. This 

would include inbound traffic reentering from space. These vehicles are likely to require 

specialized datalinks tailored to their operational environments, and they will have physical 

limitations such as radio black-out periods during very high speed flight or reentry periods. 

These highly specialized considerations were deemed to be outside the scope of this study. 

For each function, at least the top three candidates were scored; in some cases as many as five 

were deemed suitable enough to merit comparison. In each case, all candidates scored 

showed ratings of 5 out of 10 or better, and for each function there were at least 2 candidates 

scoring 7 or higher. This indicates that each function could be successful supported by at least 

two of the candidates; this is the primary criterion used by the research team to define the 

presence or absence of a gap. It should be noted, however, that gaps do exist if consideration 

is limited to currently available links only; for some functions there is no current link available, 

and in others current links are not expected to be capable of suitable performance. When 

obsolescence aspects are considered, additional gaps emerge using current technologies 

alone. 

The team’s conclusion is that no technical gaps were identified in this analysis, within the 

defined scope of the study, that were not adequately addressed by previously identified 

NASA/CR—2015-218843 74
candidates. The next phase of the study included analysis of business case factors, which 

presents another source of potential gaps, but from a technical standpoint there are no gaps 

that require development of additional candidate solutions. 

 

NASA/CR—2015-218843 75
8 Business Case Analysis 

This section summarizes the process used, and results of, the business case analysis phase of 

the research. Of the twelve link candidates compared technically, nine were included in the 

business case analysis, along with two underlying enabling technologies that facilitate or 

enable all of the link candidates. Three of the lowest-scored candidates technically were 

determined not to merit further consideration, both because of their low technical ratings and 

because they would be fully supplanted by other, enhanced equivalents: current ADS-B, UAT, 

and current VDL-2.

8.1 Business Case Analysis Approach 

The research resulted in a new approach to the underlying architecture of ground-air and airair communications as a whole, rather than simple “link to function” paired solutions. Two 

foundational technologies of that architecture are in fact not link-specific at all: The Delivery 

Manager (DM) and Broad Band Software Defined Radio (BBSDR) technologies. 

The rapidly maturing BBSDR technology allows broadband reception and processing of various 

disparate signals and modulation/encoding techniques in proximate bands using an 

architecture that allows a single receiver to receive and process a broad band of frequency 

spectrum to decode/encode multiple link technologies for different purposes. This 

significantly increases the flexibility of individual communication devices and the system as a 

whole. The BBSDR concept reduces overall cost, weight of multiple radios and associated 

wiring, and complexity. All of these improvements become an important enabling factor in the 

migration from current to future technologies. 

The Delivery Manager enables multiple individual links or link technologies to be harnessed as 

a group, routing information dynamically across the most favorable link at any given time 

based on required availability, integrity, capacity, security, cost Required Communication 

Performance (RCP), and other criteria. This allows seamless ground-air and air-air information 

transfer using a number of disparate links, including facilitating the use of non-aviation 

protected spectrum for certain functions, while still utilizing aviation protected spectrum 

where criteria require it. 

BBSDR and DM are core technologies that facilitate or enable a number of “link technologies” 

even though they are not links per se. In addition, a number of specific link candidates were 

identified and determined to meet the functional requirements from a technical standpoint, 

including ADS-B Next, VDL-Next, and AeroWAN. These technologies and candidates are 

described in detail in previous reports. In most cases, specific link candidates are implemented 

by means of software configurations in the BBSDR hardware, making their link-specific costs of 

NASA/CR—2015-218843 76
deployment very low, but each requiring the BBSDR platform to be useful. Thus in a sense, if 

taken separately, the BBSDR platform business case requires justifying a hardware installation 

that in itself meets no functions, and each link-specific software definition business case looks 

very attractive but are all predicated on the need for the platform. 

This produced a set of potential solutions that in many cases are based on the intersection of 

multiple technologies, such as DM/BBSDR (L-Band for example), and ADS-B Next, as a group 

that together offer significant technical and business advantages. Further, within the BBSDR 

concept each specific link implementation is essentially a software-only addition, which to a 

large degree renders the normal business case analysis inapplicable; the link-specific business 

case that “buys on” the BBSDR avionics would appear to be much more expensive than 

subsequent software-based added links. Any one link could be placed in that position, 

depending on deployment, operational, development/approval, and other future factors that 

cannot be fully predicted at this time. Any arbitrary assignment of such order of deployment 

would unduly skew the business case results for a particular link candidate in relation to the 

others. 

For these reasons, the team developed an architecture-based approach to business case 

analysis, wherein a reference architecture was defined for representative user groups based 

on a combination of technologies harmonized within the new architectural approach, and 

guided by the technical rankings resulting from the previous candidate comparison process. 

Three user groups were selected as reference architecture platforms, to characterize the range 

of business case factors that need to be considered: General Aviation, Domestic Air Transport, 

and Oceanic Air Transport aircraft. 

A reference data exchange architecture was defined for each of those groups, which 

successfully addresses all future NAS data exchange functions identified previously in the 

research, with appropriate levels of redundancy and reversion modes based on the type of 

operations and regulatory requirements analogous to those applied today. The team then 

established typical costs for both current and future communication solutions for each user 

group within the context of each reference architecture. For the future architectures, costs 

were estimated based on the combination of the avionics itself (i.e., the BBSDR 

hardware/installation, DM), and each added link-specific upgrade (i.e., ADS-B Next software on 

L-Band BBSDR). Then the overall solution sets were compared from the business case 

perspective, for each representative user group. 

The team assessed the relative costs of current and anticipated future certification and 

production approval processes, as a function of final product pricing. The conclusion was 

reached that, while future processes may be based on different criteria and implemented in 

NASA/CR—2015-218843 77
different ways, the relative cost of such approvals and related engineering, documentation, 

testing, and time are expected to be similar as a percentage of final product pricing. 

Installation costs were also analyzed as a comparison of current and future architectures. The 

future architectures offer some installation advantages based on a lower number of LRUs and 

LRU types, and allowance was made on that basis. Some economies of scale are possible in 

antenna installations, but for the most part physical limitations of intermodulation distortion 

and other proximity issues will require provisions for antenna diversity between systems. 

Coupled with the need to address possible failure or impairment of an antenna in flight, the 

research team elected to assume that the antenna installation requirements will not differ 

significantly between the current and the future architecture. 

Recommendations for most promising technology alternatives were then formulated based on 

the comparative costs of the future versus the current architecture for each user group, on an 

overall solution basis. Recommendations are summarized in Section 9. 

8.2 Underlying Assumptions 

A number of underlying assumptions were adopted to guide and clarify the analysis process. 

The following three sections summarize significant underlying assumptions applied to the 

analysis. 

8.2.1 Future Technology Assumptions 

 Advances in basic enabling technologies such as microprocessor capabilities, memory 

capacity and density, Digital Signal Processors, and RF device performance, all versus 

cost, size and power consumption, will continue to evolve at a pace close to that of the 

past 30 years; 

 RF power components will continue to advance in terms of power capacity and cost 

reduction; 

 End-product avionics designs will continue to see reductions in size, weight, power 

consumption, and cost, along with increasing technical capabilities, at a rate 

approaching that of the past 30 years; 

 RF filtering capabilities will advance, but at a slower rate than digital technologies due 

to physical considerations of RF energy propagation and interaction; 

 Antenna considerations will require similar allowances for locations diversity, 

directionality, and time-coordinated transmit and receive operations for certain 

operations, due to physical considerations of RF energy propagation and interaction 

NASA/CR—2015-218843 78
combined with the inherently limited physical separation distances possible within a 

single aircraft; 

 Aviation communications will continue to be an especially challenging field in which to 

realize major gains in link performance, due to the inherently wide ranges of distance, 

speed, antenna limitations and other factors that affect system link margins, dynamic 

ranges, and the relatively limited production volumes for aviation-specific components; 

 Advances in GA and VLJ aircraft design and capabilities will increase demand for 

oceanic communications in smaller aircraft; 

 Future high bandwidth satellite communication systems (i.e., Ku- or Ka-band) will 

increasingly migrate to LEO-type configurations, allowing smaller link margins and the 

use of smaller, simpler and less expensive antennas for aviation applications. This will 

allow increased GA use of Ku- and Ka-Band links, and allow air transport aircraft to 

utilize multi-satellite links for higher bandwidth, increased reliability, and lower cost 

than current systems. 

 In the future all aircraft will have a much higher level of onboard navigation capability, 

both from external and internal (i.e., inertial) sources, than they do today. This is 

relevant to communications because loss of GNSS guidance can be mitigated for 

substantial periods of time without the need for Minimum Operating Networks (MON) 

of current-technology ground-based navigation aids such as VOR, DME, or ILS. This in 

turn allows re-use of that aviation spectrum for more broad-based communication 

purposes such as AeroWAN and VDL-Next. 

 The future architectures assume than the AeroWAN systems will include as a core 

capability a form of “Pseudolite” capability in AeroWAN ground stations that serves as 

a backup to GNSS navigation reception in events of GNSS failure, jamming, or other 

interruptions. This is also relevant to the cost analysis because it increases the 

importance of the L-Band BBSDR in maintaining safety; as a result a minimum 

configuration requiring two or more L-Band BBSDRs was applied. 

 The future architectures include an inherent radio altimetry function as one of the 

software-defined capabilities of the 4 – 5 GHz BBSDR; this is to allow precision landing 

capabilities using a combination of either non-augmented GNSS (i.e., without GLS-type 

differential correction) or onboard inertial navigation position data, augmented by 

radio altimetry for vertical position accuracy. This allows both a primary and backup 

precision landing capability (equivalent to Category 3c) without the need for legacy ILS 

NASA/CR—2015-218843 79
or MLS landing systems, and helps to increase aviation RF spectrum available for 

broader-purposed communication applications. 

8.2.2 Future Technology Cost Assumptions 

 Cost comparisons were made in terms of current rather than future dollars, to focus 

the results on comparative costs without the unnecessary variables of economic 

projections of future currency fluctuations. Factors were applied to allow for future 

changes in costs of components, production, product volumes and other factors. 

 A nominal avionics LRU price point of five times direct cost, including hardware and 

software development, certification, and production, will continue to be a valid basis 

for end product pricing; 

 Components developed and produced in large volume for mass-market applications 

(i.e., cell phones) can facilitate significant reductions in avionics cost where they are 

applicable; conversely, it is assume that aviation-specific components, where they 

remain necessary for certain functions, will continue to be severely production volume 

limited and cost reduction potential; 

 Pricing for air transport aircraft on a fleet-wide basis assumed a median discount level 

of 30% under list pricing, based on current market practice 

 The relative cost of regulatory approvals, certifications, and production approvals are 

expected to be similar as a percentage of final product pricing. 

 For air transport, a typical major airline in current terms was modeled, based on a total 

fleet of 700 aircraft, and a mix of domestic and international operations. Typical 

current equipage costs were defined, along with spares allowances and other factors. 

It should be noted that the same allowance for spares (10%) per LRU type was assumed 

for the future configuration, on the basis that the underlying logistics of fleet support, 

spares positioning across the system, and dispatch reliability drivers will remain similar 

over time. However, it can be seen that the future architecture requires a smaller 

number of different LRU types, and that is reflected in the model as well. 

 For the purposes of cost analysis, Business Aviation was deemed to be similar to Part 

121 operators in terms of reference architecture and current versus future equipage 

cost. Decision drivers are somewhat different, but in practice the typical business 

aircraft is equipped with similar, or in many cases better, capabilities than the typical 

air transport aircraft. This trend was deemed to be unlikely to change significantly over 

the study period. 

NASA/CR—2015-218843 80
8.2.3 UAS Considerations 

UAS aircraft and operational situations vary widely in size, weight, power, and cost, and their 

sensitivity to those factors in avionics equipage are equally diverse. In addition, basic NAS 

requirements and still being determined, making it very difficult to analyze quantitatively at 

this time. General Aviation is a good representative case for all but very small UAS, which also 

may have additional functional and communication requirements for operations at very low 

altitudes, which are also still in flux. Further study is needed into the probable NAS functional 

requirements in the UAS sector as regulatory guidance and UAS technology mature; both have 

undergone significant movement during the course of the research. 

8.3 Reference Architectures 

Future reference architectures were defined for three representative user groups: Air 

Transport – Domestic, and Air Transport – Oceanic, and General Aviation. A “current practice” 

architecture was developed for comparison with each future reference architecture. For 

clarity, these current and future architectures are summarized in the following sections, and 

both architectures and costs are shown in tabular form in Appendices 2 – 5. Each table 

includes reference architecture components, costs and LRU count for each current and future 

architecture, and relative comparisons between them. 

All future architectures share some common features, as depicted in Fig. 3. One or more DMs 

communicate with each other and with various BBSDRs and other link-specific LRUs via a 

Communications Bus, and one or more of each type of BBSDR (i.e., L-Band, VHF, SATCOM, etc.) 

implement various specific software-defined links and connect to appropriate antennas. In 

practice, the DM function will be physically integrated with BBSDR units, with one designated 

as primary and the other(s) as backup. Definitions for abbreviations and acronyms can be 

found in Appendix 8, Acronyms and Terms. 

The future air transport analysis is based on a hypothetical carrier consisting of a total fleet of 

700 aircraft, with 400 of those considered to be domestic use only, and the remaining 300 

configured for oceanic operations as well. This fleet size and breakdown resembles the 

structure of a number of major air carriers today. It allowed the analysis to consider the 

differences in number of LRUs, and related spares cost, for the two different sub-fleets, as well 

as the overall costs of the “blended” fleet of a typical carrier of today and the future. 

It should be stressed that these reference architectures are intended to be a tool for relative 

comparison of current and proposed future solution sets, and capture a notional 

implementation of the proposed future data link technologies. These reference architectures 

are not meant to address the many optimizations and tailored architectures that will 

undoubtedly be part of any actual implementation. 

NASA/CR—2015-218843 81
8.3.1 Air Transport – Domestic Architecture 

Domestic air transport aircraft differ from their oceanic counterparts in communication 

terms primarily due to the need for oceanic aircraft to have robust capabilities for 

communicating without ground infrastructure to support various links. Thus oceanic 

aircraft require one or more high-bandwidth satellite links, where domestic aircraft 

may not. In addition, in some cases domestic air transport aircraft (regional jets, for 

example) may be small enough in size to pose significant installation problems for GEO 

satellite communication systems. 

The Domestic Air Transport reference architecture for the future system includes four 

primary components, which between them comprise all required data link functions 

identified in the course of this research: VHF BBSDR, L-Band BBSDR, AeroMACS/5 GHz 

BBSDR, and a Commercial Cellular Datalink. Table 3 below correlates the future 

architecture and its LRUs to the current typical configuration of a domestic air transport 

aircraft. 

NASA/CR—2015-218843 82
Table 8-1: Air Transport Domestic, Current and Future Architecture 

Domestic Air Transport –

Future Architecture

Domestic Air Transport –

Current Architecture

LRU Type LRU Configuration LRU Type LRU Configuration

VHF BBSDR / DM 2 LRUs, primary and 

backup

CMU Individual LRU

DMU Individual LRU

VHF Comm 1 Individual LRU

VHF Comm 2 Individual LRU

VHF Comm 3 Individual LRU

VDL-2 #1 Individual LRU

VDL-2 # 2 Individual LRU

VOR / ILS / GS - 1 Individual LRU

VOR / ILS / GS 2 Individual LRU

L-Band BBSDR 3 identical LRUs Transponder 1 Individual LRU

Transponder 2 Individual LRU

GPS / All GNSS Individual LRU

GPS 2 / MMR 2 Individual LRU

DME 1 Individual LRU

DME 2 Individual LRU

TCAS Individual LRU

AeroMACS 5 GHz 

BBSDR

2 identical LRUs Radar Altimeter Individual LRU

Commercial Cellular 

Datalink

1 LRU Cellular IFE System Individual LRU

8 LRUs 4 LRU Types 18 LRUs 11 LRU Types

NASA/CR—2015-218843 83
In addition, these LRUs are interconnected by the Communications Bus, and each 

requires its own antennas and other support provisions. 

VHF BBSDR / DM 

The VHF BBSDR performs all current VHF radio functions, both digital and analog. This 

includes current voice and VDL communication, to the extent that they are still needed 

in the future, along with the proposed VDL-Next. Dual, independent software defined 

receivers process the entire VHF band, coupled with two or more 50 watt, frequency 

agile, software defined transmitters. It should be noted that a single VHF BBSDR 

provides at least two independent voice and VDL capabilities; in addition, hardware 

redundancy in the form of dual VHF BBSDRs is included in the reference architecture 

for air transport, essentially providing 4 VHF voice and 4 VDL datalink systems under 

normal conditions. VHF-based GLS functions will also be performed by the VHF BBSDR. 

The Delivery Manager function is also integrated into the VHF BBSDR. This function is a 

key component of the future system, but does not need to be a stand-alone LRU from 

an architecture standpoint, and is expected to be integrated into other LRUs for 

efficiency and cost reasons. It could be integrated into either the VHF or L-Band BBSDR, 

and in fact was included in both in the reference architecture to provide triple 

redundancy provisions. The DM, as part of its overall function, also addresses all 

functions currently performed by the CMU and DMU in the current architecture. 

L-Band BBSDR 

The L-Band BBSDR serves a broad range of current and future functions, and is of 

sufficient importance in the future architecture to merit assignment of triple 

redundancy, particularly in the oceanic configuration. Each L-Band BBSDR includes four 

software defined receivers, each processing the entire L-Band simultaneously, including 

all GNSS signals, ADS-B, ADS-B Next, legacy UAT for GA aircraft), multiple DME 

channels, SDARS, and AeroWAN. Dual 300 watt software defined, frequency agile 

transmitters with switchable antennas are managed by the DM to provide bidirectional 

communication as needed. 

 It should that noted that from a technical standpoint legacy TCAS could also be 

accommodated, but due to the requirement for specialized antennas and other 

provisions, this is not included in the 50-year future cost analysis. It is assumed that by 

that point, over 40 years after the global mandates for universal ADS-B equipage, TCAS 

in its current form will have become fully obsolete. As discussed in the previous 

section, DM capability can also be integrated into the L-Band BBSDRs, and was included 

in the reference architecture. 

NASA/CR—2015-218843 84
AeroMACS 5 GHz BBSDR 

The 5 GHz BBSDR platform is designed to host two currently known functions that 

reside within that spectral band, and also to accommodate additional future functions 

that may be defined in that band. Each AeroMACS BBSDR LRU includes dual software 

defined receivers and transmitters to support simultaneous and independent 

AeroMACS and Radio Altimeter (RA) operations. For air transport two separate LRUs 

are included for redundancy due to the importance of both RA and AeroMACS to 

effective flight operations. 

It should be noted that AeroMACS capability could have been incorporated into the 

system in a number of ways. For example, it is likely that in the future there will be 

small, inexpensive stand-alone AeroMACS transceivers or chipsets that could be 

integrated into any of the other LRUs. However, for air transport applications the team 

decided to define a separate LRU for various 5 GHz software defined functions, 

including AeroMACs. The primary reasons were: 

 Hosting AeroMACS on a BBSDR platform provides maximum forward flexibility 

for addition of bandwidth as additional spectrum becomes available and system 

needs grow; 

 This allows the RA function to also be hosted on the same platform, further 

reducing LRU/type count and again providing software-defined forward 

flexibility for technical advancements in RA technology; 

 Defining a 5 GHz BBSDR platform lays the groundwork for implementation of 

other, future software defined functions using 5 GHz spectrum; 

 A more conservative approach to future architecture cost estimation. 

Commercial Cellular Datalink 

As discussed in previous reports, the future of commercial cellular systems as far as 50 

years in the future, as they might apply to aviation applications, is difficult to define in 

detail from a technical standpoint. The large numbers of potential subscribers, 

associated revenue, and their effect on regulatory processes can bring about significant 

changes in the communications landscape as a result of the powerful commercial and 

political forces they influence – far more so than the comparatively miniscule numbers 

of aviation users, even when large numbers of UAS are included. As a result, it is 

difficult to predict even fundamental technical drivers such as frequency bands of 

future cellular operations, modulation and bandwidth management techniques, etc. 

Current air transport uses of commercial cellular technology are typically focused 

primarily on In Flight Entertainment (IFE) uses rather than flight deck functions. Thus 

IFE equipage today is typically based on a commercially-driven business case, justified 

NASA/CR—2015-218843 85
by a revenue segment comprising significant numbers of potentially fee-paying cabin 

users, rather than a small number of flight deck users who represent an operator cost 

center rather and a revenue segment. This is unlikely to change over time. It is difficult 

to predict what the future cabin entertainment / passenger communication 

marketplace will look like, beyond the fact that it will continue to demand higher 

bandwidth, in keeping with the terrestrial mobile sector. For modeling purposes, the 

research team based those architecture elements on a market-driven demand for 500% 

increase over current capabilities. 

Another important business consideration is the degree to which terrestrial commercial 

cellular operators may be motivated to make accommodations for capturing aviation 

users in their revenue base. Cellular operators, by their technical definition, base their 

business on the principle of re-using their RF bandwidth again and again, on a 

geographical basis, to provide services to large numbers of terrestrial users equipped 

with purposely range-limited transceivers. Aviation applications are in many ways 

diametrically opposite to this, with fast-moving platforms covering large distances, with 

relatively long-range antenna footprints, occupied by relatively small numbers of 

potential service users. While there are some technical options for mitigating these 

divergent service models, as discussed in previous reports, the ultimate business 

consideration is the cost / benefit relationship for both provider and subscriber. In 

particular, the provider will be required to invest in initial and ongoing provisions to 

capture and serve airborne users, including technical development, regulatory change, 

capital asset investments and maintenance, and some concessions of RF bandwidth to 

a smaller pool of potentially higher-rate service users. Of these, the business cost of 

allocating scarce bandwidth to aviation users may be the most difficult to justify. 

From the subscriber side, and the aircraft operator side, there are also hurdles to 

overcome. Avionics must be developed and capitalized that are suitable for air 

transport aircraft, at much higher development and production cost than consumer 

equipment. Typical cellular systems only serve land-based users, so either alternate 

sources of oceanic service will be needed, or the development of oceanic provider 

systems (i.e., satellite) – to serve a miniscule subset of the population that justifies 

terrestrial system development. And finally, providers must have a revenue model that 

justifies the opportunity costs for serving the aviation population, resulting in 

potentially much higher service costs for aviation than that for terrestrial users. 

Despite those hurdles, it is likely that commercial cellular systems will be adapted to 

aviation to some extent; in fact, that is happening to some degree today. For example, 

several airlines use cellular technology to update navigation charts, download FOQA 

NASA/CR—2015-218843 86
data, or engine health monitoring data, but it is quite limited today and geographically 

constrained. It is difficult to predict a compelling enough business case for the provider 

to postulate adapted cellular systems becoming any sort of primary or backbone link 

for aviation operations. As a result, the air transport reference architectures assumed 

that the future cellular system would be a stand-alone system, rather than being 

integrated into other BBSDR avionics, and would primarily serve passenger 

entertainment rather than flight deck needs. It was also assumed that these systems 

would primarily serve domestic rather than oceanic operations. 

8.3.2 Air Transport – Oceanic Architecture 

Oceanic air transport architectures are essentially identical to those for domestic 

operations, with one key addition: the need for added connectivity when out of range 

of land-based communication systems, as illustrated in Table 4. This includes both fight 

deck and passenger applications. Aside from these additional oceanic connectivity 

solutions, oceanic and domestic air transport aircraft are essentially identically 

equipped, and were covered in the previous section. This section covers only the 

differences between oceanic and domestic air transport aircraft. 

Table 8-2: Air Transport Oceanic, Current and Future Architecture 

Oceanic Air Transport –

Future Architecture

Oceanic Air Transport –

Current Architecture

LRU Type LRU Configuration LRU Type LRU Configuration

VHF BBSDR / DM 2 LRUs, primary and 

backup

CMU Individual LRU

DMU Individual LRU

VHF Comm 1 Individual LRU

VHF Comm 2 Individual LRU

VHF Comm 3 Individual LRU

VDL-2 #1 Individual LRU

VDL-2 # 2 Individual LRU

VOR / ILS / GS - 1 Individual LRU

NASA/CR—2015-218843 87
VOR / ILS / GS 2 Individual LRU

L-Band BBSDR 3 identical LRUs Transponder 1 Individual LRU

Transponder 2 Individual LRU

GPS / All GNSS Individual LRU

GPS 2 / MMR 2 Individual LRU

DME 1 Individual LRU

DME 2 Individual LRU

TCAS Individual LRU

Iridium LEO L-Band Individual LRU

AeroMACS 5 GHz 

BBSDR

2 identical LRUs Radar Altimeter Individual LRU

Ku- or Ka-Band 

BBSDR

1 LRU HF Radio / SATCOM Individual LRU

Ku-Band or 

INMARSAT

Individual LRU

Commercial Cellular 

Datalink

1 LRU Cellular IFE System Individual LRU

9 LRUs 5 LRU Types 21 LRUs 14 LRU Types

To some degree it will be possible to use AeroWAN, VDL-Next and other systems for 

oceanic communications via aircraft-to-aircraft relay to land-based systems. This is 

included in the proposed capabilities for both AeroWAN and VDL-Next for this reason. 

However, this is dependent on the presence of proximate aircraft located at suitable 

distances between a given oceanic aircraft and a land-based system. Although the DM 

function is expected to leverage these conditions when available, it is not reliable enough 

to be considered a primary mode of operation, and is expected to serve more as a cost 

reduction opportunity and backup capability when conditions permit. 

Satellite communications offer the most practical solution today, and are expected to 

increase in technical capabilities, cost effectiveness, and availability in the future. LEObased L-band communications, such as the current Iridium and Iridium/Next, are 

accommodated in the future architecture by the addition of software defined functions in 

the L-Band BBSDRs. Ku- and Ka-Band satellite links are in use today for oceanic 

communications, and are expected to grow in availability, capabilities and cost 

NASA/CR—2015-218843 88
effectiveness over the research period. Ku- and Ka-Band systems today are based on GEO 

orbital orientations and relatively high gain, steerable antennas on aircraft large enough to 

accommodate them. Similar future systems are expected to be available, and offer greater 

bandwidth, lower cost, and compatibility with BBSDR architectures. 

In addition, it is expected that future Ku- and Ka-Band satellite systems will be available 

using LEO orbital configurations, allowing much more favorable link margins, lower power 

uplink transmitter requirements, and use of small, lightweight, and low-drag patch type 

antennas. The Iridium/Next system currently beginning its deployment will offer this 

capability to a limited degree, and it is expected that in 50 years there will be multiple, 

high-bandwidth options available. For this business case analysis, future cost estimates 

were limited to GEO-based systems with antenna systems similar to current offerings, in 

the interest of conservative cost estimation. 

It is also technically feasible to serve Ku/Ka-Band communications using the L-Band BBSDR 

rather than an additional, dedicated Ku/Ka-Band LRU. It is not uncommon today for Ku/KaBand systems to actually use antenna-mounted amplifiers and downconverters to convert 

received signals to L-Band for coaxial cable transmission to the avionics, and to use L-Band 

modulators and upconverters to separate High Power Amplifier (HPA) units. Adding two 

additional software defined, frequency agile transmitters to the L-Band BBSDR for example, 

along with additional antenna switching, would provide further flexibility, cross-functional 

redundancy, and low additional cost. Upconverters and HPA’s could even be antennamounted, as downconverters frequently are today, decreasing cable losses and HPA power 

requirements. However, for several reasons this analysis placed the Ku/Ka-Band avionics in 

its own LRU. Those reasons included cost conservatism, maintenance simplicity, and 

separation of commercial provider systems from conduct-of-flight systems. 

8.3.3 General Aviation Architecture 

The GA architecture is similar to domestic air transport in many ways, with some 

important differences. Less redundancy is required, and simpler aircraft systems and 

architecture reduce overall LRU complexity. GA avionics packaging is typically different 

from air transport, emphasizing smaller, lighter enclosures, different mounting 

provisions, and a growing trend toward integration of communication functions into 

other avionics, such as panel-mounted multifunction displays, due to economies of 

scale and limitations of space, weight and power capacity. In order to avoid the 

complications of attempting to predict the 50-year future of GA panel displays and 

other non-communication products, which is beyond the scope of this research, the GA 

NASA/CR—2015-218843 89
reference architecture assumes that all communications functions will be packaged as 

individual LRUs as summarized in Table 5 below, which communicate with other 

systems and control/display interfaces via a data bus. In keeping with the team’s 

conservative approach to future cost estimates, the chosen approach does not include 

potential further reductions in communication LRU cost through integration with other, 

non-communication avionics. 

For a typical installation, a single VHF BBSDR provides the functions of current dual 

Nav/Com units, including VHF voice, VDL, VOR, ILS and Glide Slope functions to the 

extent that they are still needed 50 years in the future. The proposed future VDL-Next, 

GLS, and other VHF functions are also supported as software defined capabilities. Dual 

software defined receivers and dual 10 watt software defined transmitters emulate 

current dual Nav/Com operation when needed, in addition to supporting the additional 

future capabilities. The software defined architecture facilitates transitional operations 

as the current VHF systems migrate to the future VDL-Next structure while maintaining 

legacy capabilities as needed. 

It is anticipated that by the 50-year future point, very little analog voice 

communication, VOR, or ILS systems will remain in common use, with even Minimum 

Operating Networks (MON) of VORs having been replaced with other, more efficient 

and effective means such as CPDLC, digital voice, all-GNSS reception, highprecision/long persistence onboard inertial navigation systems, and AeroWAN-based 

navigation “pseudolites”. For this reason the reference architecture only includes a 

single VHF BBSDR LRU with its dual Nav/Com capabilities. In event of the loss of that 

VHF BBSDR LRU from service, the broad capabilities of the dual L-Band BBSDRs are 

expected to provide adequate backup communication capability. 

Two L-Band BBSDR LRUs are included, due to their importance in primary and backup 

navigation, surveillance, and communication. Each LRU includes dual software defined 

receivers and dual 100 watt software defined transmitters. Each LRU can provide all LBand functions, including GNSS, ADS-B, ADS-B Next, AeroWAN, multi-channel DME, LBand LEO communications (e.g., Iridium/Next), SDARS, and legacy UAT and Mode S 

Transponder functions. For GA, the DM function is integrated into the L-Band BBSDR 

due to its typical dual LRU hardware redundant configuration; a third DM is not 

expected to be needed for GA, so no DM function is included in the VHF BBSDR for the 

reference architecture. 

The GA architecture also includes a single AeroMACS 5 GHz BBSDR, providing both the 

ground-ground AeroMACS digital connectivity and the radio altimeter function similar 

NASA/CR—2015-218843 90
to air transport. Based on the low price point expected for this SDR-based unit, the 

addition of RA capability to a GA aircraft, equipped with all-GNSS navigation, adds 

significant new options for precision landing aids as discussed in previous reports. As in 

air transport, this also positions the future GA aircraft to take advantage of other 

potential future communication and/or navigation options that may be developed in 5 

GHz aviation spectrum as it is re-used. 

The application of commercial cellular networks for common aviation use is most likely 

to take place in GA, where aircraft operate at lower altitudes, at lower speeds, and 

users more closely resemble the mainstream ground-based mobile customer. For this 

reason the architecture also includes a commercial cellular link LRU, which can provide 

various data and digital audio communication functions. By linking it to the DM in the 

L-Band BBSDR, its availability can be monitored and leveraged by the DM along with 

other available links as appropriate. 

Table 8-3: General Aviation, Current and Future Architecture 

General Aviation –

Future Architecture

General Aviation –

Current Architecture

LRU Type LRU Configuration LRU Type LRU Configuration

VHF BBSDR / DM Single LRU, provides 

Dual Nav/Com

VHF Nav/Com #1 Individual LRU

VHF Nav/Com #2 Individual LRU

L-Band BBSDR 2 identical LRUs GPS / All GNSS Individual LRU

Mode S 

Transponder or UAT

Individual LRU

GPS / All GNSS Individual LRU

GPS 2 / VOR-2 / 

DME/DME 

Substitute for sole 

means navigation

Individual LRU

DME 0 LRUs: 

GA A/C typically 

not equipped 

NASA/CR—2015-218843 91
today

AeroMACS 5 GHz 

BBSDR

1 LRU No Equivalent Link

Commercial Cellular 

Datalink

1 LRU Handheld Cellular 

WX Service

Individual LRU

5 LRUs 4 LRU Types 7 LRUs 6 LRU Types

8.4 Business Case Findings 

Business case inputs and numerical outputs are shown in tabular form in Appendix 3 - 5 for 

reference. Estimated costs were assigned to each current and future LRU in the context of 

each current and future reference architecture, and results compared in various ways. This 

section summarizes the results of each comparison, organized by reference architecture 

groups. 

In general, future architectures were deemed to be feasible from a business case perspective if 

the overall cost of the future architecture is equal to or less than the current architecture. 

Simply put, if the new solution costs less than the current solution, it will probably constitute a 

workable value proposition; the greater the reduction in cost, the more compelling the 

equipage case will be. 

Conversely, if the future solution costs more, there needs to be a significant functional, safety 

or operational efficiency benefit to offset the difference; in particular, measurable financial 

gains in operating efficiency would constitute the most effective mitigator to higher cost. The 

greater the increase in future over current cost, the harder the value proposition will be to gain 

industry acceptance. In general, an increase of more than 10 – 15% over current conditions 

was deemed to be a very difficult value proposition for which to gain acceptance. 

It should be noted that antenna considerations were considered to be a similar for both 

current and future architectures, due to governing physical limitations; for example, in some 

cases different software-defined systems in the same band would require different antennas, 

potentially mounted in different locations on the aircraft, to function correctly. Future 

transmit antennas will need to be located as far away from reception systems as possible, as 

they are today. One possible exception to this is future LEO-based Ku/Ka systems, which could 

use a patch versus beam-steered antenna. For cost conservatism though this case was not 

included. 

NASA/CR—2015-218843 92
8.4.1 Number and Type of LRUs 

The total number of LRUs included in an overall avionics package, along with the number of 

different types of LRUs, can have a significant impact on both initial and long-term costs of the 

package to the operator. Obviously if initial equipage requires the purchase of a larger number 

of LRUs, the cost can be expected to be higher, but other less apparent factors come into play 

as well. Each additional LRU also adds weight, size, cost, and maintenance complexity to the 

overall system. In addition, for air transport, each LRU type drives the business planning for 

purchase, positioning, and repair strategies for spares across and air carrier’s system. Typically 

the more LRU types there are in the package, the more spares must be purchased and 

maintained to achieve the necessary dispatch reliability. These factors were included in the 

business case analysis, for both the current and future architectures. 

Another consideration is the expected reliability of each LRU, along with its contribution to 

overall maintenance costs and strategy. Historically, a higher level of functional integration 

per LRU has had both advantages and disadvantages in real-world maintenance: an LRU that 

performed multiple functions reduced the number of spares needed, but a failure of any one 

function required the replacement and off-line repair of the entire unit. However, in the 

proposed future architecture there are important mitigators to this consideration. First, the 

overall reliability of components and systems has steadily increased over the years, and is 

expected to continue to improve over the 50-year research period. Both the number of 

components subject to failure, and the technical factors that influence failure, will continue to 

decrease. From vacuum tubes to transistors, to integrated circuits, to the highly integrated 

microprocessor and DSP based architecture of the future, each LRU contains fewer and fewer 

components that are subject to failure. In parallel, the factors of heat, voltages, currents, and 

vibration susceptibility will continue to decrease; typical circuit voltages, for example, have 

gone from hundreds of volts to tens, to five, to three and below over the past 50 years. 

Finally, it should be remembered that the future architecture will derive its ability to deliver 

multiple functions from a single LRU primarily through the SDR technology, wherein additional 

functions are primarily software enabled, rather than requiring significant additional hardware 

for each function. In this context the reliability of the underlying SDR platform becomes the 

controlling factor, rather than the number of functions performed. 

The team’s research concluded that no additional sparing allowance would be needed based 

on the number of functions performed by each LRU, in light of the expected reliability levels of 

the future technologies and the levels of redundancy built into the reference architectures. As 

a result, the future architecture offers some significant cost advantages over the current norm 

due to a smaller number of spares required for both initial purchase and ongoing maintenance 

NASA/CR—2015-218843 93
and repair. For air transport, a common current industry allowance of 10% was applied to 

both current and future architectures. 

For GA, no direct spares allowance was included in the model. Typical GA aircraft are either 

individually owned and operated, or are operated in relatively small fleets. Maintenance is 

typically done on an outsource basis to third-party shops, who either stock spares or order as 

needed. In few cases is the quantity of owner/operator maintained spares large enough to 

affect the overall value proposition of equipage significantly. 

8.4.2 Air Transport, Domestic 

The proposed future solution set compared very favorably to the current architecture for 

domestic air transport aircraft. Eight different comparisons were made, examining various 

aspects of the overall system, culminating in an overall aggregate comparison of total costs for 

equipage and spares for a reference fleet of 400 domestic aircraft. 

 The first four comparisons examined the comparative cost of the four major types of future 

LRU: 

 VHF functions only; 

 L-Band functions only; 

 5 GHz functions only, and; 

 Cellular datalink. 

The other four comparisons focused on different aspects of the overall cost of equipage and 

support: 

 Cost of equipping a single aircraft; 

 Cost of equipping a 400 aircraft “reference fleet” of domestic aircraft, including typical 

quantity discounts; 

 Spares cost for the 400 aircraft reference fleet, including typical quantity discounts; 

 Aggregate total cost for the reference fleet, including both aircraft equipage and 

spares. 

Industry-typical quantity discounts of 30% below list price for fleetwide purchases were 

applied, and similarly typical spares allocations of 10% per LRU type were also used. Note that 

sparing is based on LRU type, rather than number of LRUs – for example, if the architecture 

(current or future) includes 3 VHF LRUs, each LRU is allocated a spares allowance of (10% / 3), 

in accordance with current practice. Based on the technical assessment that the future LRUs 

are likely to have a higher overall reliability that current systems, and that the smaller number 

of LRUs and LRU types will have a positive impact on MTBF and MTTR, these assumptions were 

considered to be conservative. 

Table 6 summarizes the eight comparison criteria and their results: 

NASA/CR—2015-218843 94
Table 8-4: Air Transport – Domestic Business Case Results 

Air Transport – Domestic Business Case Results

Criterion Current Architecture Future Architecture Future / Current %

Total LRU Count 18 8 44 %

Total LRU Types 11 4 36 %

VHF Functions Only $135,408,000 $6,160,000 5 %

L-Band Functions Only $148,176,000 $53,760,000 36 %

5 GHz Functions Only $24,640,000 $6,160,000 25 %

Cellular Datalink $12,320,000 $9,240,000 75 %

Single A/C Equipage $1,021,000 $250,000 24%

400 A/C Fleet Equipage $285,880,000 70,000,000 24%

Spares Cost $17,892,000 $3,080,000 17%

Aggregate Total Cost $303,772,000 $73,080,000 24%

 

The future architecture shows a very significant cost improvement over the current 

architecture in every category, with and without quantity discounts. The overall aggregate 

cost of the future system is less than one fourth of the current architecture, and this trend is 

consistent across the single aircraft and fleet equipage as well. Future spares cost is slightly 

lower, at 17% of current cost, due to the smaller number of LRU types required. 

The future system has a significantly lower number of total LRUs (44%), but of commensurate 

importance is the lower number of LRU types (36%). The decreased number of LRUs directly 

decreases the purchase cost of spares; the smaller number of LRU types also will have a 

beneficial effect on more indirect cost drivers, such as reduced maintenance training (fewer 

LRUs to train for), reduction in test equipment, decreased MTTR due to simpler diagnostics, 

and reduced spares storage and logistics costs. These benefits were not quantified in the 

analysis, but can be expected to have a significant positive effect on overall life cycle cost for 

the aircraft operator. 

NASA/CR—2015-218843 95
When viewed in terms of individual LRU types, the future architecture again shows clear cost 

improvement, but more variation is seen. For VHF, the incorporation of the currently 

expensive CMU and DMU functions into the overall DM, coupled with the lack of need to triple 

redundant, limited-function LRUs as in the current architecture, produce a sharp cost 

difference: 5% of current cost. This is less visible for L-Band (36%), partly because the cost 

reduction benefit for eliminating the stand-alone CMU and DMU has already been ascribed to 

VHF. In addition, the future L-Band LRU requires triple redundancy due to the breadth of its 

functions, whereas the future VHF only requires two. 

Of note is the AeroMACS 5 GHz LRU which, even with dual redundant LRUs, shows a very 

favorable 25% of current system cost. This is interesting, since there is currently no AeroMACS 

equipage at all in the current architecture; integrating the current Radio Altimeter function, in 

itself, produces the future vs. current cost reduction. 

Regarding cellular data links, a number of unknowns are involved which make the comparison 

less meaningful than the other LRU types. As previously discussed, the future commercial 

environment for adaptation of high-volume, low cost personal cellular products to aviation are 

difficult to predict as far as 50 years in the future. The future business model for equipage is 

uncertain, and may involve commercial provider owned equipment onboard aircraft, leasing of 

aircraft equipment, and other factors that make comparison to current systems difficult. For 

the purpose of comparison, the research team assumed that the carrier would own the aircraft 

equipment, and estimated future cost based on extrapolation of current cost and capability 

trends toward a future performance goal of a 500% improvement over current system 

performance. This analysis yielded a future cost of 75 % of current architecture cost. 

8.4.3 Air Transport, Oceanic 

As discussed in previous sections, the oceanic air transport architecture is identical except for 

three additional LRU categories for enhanced communications in areas distant from landbased communications. The same comparison criteria were used with one category added for 

Ku/Ka-Band satellite communications, and the same industry-typical discount and spares 

allowance were applied. An oceanic reference fleet of 300 aircraft was used. 

Table 7 summarizes the eight comparison criteria and their results: 

NASA/CR—2015-218843 96
Table 8-5: Air Transport – Oceanic Business Case Results 

Air Transport – Oceanic Business Case Results

Criterion Current Architecture Future Architecture Future / Current %

Total LRU Count 21 9 43 %

Total LRU Types 14 5 36 %

VHF Functions Only $101,556,000 $4,620,000 5 %

L-Band Functions Only $132,846,000 $40,320,000 30 %

5 GHz Functions Only $18,480,000 $4,620,000 25 %

Ku/Ka-Band SATCOM $47,040,000 $5,145,000 11 %

Cellular Datalink $9,240,000 $6,930,000 75 %

Single A/C Equipage $1,315,000 $270,000 21 %

300 A/C Fleet Equipage $276,150,000 $56,700,000 21 %

Spares Cost $16,506,000 $2,730,000 17 %

Aggregate Total Cost $292,656,000 $59,430,000 20 %

The future architecture again shows significant reductions in cost relative to the current 

architecture. LRU Count and LRU types are similar to air transport domestic. VHF, 5 GHz and 

cellular all are based on the same data as domestic air transport, and show the same 

improvement. L-Band scores slightly better at for oceanic than domestic, at 30%, due to the 

incorporation of additional software defined functions that replace current LRUs for HF radio 

and/or L-Band Iridium oceanic communications. 

The added LRU type, Ku/Ka-Band satellite communications, also shows a significant reduction 

from current to future cost. This is due to several factors, including the application of BBSDR 

technology, anticipated advances in component performance versus cost, and the probable 

advent of LEO-based Ku- and Ka-Band communication systems, which reduce required 

transmit power levels, antenna complexity, and other design requirements. As noted 

previously, a further reduction on future cost could be realized by incorporating Ku/Ka Band 

communications into the L-Band BBSDR, which was not included in this analysis. 

NASA/CR—2015-218843 97
Overall cost reduction from current to future is slightly better than domestic air transport, due 

to the added improvements in L-Band and Ku/Ka-Band previously discussed. Aggregate total 

cost for the 300 aircraft reference fleet is 20% of current architecture, an improvement from 

the 24% for domestic air transport. 

8.4.4 General Aviation 

The General Aviation analysis is much simpler than those for air transport, due to the simpler 

reference architecture, reduced number of required functions and redundancies, the lack of 

fleet discounts and oceanic configurations, and the absence of spares as a significant 

consideration. As previously discussed, spares were not considered because GA fleets are 

typically single aircraft or small fleets with limited opportunities for economies of scale; typical 

maintenance scenarios often rely on outsourcing to a commercial provider who orders on 

demand or handles stocking in accordance with their overall business model. On that basis 

spares were deemed to be a negligible business case consideration for GA. 

A total of seven comparison criteria were used, structured similar to the air transport analyses. 

LRU count is compared in the same way as air transport, although with the lack of sparing as a 

consideration, the impact of the number of LRU types is insignificant. Only five cost criteria 

were used: the four major LRU types, and aggregate total cost for a single aircraft. 

Table 8 summarizes the eight comparison criteria and their results: 

Table 8-6: General Aviation Business Case Results 

General Aviation Business Case Results

Criterion Current Architecture Future Architecture Future / Current %

Total LRU Count 7 5 71 %

Total LRU Types 6 4 67 %

VHF Functions Only $11,000 $5,000 45 %

L-Band Functions Only $31,500 $24,000 76 %

5 GHz Functions Only N/A $5,000 N/A

Cellular Datalink $1,700 $1,000 59 %

Aggregate Total Cost $44,200 $35,000 79 %

NASA/CR—2015-218843 98
99

All categories except one show a clear and advantageous reduction in cost over the current 

architecture, although of a much lesser magnitude than seen in air transport. This is due in 

large part to the much smaller number of LRUs, functions, and costs for GA equipage than air 

transport. The reduced LRU count (71 %) does offer advantages in reduced space, weight, and 

power consumption, in addition to the simple reduction in cost from acquiring fewer devices. 

The one exception to clear cost reduction is due to the lack of any current analogous function 

or system in GA to the AeroMACS/5 GHz technology, so no direct comparison could be made. 

Most GA aircraft are not equipped with Radio Altimeters, and AeroMACS has not yet been 

deployed operationally in any user segment. 

The largest LRU-specific improvement is the VHF BBSDR, at 45% of current architecture. 

Although the future VHF LRU does provide all functions of dual Nav/Com installations, 

however, it should be noted that the cost reduction is largely due to its single hardware LRU 

configuration, whereas the current architecture includes dual LRUs. It should also be noted 

that the VHF BBSDR also provides dual VDL Mode 2 and/or VDL-Next capability, along with 

expandability to include other software-defined future VHF functions, which the current 

architecture does not. 

The L-Band BBSDR shows a cost of 76% of the current architecture, even though there is no 

equivalent to several future L-Band functions in the current architecture. It is worthy of note, 

for example, that fully compliant ADS-B capability is not included in the current architecture, as 

most GA aircraft have yet to be equipped. An intermediate assumption was made instead, 

including a Mode S transponder. Also, as previously discussed, the current GA architecture 

does not include DME, another L-Band function. Nonetheless, the L-Band BBSDR is able to 

address multiple current functions including GNSS, Transponder (or UAT, which is also L-Band), 

and SDARS, along with providing backup functions to the single VHF LRU via AeroWAN. The 

necessity for dual redundant L-Band LRUs, due to the broad scope of its functions, offsets the 

artificially favorable cost score of the VHF BBSDR. 

As with air transport, definitive comparison of commercial cellular links is hampered by both 

future uncertainties and the lack of current equipage with avionics-type cellular links. In the 

current architecture, cellular communication is often accomplished using handheld personal 

devices using existing ground-based infrastructure, mainly at low altitudes. Some derivatives 

of corporate and airline-oriented cellular services are becoming available, but volumes are low, 

making cost estimates unreliable. It should be noted that the potential for adoption of 

commercial cellular links for conduct of flight related functions may be much greater for GA 

than for air transport. Future cellular systems may require smaller adaptations for airborne 

use at low altitude, and usage-measured commercial fees may be more palatable for 

occasional GA operations than for continuous, high-volume airline operations. Thus the 

NASA/CR—2015-218843 99
likelihood of aircraft-resident devices, connected via the DM to other flight deck systems, may 

be much greater for GA than air transport. For these reasons the research team felt it was 

important to define estimates for both current and future costs. A production volume of 

100,000 units, developed primarily using components and subsystems from consumer 

products with volumes in the millions of units, was used to compare current and future costs. 

A modest reduction of 59% resulted from that analysis. 

Overall, the future architecture GA business case is quite favorable compared to the current 

architecture, indicating over 20% reduction over current cost (future cost being 79% of current 

cost) – a favorable shift for a very cost-conscious market segment. In addition, the future 

architecture offers significant non-financial gains over the current system. There is also a 

significant gain in capabilities, functions, and redundancy gained for that decreased cost; 

examples include: 

 VHF datalink (redundant) – VDL M2 and/or VDL-Next 

 Dual all-GNSS navigation reception 

 Functional equivalent of 4 VHF Nav/Coms during normal conditions 

 Dual DME capability; ability to participate in DME/DME backup of GNSS 

 Dual ADS-B systems 

 Fully interoperable ADS-B system with all other user segments, no ground segment 

needed 

 AeroWAN capability (does not exist now) 

 AeroMACS capability (does not exist now) 

 Radio Altimeter – improved safety, potential for new precision approach options 

 All major functions software defined for upgradability and transitional compatibility 

8.4.5 Commercial Considerations for Cloud Communication 

The use of commercial services for aviation applications requires that both technical and 

commercial criteria be satisfied to achieve success. In some cases the commercial or business 

case considerations needed to induce a commercial (i.e., terrestrial services) operator to make 

the service available for aviation users may be more challenging than the technical ones. 

Commercial communication service providers are fundamentally in the business of turning 

bandwidth and infrastructure into revenue, and generally allocate those resources where the 

greatest revenue is expected, at the lowest risk. Aviation applications may not compare 

favorably to other market segments from a business case standpoint for several reasons, 

including: 

 Limited market size compared to “core” terrestrial markets

NASA/CR—2015-218843 100
 Perceived high liability of aviation applications for conduct-of-flight use 

 Cost of specialized infrastructure to access aviation markets 

 Relative Market Size 

On average, the number of aircraft that are likely to be served by any given cellular station is 

very small compared to the number of ground-based mobile phone users served by the same 

station. To accommodate the aviation users, some business incentive is needed to balance 

that disparity, and this is exacerbated by any specialized infrastructure needed to access those 

users. The most obvious incentive is rate structure, charging proportionately higher rates for 

the “minority” airborne users. While this may solve the problem, it also changes the incentive 

basis for aviation to use cellular services – the more expensive it is, the less attractive it may 

become relative to other link options. 

8.4.5.1 Aviation Liability Climate 

Aviation applications, particularly where any connection with conduct of flight is involved, are 

perceived to have a significantly higher potential legal liability risk than service to terrestrial 

mobile users, and this is unlikely to change substantially over the study period, without 

focused effort. There are electronic component manufacturers today who prohibit the use of 

components from use in avionics for this reason. Commercial service operators will need to 

either justify this higher risk profile from a business perspective, or find ways to mitigate the 

risk through legal, legislative, or other means. Limiting use to passenger services is an option, 

but does not serve the goal of having the service as a “cloud communication” link option from 

an overall aviation industry standpoint. 

8.4.5.2 Infrastructure Costs 

The technical steps required to adapt current cellular systems to aviation use include addition 

of antennas and associated equipment, and alterations to system software and algorithms. 

These infrastructure needs represent a capital investment by the service provider, who again 

needs to justify that investment with commensurate revenue; aviation user service fees may 

need to be adjusted accordingly. 

For high altitude use, such as by commercial aircraft, a relatively small number of ground 

stations would need to be modified. Amortizing a relatively small infrastructure investment 

against the number of airline passenger users could well provide a workable fee structure and 

business case. However, for GA use a much larger number of modified ground stations would 

be needed, due to the lower altitude operations. Unfortunately, this higher infrastructure 

investment would need to be amortized across a much smaller population of GA users than 

the airline case, resulting in a much less attractive business case. This is particularly 

unfortunate since GA aircraft have fewer alternatives than airline aircraft (e.g., Ku/Ka-Band 

NASA/CR—2015-218843 101
SATCOM) due to limitations on antenna size, equipment size/weight, and cost, and would 

arguably benefit more from having the cellular option. 

These business case considerations may be the deciding factor in determining which links are 

actually practical for future aviation use, and must be carefully weighed in the evaluation 

process. 

8.5 Business Case Gap Analysis 

Overall, the business case analysis established the financial viability of the future architectures 

in comparison to current practice. This was established as the primary criterion: if the industry 

has demonstrated the viability of the current business case through widespread adoption of 

the current architectures, and the future architectures are less expensive while delivering 

equivalent or greater capabilities, then the proposed future architecture has a high probability 

of business acceptance. In most cases the future architectures were found not only to be less 

expensive, but substantially less expensive. Previous research has established that the 

proposed future architectures analyzed herein are fully viable from various technical 

standpoints as well. In all cases, the future architectures also deliver not only equivalent or 

greater capabilities, but significantly greater capabilities. Thus it is reasonable to conclude that 

the proposed future solution set can be expected to be viable from a business perspective, and 

leaves no gaps or unmet needs within the construct of the defined future NAS functions. 

NASA/CR—2015-218843 102
9 Most Promising Technologies 

The convergence of the technical and business analysis efforts research allows the 

identification of the overall most promising technology alternatives for the ground-air and airair datalink needs of the future aviation system. The candidates best suited technically were 

identified in the Section 7, and were used as inputs to the business case analysis summarized 

in Section 8. The business case analysis validated the viability of those candidates from a 

business perspective, and does not identify any gaps, unmet needs, or unworkable solutions. 

Among those candidate solutions, the most promising have been identified based on several 

criteria: 

 Breadth of applicability; 

 Scope of impact on future system capabilities; 

 Cost / benefit contribution to the overall business case, and 

 Long-term growth and adaptation potential. 

Identifying and ranking the most promising of the technologies that make up the future 

architectures is complicated by the fact that some represent specific links, and others are 

enabling or underlying technologies that enable or facilitate multiple different links. The 

results are summarized in this section, and comprise a mix of both types. Each technology 

described in this section has been summarized in technical detail in previous sections. This 

section is focused on the reasons for their “most promising” selection and ranking.

Two enabling technologies top the list, based on their breadth of applicability and fundamental 

contributions to system capabilities, cost/benefit, and future growth and adaptability: BBSDR, 

and DM. Following those are a number of specific link types: AeroWAN/AeroMACS/Merged 

WiFi, ADS-B-Next, VDL-Next, and LEO Ku/Ka-Band SATCOM. 

9.1 BBSDR 

Broad Band Software Defined Radio (BBSDR) technology is a key enabling platform for early all 

of the proposed future link candidates. BBSDR enables a single LRU to host multiple software 

defined links simultaneously, establishing a new and much lower price/capability point for 

avionics. Its software-driven nature also greatly facilitates both initial transition from current 

to future architectures, and future growth and adaptation to added bandwidth, the advent of 

new and more efficient modulations and other link innovations, and protection from 

obsolescence. BBSDR technology is applicable to all frequency bands currently allocated to 

aviation, including VHF, L-Band, and the 5 GHz band used by AeroMACS and radio altimetry. It 

is also applicable to Ku/Ka-Band applications and various commercial cellular bands. 

NASA/CR—2015-218843 103
9.2 Delivery Manager 

The DM provides the cohesion between the various specific links and the ultimate goal of all 

communication systems: the effective, timely and secure transfer of information. The DM 

function addresses link selection and prioritization, based on link availability, information 

transfer needs, timeliness requirements, security and integrity considerations, cost 

considerations, Required Communication Performance (RCP), and many other factors. The DM 

function The DM function takes place both on the ground and onboard the aircraft, and has 

both distributed and centrally managed elements. The existence of an effective DM function 

allows the seamless, coordinated use of both aviation-specific and commercial links, allows 

access by a broad range of ground and air users while maintaining system security, and 

minimizes user costs by selecting the lowest cost suitable link for any given information 

transaction and any given time. 

9.3 AeroWAN, AeroMACS, and Merged WiFi 

AeroWAN is a new link proposed as part of this research effort, and is technically based on the 

existing AeroMACS concept and much of its underlying technology. AeroWAN is designed to 

function as an in-flight implementation of the AeroMACS concept, using L-Band aviation 

spectrum repurposed from DME and other previous aviation services. AeroMACS, conversely, 

is designed only for communication to and from aircraft when they are on the ground, and 

resides in repurposed 5 GHz aviation spectrum. Each in its own area of use offers the broadest 

and most flexible range of applications, highest bandwidth, and networking capabilities, and 

together serve as the “work horse” communication resource of the future system. 

If the AeroMACS and AeroWAN technologies, being so similar in underlying architecture and 

components, could somehow be merged into a single system that served both air and ground 

operations with a single system, the advantages of the resulting system would be even more 

compelling, This concept, called “Merged WiFi” was discussed in more detail in a previous 

report. 

9.4 ADS-B Next 

The re-architected ADS-B Next link offers significant advantages over the current ADS-B 

system. Advantages include higher initial bandwidth and improved performance, much more 

efficient bandwidth usage, significant expansion capability, and universal service to all aircraft 

without ground relays. Its applicability includes aircraft from large transports, to GA, to small 

UAS, and potential functions include air to air and air to ground communications for 

surveillance, collision avoidance, wake turbulence and other environmental data, and spacebased ADS-B. 

NASA/CR—2015-218843 104
9.5 VDL-Next 

VDL-Next is similar to ADS-B Next in that is based on a re-architecture of the existing aviation 

VHF spectrum to allow much greater efficiency, bandwidth, and range of applications than 

current VHF systems. VDL-Next is ranked below ADS-B Next primarily because its range of uses 

are primarily communication versus aircraft separation, and because those communication 

functions in the future could in many cases also be fulfilled by AeroWAN and AeroMACS. 

However, the propagation characteristics of VHF versus L-Band (or 5 GHz for AeroMACS) give it 

some specific advantages for some communication applications, particularly for GA. 

9.6 Commercial Cellular Data Systems 

Commercial Cellular links offer an impressive range of capabilities today to mobile users, and 

can only be expected to improve enormously over the 50 year research period. However, even 

with the DM to address security and other robustness considerations, the business case for 

commercial providers to make technical and commercial concessions to capture relatively 

limited numbers of aviation users is unclear at best. As discussed previously, however, cellular 

systems are already in limited use for air transport IFE applications, and offer a clearer path to 

even broader use in GA. Commercial cellular systems may also prove to be a vital link option 

with small, low altitude UAS aircraft – potentially the largest aviation user segment by far by 

2064, subject to the business case caveats discussed in Section 8. 

NASA/CR—2015-218843 105
10 Conclusions and Recommendations 

The research team has identified a combination aviation-specific and commercial links which 

can provide a practical, cost-effective communication solution for the aviation needs circa 

2063. Harnessed and organized by a Delivery Manager function, such a combination will allow 

robust, high speed connectivity in all phases of flight, at or above required levels of reliability, 

latency, and integrity, at lowest practicable cost. 

Practical opportunities exist to make much more efficient use of aviation RF spectrum using 

technology that will be readily available long before 2063. Transitions from current to these 

future solutions can be made practical and cost-effective, with compelling business cases to 

stimulate equipage during the transition period. New and legacy systems can be made fully 

interoperable throughout the transition phase. 

This section summarizes the research team’s conclusions in three groups: overall conclusions, 

specific recommendations for the industry to consider in preparing for the future, and 

recommendations for follow-on research to facilitate progress toward meeting the future 

industry datalink needs. 

10.1 Overall Conclusions 

This section summarizes eighteen conclusions reached by the research team during the course 

of the two-year research effort, which may offer insights into issues and considerations that 

affect the industry’s progression from the current state of aviation data link to meeting its 

future needs: 

1) Current communication systems are not adequate to meet the needs of the 50-year 

future NAS; 

2) Current systems make very inefficient use of scarce aviation spectrum in the context of 

current and future technical capabilities; 

3) Currently allocated aviation spectrum can serve nearly all future NAS CNS needs if used 

effectively; 

4) Communication, Navigation, and Surveillance are all interrelated in any study of future 

communication optimization; 

5) Viewing all RF-enabled systems as software functions on a BBSDR platform facilitates 

optimization, integration, simplification of avionics architectures, and reduced cost / 

size / weight / power footprints; 

6) Future onboard navigation capabilities can be an important facilitator to bandwidth 

optimization; 

NASA/CR—2015-218843 106
7) The proposed future architecture offers significant functional and cost advantages over 

current systems; 

8) The proposed future architecture offers a workable transition path from current to 

future configuration; 

9) BBSDR technology is a key enabler of re-architecting existing aviation spectrum; 

10) DM-enabled “cloud communications” architecture offers a compelling combination of 

performance, robustness, and cost; 

11) AeroWAN and AeroMACS could serve as the “workhorse” of future systems; 

o Aviation protected spectrum 

o High bandwidth 

o Broadcast and interactive 

o Ground – Air – Ground 

o Air – to – Air, including aircraft to aircraft message relay 

o Suitable for C, N and S functions 

12) A “Merged WiFi” system would offer even greater advantages than AeroMACS and 

AeroWAN Separately; 

13) Expanded use of commercial communication links is feasible for aviation, but will 

require a DM-type manager to realize significant use outside of passenger 

entertainment applications; 

14) The security issues associated with use of cellular links for aviation applications should 

not be underestimated; 

15) Cellular providers will need to realize sufficient revenue volume to justify system 

changes and bandwidth diversion from their primary customer base; 

16) Cellular links may be best suited to passenger communications, use as backup links, and 

other non-critical information applications; 

17) GA and small UAS operations could be good potential applications of cellular links; 

o Low altitude operations in particular 

o Widespread coverage 

o Usage-metered cost structure could be attractive to occasional users 

o Security & lost-link management will be necessary 

o Commercial provider cost/benefit considerations must be addressed 

18) UAS revenue volume could become the enabler for commercial service provider 

interest in pursuing aviation markets. 

NASA/CR—2015-218843 107
10.2 Recommendations 

The research team formulated six recommendations based on the research and its results, 

which could help guide the aviation industry toward a successful long-term communication 

strategy: 

I. An overall strategy for efficient use of aviation bandwidth should be developed, 

encompassing communications, navigation, and surveillance functions; 

II. Serious consideration should be given to a carefully planned, strategic realignment of 

RF spectrum currently allocated to aviation use for optimum benefit to the aviation 

industry over the long term. Objectives should include: 

o Efficiency of spectrum utilization; 

o Mmultipurpose usage of bandwidth and avionics where practicable; 

o Cost/benefit considerations for all aviation user segments, and 

o The preservation of aviation spectrum against efforts to reassign portions of it 

to non-aviation usage; 

III. Focused effort should be undertaken toward the development of a multi-purpose, 

ground-air and air-air aviation communications network, residing in aviation protected 

RF spectrum, analogous to the AeroWAN concept proposed in this research; 

IV. The aviation industry should explore the concept of implementing a privately funded, 

commercial technology based multipurpose communications system residing within 

unused aviation spectrum; 

V. Additional research should be conducted into options and development strategy for a 

“Merged WiFi” system combining the AeroMACS and AeroWAN concepts into a single 

system using a single LRU; 

VI. The aviation industry should explore ways of accomplishing future oversight and 

approvals of communication capabilities on a functional performance assurance basis, 

rather than a link- and hardware-specific basis as is often the case today. 

10.3 Follow-On Research Recommendations 

The scope of the research undertaken under this contract was necessarily limited, by program 

definition, and by available time and resources. A number of areas were identified in the 

course of the research that merit additional exploration. In particular, the following eight 

areas should be considered for follow-on research. 

NASA/CR—2015-218843 108
A. Propose, characterize and quantify benefits of specific methods and opportunities for 

greatly improved efficiency in the use of existing aviation spectrum; 

B. Develop a holistic approach and strategy for future CNS in terms of RF spectrum 

utilization, particularly current aviation spectrum; 

C. Develop a conceptual transition path from current to future RF systems architecture, 

allowing seamless implementation of new systems while maintaining full operational 

capability for existing systems over ample periods for industry equipage, infrastructure 

deployment, and other factors; 

D. Examine interim points between current and 50-year architectures, and develop 

“snapshot” scenarios illustrating what is achievable in the nearer term; 

E. Develop detailed definition, requirements, and performance models for a multi-link 

communications system management function similar to the “Delivery Manager” 

function cited in this research. Include bot air and ground systems, functional 

considerations including communication and security aspects, and the capability to 

utilize both aviation-specific and commercial communication links. Develop a concept 

design to enable the safe, effective, and secure use of cloud communications for 

aviation; 

F. Focused research is needed into the near- and long-term needs, and potential 

solutions, for communications to, from, and between UAS operating at low altitudes; 

G. Develop detailed definition, requirements, and performance models for a potential 

ground-air and air-air wireless utility network similar in concept to the AeroWAN 

technology proposed in this research; include potential merging of AeroWAN and 

AeroMACS functions into a single network; 

H. In-depth analysis of the requirements and options for the adaptation of terrestrial 

cellular communication systems to aviation applications, particularly GA and lowaltitude UAS, is needed; commercial provider business case considerations and 

potential motivators must be considered equally with technical aspects. 

10.4 Summary 

The combination of overall architectures and specific link solutions proposed by this research 

constitute a viable and effective solution to the future communication needs of the NAS in the 

target 50-year future. The research illustrates the significant potential service capabilities that 

can be attained by more effective use of currently allocated aviation spectrum, and the 

advantages of moving to a multi-purpose approach to link design and utilization in contrast to 

NASA/CR—2015-218843 109
the current paradigm. By combining optimized aviation links with commercial systems, 

carefully managed by a Delivery Manager function, aviation can leverage true cloud 

communications to meet its needs 50 years, and much farther, into the future. By leveraging 

BBSDR technology as a multi-link, software modifiable platform, tremendous capabilities can 

be provided to the future aircraft at a fraction of today’s cost.

The research team maintained a conservative approach throughout the effort, and believes 

that the concepts, capabilities, and costs proposed are not only feasible within the nominal 50-

year period, but are in fact possible in a shorter time, from a technical standpoint. The 

controlling factors to realizing what is technically practicable are primarily issues of planning, 

consensus, regulatory adaptation, and initiative. Looking back at the progress of aviation over 

the past 50 years suggests that the industry is certainly capable of achieving the degree of 

progress that would be required. It is the XCELAR research team’s hope that its presenting an 

example of what the future can be will help stimulate progress toward a reality that far 

eclipses the proposals in this report. 

NASA/CR—2015-218843 110
11 Appendices 

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NASA/CR—2015-218843 111
11.1 Future Datalink Technology Candidate Matrix 

Table 11-1: Airborne Aircraft, Inbound Communications 

1A	(1	of	2)

Function	

Code

Function Type

A/C type-

(U, G, A,

H)

Information Use Content

(examples)

Update

Rate Range

Ack/Nac

or

Broadcast

From:

Air (A),

Ground

(G)

Priority: Must (1),

HD (2),

Nice (3)

Origin

of data

Latency File

Size

Best Datalink Candidates and Merit Scores (1-10)

Other proximate

vehicle status /

Info

Other

aircraft

AAI-1

Location / State

Vector / Intentions G, A, U

Flight path

deconfliction,

collision avoidance,

wake vortex

avoidance

Present position,

velocity vector,

Flight ID, Aircraft

category

6/sec 80 miles B A 1 Other

aircraft <1 sec 112 b

9- ADS-B Next

9- AeroWAN

7- ADS-B

5- UAT

AAI-2

Other vehicle

information

G, A, U General information

Vehicle

type/size/major

characteristics

Every 30 sec 80 miles B A 2 Other

aircraft

5 sec 112 b

9- ADS-B Next

9- AeroWAN

7- ADS-B

5- UAT

AAI-3

Other vehicle

information

G, A, U

Vehicle information

necessary to predict

the intensity,

transport, and decay

of wake vorticies

Gross weight,

initial vortex

cirulation, flap

setting

Every 30 sec 80 miles B A 2 Other

aircraft

5 sec 112 b

9- ADS-B Next

9- AeroWAN

7- ADS-B

5- UAT

AAI-4

Atmospherics (e.g.

Turbulence)

G, A, U

Hazard avoidance,

wake vortex

avoidance, weather

forecasting

AMDAR reports

(terminal), wind

speed and

direction, Eddy

Dissipation Rate

(EDR),

temperature,

humidity level

Every 10 sec 20 miles B A 2 Other

aircraft

5 sec 2 Kb

9- ADS-B Next

8- UAT FIS-B with Revisions

8- AeroWAN

AAI-5

Atmospherics (e.g.

Turbulence)

G, A, U

Flight management

of ride quality,

hazard avoidance,

wake vortex

avoidance, weather

forecasting

AMDAR reports

(Enroute), wind

speed and

direction, Eddy

Dissipation Rate

(EDR),

temperature,

humidity level

Every 60 sec 80 miles B A 2 Other

aircraft

5 sec 2 Kb

9- ADS-B Next

8- UAT FIS-B with Revisions

8- AeroWAN

AAI-6

Atmospherics (e.g.

Turbulence)

G, A, U

Avoidance of

immediate weather

hazards (i. e. moderate or greater

turbulence, icing,

volcanic ash)

EDR, icing

information,

volcanic ash

concentration

As needed 80 miles B A 2 Other

aircraft

1 sec 112

bit

9- ADS-B Next

8- UAT FIS-B with Revisions

8- AeroWAN

AAI-7

Enroute G, A

Traffic flow management to

allow for flight and

systemic

optimization

Arrival slot times,

arrival capacity

versus demand,

flow constrained

areas

As needed Regional Ack G 1 ATM 2 min 10KB

9- AeroWAN

7- VDL-2 Next

5- UAT

AAI-8

Destination G, A Delay probability,

NOTAMS, ATIS

Every 5 min Regional B G 2 ATM 2 min

<10K

B

9- AeroWAN

7- SDARS

6- VDL-2 Next Broadcast

5- UAT FIS-B

NASA/CR—2015-218843 112
Table 11-2: Airborne Aircraft, Inbound Communications (Cont’d)

 

1A	(2	of	2)

Function	

Code

Function Type

A/C type-

(G, A, U)

Information Use

Content

(examples)

Update

Rate

Range

Ack/Nac

or

Broadcast

From:

Air (A),

Ground

(G)

Priority:

Must (1),

HD (2),

Nice (3)

Origin

of data

Latency

File

Size

Best Datalink Candidates and

Merit Scores (1-10)

Atmospherics

AAI-9

Atmospherics G

Flight crew

situational

awareness

information

regarding weather

along their flight

path

Graphical or 4-D

Matrix Wx for

manned aircraft

(pilot/crew use)

Every 60 sec 500 miles B G 2

Ground

nonATM2 min 50KB

9- AeroWAN

8- SDARS

8 - Cellular

AAI-10

Atmospherics A

Flight crew

situational

awareness

information

regarding weather

along their flight

path

Graphical or 4-D

Matrix Wx for

manned aircraft

(pilot/crew use)

Every 5 min

1,000

miles

B G 2

Ground

nonATM2 min

100K

B

9- AeroWAN

8- SDARS

6- Ka band broadcast

AAI-11

Atmospherics A

Textual Wx for flight

management use Wind, temp, EDR

Every 30

min

Global B G 2

Ground

nonATM2 min 2KB

9- AeroWAN

8- SDARS

6- Ka band broadcast 

6 - VDL-2 Next

AAI-12

Atmospherics G

Textual Wx for flight

management use Wind, temp, EDR

Every 30

min

500 miles B G 2

Ground

nonATM2 min 2KB

9- AeroWAN

8- SDARS

6- VDL-2 Next

AAI-13

Atmospherics U

Numerical Wx for

flight management

use

Wind, temp, EDR

Every 30

min

variable B G 3

Ground

nonATM2 min 20KB

9- AeroWAN

8- SDARS

7- VDL-2 Next 

6- Ka band broadcast

AAI-14

ATM guidance &

Relevant

Information

A, G, U

Separation &

optimization

Routing, RTA,

hazards

As required Regional Ack G 1 ATM 2 min

100K

B

9- AeroWAN

8- Iridium / Iridium Next

6- VDL-2 Next

6- Ka band ACK/NAC

6- Cellular

AAI-15

Non-ATM A

AOC (airline

operational control)

Gate assignment,

non-routine crew

scheduling, flight

planning, irregular

operations

As required Individual Ack G 3

NonATM

2 min 50KB

9- AeroWAN

8- Iridium / Iridium Next

6- VDL-2 Next

6- Ka band ACK/NAC

6- Cellular

AAI-16

Non-ATM A

AAC (airline admin.

Comm)

Connecting flights As required Individual 3

NonATM

2 min 50KB

9- AeroWAN

8- Iridium / Iridium Next

6- VDL-2 Next

6- Ka band ACK/NAC

6- Cellular

NASA/CR—2015-218843 113
Function	

Code

Function Type

A/C

type-

(G, A,

U)

Information Use

Content

(examples) Update Rate Range

Ack/nac

or

Broadcast

From:

Air (A)

Ground

(G)

Priority

Must (1),

HD (2),

Nice (3)

Destinat

ion

(V, ATM,

N-ATM)

Datalink Candidates and Merit

Scores

Vehicle Status

AAO-1

G, A, U

Flight path

deconfliction,

collision avoidance,

wake vortex

avoidance

Present position,

velocity vector,

Flight ID, Aircraft

category

6/sec

80

miles

B A 1 V, ATM

9- ADS-B Next air-to-air

7- ADS-B present implementation

7- UAT air-to-air

5- AeroWAN

AAO-2

G, A, U

General

information

Vehicle

type/size/major

characteristics

Every 30 sec

80

miles

B A 2 V, ATM

9- ADS-B Next air-to-air

7- ADS-B present implementation

7- UAT air-to-air

5- AeroWAN

AAO-3

G, A, U

Vehicle information

necessary to

predict the

intensity, transport,

and decay of wake

vorticies

Gross weight,

initial vortex

cirulation, flap

setting

Every 30 sec

80

miles

B A 2 V, ATM

9- ADS-B Next air-to-air

7- ADS-B present implementation

7- UAT air-to-air

5- AeroWAN

AAO-4

U

Flight path

deconfliction,

collision avoidance,

wake vortex

avoidance

Present position,

velocity vector,

Flight ID, Aircraft

category

1 / sec

10

miles

B A 1

V, ATM NATM

9- ADS-B Next air-to-air

7- ADS-B present implementation

7- UAT air-to-air

5- AeroWAN

AAO-5

G, A

Maintenance data /

System Status /

Departure from

normal

Engine data, other

vehicle health

data

As required,

Departure

from normal

(Tx

immediately

)

as

needed

A A 2 N-ATM

8- AeroWAN

8- Iridium / Iridium Next

7- Ka band ACK/NAC

6- Cellular

AAO-6

U

Maintenance data /

System Status /

Departure from

normal

Engine data, other

vehicle health

data

As required,

Departure

from normal

(Tx

immediately

)

as

needed

A A 1 N-ATM

8- AeroWAN

8- Iridium / Iridium Next

7- Ka band ACK/NAC

6- Cellular

AAO-7

Local

Environment

Atmospherics

G, A, U

Terminal Area

Atmospheric

reports for highgranularity model

input

Wind, Temp, EDR,

shear-layers

Every 15 sec

(Terminal)

as

needed

B A 3

V, ATM,

N-ATM

8- VDL-2 Next

8- AeroWAN

7- ADS-B Next

7- UAT downlink

AAO-8

Local

Environment

Atmospherics,

higher update

rate

G, A, U

Enroute tmospheric

reports

Wind, Temp, EDR,

shear-layers

Every 5 min

(Enroute)

as

needed

B A 3

V, ATM,

N-ATM

8- VDL-2 Next

8- AeroWAN

7- ADS-B Next

7- UAT downlink

AAO-9

Special Requests A Medical As needed

as

needed

A A 2

ATM, NATM

9- AeroWAN

8- Iridium Next

6- Cellular

Appendix	1B:	Future	Datalink	Matrix	-	Airborne	Aircraft,	Outbound	Communications

Table 11-3: Airborne Aircraft Outbound Communications

NASA/CR—2015-218843 114
Table 11-4: Aircraft on Ground, Inbound Communications 

1C (1 of 2)

Function	

Code

Function

Type

A/C

type

U,G,A

Information Use

Content

(examples)

Update Rate Range

Ack/nac

or

Broadcas

t

From:

Air (A)

Groun

d (G)

Priority

Must (1),

HD (2),

Nice (3)

Origin of

Data

DatalinkCandidates

AGI-1

Airspace System

Information

(From Ground)

G, A

General / noninstructional,

Situational

Awareness

NOTAMS, Traffic

Flow

Management

restrictions

As needed

/ variable

Local B G 1

ATM, NATM

9- AeroMACS

8- Cellular (4GLTE)

6- VDL-2 Next

5- SDARS

Atmospherics

ATM &

N-ATM

AGI-2

G

Graphical or 4-D

Matrix Wx for

manned aircraft

(pilot/crew use)

4-D Numerical

Matrix, NEXRAD,

Maps, Sat

imagery, etc.

Every 5

min

Local B G 1

N-ATM;

Pvt or

Gov

Source

9- AeroMACS

8- Cellular (4GLTE)

5- SDARS

AGI-3

A

Graphical or 4-D

Matrix Wx for

manned aircraft

(pilot/crew use)

Includes LLWAS,

TDWR, WSDM,

Etc.

Every 5

min

Local B G 1

N-ATM;

Pvt or

Gov

Source

9- AeroMACS

8- Cellular (4GLTE)

5- SDARS

AGI-4

A

Textual Wx for

flight

management use

Wind, temp, EDR

Every 15

min

Local B G 2

N-ATM;

Pvt or

Gov

Source

9- AeroMACS

8- Cellular (4GLTE)

5- SDARS

AGI-5

G

Textual Wx for

flight

management use

Wind, temp, EDR

Every 15

min

Local B G 2

N-ATM;

Pvt or

Gov

Source

9- AeroMACS

8- Cellular (4GLTE)

5- SDARS

AGI-6

G, A,

U

Numerical Wx for

flight

management use

Wind, temp, EDR

Every 15

min

Local B G 1

N-ATM;

Pvt or

Gov

Source

9- AeroMACS

8- Cellular (4GLTE)

5- SDARS

Appendix	1C:	Future	Datalink	Matrix	-	Aircraft	on	Ground,	Inbound	Communications

NASA/CR—2015-218843 115
Table 11-5: Aircraft On Ground, Inbound Communications (Cont’d)

1C	(2	of	2)

Function	

Code

Function

Type

A/C

type

U,G,A

Information Use

Content

(examples)

Update Rate Range

Ack/nac

or

Broadcas

t

From:

Air (A)

Groun

d (G)

Priority

Must (1),

HD (2),

Nice (3)

Origin of

Data

DatalinkCandidates

ATM guidance &

Relevant

Information

Separation &

optimization

ATM

AGI-7

Taxi

U, G,

A

Taxi separation

and optimization

Taxi instructions As needed Local Ack G 1

ATM,

UAS

Operat

or

9- AeroMACS w/ACK/NAC

8- VDL-2 Next 7-

VDL-2

5- Iridium

AGI-8

Departure

Information

U, G,

A

Departure

planning

Departure

instructions /

clearance, queue

assignment,

ground hold, deicing

As needed Local Ack G 1

ATM,

UAS

Operat

or

9- AeroMACS w/ACK/NAC

8- VDL-2 Next 7-

VDL-2

5- Iridium

AGI-9

Non-ATM A

AOC (airline

operational

control)

Gate assignment,

non-routine

crew scheduling,

flight planning,

irregular

operations, Deicing

As

required

Local Ack G 3 N-ATM

9- AeroMACS w/ACK/NAC

8- VDL-2 Next 7-

VDL-2

5- Iridium

AGI-10

A

AAC (airline

admin. Comm)

Connecting

flights

As

required

Local Ack G 3 N-ATM

9- AeroMACS w/ACK/NAC

8- VDL-2 Next 7-

VDL-2

5- Iridium

Other proximate

vehicle status /

Info

V

AGI-11

U, G,

A

Relative Distance

/ Path

convergence /

Intentions

Includes a/c,

ground vehicles

and equipment

1/sec Local B A 1

Other

vehicle

s, ATM

9- ADS-B Next

7- AeroMACS

5- ADS-B/Low Power

5- UAT

AGI-12

U, G,

A

Vehicle

type/size/major

characteristics

1/ sec Local B A 3

Other

vehicle

s, ATM

9- ADS-B Next

7- AeroMACS

5- ADS-B/Low Power

5- UAT

AGI-13

U, G,

A

Pre-takeoff Wake Vortex

Every 30

sec

Local B A 2

Other

vehicle

s, ATM

9- AeroMACS

8- ADS-B Next

8- VDL-2 Broadcast

7- Cellular

AGI-14

U, G,

A

Hazard avoid

AMDAR reports,

(terminal)

Every 30

sec

Local B A 2

Other

vehicle

s, ATM

9- AeroMACS

8- ADS-B Next

8- VDL-2 Broadcast

7- Cellular

NASA/CR—2015-218843 116
Table 11-6: Aircraft on Ground, Outbound Communications 

Function	

Code

Function

Type

A/C

type

Information Use

Content

(examples)

Update Rate Range

Ack/nac

or

Broadcas

t

From:

Air (A)

Groun

d (G)

Priority

Must (1),

HD (2),

Nice (3)

Destination

(V, ATM,

N-ATM)

Datalink Candidates and Merit

Scores

Vehicle Status

AGO-1

G, A

Location / State

vector / Aero /

Intentions

Non-UAS 1 / sec Local B A 1 V, ATM

9- ADS-B Next

7- AeroMACS

5- ADS-B/Lo Power

5- UAT

AGO-2

U

Location / State

vector / Aero /

Intentions

UAS 1 / sec Local B A 1

V, ATM?,

N-ATM

9- ADS-B Next

7- AeroMACS

5- ADS-B/Lo Power

5- UAT

AGO-3

G, A

Maintenance data

/ System Status /

Departure from

normal

Engine data, other

vehicle health data

As required,

Departure

from normal

(Tx

immediately

)

Local A A 2 N-ATM

9- AeroMACS

8- Cellular w/ ACK/NAC proto

7- VDL-2 Next

5- VDL-2

AGO-4

U

Maintenance data

/ System Status /

Departure from

normal

Engine data, other

vehicle health data

As required,

Departure

from normal

(Tx

immediately

)

Local A A 1 N-ATM

9- AeroMACS

8- Cellular w/ ACK/NAC proto

7- VDL-2 Next

5- VDL-2

AGO-5

A Wake Turbulence

Information

Initial circulation

strength

Every 10 sec

(Terminal)

Local B A 2 V, ATM

9- AeroMACS

8- VDL-2 Next

8- Cellular

6- VDL-2

Every 5 min

(Enroute)

AGO-6

Local

Environment

Atmospherics

U, G,

A

Wind, Temp, EDR

Every 15 sec

(Terminal)

Local B A 3

V, ATM,

N-ATM

9- AeroMACS

8- VDL-2 Next

8- Cellular

6- VDL-2

AGO-7

Special

Requests

A Medical As needed Local A A 1

ATM, NATM

9- AeroMACS

8- Cellular w/ ACK/NAC proto

6- VDL-2 Next

Appendix	1D:	Future	Datalink	Matrix	-	Aircraft	on	Ground,	Outbound	Communications

NASA/CR—2015-218843 117
11.2 Appendix 2 - Literature Review Bibliography and Notes 

Table 11-7: Literature Review Bibliography and Notes 

Document Pub date Notes Location

Lead Reviewer

NASA/CR–2012-

217590

Surveillance and 

Datalink 

Communication 

Performance 

Analysis for 

Distributed 

Separation 

Assurance System 

Architectures

July, 2012 Investigates effects of two 

technical enablers: ADS-B 

and digital datalink 

communication in the FAA 

NextGen. Report is done 

under both ground based 

and airborne System 

Architectures (SA).

Datalink performance is 

examined under various 

operational conditions. 

Required SA performance 

is evaluated as a function 

of subsystem performance 

to establish overall 

required separation 

assurance performance, 

under normal and offnominal conditions.

65 pages as a PDF file

http://ntrs.nasa.gov/arc

hive/nasa/casi.ntrs.nasa

.gov/20120013095_201

2013878.pdf

Abstract and document 

info

http://ntrs.nasa.gov/sea

rch.jsp?R=20120013095

Jarrett

OPTIMIZING 

AIRPORT SURFACE 

OPERATIONS 

USING DATALINK 

AND THE

TAXIWAY 

NAVIGATION AND 

SITUATION 

AWARENESS (TNASA) DISPLAY 

SUITE

Not 

provided in 

document

Report on T-NASA s 

efficiency and safety 

benefits for surface 

operations with potential 

for taxi efficiency 

improvement by 

implementing changes to 

current procedures that 

include airborne taxi 

clearances and datalink 

communications.

Single page PDF

http://humanfactors.arc.nasa.gov/gro

ups/HCSL/publications/

Hooey_hfesposter_00.p

df

Jarrett

NASA/CR—2015-218843 118
Document Pub date Notes Location

Lead Reviewer

Taxiway 

Navigation and 

Situation 

Awareness (TNASA) Display 

Suite

Not 

provided in 

document

List of additional 

documents and reports 

generated by the T-NASA 

Display Suite

T-NASA Web site

http://humanfactors.arc.nasa.gov/ihi/

hcsl.inactive/TNASA_studies.html

http://humanfactors.arc.nasa.gov/ihi/

hcsl.inactive/publication

s.html#TNASA

Jarrett

AIAA-2012-5618 

Simulation 

Evaluation of 

Conflict Resolution 

and Weather

September 

2012

Pilot- and controller-in-theloop simulation of conflict 

resolution using near term 

equipped systems

12 page PDF

http://www.aviationsyst

emsdivision.arc.nasa.go

v/publications/2012/AIA

A-2012-5618.pdf

Jarrett

Flight Deck 

Procedural 

Guidelines for 

Datalink 

Trajectory

Negotiation

Based on 

content 

post 2007

4-D trajectory information 

exchange for conflict 

resolution using datalink

19 page PDF

http://www.aviationsyst

emsdivision.arc.nasa.go

v/publications/modeling

/ATIO_Mueller_Lozito_f

inal.pdf

Jarrett

CONVEYING 

MESSAGE 

CRITICALITY VIA 

DATALINK

Appears 

around 

2003 

Establishing notification 

priority from a Psychology 

perspective

6 page PDF

http://humansystems.ar

c.nasa.gov/groups/hcsl/

publications/Andre_AvP

syc03.pdf

Jarrett

Controller-pilot 

communications 

using a VDL Mode 

Tests 

conducted 

in 2000 at 

NASA Runway Incursion 

Prevention System (RIPS) 

test report using 

PDF available to 

members of IEEE at 

http://ieeexplore.ieee.o

Jarrett

NASA/CR—2015-218843 119
Document Pub date Notes Location

Lead Reviewer

2 datalink for the 

NASA runway

incursion 

prevention system

the Dallas 

Fort-Worth

Internation

al Airport 

(DFW)

Controller-Pilot Datalink 

Communications

(CPDLC)

rg/xpl/freeabs_all.jsp?re

load=true&arnumber=9

63335 for $10.00 

($31.00 for non 

members)

Has NOT been 

downloaded yet

AOS SPACE DATA

LINK PROTOCOL

September 

2005

Draft Recommendation for

Space Data System 

Standards. NOT a final 

document but a 

recommendation for 

standards

http://standards.gsfc.na

sa.gov/reviews/450-

502/450-502.pdf

Jarrett

AN OPERATIONAL 

CONCEPT FOR 

FLYING FMS 

TRAJECTORIES IN 

CENTER AND 

TRACON

AIRSPACE

Post 2000 Near and far term 

operational concepts for 

how an ATM automation 

system like CTAS

could work more 

effectively with the 

airborne automation in 

FMS equipped aircraft.

7 page PDF

http://humanfactors.arc

.nasa.gov/ihi/research_

groups/air-groundintegration/publication_

papers/Pa1999-

CTASFMStraj.pdf

Jarrett

THE APPLICATION 

OF SATELLITE 

COMMUNICATION

S TO THE DATA 

LINK

REQUIREMENT 

FOR UNMANNED 

GROUND 

VEHICLES

Undated, 

appears to 

be late 90s

Application of Datalink 

between satellites and 

unmanned GROUND 

vehicles

6 page PDF 

http://trsnew.jpl.nasa.gov/dspac

e/bitstream/2014/3033

9/1/95-0928.pdf

Jarrett

NASA/CR—2015-218843 120
Document Pub date Notes Location

Lead Reviewer

Is ACARS and 

FANS-1A Just 

Another Data Link 

to the

Controller?

2011 0r 

2012

Report on investigation of 

TBO procedure issues for 

current aircraft fleet when 

requesting deviations 

around weather.

10 page PDF excerpt 

from 453 page 

document

http://pdars.arc.nasa.go

v/publications/acars_fa

ns.pdf

Jarrett

RTCA DO-242, 

MASPS for 

Automatic 

Dependent 

Surveillance -

Broadcast (ADS-B)

Feb. 19, 

1998

See appendix E on other 

applications, also 

appendices D and J may be 

useful (from RTCA SC-186)

No longer in print or 

distribution from RTCA

Stone

RTCA Task Force 3 

final report

Oct. 1995 Free flight rationale RTCA Stone

RTCA Free Flight 

Action Plan

Aug. 15, 

1996

Talks about data link 

requirements, architecture 

decisions

RTCA Stone

Study of 

Alternative 

Beacon-based 

Surveillance and 

Data Link Systems, 

S.R. Jones

April 1974 FAA Report EM-74-7,11 MITRE document Stone

RTCA DO-242A, 

MASPS for 

Automatic 

Dependent 

Surveillance -

Broadcast (ADS-B)

June 25, 

2002

Revised ADS-B 

Requirements (from RTCA 

SC-186)

RTCA Stone

RTCA DO-263, 

Application of 

Airborne Conflict 

Dec. 2000 Conops for ADS-B conflict 

management and 

resolution (from RTCA SCRTCA Stone

NASA/CR—2015-218843 121
Document Pub date Notes Location

Lead Reviewer

Management: 

Detection, 

Prevention, & 

Resolution

186)

RTCA DO-317A, 

Minimum 

Operational 

Performance 

Standards (MOPS) 

for Aircraft 

Surveillance 

Applications (ASA) 

Systems

Dec. 13, 

2011

Detailed requirements for 

Interval Management 

(from RTCA SC-186)

RTCA Stone

RTCA DO-328, 

Safety, 

Performance, and

Interoperability 

Requirements for 

Airborne Spacing, 

Flight Deck 

Interval 

Management

June 22, 

2011

Detailed requirements for 

Interval Management 

(from RTCA SC-186)

RTCA Stone

Technical Link 

Assessment 

Report

March 

2001

Comparison of 1090 MHz 

Extended Squitter, UAT, 

and VDL Mode 4

FAA document Stone

RTCA DO-305A, 

Future Air 

Navigation System 

1/A (FANS 1/A) -

Aeronautical 

Telecommunicatio

ns Network (ATN) 

Interoperability 

Standard

March 

2012

ATC data link 

interoperability 

requirements (from RTCA 

SC-214)

RTCA Stone

RTCA DO-306, March FANS data link RTCA Stone

NASA/CR—2015-218843 122
Document Pub date Notes Location

Lead Reviewer

Change 1, Safety 

and Performance 

Standard for Air 

Traffic Data Link 

Services in Oceanic 

and Remote 

Airspace (Oceanic 

SPR Standard)

2012 requirements (from RTCA 

SC-214)

RTCA DO-308, 

Operational 

Services and 

Environment 

Definition (OSED) 

for Aeronautical 

Information 

Services (AIS) and 

Meteorological 

(MET) Data Link 

Services

Dec. 2007 Conops for 

weather/NOTAM data link 

services (from RTCA SC206)

RTCA Stone

RTCA DO-339, 

Aircraft Derived 

Meteorological 

Data via Data Link 

for Wake Vortex, 

Air Traffic 

Management and 

Weather 

Applications –

Operational 

Services and 

Environmental 

Definition (OSED)

June 2012 Conops for weather 

downlink from aircraft. 

(from RTCA SC-206)

RTCA Stone

RTCA DO-340, 

Concept of Use for 

Aeronautical 

Information 

Services (AIS) and 

Sept. 2012 Use cases for uplinked 

weather

RTCA Stone

NASA/CR—2015-218843 123
Document Pub date Notes Location

Lead Reviewer

Meteorological 

(MET) Data Link 

Services

RTCA DO-xxx 

(draft), AIS and 

MET Services 

Delivery 

Architecture 

Recommendations

Draft Draft architecture 

recommendations for 

weather/NOTAM data link 

communications

Draft RTCA document 

from SC-206

Stone

NextGen 

Implementation 

Plan

March 

2012

High level overview of 

NextGen plans and 

objectives

FAA document Stone

European ATM

Master Plan, 

edition 2

October 

2012

High level overview of 

SESAR plans and objectives

Eurocontrol document Stone

Minimum 

Performance 

Standards -

Airborne Selective 

Calling Equipment

February 

1959

Example of an early data 

communications standard, 

valuable for looking at 

what the state of data link 

standards were 50 years 

ago

RTCA document Stone

Measurements of 

ADS-B squitter 

Performance in 

the LA basin 

region

2000 Authors: J. Bernays, 

S.Thompson & W.H. 

Harmon, MIT Lincoln Lab

www.11.mit.edu/missio

n/aviation/publications/

publication-files/m..

Haendel

Air Traffic Bulletin 2005 VP. Systems Operations 

Service, FAA

www.faa.gov/air_traffic

/publications/bulletins/

media/atb_aug_05

Haendel

Technical Link 

Assessment 

Report

2001 RTCA Safe flight 21 Haendel

NASA/CR—2015-218843 124
Document Pub date Notes Location

Lead Reviewer

FCC frequency 

Spectrum Table 

May 2012 List of Frequency 

allocations including those 

for Aero Mobile

Haendel

ADS-B airborne 

system 

measurements

2002 Tests in Frankfort ADS-B, 

W. H. Harmon

http://www.journalogy.

net/Publication/502855

44/ads-b-airbornemeasurements-infrankfurt

Haendel

Data Fusion 

Algorithms based 

on ADS-B and 

Radar

2000 IEEE International Radar 

Conference, Madrid Spain

http://doc.mbalib.com/

view/12163729dc2baf0

5f11419e093c32e06.ht

ml

Haendel

ADS-B “out” 

performance

May 2010 Docket No. 

FAA–2007–29305; Amdt. 

No. 91-314] 

http://www.eaa.org/Ne

ws/2010/ADS-BFinalRule.pdf

Haendel

Third Meeting of 

ADS-B Study and 

implementation

25 March 

2005

ICAO 

ADS-B TF/3

http://legacy.icao.int/ic

ao/en/ro/apac/2005/AD

SB_ADSB_TF3/ip15.pdf

Haendel

ADS-B Technical 

Issues

2011 W. A. Thedford

PhD, USAF

Tradeoff Analyses, 

different systems

http://www.afceabosto

n.com/documents/even

ts/cnsatm2011/Briefs/0

2-Tuesday/TuesdayPM%20Track-1/03-

Thedford-ADSB%20In_Out%20Tech%2

0IssuesTuesday%20Track1.pdf

Haendel

ADS-B 1090 MOPS Jan 2003 RTCA SC-186 http://adsb.tc.faa.gov/

WG3_Meetings/Meetin

g17/1090-WP-17-03.pdf

Haendel

Worlds Busiest Feb 2013 http://en.wikipedia.org/

wiki/World%27s_busiest

Haendel

NASA/CR—2015-218843 125
Document Pub date Notes Location

Lead Reviewer

Airports _airports_by_aircraft_m

ovements

LAX aircraft 

Movements

2012 19 year summary http://www.lawa.org/w

elcome_LAX.aspx?id=80

6

Haendel

LAX Departures 2013 Scheduled departure

Times

http://www.airportla.com/lax/departures?t

=1

Haendel

VDL Mode 4 2011 http://www.eurocontrol

.int/services/vhf-digitalmode-4

Haendel

VDL/4 ADS-B 1999 Overview by AAT Ltd http://www.aatl.net/pu

blications/implementing

ADS-B.htm

Haendel

VDL/4 

presentation

2000 J. Nilsson Swedish Aviation 

Authority

http://staffwww.itn.liu.s

e/~annli/JN-art2.pdf

Haendel

VDL Mode 4 

Concerns

2000 R. Jones, B. Phillips AMCP 

WG-C

legacy.icao.int/anb/pan

els/acp/WG/M/M1wp/

WP/M1-WP25.doc

Haendel

NASA/CR—2015-218843 126
11.3 Appendix 3 – Air Transport – Domestic Business Case Analysis 

Table 11-8: Air Transport – Domestic Business Case Analysis 

Future 

Architecture 

Components

Future Architecture 

Includes

Notes

Future 

Architecture 

LRU Cost

Future 

Architecture - 

400 A/C Fleet 

Cost

Future 

Architecture 

Fleet Spares 

Cost/Discount

Current 

Architecture 

Components

Current 

Architecture 

LRU List Price

Current 

Architecture - 

400 A/C Fleet 

Cost

Current 

Architecture 

Spares

VHF BBSDR / 

DM (Primary)

VHF / DM LRU; Dual 

SDR, Dual TX (50W), 

All VHF voice and VDL, 

LAAS

DM integrated into VHF 

BBSDR LRU; will perform all 

current CMU and DMU 

functions

$10,000 $2,800,000 $140,000 CMU

$ 72,000 $ 20,160,000 $ 2,016,000

$ 0 $ 0 $ 0

DMU 

(Redundancy 

Included) $ 85,000 $ 23,800,000 $ 2,380,000

$ 0 $ 0 $ 0 VHF Comm 1 $ 40,000 $ 11,200,000 $ 373,333

A single VHF BBSDR will 

provide dual VHF voice LRU 

capabilities; Backup VHF 

BBSDR provides equivalent 

of Comm 3 and a 4th VHF 

LRU not in current 

architecture

$ 0 $ 0 $ 0 VHF Comm 2

$ 40,000 $ 11,200,000 $ 373,333

$ 0 $ 0 $ 0 VDL-2 #1 $ 40,000 $ 11,200,000 $ 560,000

$ 0 $ 0 $ 0 VOR / ILS / GS - 1 36,000 $ $ 10,080,000 $ 504,000

VHF BBSDR / 

DM (Backup)

VHF / DM LRU; Dual 

SDR, Dual TX (50W), 

All VHF voice and VDL, 

LAAS

VHF functions will be 

important enough to require 

AT redundancy

$10,000 $2,800,000 $140,000 VDL-2 # 2

$ 40,000 $ 11,200,000 $ 560,000

$ 0 $ 0 $ 0 VOR / ILS / GS 2 36,000 $ $ 10,080,000 $ 504,000

$ 0 $ 0 $ 0 VHF Comm 3 $ 40,000 $ 11,200,000 $ 373,333

L-Band BBSDR

L-BBSDR; Quad SDR, 

Dual TX (300W), All LBand - ADS-B-NEXT, 

AeroWAN, GNSS, DME, 

SDARS, Legacy TCAS

Note that if a third backup 

DM is required it will be 

embedded into L-Band BBSDR

$60,000 $16,800,000 $560,000 GPS / All GNSS

$ 65,000 $ 18,200,000 $ 910,000

$ 0 $ 0 DME 1 $ 55,000 $ 15,400,000 $ 770,000

$ 0 $ 0 DME 2 $ 55,000 $ 15,400,000 $ 770,000

$ 0 $ 0 SDARS $ -

$ 0 $ 0 Transponder 1 66,000 $ $ 18,480,000 $ 924,000

TCAS Function 

embedded in L-Band 

BBSDR ADS-B Next

$ 0 $ 0 TCAS

$ 100,000 $ 28,000,000 $ 2,800,000

L-Band BBSDR - 

Backup L-Band BBSDR #2 $60,000 $16,800,000 $560,000 GPS 2 / MMR 2

$ 65,000 $ 18,200,000 $ 910,000

L-B BBSDR # 3 L-Band BBSDR #3

3 L-Band BBSDRs required 

for AT due to broad range of 

important functions 

included

$60,000 $16,800,000 $560,000 Transponder 2

$ 66,000 $ 18,480,000 $ 924,000

AeroMACS 5 

GHz BBSDR

AeroMACS / Radio 

Altimeter 5 GHz BBSDR

AeroMACS + Radio Altimeter; 

AeroMACS is software 

defined to facilitate future 

upgrades, added spectrum, 

etc.

$10,000 $2,800,000 $140,000

$ -

AeroMACS 

BBSDR - 

Backup

AeroMACS / Radio 

Altimeter 5 GHz BBSDR 

(Backup)

Importance of AeroMACS for 

AT operations requires 2 

LRUs

$10,000 $2,800,000 $140,000 Radio Altimeter

$ 80,000 $ 22,400,000 $ 1,120,000

Commercial 

Cellular 

Datalink

Cellular IFE System

For IFE service during nonoceanic flight segments $30,000 $8,400,000 $840,000 Cellular IFE 

System $ 40,000 $ 11,200,000 $ 1,120,000

Future Architecture Current Architecture

Air Transport – Domestic Business Case Analysis

NASA/CR—2015-218843 127
Table 11-9: Air Transport – Domestic Business Case Analysis (Cont.) 

Future Architecture Cost $73,080,000

Current Architecture Cost $303,772,000

Future / Current Cost: 24%

LRU Comparison: Current Future %

Total LRU Count 18 8 44%

Total LRU Types 11 4 36%

VHF Functions: $6,160,000 $ 135,408,000 5%

L-Band Functions: $53,760,000 $ 148,176,000 36%

5 GHz Functions: $6,160,000 $ 24,640,000 25%

Commercial Cellular Functions: $9,240,000 $ 12,320,000 75%

Air Transport - Domestic Business Case Analysis: 

Summary of Results

Architecture Cost Comparison

Comparison by Future LRU Type

NASA/CR—2015-218843 128
11.4 Appendix 4 – Air Transport – Oceanic Business Case Analysis 

Future 

Architecture 

Components

Future Architecture Includes Notes

Future 

Architecture 

LRU Cost

Future 

Architecture - 

300 A/C Fleet 

Cost

Future 

Architecture 

Fleet Spares 

Cost/Discount

Current Architecture 

Components

Current 

Architecture LRU 

List Price

Current Architecture 

- 300 A/C Fleet Cost

Current 

Architecture 

Spares

VHF BBSDR / DM 

(Primary)

VHF / DM LRU; Dual SDR, Dual 

TX (50W), All VHF voice and 

VDL, LAAS

DM integrated into vHF BBSDR 

LRU; will perform all current 

CMU and DMU functions

$10,000 $2,100,000 $105,000 CMU

$ 72,000 $ 15,120,000 $ 1,512,000

$ 0 $ 0 $ 0 DMU (Redundancy 

Included) $ 85,000 $ 17,850,000 $ 1,785,000

$ 0 $ 0 $ 0 VHF Comm 1 $ 40,000 $ 8,400,000 $ 280,000

A single VHF BBSDR will provide 

dual VHF voice LRU capabilities; 

Backup VHF BBSDR provides 

equivalent of Comm 3 and a 4th 

VHF LRU not in current 

architecture

$ 0 $ 0 $ 0 VHF Comm 2

$ 40,000 $ 8,400,000 $ 280,000

$ 0 $ 0 $ 0 VDL-2 #1 $ 40,000 $ 8,400,000 $ 420,000

$ 0 $ 0 $ 0 VOR / ILS / GS - 1 $ 36,000 $ 7,560,000 $ 378,000

VHF BBSDR / DM 

(Backup)

VHF / DM LRU; Dual SDR, Dual 

TX (50W), All VHF voice and 

VDL, LAAS

VHF functions will be important 

enough to require AT 

redundancy

$10,000 $2,100,000 $105,000 VDL-2 # 2

$ 40,000 $ 8,400,000 $ 420,000

$ 0 $ 0 $ 0 VOR / ILS / GS 2 $ 36,000 $ 7,560,000 $ 378,000

$ 0 $ 0 $ 0 VHF Comm 3 $ 40,000 $ 8,400,000 $ 280,000

L-Band BBSDR

L-BBSDR; Quad SDR, Dual TX 

(400W), All L-Band - ADS-BNEXT, AeroWAN, GNSS, DME, 

SDARS, Legacy TCAS

Narrow Body 2, Widebody 3; 

Note that if a third backup DM 

is required it will be embedded 

into L-Band BBSDR

$60,000 $12,600,000 $420,000 GPS / All GNSS

$ 65,000 $ 13,650,000 $ 682,500

$ 0 $ 0 DME 1 $ 55,000 $ 11,550,000 $ 577,500

$ 0 $ 0 DME 2 $ 55,000 $ 11,550,000 $ 577,500

$ 0 $ 0 SDARS $ -

$ 0 $ 0 Transponder 1 $ 66,000 $ 13,860,000 $ 693,000

TCAS Function embedded in LBand BBSDR ADS-B Next $ 0 $ 0 TCAS $ 100,000 $ 21,000,000 $ 2,100,000

Iridium-NEXT LEO L-Band (No 

LRU Required - integral to LBand BBSDR)

Polar & Oceanic data / vox 

comm for ATC & AOC

$ 0 $ 0 Iridium LEO L-Band 

Comm

$ 35,000 $ 7,350,000 $ 367,500

Space-Based ADS-B (NO LRU 

required) Oceanic surveillance $ 0 $ 0 HF Radio / SATCOM $ 100,000 $ 21,000,000 $ 1,050,000

L-Band BBSDR - 

Backup

L-Band BBSDR #2 $60,000 $12,600,000 $420,000 GPS 2 / MMR 2 $ 55,000 $ 11,550,000 $ 577,500

L-B BBSDR # 3 L-Band BBSDR #3

3 L-Band BBSDRs required for AT 

due to broad range of important 

functions included

$60,000 $12,600,000 $420,000 Transponder 2

$ 35,000 $ 7,350,000 $ 367,500

AeroMACS 5 GHz 

BBSDR

AeroMACS / Radio Altimeter 5 

GHz BBSDR

AeroMACS + Radio Altimeter; 

AeroMACS is software defined to 

facilitate future upgrades, 

added spectrum, etc.

$10,000 $2,100,000 $105,000 No Equivalent Link

$ - $ -

AeroMACS BBSDR 

- Backup

AeroMACS / Radio Altimeter 5 

GHz BBSDR (Backup)

Importance of AeroMACS for AT 

operations requires 2 LRUs $10,000 $2,100,000 $105,000 Radio Altimeter $ 80,000 $ 16,800,000 $ 840,000

Ku- or Ka-Band 

BBSDR

Simultaneous multiple 

channel capability; single 

beam-steered antenna or 

future LEO Ku/Ka satellite 

system

ATC & AOC Datacom, IFE; BBSDR 

allows multiple channel 

operations with single LRU, but 

may not address antenna 

steering issues

$20,000 $4,200,000 $420,000 Ku-Band or INMARSAT

$ 200,000 $ 42,000,000 $ 2,100,000

Commercial 

Cellular Datalink

Cellular IFE System $30,000 $6,300,000 $630,000 Cellular IFE System

$ 40,000 $ 8,400,000 $ 840,000

 Future Architecture Current Architecture

Air Transport - Oceanic Business Case Analysis

Table 11-10: Air Transport – Oceanic Business Case Analysis

NASA/CR—2015-218843 129
Table 11-11: Air Transport – Oceanic Business Case Analysis (Cont.) 

Future Architecture Cost $59,430,000

Current Architecture Cost

$292,656,000

Future / Current Cost: 20%

LRU Comparison: Current Future %

Total LRU Count 21 9 43%

Total LRU Types 14 5 36%

VHF Functions: $4,620,000 $101,556,000 5%

L-Band Functions: $40,320,000 $132,846,000 30%

5 GHz Functions: $4,620,000 $18,480,000 25%

Ku/Ka-Band Functions: $5,145,000 $ 47,040,000 11%

Commercial Cellular Functions: $6,930,000 $ 9,240,000 75%

Air Transport - Oceanic Business Case Analysis: 

Summary of Results

Architecture Cost Comparison

Comparison by Future LRU Type

NASA/CR—2015-218843 130
11.5 Appendix 5 – General Aviation Business Case Analysis 

Table 11-12: – General Aviation Business Case Analysis 

Future 

Architecture 

Components Future LRU Includes Notes

Future LRU 

COST

Current Architecture 

Components Notes

Current Architecture: 

Cost of LRU (Light 

Twin)

VHF BBSDR

VHF BBSDR: Dual SDR, 

Dual TX (10 W); all VHF, 

LAAS

Single LRU includes all functions of 

current dual VHF Nav/Com, and VDL $5,000 VHF Nav/Comm/VOR/ILS 1 No VDL capability today

$5,500

Only 1 GA VHF BBSDR required; 

performs current functions of 2 (or 

more) VHF Nav/Coms with single LRU, 

L-Band BBSDR serves as failure 

backup

VHF Nav/Comm/VOR/ILS 2

$5,500

L-Band BBSDR / DM 

#1

L-BBSDR; Dual SDR, Dual 

TX (100W), ADS-B NEXT, 

GNSS, AeroWAN, DME, 

SDARS

2 redundant LRUs required for GNSS, 

ADS-B, & VHF Backup; For GA, DM is 

built into the necessarily redundant 

LRU – the L-Band BBSDR.

$12,000 GPS GPS / All GNSS

$8,000

Expect DME becoming obsolete in 50 

years, but FAA GNSS backup plan is 

based on DME/DME, so softwaredefined legacy capability included

DME 

 (Note: although FAA GNSS backup 

plans include use of DME/DME 

navigation, few GA aircraft currently 

are DME equipped, and very few new 

OEM products are available, so no 

current cost is incuded)

Provider subscription control 

provisions in generic BBSDR – similar 

to SIM Card

SDARS XM/Sirius

$15,000

AeroWAN performs very wide range of 

functions and backups for others No Current Equivalent

Re-architected 1030/1090 is integral 

part of L-BBSDR Mode S Transponder Most GA A/C not fully ADS-B equipped today $3,500

UAT becomes obsolete with new 

1030/1090 ADS-B UAT Cost analysis used Mode S TXPDR

AeroWAN will be capable of 

providing digital audio backup to VDL 

(or primary) as needed

VHF Comm –(Backup) Typically no third comm backup 

today unless handheld

L-Band BBSDR / DM 

- #2 (Backup)

Identical to Unit #1 $12,000

GPS 2 / VOR-2 / DME/DME 

Substitute for sole means 

navigation

No current equivalent for GNSS sole 

means; estimated cost for 

compliance $5,000

ILS-2 Substitute (GNSS) No Current Equivalent

AeroMACS BBSDR

5GHz BBSDR, AeroMACS, 

Integral Radio Altimeter

AeroMACS includes ground-ground 

datalink; Radio Altimeter function 

provides vertical approach guidance 

for additional precision approach 

capability

$5,000 No Current Equivalent

Commercial 

Cellular Datalink

Too many variables about future 

spectrum allocations to speculate on 

integration into other BBSDR units – 

analyze as separate unit

$1,000 Handheld Cellular WX 

Service

$1,700

General Aviation Business Case Analysis

Future Architecture Current Architecture

NASA/CR—2015-218843 131
Table 11-13: – General Aviation Business Case Analysis (Cont.) 

Future Architecture Cost $35,000

Current Architecture Cost $44,200

Future / Current Cost: 79%

LRU Comparison: Current Future %

Total LRU Count 7 5 71%

Total LRU Types 6 4 67%

VHF Functions: $5,000 $11,000 45%

L-Band Functions: $24,000 $31,500 76%

5 GHz Functions: $5,000 $ - N/A

Commercial Cellular Functions: $1,000 $ 1,700 59%

Comparison by Future LRU Type

Architecture Cost Comparison

General Aviation Business Case Analysis: 

Summary of Results

NASA/CR—2015-218843 132
11.6 Appendix 6 – Candidate Comparison Detail 

11.6.1 Baseline Analysis Results 

The Baseline Analysis step employed a numerical analysis of the initial suitability ratings for 

each link, by function, for both the aggregate and average scores, using as input the ratings 

summarized in Appendix 2. Results are shown in Figs. 1 and 2 below. 

Figure 11-1: Aggregate Link Scores - Baseline 

The aggregate scores highlight those links that would be suitable for a wider range of 

functions, with higher scores due on part to having been scored for more functions. This is 

particularly true for AeroWAN and AeroMACS, whose mobile network capabilities are 

compatible with many different tasks. VDL-2 Next and Cellular also feature applicability to a 

variety of functions. The average scores (Fig. 2) show a different perspective, with systems 

optimized for a specific type of function, and only scored for those functions, ranking higher 

than their aggregate scores. ADS-B Next has the highest score, essentially because it is 

intended to do one very important type of job, and does it well. 

0.0 50.0 100.0 150.0 200.0 250.0

ADS-B Next

ADS-B/ Lo Pwr

AeroMACS

AeroWAN

UAT

VDL-2 Next

VDL-2

SDARS

Cellular

Iridium/Next

Ka Bcast

Ka Inter

Aggregate Link Score - Baseline

Total Link Score

NASA/CR—2015-218843 133
Figure 11-2: Average Link Scores - Baseline 

11.6.2 Function Priority Weighted Analysis Results 

Each future NAS function was identified early in this research effort, and its datalink-related 

characteristics defined. One parameter defined for each function is its relative priority, 

defined in terms of “Must Have, Highly Desirable, or Nice-to-Have”. These priorities were used 

in the next analysis step, to establish weighting factors for each candidate link based on the 

priority of each function that it serves. A 5-point scale was used, with weightings of 1, 3 and 5 

respectively for each increasing level of priority. This helps to differentiate those candidates 

which potentially contribute the most to the conduct of flight. Fig. 3 shows the aggregated 

scores, which also illustrate broadest range of functions served, and shows that AeroMACS and 

AeroWAN not only serve a range of functions, but of high priority functions as well. 

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

ADS-B Next

ADS-B/ Lo Pwr

AeroMACS

AeroWAN

UAT

VDL-2 Next

VDL-2

SDARS

Cellular

Iridium/Next

Ka Bcast

Ka Inter

Average Link Score - Baseline

Average Link Score

NASA/CR—2015-218843 134
Figure 11-3: Function Priority Weighted Aggregate Scores 

0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0

ADS-B Next

ADS-B/ Lo Pwr

AeroMACS

AeroWAN

UAT

VDL-2 Next

VDL-2

SDARS

Cellular

Iridium/Next

Ka Bcast

Ka Inter

Function Priority Weighted Aggregate Score

Total Link Score

NASA/CR—2015-218843 135
Figure 11-4: Function Priority Weighted Average Scores 

Fig. 4 considers the function priority in the context only of each candidate’s level of service to 

those functions it enables. Results are similar to the Baseline average in that ADS-B Next still 

scores well, as a system focused on a specific and important job. AeroMACS also scores well, 

in part because of its potential application as an alternative to ADS-B Next for ground 

operations. 

11.6.3 Weighted Scores with Obsolescence 

Adding the consideration of potential obsolescence to the preceding analysis steps required 

developing ratings for each candidate link’s susceptibility or resistance to obsolescence going 

forward from the 50-year future reference time point. It should be noted that this area of 

consideration has both technical and business case elements; this report is focused on the 

technical aspects of candidate comparison, with business case analysis to follow in the next 

phase of this research. Accordingly, the team approached the analysis of potential 

obsolescence as a transition point, with primary focus on technical considerations but 

including some preliminary assessment of related business factors as well. A number of factors 

were applied by the team to assign obsolescence ratings, using a scale of 1 (most susceptible) 

to 5 (least susceptible), including: 

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0

ADS-B Next

ADS-B/ Lo Pwr

AeroMACS

AeroWAN

UAT

VDL-2 Next

VDL-2

SDARS

Cellular

Iridium/Next

Ka Bcast

Ka Inter

Function Priority Weighted Average Score

Average Link Score

NASA/CR—2015-218843 136
 Adaptability to Evolving Technology 

 Adaptability to Future Functions 

 Bandwidth Expansion Capability 

 Acquisition and Operation Cost Trends 

 Potential Trends in Underlying Business Model (commercial candidates) 

 Uncertainty of Long-Term Stability (technical or business model) 

Ratings assigned were: 

ADS-B Next 5 

ADS-B 2 

AeroMACS 4 

AeroWAN 5 

UAT 2 

VDL-2 Next 4 

VDL-2 1 

SDARS 3 

Cellular 4 

Iridium 4 

Ka-Band Broadcast 5 

Ka-Band Interactive 5 

As discussed previously in Section 4.8, the application of SDR technology to the overall 

architecture of the future system enhances the resistance of all candidate links to potential 

obsolescence, but to varying degrees in some cases. Current links such as VDL-2 and ADS-B 

were evaluated without any benefit of SDR technology; the future versions proposed in this 

research (e.g., ADS-B Next, VDL-2 Next) are in part defined by that specific difference. 

Conversely, the AeroWAN concept is an entirely new link, with SDR and DM as integral 

elements of its architecture, and can be expected to derive maximum benefit from those 

technologies as a result. Other candidates fall in between. 

In general, the SDR architecture for any given avionics unit applies to a group of contiguous 

frequencies, such as the entire L-Band, but not to more disparate frequency ranges such as LBand (nominally 1 GHz) versus the 5 GHz range where AeroMACS currently resides. For this 

reason AeroMACS is rated slightly lower than AeroWAN due to its location in the former MLS 

NASA/CR—2015-218843 137
band, where there is a limited scope of other aviation services or link types within SDR range, 

whereas AeroWAN resides in the L-Band along with many other aviation-related services. Thus 

an L-Band SDR could potentially be adapted and used for many more future functions than an 

SDR designed for AeroMACS. 

It is anticipated that bandwidth demand will continue to escalate beyond the 50-year 

reference point, and links whose capability to adapt as needed in order to access additional 

bandwidth will be more susceptible to obsolescence. Current VDL-2 is limited by its available 

bandwidth and channel structure, and its modulation, and offers limited resistance to future 

obsolescence in its current form. With the implementation of SDR and DM technology in the 

future, VDL (or other VHF band avionics) will allow restructuring of many of those limiting 

factors, greatly increasing capabilities and allowing various future enhancements and 

functions. Since this is the underlying premise of the VDL-2 Next candidate proposed as part of 

this research versus current VDL-2, VDL-2 Next is scored much higher in its resistance to 

obsolescence. However, as a VHF system, with its growth presumably limited to the scope of 

the currently defined VHF aviation band, its potential for growth is more limited than some 

other candidates and was scored at less than maximum rating as a result. 

The same rationale applies to current ADS-B relative to the ADS-B Next candidate proposed as 

part of the current research. Current ADS-B’s inherent purpose-built hardware and singlechannel spectral structure place firm constraints on both its bandwidth expansion and 

adaptability to future architectures. In contrast, ADS-B Next has inherent SDR architecture and 

access to additional spectrum for future growth, and a straightforward path to implementing 

enhancements on a configuration-based or software-only basis using the same hardware. 

UAT has similar limits to those of current ADS-B, with single-purpose hardware and a single, 

firmly defined bandwidth for current and future needs. The research team opted not to 

propose a “UAT-Next” candidate; the proposed ADS-B Next system has ample bandwidth and 

expansion capacity to accommodate all potential ADS-B user segments, without the need for 

segregated solutions for different types of users. Merging the General Aviation and lowaltitude users into the overall ADS-B community increases interoperability, standardization, 

production volumes, and efficiency of bandwidth use, while simplifying system architecture 

and removing the need for an external infrastructure to relay ADS-B data between the 

different systems. As a result, UAT was scored lower than ADS-B Next in Obsolescence 

susceptibility. 

SDARS is a commercial system designed for a very different purpose, and capitalized by a very 

different core subscriber base, than its current aviation users. This “piggyback” model for 

serving aviation is more difficult to predict going forward, particularly over long periods of 

time. If the market dynamics of the much larger core use base change, niche users such as 

NASA/CR—2015-218843 138
aviation have little control over the future of the link, be it technically or financially. As a 

result, SDARS was assigned a lower rating than some candidates based in part on this 

uncertainty. In addition, its commercial architecture allocates limited bandwidth to special 

applications such as aviation. If the current commercial business model does remain stable, 

there is a likelihood that limited additional bandwidth will be allocated away from other 

competing user functions to aviation. In that event its ability to keep pace with anticipated 

increasing aviation bandwidth needs becomes a form of obsolescence in itself. 

In general, it is anticipated that cellular systems will continue to thrive and progress for the 

foreseeable future, both technically and commercially. What is less certain is the evolution of 

the commercial business model in relation to the relatively small aviation user base. The 

aviation base is expected to grow over time, but whether it will grow in relation to the groundbased revenue segments competing for bandwidth and capital for infrastructure adaptations is 

much less clear. As discussed previously, this report is focused on technical considerations, 

with business case aspects the subject of the next phase of research, and obsolescence is an 

area that relates to both areas. For the purpose of this report, cellular candidates were given a 

relatively high score on technical grounds, but reduced by one point based on the 

uncertainties of bandwidth access and other expansion and adaptation prospects. 

The obsolescence prospects of Iridium and similar future systems was based on the 

assumption the that planned Iridium/Next system will be fully implemented as planned over 

the next 10 years, including the planned space-based ADS-B capability. Although Iridium is 

fundamentally a commercial system being used in part by aviation applications, the advent of 

space-based ADS-B increases the likelihood of long-term access to bandwidth and adaptation 

by aviation users. Conversely, the prospects for allocation of significant additional spectrum to 

such services are questionable based on the current climate, which could limit this candidate’s 

long-term bandwidth growth access. For these reasons a relatively high rating was assigned, 

but not the highest possible rating. 

Both Ka-band systems (broadcast and interactive) were considered primarily in the context of 

services to larger aircraft in oceanic regions, where they offer the strongest benefits. Due to 

considerations of antenna size limitations on smaller aircraft, and weather induced signal 

attenuation at low altitudes, their obsolescence potential was based primarily on air transport 

applications. In this user segment Ka-band candidates were deemed to have strong long-term 

potential for availability, as well as potential bandwidth growth through both spectrum access 

and various technical enhancements. With these caveats, both were assigned high ratings. It 

should be noted that satellite systems will also benefit from the application of SDR technology, 

and those requiring steerable antennas can be expected to benefit less SDR than other 

candidates. 

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Fig. 5 shows the aggregate scores for all candidates with obsolescence taken into account. This 

may provide the clearest “investment case” perspective of which candidates merit the most 

investment in research and development to realize the maximum future benefit. It can be 

seen that the highest ranking candidates, in order, are AeroWAN, VDL-2 Next, and ADS-B Next. 

ADS-B Next and AeroMACS have nearly identical rankings; AeroMACS is already the subject of 

significant research, the merit of which is reinforced by this analysis. 

Figure 11-5: Aggregate Weighted Score with Obsolescence 

The average scores with obsolescence considered, shown below in Fig. 6, show a somewhat 

different order of the same four top candidates, with ADS-B Next scoring highest in is 

performance within its more limited areas of application, followed by AeroMACS, AeroWAN, 

and VDL-2 Next. Note that both cellular and interactive Ka-band also score well. 

ADS-B Next, AeroMACS, and interactive Ka-Band all score well in part because they are 

purposely designed to address specific needs in specific areas rather than broad-based links for 

many applications. AeroMACS only serves ground-based users in close proximity to airports, 

Ka-Band serves large aircraft at cruise altitudes in oceanic areas, and ADS-B Next is purposely 

0.0 500.0 1000.0 1500.0 2000.0 2500.0 3000.0 3500.0

ADS-B Next

ADS-B/ Lo Pwr

AeroMACS

AeroWAN

UAT

VDL-2 Next

VDL-2

SDARS

Cellular

Iridium/Next

Ka Bcast

Ka Inter

Aggregate Weighted Score with Obsolescence

Total Link Score

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limited to proximate aircraft awareness functions. Cellular candidates also score well for 

similar reasons, being primarily applicable to ground-based uses not unlike AeroMACS. 

Figure 11-6: Average Weighted Score with Obsolescence 

11.6.4 Merged AeroWAN and AeroMACS Rankings 

One special case was also analyzed by the team. AeroWAN and AeroMACS, as currently 

defined, are based on similar technology and operate in similar ways, but serve two disparate 

operational contexts using two relatively divergent frequency bands. As a result, each scores 

very well in its own area: AeroWAN in airborne functions only, and AeroMACS in ground 

functions only. If these two systems could be merged over the 50 year research period, a 

number of advantages would be gained, including commonality of both ground and airborne 

equipment, potentially one less avionics device needed, and increased coverage by using 

airport systems to also serve as AeroWAN ground stations and vice versa. 

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0

ADS-B Next

ADS-B/ Lo Pwr

AeroMACS

AeroWAN

UAT

VDL-2 Next

VDL-2

SDARS

Cellular

Iridium/Next

Ka Bcast

Ka Inter

Average Weighted Score with 

Obsolescence

Average Link Score

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The team opted to model a merged system in the AeroWAN L-Band spectrum primarily due to 

the broader applications of an L-Band SDR; however, more detailed technical analysis would be 

needed to formulate a considered recommendation. The aggregate scores under this scenario 

are shown in Fig. 7 below. It can be seen that the “Merged WiFi” system outscores all other 

candidates by a significant margin, due to its broad-based range of functions served coupled 

with its high bandwidth and other strong performance rankings. This suggests that, of all the 

candidates analyzed, such a merged AeroWAN/AeroMACS system would serve the most users 

and functions, across all user segments, of any single link studied. 

Figure 11-7: Aggregate Weighted Scoring Showing Merged AeroWAN / AeroMACS Score 

11.6.5 Aggregate versus Average Rankings 

The Gap Analysis input downselect step identified the top three or more candidates for each 

function; in each case there were at least 2 candidates with scores of 7 or higher. This 

indicates that suitable performance can be obtained for each function from two or more 

candidates. As a result, while the Average rankings provide an interesting look at optimum 

options, which may be useful in choosing between two qualified candidates, in general the 

team concluded that the Aggregate Rankings provide a more compelling measure of overall 

candidate merit, particularly in the context of defining development priorities going forward. 

0.0 1000.0 2000.0 3000.0 4000.0 5000.0 6000.0 7000.0

ADS-B Next

ADS-B/ Lo Pwr

AeroMACS

AeroWAN

UAT

VDL-2 Next

VDL-2

SDARS

Cellular

Iridium/Next

Ka Bcast

Ka Inter

Aggregate Weighted Score - Merged WiFi

Total Link Score

NASA/CR—2015-218843 142
A link candidate that provides suitable service for many functions may well support a better 

business case for development investment than one that provides somewhat better service for 

a small number of functions. Using the Aggregate Weighted Score with Obsolescence analysis 

results, as shown in Fig. 5, the highest-ranking candidates are AeroWAN, VDL-2 Next, ADS-B 

Next, and AeroMACS. 

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11.7 Appendix 7 – SDR and DM Conceptual Architecture 

Figure 11-8: Conceptual Architecture of Delivery Manager and BBSDR in Aircraft Systems 

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Figure 11-9: Conceptual Architecture of Delivery Manager and BBSDR in Aircraft Systems 

(Cont.) 

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11.8 Appendix 8 – Acronyms and Terms 

Table 11-14: Acronyms and Terms 

4G LTE Fourth Generation, Long-Term Evolution

AAC Airline Administrative Communications

AAI Aircraft Airborne, Inbound communications

AAO Aircraft Airborne, Outbound communications

ACARS Aircraft Communications Addressing and 

Reporting System

ACK/NAC Acknowledge / No Acknowledgement

A/D Analog to Digital

ADS-B Automatic Dependent Surveillance – Broadcast

AeroMACS Aeronautical Mobile Airport Communication System

AeroWAN Aeronautical Wide Area Network

AGATE Advanced General Aviation Transport Experiment

AGI Aircraft on Ground, Inbound communications

AGO Aircraft on Ground, Outbound communications

ALOHA Protocol: precursor to Ethernet

AM Amplitude Modulation

AMDAR Aircraft Meteorological DAta Relay

ANSP Air Navigation Service Provider

AOC Airline Operational Communications

ATC Air Traffic Control

ATIS Automatic Terminal Information Service

ATM Air Traffic Management

BBSDR Broad Band Software Defined Radio

CMU Communication Management Unit

CO-I Co-Investigator

CPDLC Controller Pilot Data Link Communications

DBA Doing Business As

DM Delivery Manager

DME Distance Measuring Equipment

DMU Data Management Unit

DSP Digital Signal Processor

FAMS Federal Air Marshall Service

FCC Federal Communication Commission

FEC Forward Error Correction

FIS-B Flight Information Services - Broadcast

FOQA Flight Operations Quality Assurance

GA General Aviation

GEO Geosynchronous Satellite

GHz GigaHertz

GLS GNSS Landing System

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GNSS Global Navigation Satellite System

GPS Global Positioning System, a U.S. operated GNSS

HPA High Power Amplifier

ICNS International Communication, Navigation, and Surveillance conference

IFE In Flight Entertainment

IFR Instrument Flight Rules

ILS Instrument Landing System

IP Internet Protocol

Ka-band 26.5–40 GHz

Km Kilometers

KOM Kick-Off Meeting

Ku-Band 12-18 GHz

LAAS Local Area Augmentation System

LEO Low Earth Orbit

LRU Line Replaceable Unit

MLS Microwave Landing System

MON Minimum Operating Network

MSK Minimum Shift Keying

MTBF Mean Time Between Failures

MTTR Mean Time To repair

NAS National Airspace System

NIA National Institute of Aerospace

NOTAM NOtice To AirMen

OPD Optimum Profile Descent

PI Principle Investigator

PTM Pair-wise Trajectory Management

PTT Push-To-Talk

QAM Quadrature Amplitude Modulation

QPSK Quadrature Phase Shift Keying

RA Radio Altimeter

RCP Required Communication Performance

RF Radio Frequency

RX Receiver

SATCOM Satellite Communication

SATS Small Aircraft Transportation System

SDARS Satellite Digital Audio Radio Systems

SDR Software Defined Radio

SME Subject Matter Expert

Squitter Transponder Message Transmission 

SSID Service Set IDentifier

SUAS, sUAS Small Unmanned Aircraft System

SWAT Special Weapons And Tactics team

TACAN TACtical Air Navigation

TCAS Traffic alert and Collision Avoidance System

TDMA Time Division Multiple Access

TOC Top Of Climb

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TSA Transportation Security Administration

TX Transmitter

UAS Unmanned Aerial System

UAT (ADS-B) Universal Access Transceiver

UPR User Preferred Routing

VDL- Mode 2 VHF Data Link – Mode 2

VFR Visual Flight Rules

VHF Very High Frequency

VLJ Very Light Jet

WAN Wide Area Network

Wi-Fi Wireless Network Technology

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