Spectrum Depletion Analysis

MP 03W0000068 MITRE PRODUCT White Paper: Spectrum Depletion Analysis

April 2003

Melvin J. Zeltser Frederick R. Morser Philip Long Frank Box

Sponsor: Federal Aviation Administration Contract No.: DTFA01-01-C-00001 Dept. No.: F066 Project No.: 02033102-AB

Center for Advanced Aviation System Development McLean, Virginia

Table of Contents

Section Page

1. Introduction 1 2. Circuit Demand 3 2.1 Historic Demand Data Sources 3 2.1.1 Historic Circuit Assignments (25-year Span) 4 2.1.2 Historic Circuit Assignments (5-year Span) 5 2.1.2.1 Assessment of Circuit Assignments (42-month Span) 5 2.2 Estimated Circuit Requirements to Support NAS Modernization Plans 8 3. Circuit Supply 11 3.1 Increasing Supply via 25 Initiatives 11 3.1.1 Expansion of ATS Frequency Resources 12 3.1.2 Frequency Pooling 15 3.1.3 Physical Mitigation Measures 15 3.1.4 Database and Tool Improvements 16 3.2 Initiatives Summary 16 3.3 Sensitivity of Supply Results to Assumptions 17 4. Managing Spectrum 19 5. Conclusions and Recommendations 21 List of References 23 Appendix A. Spectrum Prospector 25 Appendix B. Impact of Delaying the Transition to VDL3 29 Appendix C. Circuit Assignment Changes from 1998 to 2002 by Region and Altitude 33 Appendix D. Programmatic Circuit Demand Estimate Through 2020 37 Glossary 45

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List of Figures

Figure Page

1. ATS Historic Circuit Growth 4

2. FAA Circuit Changes by Flight Level 7

3. Other Circuit Changes by Flight Level 7

4. High Altitude Sector Concept Diagram 42

List of Tables

Table Page

1. Short-Term Circuit Growth 5

2. Programmatic Based Circuit Demand Estimate Through Year 2020 8

3. Annual Circuit Demand Forecast Summary 9

4. Overview of the 25 Initiatives for Extending 25AM System Life (Page 1 of 2) 13

Circuit 5and for Terminal Area Redesign Through 2015 41

AWOS Installation Forecast by Region 43

Executive Summary

The Very High Frequency (VHF) spectrum for air/ground (A/G) communications has become very crowded. It is becoming difficult for the Federal Aviation Administration (FAA) to find frequency assignments to meet the demand for additional air traffic control (ATC) services. The unit of measure for A/G voice supply and demand is a circuit.1 In September 2001, the Next-Generation Air/Ground Radio Communications System (NEXCOM) Aviation Rulemaking Committee (NARC) recommended that the FAA extend the life of the existing 25-kilohertz (kHz) amplitude-modulation (25AM) system. Recommendation 1 in the NARC report stated the FAA should “Continue to aggressively manage frequency assignments to prolong the useful life of the 25AM circuit allocation in support of Air Traffic Services.” The need to aggressively manage the spectrum is more pertinent than ever because of the economic factors the aviation industry is facing since the events of September 11, 2001. The economic impact of transition to new avionics will be relatively great for an industry that is struggling to remain economically viable. The objective of this paper is to support FAA A/G voice communication modernization efforts by determining, based on estimates of remaining circuit supply and future circuit demand, when the FAA and the aviation industry should plan to transition to a new radio to accommodate future additional ATC services. The estimated year when demand will exceed supply is sensitive to several factors including assumptions about: frequency use and circuit assignments (protected altitude, geographic location, etc.), demand data projections, and supply projections. The approach The MITRE Corporation’s Center for Advanced Aviation System Development (CAASD) used in this analysis was to develop a range of values for both ATC services (i.e., demand) and circuit supply that can be supported (i.e., circuits can be provided before the 25 AM system will start to experience service denials) based on an engineering analysis. Denials occur when new services are required in already congested airspace, and there is no way to assign a frequency to support the circuits needed to provide the new services. (It should be recognized that growth could still be available in noncongested areas.)

1 A circuit is defined as the physical facilities (including one or more ground radios operating on a common frequency) that enable a controller or uplink-broadcast station to communicate with pilots within a given portion of airspace known as a “service volume” (SV). In some cases, more than one frequency assignment may support a single circuit. In February 2002 the average number of assignments per circuit was 1.22. FAA uses frequency channel assignments as the measure of supply.

Future circuit demand estimates are based on three values representing data described below: 1. 80 circuits per yearbased on CAASD’s projections of future demand for services derived from the Aviation Capacity Enhancement (ACE) plan, Operational Evolution Plan (OEP), previous CAASD studies, and conversations with FAA air traffic personnel. A detailed breakdown is presented in Section 2.2. This estimate is likely to be on the low side because some future applications (e.g., data link services and air-to-air voice and data communication) are not considered. 2. 100 circuits per yearbased on an analysis of the 5 most recent years of data from the Government Master File (GMF). This estimate is likely to be on the low side because the interval included the 5 months after 9/11 when traffic demand and service demand were very low. Nonetheless, the low growth during this interval is seen during other periods that are followed by a period of high growth. 3. 190 circuits per yearbased on an analysis of GMF data covering the 28-year period from 1974 to 2002. This estimate is likely to be high because the main reasons for past growth (e.g., significant increases in the Automated Weather Observing System [AWOS] and Automated Surface Observing System [ASOS]) have for the most part already occurred. Future circuit demand is expected to be more modest and subjected to saturation effects (e.g., sector splits in congested airspace are approaching minimum practical size and limited growth in the controller work force is anticipated). A demand value of about 160 circuits per year appears reasonable to assume for planning purposes. This target is double the estimated number of 80 and is 60 percent greater than the circuit assignment growth over the past 5 years and only 16 percent below the 28-year average. On the supply side, increasing the number of circuits is a function of the frequency assignment models, FAA initiatives of freeing up frequencies currently assigned for other ATC purposes, eliminating co-site constraints by adding sites2 where necessary, and selective “repacking” (i.e., reassigning frequencies for existing circuits as necessary to create spectral room for new circuits). CAASD selected three states of increased supply: 1,000, 1,500, and 2,200 additional circuits. 1. The low estimate of 1,000 additional circuits considers pending frequency assignments already set aside for future circuits in the GMF, plus additional circuits that can be accommodated by eliminating cosite constraints, with no need for repacking. This capacity/supply increase estimate has low risk. The major risk is

2 Although it is technically feasible to add sites, the costs of doing so must be considered.

associated with the allocation of resources to remove cosite constraints at congested sites. 2. The middle estimate of 1,500 additional circuits includes the 1,000 circuits identified above, plus 500 accommodable through repacking using CAASD’s Spectrum Prospector™ tool or the more promising of the 25 FAA Initiatives to extend the 25AM system. This supply estimate seems achievable from an engineering view, but involves some operational and institutional hurdles. 3. The high estimate of 2,200 additional circuits considers the 1,000 circuits plus 1,200 accommodable through repacking and the more promising of the 25 FAA Initiatives, and is the most risky of the three. The acceptable level of repacking and the ability of Spectrum Prospector to do the repacking should be assessed if these levels are to be achieved. The four circuit demand scenarios and three circuit supply scenarios create a wide range of possible years in which the first service request denial may occur. For planning purposes an annual circuit demand of 160 new circuit assignments and a supply increase of 1,500 to 2,200 circuits seem reasonable. The intersection of these two scenarios puts the anticipated spectrum depletion time frame between 2011 and 2016. Under these circumstances the first service request denial would occur during this period. The historically based demand scenarios, our planning demand scenario, the three supply scenarios and the estimated year of spectrum depletion are shown in Table ES-1.

Table ES-1. Estimated Year of Spectrum Depletion Demand Per Year Circuit Supply 1,000 1,500 2,200 100 2012 2017 2024 160 2008 2011 2016 190 2007 2010 2014

Managing and executing a repacking campaign and the 25 Initiatives has uncertainty and can mean considerable cost for the FAA. Not pursuing a managed approach to spectrum may mean considerable cost for users who must transition to new radios before older aircraft are retired. The demand and supply scenarios create the range of spectrum extension given various management approaches and possible future demand levels. For planning purposes, it seems reasonable to target 2011 to 2016 from a spectrum depletion viewpoint. The question of how to narrow this range involves balancing the economic and operational costs of running out of spectrum with the economic costs of avionics equipage and ground system development, the technical risks of meeting schedules, and the operational need for future

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data-link capabilities. An update of this estimate should be reviewed every year using the latest demand data and calibrating FAA tools, including Spectrum Prospector, to reduce the uncertainties in the demand and supply projections. In any event, a tool such as Spectrum Prospector will likely be required to repack circuits for transition to the new communication system. CAASD analyses (see Appendix B) using Spectrum Prospector show that a circuit transition allowance is not required.

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Section 1 Introduction

The Very High Frequency (VHF) Spectrum from 118−137 MHz is divided into 760 channels spaced 25 KHz apart. Most (i.e., 524) of these frequencies are used for air/ground communications in the National Airspace System (NAS). Forecasting when spectrum depletion is expected to occur involves estimating both circuit3 demand and circuit supply. Demand for circuits is not uniformly spread throughout the NAS. This asymmetry is manifested in both geographic location and protected altitude, which affects the footprint of assigned frequencies. The 524 available frequencies are reassigned throughout the NAS. The number of times a frequency can be reassigned depends on its use and its protected altitude. For propagation reasons, high-altitude circuit growth will deplete the available spectrum faster than low altitude assignments. The MITRE Corporation’s Center for Advanced Aviation System Development’s (CAASD’s) Spectrum Prospector tool (described in Appendix A) takes account of assignment constraints and the current frequency assignment database, which includes geographic locations and altitudes of airspace sectors. It has been used in this study to build future circuit-growth scenarios assuming “proportional” growth.4 Spectrum Prospector models proportional growth and assigns frequencies until a frequency request is unassignable because one or more assignment rules are violated. Spectrum Prospector is able to perform neighbor-repacking (i.e., making selective changes to neighboring frequency assignments in an attempt to create spectral room for the circuit in question). At some point the options are exhausted and a frequency-assignment denial results. The proportional growth assumption accounts for the already dense spectrum use and produces early occurrences of frequency assignment denials. Allowing unconstrained repacking is optimistic since operational considerations may limit repacking. Proportional growth (in essence, “cloning” the existing circuit requirements) creates a demand pattern that duplicates historical demand for services.

3 A circuit is defined as the physical facilities (including one or more ground radios operating on a common frequency) that enable a controller or uplink-broadcast station to communicate with pilots within a given portion of airspace known as a “service volume” (SV). In some cases, more than one frequency assignment may support a single circuit. In February 2002 the average number of assignments per circuit was 1.22. FAA uses frequency channel assignments as the measure of supply.

4 Proportional growth is an increase in circuits in proportion to the current geographical frequency assignment distribution and operational environment.

There are nine Federal Aviation Administration (FAA) administrative regions. Circuit assignment counts for each region are tracked and growth estimates by region are calculated against Spectrum Prospector analysis results until one of these regions reaches spectrum saturation. At this point, any new frequency requests that cannot be accommodated through frequency repacking will be counted as a denial of service. The model identifies the demand and region where the denial occurs.

Section 2 Circuit Demand

The future circuit demand estimate is derived in three different ways to establish a range of values for sensitivity. First, there is a programmatic estimate that compiles the expected circuit requirements of planned NAS improvements to the year 2020. This estimate draws from information in the Aviation Capacity Enhancement (ACE) plan, the Operational Evolution Plan (OEP), and prior CAASD analyses. Planned NAS improvements include, among other things, new and redesigned sectors to accommodate chokepoints, new runways, and broadcast applications. The second and third estimates use historic circuit assignment data. One historic estimate is based on frequency assignment records in Government Master File (GMF) history files covering the 5-year period from February 1997 to February 2002. The other historic estimate is based on GMF records dating back to 1974. It is a count of annual circuit assignments without regard to distinguishing characteristics from one circuit assignment to the next. Considerable discussion has gone into the need for a Very High Frequency (VHF) Digital Link Mode 3 (VDL3) “transition allowance.” The allowance would consist of spectral resources held in reserve for use during the switch from 25-kHz amplitude modulation (25AM) to VDL3. A recent CAASD analysis confirmed this allowance is unnecessary and is not included in this study. See Appendix B for further explanation. This paper presents the results of these three circuit-demand forecasts and provides a planning date (based on the proportional growth assumption) for the transition to a new air/ground (A/G) communications system.

2.1 Historic Demand Data Sources The data used in this analysis came from three sources: the GMF containing FAA and other Government agencies’ assignments in the 117.975–137 megahertz (MHz) band, ARINC data in the 128.800–132 MHz sub-band, and Canadian data in the 117.975–137 MHz band. The GMF contains records of both current and “pending” frequency assignments. (The latter are frequency assignments that are “in process” or expected for the future.) Each record is ordinarily associated with an individual ground radio. In large sectors, several frequency assignments are needed so Spectrum Prospector aggregates their individual records before analyzing spectral interactions. For example, in the most recent version of the database, 8,171 non-pending Air Traffic Services (ATS) assignment records were aggregated into 6,675 circuits, yielding an average ratio of 1.22 assignment records per circuit. This ratio is used to establish the vertical axis in Figure 1.

8000

7000

6000

5000

4000

3000 191 Average Circuit Assignments Per Year 2000 Number ofNumber Circuit Assignments 1000

1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 (Source: ATS-1, 30 September 2001, Radio Spectrum Plan for 2001−2010)

Figure 1. ATS Historic Circuit Growth To estimate circuit growth over time, five snapshots of the GMF dated February 1997, September 1998, December 1999, April 2001, and February 2002 were used. These databases were compared to ascertain net circuit growth. Because circuits do not have identification tags in the database an association algorithm was used to identify unchanged circuits from one year to the next and among different databases. The output of this process was a list of circuits that were apparently “new” or “deleted” in one version of the database compared to an earlier or later one. Further analysis comparing the “new” and “deleted” circuits by region and altitude, and discussions with the FAA’s Office of Spectrum Management and Policy (ASR) technical personnel, showed that the overwhelming majority of circuits apparently “deleted” from any older database could actually be matched to “new” assignments in the subsequent database—indicating the continued existence of the circuit in question, perhaps with some slight changes in ground-radio location or service-volume size.

2.1.1 Historic Circuit Assignments (25-year Span) Data from the GMF on ATS circuit assignments from 1974 to 1999 give an initial look at circuit assignment growth trends. There is reason to exercise caution when using the GMF for establishing historic trends. There is strong evidence to support the suggestion that circuit assignments contained in the GMF may not have been recorded in a timely manner. For example, actual circuits in use in a given year were not recorded until subsequent years. It also appears that frequency retirements were not recorded in a timely fashion in the GMF.

The lack of time stamps means it is not possible to determine exact historic growth. However, it is a reasonably good estimate of historic long-term circuit growth. It is believed that frequency assignments were recorded in a more timely fashion and there were few unrecorded entries for active circuits in more recent years. If there were a bias in the GMF data, it would lead to an overstatement of circuit growth rates. Figure 1 highlights the fact that circuit assignment growth is linear over the long term, but growth spurts and flat periods exist. The growth spurts can be linked to major FAA programs, such as automated weather services in the mid 1990s and activities associated with pent-up demand after the controller strike in the 1980s.

2.1.2 Historic Circuit Assignments (5-year Span) Recent GMF data records offer more details for comparing the total circuits in use from one year to the next. Data on circuit counts from February 1997 through February 2002 was obtained. Table 1 summarizes the net circuit assignments over the period. The files from September 1998 through February 2002 contain both circuit assignments (i.e., additions) and removals. In some cases, “new” circuits are balanced by removals (i.e., when it can be shown the coverage volume was similar). Between September 1998 and February 2002, new circuit assignments by region, location, and altitude were analyzed. The data include new circuit additions and removals showing the net increase in circuit assignments for the period. In addition to the circuit-assignment files, there are “pending” files. These are circuit assignment changes awaiting paperwork completion, for which specific frequency assignments were identified. They include additions and removals (i.e., subtractions). Over the 42-month period from September 1998 to February 2002, 999 new circuits were added and 711 removed, for a net difference of 288, or about 82 circuits per year. Over the entire 5-year period from February 1997 to February 2002, the 285 net new GMF circuits added to the GMF represents an average rate of 57 circuits per year. Table 1. Short-Term Circuit Growth GMF Net Circuit Circuits Growth February 97 6,390 September 98 6,387 -3 December 99 6,506 119 April 01 6,660 154 February 02 6,675 15 Total 285

2.1.2.1 Assessment of Circuit Assignments (42-month Span) Of the 285 net new circuit assignments, 67 percent were for use by non-FAA airports and other non-aviation government agencies. Net circuit assignments for high altitude

(24,000 feet and above) declined while growth occurred at lower altitudes. By region (Alaska − AL, Central − CE, Eastern − EA, Great Lakes − GL, New England − NE, Northwest Mountain − NM, Southern − SO, Southwest − SW, Western Pacific − WP), the most active regions were GL, SO, SW, and WP, with the SO region accounting for most of the circuit additions and removals. These regions account for 69 percent of the total net new circuit assignments. Data on circuit changes by region can be found in Appendix C. Figures 2 and 3 show circuit additions and removals for the FAA and “Other” categories, for the 42-month period from September 1998 to February 2002, broken into groups above and below flight level 240. Over this period, low altitude circuit growth predominated. In fact, for the period, high-altitude circuits had a net decrease of 68 circuits (most likely due to a concerted database cleanup effort). The majority of the growth occurs from non-FAA-provided services and occurs below flight level 240 indicating a possibility of managing future demand. To gain insight into circuit assignment changes for the 5-year period from February 1997 to February 2002, discussions were held with ASR technical personnel in March 2002. It has been established that this was a period where a conscious effort to “clean up” frequency assignments and associated records was underway. The ASR personnel offered an excellent perspective on the record cleanup activities, estimating that 60 records from three branches of the military and 30 other non-FAA records had been removed from the GMF over the previous 2 years. Likewise, another 80 FAA records were removed over the same period as well as 50 en route records in 1998-1999. Thus, the removal of 220 records was masking true growth over a 4-year period. The 285 net growth number for circuit records over the same period actually indicates a total of 505 new circuits for the 5-year period, for an annual rate of 101 new circuits per year. It is possible that this interval was not representative of future demand. Without question, having more years with data of this quality would improve the analysis. As monitoring spectrum is an annual function, incorporating GMF updates into this analysis will improve future demand trend estimates.

FAA Circuit Changes 98 to 02 by Flight Level (No Pending Records)

FAA Additions Masked by Cleanup 240 and Above -98 104 50 FAA Deletions

0-239 -136 225 80

-600 -400 -200 0 200 400 600 800

Figure 2. FAA Circuit Changes by Flight Level

Other Circuit Changes 98 to 02 by Flight Level (No Pending Records)

Other Additions Masked by Cleanup 240 and Above -86 12 Other Deletions

0-239 -391 658 90

-600 -400 -200 0 200 400 600 800

Figure 3. Other Circuit Changes by Flight Level

2.2 Estimated Circuit Requirements to Support NAS Modernization Plans The FAA’s many modernization programs, past, present and future, impact the demand for VHF frequencies. The OEP and ACE Plan identify some of the NAS changes for the next 10 to 20 years. Improvement plans requiring more circuits include new runways, terminal and transition airspace redesign, choke points, and en route airspace redesign. As spelled out in the OEP and in other planning documents, all have anticipated ramifications for VHF spectrum use. FAA programs (such as Automatic Dependent Surveillance Broadcast (ADS-B), Reduced Vertical Separation Minima (RVSM) and Controller/Pilot Data Link Communications (CPDLC)) are not considered to require more ATS spectrum for the following reasons: ADS-B because current systems operate at L band; CPDLC because the spectrum for Build 1A comes out of the allocation to ARINC; and RVSM because, if interference occurs, it is expected to be transitory. Programs such as automated weather- related services, including Automated Surface Observing System (ASOS), Automated Weather Observing System (AWOS) and Automatic Terminal Information Service (ATIS), will also place demand on the available spectrum. These types of installations have consumed a considerable portion of frequency assignments, and AWOS installations are expected to continue to demand frequencies through 2015 as local airports implement Wide Area Augmentation System (WAAS) precision approach procedures. CAASD undertook an analysis of these planned changes and estimated the impact on circuit demand through 2020. Where ranges were given, the conservative high value was selected. Table 2 provides estimates of the total circuits required to complete the identified tasks.

Table 2. Programmatic Based Circuit Demand Estimate Through Year 2020

Requirement En Route Terminal Broadcast Total New Runways − 224 − 224 En Route Sector Splits 60 − − 60 Terminal Airspace Redesign − 35 − 35 Super High Altitude Sectors 6 − − 6 AWOS/ASOS/ATIS − − 1,130 1,130 Chokepoints 62 46 − 108 Total 128 305 1,130 1,563 (Source: see Appendix D) Counts of future circuit requirements to accommodate new sectors (e.g., to address choke points), terminal airspace redesign to increase throughput and airspace use in high volume terminal areas, and the addition of new super high altitude sectors are reflected in the above

table. Because these estimates came from plans with 10-, 15- and 20-year time frames, the table was standardized on 10-year estimates and doubled. For example, the FAA’s 2001 ACE Plan specifies construction of 40 new runways from the year 2000 on. The Plan calls for 24 runways to be built by the end of 2010. There were eight runways where the completion date was still “to be determined (TBD).” These TBD runways were split between the periods before and after 2011 for 28 total planned new runways by the end of 2010. This planning figure was doubled to 56 new runways by 2020 and a conservative average number of circuits per runway requirement of four was applied resulting in new runways projected to consume a total of 224 circuits. In the analysis, future demand estimates are zero for Data Link, ADS-B, and RVSM. It is possible they will have an impact on future circuit demand, but that impact is still not clearly understood and does not appear likely to become a factor for more than a decade. Navigation technologies such as WAAS and the Local Area Augmentation System (LAAS) are not expected to use bandwidth between 117.975−137 MHz. It is expected that the 112−117.975 MHz frequencies will accommodate all LAAS navigational needs and consequently are not included in the A/G demand forecast. The programmatic estimate results in an average annual growth rate of 78 circuits per year (1,563 new circuits/20 years = 78.1 circuits per year) and has been recorded in Table 4 as 80. Table 3 compiles and rounds the various demand estimates of future circuit demand. These estimates are designed for planning to enable the FAA to continue to manage spectrum as the 25AM system nears saturation and to plan for and execute a smooth transition to the next generation communications system.

Table 3. Annual Circuit Demand Forecast Summary

Annual New Circuit Forecast Source Estimate 20-year historical average 190 February 1997 – February 2002 100 Programmatic estimate 80

On the low end, the programmatic estimate calls for 80 circuits per year while on the high end the 20-year historic average gives an estimate of 190 new circuits per year. We recognize the demand of 190 circuits per year could occur, but it is expected that the number of new AWOS/ASOS circuits in the VHF communication band can be managed by using part of the VHF Omnidirectional Range (VOR) frequencies from 112–117.975 MHz. We believe that an intermediate estimate of 160 circuits a year provides an adequate degree of conservatism for the purpose of planning to avoid the exhaustion of spectral resources for future circuits.

Section 3 Circuit Supply

Estimating the spectrum-limited capacity, or supply, of the existing 25AM A/G radio system is a fundamental issue in scheduling the phase-in of the successor system that will utilize the 8.33-kHz-spaced channels or VDL3. A reasonable definition of system capacity is the maximum size to which the system’s population of ATS radio circuits can grow before the first “frequency denial”—i.e., the first instance in which no acceptable frequency can be found to satisfy a new circuit requirement. Analyses such as those documented in [Box et al., 2001] have recently been done with the aid of MITRE’s Spectrum Prospector tool (described in Appendix A) to estimate system capacity for both the 25AM and VDL3 architectures. There are several ways the supply can be extended. These include selective frequency repacking (discussed earlier) and four categories of alleviatory measures pursued under the FAA’s 25 Initiatives: expansion of ATS frequency resources, frequency pooling, physical mitigation measures (including cosite mitigation), and improvements to databases and associated tools. The Prospector simulations performed to support the analyses have employed the “proportional growth” scenario explained earlier, together with ad hoc neighbor-repacking to create spectral room for postulated future circuits. The simulations have also assumed that interference mitigation techniques, such as filtering, will be used whenever necessary to prevent intermodulation or cosite interference problems from blocking an otherwise acceptable frequency assignment. The results depend to some extent on parameters such as the randomly selected order of arrival of hypothetical future requirements, and the amount of neighbor-repacking deemed acceptable when accommodating any new circuit. When 524 frequencies are available to ATS, as assumed in the Prospector simulations, the resultant 25AM capacity estimates generally fall within the 8,000−8,500 range, with a median value near 8,200. In other words, the first denial is likely to occur when the (approximately) 8,200th circuit is added to the 25AM system. Since the system in February 2002 contained 6,675 circuits (not counting the records of 761 pending circuits to which frequencies had already been assigned), these results seem to indicate that the present population of operational ATS circuits could grow by about 1,500 circuits before the first unavoidable frequency denial. Subsequent denials would occur infrequently for a while, but their occurrence rate would accelerate as additional assignment requests are generated.

3.1 Increasing Supply via 25 Initiatives Seeking to postpone the first denial as long as possible, ASR in 2000 launched a major effort to identify and implement measures for increasing the supply of frequencies available to ATS A/G circuits, and improving the effectiveness with which those frequencies are used. This effort resulted in the so-called “23 Initiatives,” to which two more were later added,

bringing the total to 25. The status of the 25 Initiatives as of 30 September 2001 is documented in [ATS-1, 2001]. The current status of the 25 Initiatives is summarized in Table 4, which groups them into four categories and provides our engineering judgement of their effectiveness in postponing the advent of frequency denials in the existing 25AM system. The supply increase estimated for each of four Initiative categories in the table is discussed below.

3.1.1 Expansion of ATS Frequency Resources This first and largest category of initiatives involves discussions between ASR and regulatory bodies and/or other agencies to increase the number of frequencies available to ATS. Through extensive experience with Spectrum Prospector, it has been found that when cosite mitigation measures are presupposed (as they have been in all the simulations discussed in this report), the first-denial capacity of the system increases fairly linearly with increasing spectrum resources, provided that those resources are available for general use. Since the simulations indicate a capacity of some 8,200 ATS circuits when 524 frequencies are available—about 15.6 circuits per frequency—a reasonable first-order approximation is that each additional unencumbered frequency made available to ATS will increase the capacity by 15–16 additional circuits. That assumption underlies the estimate of the probable effectiveness of Initiatives 1–6. The linearity assumption may not apply to Initiatives 13 and 14, which may provide few if any completely unencumbered frequencies to ATS; for those initiatives, capacity-improvement estimates provided by ASR technical personnel were used. The great unknown in the capacity equation is the effectiveness of Initiative 16: the proposed use of frequencies in the 112−117.975 MHz band to accommodate what may be a large future increase in requirements for AWOS and ASOS stations. The development of many WAAS procedures at airports, and the low cost and the high operational and competitive value of having an AWOS or ASOS, may eventually induce most of the airports in the United States to install one and then to ask ASR for a frequency on which to operate it. Even though AWOS and ASOS have no airborne transmitters, and hence have relatively small radio horizons that make it easier for them to reuse frequencies and thereby reduce their “spectrum consumption,” the appearance of large numbers of new AWOS and ASOS requirements could stress the system in ways that are currently unknown. It is estimated in [ATS-1, 2001] that Initiative 16 might provide spectrum for as many as 1,342 new AWOS/ASOS stations. Further analysis is needed because of the magnitude of the potential capacity gain, the pressing need for that gain, and the complexity of implementing the initiative. The VOR system already occupies the 112−117.975 MHz band, and although some VORs will be decommissioned as WAAS availability increases, about 200 LAAS stations soon will be installed in the same band. If any neighbor-repacking of VORs proves necessary, it would also require the retuning of associated navigational aids in

Table 4. Overview of the 25 Initiatives for Extending 25AM System Life (Page 1 of 2)

Increase in Remaining Circuit Initiative Action Considered Status/Prospects Capacity Beyond Current Prospector Predictions Category No. Est. Explanation Expansion 1 Taking 4 frequencies from 121.5-MHz Federal AWOS & ASOS 60 15 circuits/frequency of ATS emergency frequency’s guardbands likely to get all 4* Frequency 2 Taking 2 frequencies now used for flight- ATS likely to get both 30 15 circuits/frequency Resources check circuits frequencies* 3 Reclaiming 2 frequencies lent to law- ATS likely to get one or 15–30 15 circuits/frequency enforcement agencies both frequencies* 4 Acquiring some of the 37 non-ATS ATS may get some of 17 0–255 15 circuits/frequency frequencies controlled by FCC in 121.950– flight-test frequencies, 123.575 MHz subband but outcome uncertain** 5 Acquiring some of the FSS frequencies in ATS likely to get 3–4 of 45–60 15 circuits/frequency 121.975–123.650 MHz subband the frequencies* 6 Utilizing 17 frequencies in the 136– ATS may get all 17*; 0–30 15 circuits for each new 136.475 MHz subband Prospector simulations frequency not already assume 15 previously assumed 13 Time-sharing of air-show frequencies, to Being implemented 10* enable ATS to use some of them 14 Reclaiming some ATS frequencies Being implemented 50* currently used for aerial firefighting 15 Reclaiming 2 ATS frequencies currently Unlikely to occur 0 used by DoD for non-ATS purposes 16 Using 112–117.975 MHz VOR band for Being pursued VOR-band capacity TBD new AWOS and ASOS requirements simulations needed 24 Combining HIWAS and AWOS/ASOS Abandoned 0 service on VOR channels Frequency 9 Pooling ATIS/AWOS/ASOS frequency Done; assumed in 0 Pooling resources with rest of ATS resources Prospector simulations 10 Adding ground-control (GC) and Done; used in Prospector 0 clearance-delivery (CD) frequencies to simulations pool

Table 4. Overview of the 25 Initiatives for Extending 25AM System Life (Page 2 of 2)

Increase in Remaining Circuit Initiative Action Considered Status/Prospects Capacity Beyond Current Prospector Predictions Category No. Est. Explanation Physical 8 Selectively using directional antennas Abandoned 0 Mitigation 12 Cosite-interference mitigation measures at Being done; used in 0 Measures ground sites Prospector simulations 17 Using offset carriers to cover big sectors May allow elimination of 20* now using multiple frequencies 20 assignments 18 Using selective keying and voting to cover Initiative 17 currently 0 big sectors now on multiple frequencies preferred 19 Selectively lowering GC/CD ground Unlikely to occur 0 antenna heights and output powers 20 Optimizing ground-radio locations within Unlikely to occur 0 selected service volumes (SVs) Database 7 Selectively reducing sizes of frequency- Being done 250* and Tool protected service volumes Improve- 11 Auditing spectrum database in order to 220 deleted since 1998; 220 Prospector simulations ments delete unused frequency assignments another 220 possible by took the 220 initial end of 2003* deletions into account 21 Upgrading database and FAA assignment Being done; assumed in 0 tools to reflect cosite-interference Prospector simulations mitigation measures 22 Improving accuracy of ARINC data in Limited improvement 0 assignment database likely 23 Upgrading FAA assignment tools to Done 0 consider vertical SV separations 25 Modifying FAA tools to facilitate tighter Automation of existing 0 packing of assignments manual procedure Total 700-1015

*3/26/02 ASR estimate. **6/21/02 ASR update.

other frequency bands. Those activities would also be interacting with simultaneous growth of general ATS requirements in the neighboring A/G radio band. International regulatory considerations and the need to forge an operational, institutional and political consensus for this new use of the VOR band are also factors potentially limiting the effectiveness of Initiative 16. Such issues create great uncertainty about how much capacity Initiative 16 would add to the system. This analysis has used “TBD” for the capacity gain prediction until a more detailed analysis can quantify a sufficiently conservative planning estimate. The analysis would involve the present A/G radio band, the VOR band, and (if necessary) the other navigational bands whose channelization plans are interdependent with those of VOR. Recent upgrades described in [Box et al., 2002] enable Spectrum Prospector to support such an analysis, but it would first be necessary to develop frequency-assignment criteria for band-sharing by the several systems involved.

3.1.2 Frequency Pooling Past Spectrum Prospector simulations have shown that it significantly improves system capacity to combine the previously separated frequency lists of ATIS, AWOS, and ASOS ground stations, and those of ground-control and clearance-delivery circuits, with the general frequency resources of ATS circuits. In 2001, ASR merged these lists into a single pool whose existence has been assumed in all our recent simulations, and so no additional capacity gain beyond what we have already assumed in our basic predictions is shown for this category of initiatives in the summary table.

3.1.3 Physical Mitigation Measures Initiative 12, perhaps the most crucial of all of the initiatives, involves cosite-mitigation measures at ground sites. These include filters, multicouplers, antenna separations, and radio relocations. For reasons discussed in Appendix B, it was assumed throughout the simulations discussed in this report that the FAA will use such measures whenever necessary to prevent cosite interference problems, including intermodulation, from causing frequency denials. Since the benefits of such measures are already assumed in the basic capacity estimates, no additional capacity gain is shown for Initiative 12 in the summary table. However, it is important to realize that without such cosite-mitigation measures, frequency denials would already be occurring at an ever-accelerating pace at congested sites throughout the NAS. Cosite-related costs need to be identified and allocated to implement this essential initiative. Another significant activity in this category is Initiative 17. It involves the use of offset carriers to reduce the need for multiple frequencies in large sectors that depend on multiple ground radios for full coverage.

3.1.4 Database and Tool Improvements The key activity in this final category is Initiative 21, which involves upgrading the FAA’s assignment database and frequency-assignment tools to reflect the physical cosite- interference mitigation measures being carried out in Initiative 12. It is particularly important to take into account the locations of transmitting and receiving antennas, to determine whether they are sufficiently “collocated” to warrant application of intermodulation rules and the standard cosite 500-kHz frequency-separation rule. To some extent this represents automation of existing procedures, since ASR already consults maps and other paper records for the fine-grained location data often needed for analyses of this kind. Initiative 21 can potentially make this process much less time-consuming and more efficient. As with Initiative 12, the benefits of this initiative are already presupposed in all the simulations used to derive the basic capacity estimates, so that no additional benefit is shown in the summary table. Nevertheless, it is one of the most essential of the 25 initiatives, possibly second only to Initiative 12 in importance.

3.2 Initiatives Summary Initiatives 6, 9, 10, 12, and 21 have already been considered in the baseline scenario that generated assignments for about 8,200 circuits before the first denial. Implementing all the remaining initiatives (except Initiative 16 and the initiatives that have been abandoned) would increase the “first-denial” capacity by about 700−1,000 circuits, to a total of 8,900−9,200 circuits. Thus, by employing repacking and selected FAA initiatives considered probable, a supply growth of 2,200–2,500 circuits is predicted. The 2,200 value is used to provide some hedge against limits on either repacking or frequency reclamation. From an implementation perspective the supply data can be grouped into three categories. (Note that Category 2 is a subset of Category 3). • Category 1The system’s ability to absorb at least 1,000 additional circuits, on top of the current 6,675, is expected on the basis of pending circuit records (which number about 700, and for which frequencies have already been found) and the anticipated benefits of cosite filtering activities and certain other initiatives already under way that will support about 300 more circuits. • Category 2500 additional circuits can be accommodated by pursuing either repacking activities, or the remaining initiatives believed to have a high or medium likelihood of implementation. • Category 31,200 additional circuits may be accommodated by pursuing repacking activities and the remaining high/medium-likelihood initiatives. (This category includes the 500 circuits in Category 2.)

Thus, from a supply perspective, incremental increases of 1,000 (Category 1), 1,500 (Categories 1 and 2), and 2,200 (Categories 1 and 3) can be related to ascending levels of risk. Cost and implementation risk increase commensurately with the supply of circuits. The risks of Categories 1 and 2 are considered low.

3.3 Sensitivity of Supply Results to Assumptions If our assumption that Initiative 6 will be largely successfulthus allowing the 25AM system to use 15 frequencies at or above 136 MHz for ATS purposesproves incorrect, it would reduce the first-denial capacity estimates given above by about 15 circuits per frequency, or 225 circuits in all, substantially reducing the 1,000-circuit “buffer” provided by Category 1. If, contrary to our proportional-growth assumption, high-altitude circuit requirements or those in already-congested regions of the NAS proliferate more rapidly than others, the first denial is likely to occur sooner than estimated above. (If they proliferate more slowly, the first denial will probably be postponed. It can be argued (with support from the program analysis in Section 2.2) that future high-altitude demand growth will probably be less than that recorded historically.) We assumed that a “spectral reserve” would not be needed to transition to VDL3, for the reasons explained in the analysis of Appendix B. A successful transition can be accomplished even if it starts when 25AM frequency denials have begun. However, the longer the transition is postponed the more complex (and hence costly) it will become. Finally, it must be noted that the possibility of one or more frequency denials occurring before 2011 cannot be completely ruled out. Such denials might result from an unexpectedly early and strong demand surge, especially if it happened in high-altitude airspace in a congested region. Even without such a surge, a pre-2011 denial could occur if financial or other considerations prevent the FAA from establishing a new ground site or otherwise mitigating cosite interference in a situation where a particular new circuit cannot be assigned a frequency at all without such mitigation. Apart from those two contingencies, however, it appears likely that the onset of frequency denials in significant numbers within the existing 25AM system can be postponed until 2011. This date could be extended if the demand for new services is delayed in response to possible long-term effects of the events of September 11, 2001 on civil aviation.

Section 4 Managing Spectrum

The FAA has a highly successful track record of accommodating all A/G voice frequency assignments whether for its own use or for the use of other entities. Because supply has not previously been an immediate issue, all historic frequency requests were met without significant scrutiny. As the 25AM system approaches depletion, should it wish to prolong system life, the FAA can pursue, from the supply side, repacking and the 25 initiatives; and, from the demand side, allowing denials. Each has its own price tag, economic implications, and risk profile. Estimates of the economic impact of repacking frequency assignments and the associated logistics of retuning radios, or implementing initiatives, must be measured against denying a circuit for a new requirement or users agreeing to an earlier transition to a communication technology with more channels. In the near future, the FAA can judge the cost and risk of extending the existing system against the foregone benefits and risk of the lost service. One of the key considerations in managing spectrum is the user cost to transition to a new A/G communication radio and the associated technology. Older aircraft designated “classics” with analog wiring are considerably more expensive to equip with the next generation A/G radio. Retirement of these aircraft has accelerated in recent months and the older aircraft are being removed from the fleet.5 An early transition date (i.e., before 2010) would likely necessitate an approach/architecture to minimize near-term retrofit avionics costs. On the other hand, if the transition can be delayed until most analog aircraft can be retired the avionics cost to transition to a new digital radio is reasonable. A transition date in the 2011−2016 range would enable the planned retirement of most analog configured aircraft and their replacement with a digital configuration and a multimode radio that is similar in cost to an analog radio. Who benefits from communication services and who pays for them, is another aspect the FAA must consider. AWOS and ASOS services represent the single largest known source of future circuit demand. General Aviation users, the primary beneficiaries of these services, will not carry the bulk of the cost for an early transition to a new radio should spectrum depletion occur early. Initiative 16 allows the FAA to cope with a prospective large influx of future AWOS and ASOS requirements, but detailed simulations are needed to validate the feasibility of interference-free operation. A prerequisite for such simulations is the development of appropriate assignment criteria for preventing co-channel and adjacent- channel interference among the VOR, LAAS, AWOS, and ASOS stations that would all be

5 www.boeing.com

sharing the 112−117.975 MHz band. Should Initiative 16 prove technically infeasible, the FAA might consider denying AWOS/ASOS service requests under certain circumstances in congested areas to extend 25AM system life. To estimate a relative time frame for the options available to the FAA in managing spectrum, CAASD assessed three growth scenarios discussed in Section 2.2 and recommended for planning purposes the use of an intermediate estimate of 160 circuits per year. The corresponding estimate of spectrum depletion would occur between 2011−2016 as a function of supply. It is fully recognized that risk and uncertainty exist in these estimates. Program implementation risk increases when one expects the existence of greater circuit supply. Planning risk increases when one expects both lower demand and greater supply. The FAA and other stakeholders may also consider managing spectrum by accepting a limited number of frequency denials. After the first denial, some time may pass before the second, third, and subsequent denials. Assuming there are no serious NAS impacts from the early denials, this initial-denial “tolerance” could extend the life of the current 25AM system. Spectrum Prospector can model multiple denials. Simulation results indicate that if the FAA allowed spectrum growth out to the tenth denial, an additional population growth on the order of 400 circuits would be enabled, extending the year-of-depletion estimate (using the planning value of 160 new circuit assignments per year) another 2.5 years. Frequency denials are most likely to occur in congested airspace at high altitudes, and new assignments can be found for services in non-congested and low-altitude airspace. Pursuing such measures requires careful scrutiny and economic analysis before implementation.

Section 5 Conclusions and Recommendations

Given current knowledge regarding future sources of circuit demand and the supply options available to the FAA, by using a managed approach, it appears reasonable at this time to assume the current 25AM A/G communication system can be maintained, albeit at a cost to some or all concerned parties, until transition to the next generation A/G radio. The analysis described in this paper gives estimates of the spectrum-depletion time frame on the basis of differing assumptions concerning demand growth, and different assumptions on the ability to the FAA to aggressively manage the spectrum to increase supply (a function of budget and institutional changes). The FAA should pursue research on the cost and technical legitimacy of repacking and the 25 initiatives. The economic significance of denying certain frequency requests or imposing an early transition on users should be evaluated as well, since this will factor into the risk calculations with regard to transition timing. In addition, the FAA should instill an awareness of anticipated spectrum impact from planned future NAS enhancements. As programs are discussed and formulated, an awareness of anticipated spectrum needs is critical to improving future demand estimates. Another factor to consider, and one that is worthy of future study, is that there is evidence to suggest that in the aftermath of the recession of 2001 and the horrific events of September 11, the commercial airline business model may be undergoing a transition. The rise of the regional jet point-to-point service and the marked success of upstart, low cost carriers here and abroad, may point to a future shift in circuit needs away from frequency-congested regions. As recommended by the NEXCOM Aviation Rulemaking Committee (NARC), an annual assessment should be conducted of the 25AM spectrum situation. The FAA needs to monitor demand and plan to provide the resources for increased supply.

List of References

1. Air Traffic Services (ATS-1), 30 September 2001, Radio Spectrum Plan for 2001–2010, Federal Aviation Administration, Washington, DC.

2. Box, F., L. Globus, T. Kim, L. Monticone, M. Nguyen, P. Purcell, R. Snow, and M. Tran, April 2002, Development of a Spectrum Management Tool Suite for the 960–1215 MHz Band, MP 02W0000067, The MITRE Corporation, McLean, VA.

3. Box, F., Y. Hoh, P. Long, R. Snow, September 2001, Spectrum Tradeoff Analyses for the Transition to NEXCOM, MTR 01W0000079, The MITRE Corporation, McLean, VA.

4. Federal Aviation Administration, December 2001, 2001 Aviation Capacity Enhancement Plan, DOT/FAA/ASC, Federal Aviation Administration, Washington, DC.

5. Federal Aviation Administration, March 2002, FAA Aerospace Forecasts Fiscal Years 2002-2013, DOT/FAA/APO, Federal Aviation Administration, Washington, DC.

6. Federal Aviation Administration, December 2001, Operational Evolution Plan, Version 4.0, Federal Aviation Administration, Washington, DC.

7. Federal Aviation Administration, September 2001, Report of Recommendations from The NEXCOM Aviation Rulemaking Committee to Federal Aviation Administration, Federal Aviation Administration, Washington, DC.

8. Long, P., and R. E. Snow, September 1999, Detailed Spectrum Planning for the Transition to NEXCOM, MTR 99W0000107, The MITRE Corporation, McLean, VA.

Appendix A Spectrum Prospector

Managing the scarce and valuable sequenced frequency plans must interactions of large populations of frequency resources of a nation- be developed to ensure a gradual, A/G radios and potential sources wide air/ground (A/G) radio system nondisruptive transition in which of interference to those radios. is an immensely complex task. interference-prevention rules are Prospector is highly flexible and High-altitude airborne radios are followed at every step. Automated has many applications, including: mutually visible at very long support is essential for creating those transitional frequency plans. • Day-to-day frequency planning ranges, increasing their exposure to meet new circuit requirements to cochannel interference and hin- dering frequency reuse. Ground- For several years the MITRE Cor- • Spectrum planning for facility based A/G radios often share poration’s Center for Advanced relocations or airspace redesign crowded sites where the threat of Aviation System Development • Predicting the effect of proposed cosite interference greatly reduces (CAASD) has supported the Fed- radio design changes on spec- the supply of usable frequencies. eral Aviation Administration (FAA) trum-limited system capacity in planning the Next-Generation As air traffic grows, the size and A/G Radio Communications Sys- • Long-range spectrum planning complexity of the A/G radio system tem (NEXCOM). A key component for an architectural transition. grows with it, and so does the of that support has been CAASD’s development and use of Spectrum magnitude of the spectrum man- The Prospector Database ager’s task. Planned changes in Prospector, an automated system tailored to the needs of A/G radio the architecture of the worldwide The Prospector database contains VHF A/G radio system will intensify spectrum management. user-supplied environmental data, these difficulties during the pro- lists of available frequencies, and longed period of transition to the rospectorTM models the desired P frequency-assignment rules. new architecture. Detailed, time- and undesired electromagnetic Environmental data include: • The locations and dimensions of A/G service volumes • Ground-antenna locations • “Class codes” keyed to the power levels and other interference-related characteristics of ground and airborne radios • Lists of frequencies available for assignment to A/G circuits.

Assignment rules typically include: • Distance constraints for avoiding intersite cochannel and adjacent-channel interference • Frequency-separation require- Map display of frequency usage showing cochannel circuits in yellow, ments and intermodulation their radio horizons in red, and an adjacent-channel circuit in green. criteria for collocated radios.

MITRE 26 CAASD  2003 The MITRE Corporation. All rights reserved. The Assignment Engine egy, Prospector generates time- used, the engine converts the cir- sequenced, incremental frequency cuits to the new architecture one

Prospector’s assignment engine plans. At each step, it automati- by one, in a user-controllable or- cally identifies and exploits der. Each newly converted circuit automatically generates a fre- opportunities to alter preexisting is assigned a frequency and, if the quency plan for the postulated assignments whenever necessary new architecture uses TDMA, a environment in accordance with to create spectral room for a new time slot as well. No circuit is con- the rules specified by the user. It circuit. When specifically re- verted unless Prospector can has several user-selectable as- quested by the user, the engine assign it a frequency satisfying all signment strategies. can also try relocating selected the interference-prevention rules radios to one or more alternate specified by the user for the new The gapfilling strategy is the sim- sites on a user-provided list. architecture. Preexisting assign- plest. It keeps all preexisting ments are changed as necessary circuits on their old frequencies The assignment engine can also to accomplish this objective. while the engine seeks violation- develop detailed, incremental free assignments for one or more plans for converting part or all of new circuits. Gapfilling is ade- Reports the A/G radio system to a future quate in cases where spectral architecture. One such architec- congestion is not too severe. Prospector can generate a wide ture—VDL Mode 3, approved by variety of reports at the user’s op- the International Civil Aviation Or- tion. One key output is a time- However, the simple gapfilling ganization—employs a time- sequenced listing of all the radio strategy may leave one or more division multiple access (TDMA) conversions and/or retunings circuits unassigned if all their technique. It also uses a digital needed to carry out an incremental available frequencies are blocked modulation method whose inter- frequency plan. Map displays of by potential interference problems. ference characteristics differ from the radio environment, before and For such situations, Prospector’s those of today’s analog radios. much more powerful neighbor- after the Prospector-recommended repacking strategy is available. frequency changes, can also be When Prospector’s architecture- When the user selects that strat- produced. conversion assignment strategy is

A/G RADIO ENVIRONMENT

CIRCUIT ASSIGNMENT RECOMMENDATIONS AVAILABLE QUERY MAP ASSIGN- FREQUENCIES MODULE DISPLAYS MENT ENGINE MULTICIRCUIT ASSIGNMENT REPACKING AND RULES TRANSITION PLANS

Overview of Spectrum Prospector Operation

L Y SERIAL R CIRC. LINK AGNCY NUMBER SITE NAME ST SERVICE CODE OLD FREQ NEW FREQ ------2 1442 1442 AG1 554667 LINCOLN IL GND CTL 133.775 128.000 2 1300 1300 AG2 994521 DOUGLAS IL APCH CTL 133.750 118.050 2 1300 1307 AG2 994523 BIRCHWOOD IN APCH CTL 133.750 118.050 1 1601 1601 AG2 670000 FOUR-COUNTY AIRPT IL CLNC DLVY 126.775 133.750 0 1705 1705 AG2 983456 GRANT IL H-ENRT NONE 126.775 0 3345 3345 AG2 618732 TRI-CITY INTL IL DEP CTL NONE 127.400 1 8795 8795 AG1 454587 BAYOU CITY LA GND CTL 119.275 124.675 0 8675 8675 AG2 595967 TALLGRASS LA L-ENRT 127.925 119.250

Notional Example of an Incremental Frequency Plan

Appendix B Impact of Delaying the Transition to VDL3

Background Past MITRE analyses [Long and Snow, 1999] performed with the aid of MITRE’s Spectrum Prospector tool have demonstrated that an incremental transition to Very High Frequency (VHF) Digital Link Mode 3 (VDL3) can be achieved within available spectrum while observing all necessary interference-prevention rules. However, those past analyses assumed only limited growth in air/ground (A/G) circuit requirements before the VDL3 initial operational capability (IOC) date. Concerns have been expressed that delaying the transition until the existing 25 amplitude-modulation (AM) system runs out of spectrum might make the VDL3 spectrum transition too complex, costly, or interference-prone. The analysis described in this appendix was performed to ascertain whether a spectral reserve or “transition allowance” is needed to accomplish a successful transition to VDL3. Methodology and Results In this analysis it was conservatively assumed that future “proportional growth” of the U.S. population of VHF voice A/G radio circuits used for Air Traffic Services (ATS), which includes controller/pilot circuits supporting air traffic control (ATC) and uplink-broadcast stations used for the Automatic Terminal Information Service (ATIS), Automated Weather Observing System (AWOS), and Automated Surface Observing System (ASOS). (No growth was assumed for Canada or for non-ATS applications such as AOC and UNICOM.) Proportional growth means that the geographical and size distributions of the U.S. ATS circuit population are assumed to remain roughly constant as the total number of such circuits grows. Under this assumption, for example, a state or region that currently has 8 percent of all ATS ground radios would retain approximately that percentage at all future times. Similarly, the percentage of circuits with a 60-nautical mile (nmi) service volume (SV) radius and a 45,000-foot ceiling would remain basically unchanged in the future. To model the unpredictability of the chronological sequence in which the future circuit requirements will actually materialize, a random numerical sequence was used to determine their “order of arrival.” Spectrum Prospector was used in conjunction with the proportional-growth assumption to analyze several different scenarios in which various amounts of system growth takes place under today’s 25AM system architecture before the transition to VDL3 begins. Each scenario consisted of an “all-25AM” phase in which all the circuits operate 25AM as they do today, followed by a “VDL3-transition” phase. In the all-25AM phase of each scenario, Spectrum Prospector started with the actual frequency assignments of today’s 7,436 25AM circuit requirements—6,675 of which are

actually operating and 7616 of which are “pending” (planned)—and then sought a frequency for each hypothetical future circuit in succession, while obeying all intersite assignment rules applicable among 25AM radios. “Neighbor-repacking” (selective changes to preexisting frequency assignments) was used whenever necessary to find a violation-free assignment for a new circuit requirement. In the VDL3-transition phase of each scenario, Spectrum Prospector began by successively converting to VDL3 each of the 7,436 February 2002 ATS circuit requirements, as well as any hypothetical future circuits that had already been assigned (or, in a few cases, denied) frequencies as 25AM circuits before the start of the simulated VDL3 transition in the scenario in question. (For simplicity, ATS-circuit population growth that might occur after the postulated beginning of the VDL3 transition was ignored in the analysis of each scenario.) As each circuit in turn was converted, Spectrum Prospector assigned it a frequency that complied with the intersite cochannel and adjacent-channel assignment criteria associated with VDL3, and a time slot. The VDL3 assignment criteria are somewhat stricter than the corresponding 25AM assignment rules, except for VDL3’s crucial ability to “bundle” up to four circuits onto a single frequency in a given locality by putting them on separate time slots using its spectrum-conserving time-division multiple access (TDMA) technique. The VDL3-transition phase of each scenario was simulated in two different ways: with four voice slots (4V) and with two voice slots. In the four-voice-slot variation, each newly converted or activated VDL3 circuit was (if possible) made part of a 4V bundle in which all four time slots were reserved for use by preexisting and future voice circuits. Circuits that could not be put into a 4V bundle—usually because they had SVs too large to allow 4V operation without unacceptable propagation delay—were instead put into “3V” bundles that were also exclusively for voice circuits but had only three slots, with larger guard times allowing for longer propagation delays. The two-voice-slot simulations were the same, except that the usual four-slot bundles and the occasional three-slot bundles each contained only two voice slots, with the remaining slot(s) reserved for data circuits, whose future introduction and growth were not explicitly modeled in these simulations. The results are summarized in the following table.

6 This number was rounded down to 700 as a component of a conservative estimate of remaining capacity discussed in Section 3.2.

If System Converts Completely to VDL3 U.S. ATS If All-25AM System Grows to Upon Reaching the Given Size: System Size Given Size: 4 Voice Slots Assumed 2 Voice Slots Assumed (Required Circuits) Frequency Neighbors Frequency Neighbors Frequency Neighbors Denials Repacked Denials Repacked Denials Repacked 6,675a 0 0 0 c 0 c 7,436b 0 0 0 c 0 c 7,936 0 24 0 1,042 0 1,674 8,186 1 61 0 1,304 0 1,908 8,436 1 140 0 1,443 0 2,095 8,936 18 267 0 1,888 1 2,230 a. Number of existing U.S. ATS circuits (February 2002). b. Total number of existing and pending U.S. ATS circuits (February 2002). c. Not simulated.

Observations • A successful transition to VDL3 appears to be feasible, using repacking, even when the existing 25AM system has reached its spectral “capacity” (defined as the system size at which frequency denials start to become necessary under the proportional- growth assumption used throughout this analysis). • The neighbor-repacking actions needed for the transition to VDL3 grows faster as the system becomes more saturated. This will increase the cost of transition. • It does not seem necessary to allow for a spectral reserve in planning the timing of the VDL3 IOC—although, in view of the many assumptions, uncertainties, and costs involved, it would still be prudent to schedule the IOC well before the date when frequency denials are expected to start in the all-25AM system.

Appendix C Circuit Assignment Changes from 1998 to 2002 by Region and Altitude

The following charts capture circuit changes for the period from September 1998 to February 2002 by Federal Aviation Administration (FAA) region and by altitude. Each chart captures circuit additions and deletions for an FAA region. The charts on the left are FAA circuit changes and the charts on the right are circuit changes by other agencies (lumped into a single category called “Other”). The scales on all charts are equal for comparison purposes.

Regions: AL − Alaska, CE − Central, EA − Eastern

FAA Circuit Change by Flight Level 98 to 02 AL Region (No Pending Records) Other Circuit Change by Flight Level 98 to 02 AL Region (No Pending Records)

500-549 500-549

400-449 400-449

300-349 300-349

200-249 200-249

100-149 100-149

0-49 0-49 -100 -50 0 50 100 150 -100 -50 0 50 100 150

FAA Circuit Change by Flight Level 98 to 02 CE Region (No Pending Records) Other Circuit Change by Flight Level 98 to 02 CE Region (No Pending Records)

550-600 550-600 500-549 500-549 450-499 450-499 400-449 400-449 350-399 350-399 300-349 300-349 250-299 250-299 200-249 200-249 150-199 150-199 100-149 100-149 50-99 50-99 0-49 0-49 -100 -50 0 50 100 150 -100 -50 0 50 100 150

FAA Circuit Change by Flight Level 98 to 02 EA Region (No Pending Records) Other Circuit Change by Flight Level 98 to 02 EA Region (No Pending Records)

550-600 550-600 500-549 500-549 450-499 450-499 400-449 400-449 350-399 350-399 300-349 300-349 250-299 250-299 200-249 200-249 150-199 150-199 100-149 100-149 50-99 50-99 0-49 0-49

-100 -50 0 50 100 150 -100 -50 0 50 100 150

Regions: GL − Great Lakes, NE − New England, NM − Northwest Mountain

FAA Circuit Change by Flight Level 98 to 02 GL Region (No Pending Records) Other Circuit Change by Flight Level 98 to 02 GL Region (No Pending Records)

FAA Circuit Change by Flight Level 98 to 02 NE Region (No Pending Records) Other Circuit Change by Flight Level 98 to 02 NE Region (No Pending Records)

550-600 550-600 500-549 500-549 450-499 450-499 400-449 400-449 350-399 350-399 300-349 300-349 250-299

250-299 200-249

200-249 150-199

150-199 100-149

100-149 50-99 50-99 0-49

0-49 -100 -50 0 50 100 150

-100 -50 0 50 100 150

FAA Circuit Change by Flight Level 98 to 02 NM Region (No Pending Records) Other Circuit Change by Flight Level 98 to 02 NM Region (No Pending Records)

550-600 550-600 500-549 500-549

450-499 450-499 400-449 400-449

350-399 350-399

300-349 300-349 250-299 250-299 200-249 200-249 150-199 150-199 100-149 100-149 50-99 50-99 0-49 0-49 -100 -50 0 50 100 150 -100 -50 0 50 100 150

Regions: SO − Southern, SW − Southwest, WP − Western Pacific

FAA Circuit Change by Flight Level 98 to 02 SO Region (No Pending Records) Other Circuit Change by Flight Level 98 to 02 SO Region (No Pending Records)

550-600 550-600

500-549 500-549

450-499 450-499

400-449 400-449

350-399 350-399

300-349 300-349 250-299 250-299 200-249 200-249 150-199 150-199 100-149 100-149 50-99 50-99 0-49 0-49 -100 -50 0 50 100 150 -100 -50 0 50 100 150

FAA Circuit Change by Flight Level 98 to 02 SW Region (No Pending Records) Other Circuit Change by Flight Level 98 to 02 SW Region (No Pending Records)

550-600 550-600

500-549 500-549

450-499 450-499

400-449 400-449

350-399 350-399

300-349 300-349

250-299 250-299

200-249 200-249

150-199 150-199

100-149 100-149

50-99 50-99

0-49 0-49

-100 -50 0 50 100 150 -100 -50 0 50 100 150

FAA Circuit Change by Flight Level 98 to 02 WP Region (No Pending Records) Other Circuit Change by Flight Level 98 to 02 WP Region (No Pending Records)

550-600 550-600

500-549 500-549

450-499 450-499

400-449 400-449

350-399 350-399

300-349 300-349

250-299 250-299

200-249 200-249

150-199 150-199

100-149 100-149

50-99 50-99

0-49 0-49

-100 -50 0 50 100 150 -100 -50 0 50 100 150

Appendix D Programmatic Circuit Demand Estimate Through 2020

This appendix contains the derivation of circuit demand to meet the future air/ground (A/G) communication requirements implied in the Aviation Capacity Enhancement (ACE) Plan, Operational Evolution Plan (OEP), and conversations with personnel in the Federal Aviation Administration (FAA) and The MITRE Corporation’s Center for Advanced Aviation System Development (CAASD) regarding sector splits and airspace redesign.

Requirement En route Terminal Broadcast Total New Runways - 224 - 224 En Route Sector Splits 60 - - 60 Terminal Airspace Redesign - 35 - 35 Super High Altitude Sectors 6 - - 6 AWOS/ASOS/ATIS - - 1,130 1,130 Chokepoints 62 46 - 108 Total 128 305 1,130 1,563 AWOS: Automated Weather Observing System ASOS: Automated Surface Observing System ATIS: Automatic Terminal Information Service

1. New Runways The 2001 Aviation Capacity Enhancement Plan, pp. 36 – 38, 112 – 113, identifies 40 planned new runways in the year 2000 or later. The 224-required-circuits estimate was derived from the ACE Plan in the following fashion. There are 24 new runways scheduled for completion by the end of 2010, 8 scheduled for completion between 2011 and the end of 2020, and 8 whose completion date is “to be determined (TBD).” The 8 TBD runways were split before and after 2011. The scheduled runways before 2011 thus totaled 28. This estimate of new runway growth was therefore doubled to 56 new runways by the end of 2020. Each new runways was estimated to require 4 circuits. Therefore 56 new runways each requiring 4 circuits produces an estimate of 224 circuits. 2. Sector Splits A list of needed circuits was derived by CAASD in an unpublished 1998 study by applying the sector size criterion to the sector crossing times forecast using Detailed Policy Assessment Tool (DPAT) and applying the sector congestion criteria to the sector densities forecast using DPAT.

General criteria for sector splitting are noted in FAA Order 7210.46 (FAA, 1984). It was assumed that an en route sector would require splitting if it is large enough to split and sufficiently congested sufficiently often. Specifically: • A sector with a crossing time of at least six minutes is large enough to split. Splitting a sector with a crossing time less than six minutes would create a pair of sectors, one of which would have a crossing time of less than three minutes, which would be smaller than crossing times of all but one of the current en route sectors.7 • A sector is sufficiently congested to split if the number of aircraft in the sector, averaged over 15 minutes8 (which we call the sector density) exceeds the sector’s monitor alert (MA) threshold at any time during a day. • A sector is congested sufficiently often if it is sufficiently congested on 1 or more of 3 simulated (scenario) days (this is the worst-case criterion) or on 2 or more of 3 scenario days (this is the conservative criterion). Each new sector created by splitting would need a new air traffic control (ATC) A/G voice circuit. The DPAT “run script” for each scenario runs DPAT and then invokes other scripts, one of which ranks en route sectors by density, which facilitates the search for sectors that meet the density criteria for splitting. The following table, showing postulated new en route sectors, exhibits the results of the analyses using the worst-case criteria. A key to table column headings follows the table.

7 ZMP034 has an average crossing time of just under two minutes.

8 A peak instantaneous aircraft count criterion was also investigated.

Postulated New En Route Sectors (Page 1 of 2)

Sq Year Sector MA N Dnsty Diff FTime Radius Latitude Longitude Low Hgh 1 2000 ZAB038 18 3 29.00 11.00 13.86 103.4801 33.9171 110.9600 0 999 2 2000 ZKC041 8 3 11.90 3.90 19.20 149.2172 39.5099 96.4708 370 999 3 2000 ZFW093 18 2 22.20 4.20 14.82 97.1311 33.4739 101.9490 270 999 4 2000 ZFW090 18 2 21.70 3.70 11.96 113.3143 33.4273 94.2698 240 999 5 2000 ZAB068 18 1 22.50 4.50 13.01 101.6211 34.4590 107.5290 270 999 6 2000 ZOA031 18 1 20.20 2.20 16.60 146.5454 39.8626 121.8400 240 999 7 2000 ZFW082 18 1 19.30 1.30 17.47 132.5324 32.0987 101.9240 240 999 8 2000 ZID088 14 1 15.30 1.30 9.30 75.8359 40.2938 84.1586 231 329 9 2000 ZOA033 18 1 18.30 0.30 12.03 76.6902 38.0430 117.9730 240 999 10 2000 ZKC002 15 1 15.20 0.20 14.19 102.8879 38.1203 95.9003 240 369 11 2000 ZLC005 18 1 18.20 0.20 14.24 104.4678 41.9092 108.6040 0 999 12 2000 ZID087 14 1 14.10 0.10 8.73 78.3914 40.1160 82.5956 231 349 13 2005 ZOA013 15 3 25.20 10.20 11.07 93.9690 36.8653 121.2890 240 999 14 2005 ZLA033 15 2 16.20 1.20 12.29 90.0751 37.0082 113.1530 240 999 15 2005 ZLA026 14 1 19.80 5.80 7.29 85.2323 35.0804 119.5480 240 999 16 2005 ZOA015 15 1 17.90 2.90 11.00 83.0335 36.5732 119.0810 240 999 17 2005 ZLC045 23 1 25.10 2.10 19.97 111.5920 38.5987 115.2170 0 999 18 2005 ZHU042 18 1 20.00 2.00 12.96 95.5636 31.2807 91.7305 240 999 19 2005 ZMP040 21 1 22.90 1.90 21.25 150.6634 40.7855 96.0048 370 999 20 2005 ZID084 14 1 15.80 1.80 8.36 73.2732 37.0997 84.4338 231 349 21 2005 ZLC042 20 1 21.20 1.20 19.37 157.1458 40.8351 116.2270 0 999 22 2005 ZMP039 24 1 25.10 1.10 15.53 99.6280 40.6030 97.3890 240 369 23 2005 ZLA027 15 1 15.50 0.50 10.51 68.8133 35.1622 118.5700 240 999 24 2005 ZDC003 16 1 16.40 0.40 11.66 62.3890 38.9406 79.5137 241 999 25 2005 ZLC032 13 1 13.30 0.30 11.85 103.8322 40.0382 112.0440 0 329 26 2005 ZHU038 14 1 14.10 0.10 11.44 62.8538 30.6306 94.1659 0 239 27 2010 ZAB079 18 2 20.90 2.90 15.49 144.4575 35.5235 107.5100 370 999 28 2010 ZHU068 18 1 20.10 2.10 17.66 150.5932 28.9512 93.2444 240 999 29 2010 ZME021 18 1 19.30 1.30 13.85 116.0079 34.6156 92.8505 240 349 30 2010 ZSE013 16 1 16.80 0.80 21.07 113.0973 42.7514 120.1970 240 999 31 2010 ZLA036 15 1 15.50 0.50 13.84 121.9577 35.3575 113.4590 350 999 32 2010 ZLC020 18 1 18.30 0.30 24.13 147.6826 47.4367 112.0370 300 999 33 2010 ZMP030 21 1 21.30 0.30 13.98 102.6610 42.4624 94.2114 240 999

Postulated New En Route Sectors (Page 2 of 2)

Sq Year Sector MA N Dnsty Diff FTime Radius Latitude Longitude Low Hgh 34 2010 ZKC006 15 1 15.20 0.20 12.85 76.9865 36.8153 98.6533 240 389 35 2015 ZKC020 15 2 15.40 0.40 12.10 96.3228 38.4021 99.4860 240 389 36 2015 ZHU074 18 1 20.40 2.40 10.20 90.1190 30.1859 98.7806 240 999 37 2015 ZKC032 15 1 16.70 1.70 10.37 97.0115 40.2685 92.1962 240 329 38 2015 ZID083 15 1 16.50 1.50 8.84 73.3303 38.1064 83.9696 231 349 39 2015 ZDV014 18 1 19.10 1.10 13.19 118.2171 40.6436 106.6180 270 999 40 2015 ZLC006 16 1 16.90 0.90 19.64 127.6245 45.4157 112.3020 0 999 41 2015 ZSE016 15 1 15.90 0.90 13.43 102.9740 45.1397 119.9370 240 999 42 2015 ZME031 18 1 18.50 0.50 11.24 90.9236 34.7231 87.9984 240 999 43 2015 ZLA039 16 1 16.20 0.20 10.90 99.2134 34.1552 115.2170 240 999 44 2015 ZDV030 20 1 20.10 0.10 12.59 120.9623 38.1984 104.7600 350 999 45 2015 ZLC040 15 1 15.10 0.10 9.23 94.2276 40.9281 113.4640 0 999 Columns: Sq Sequence Number Year Year in which the sector is first considered congested Sector Sector Name MA Monitor Alert Threshold N Number of Scenario Days with Sector Density over Threshold Dnsty Largest Sector Density among the Scenario Days Diff Difference between Largest Sector Density and Monitor Alert Threshold FTime Average Sector Crossing Time (minutes) Radius Radius of Coverage Cylinder (nmi) Latitude Latitude of Center of Coverage Cylinder (degrees, North Latitude positive) Longitude Longitude of Center of Coverage Cylinder (degrees, West Longitude positive) Low Floor of Coverage Cylinder (100's of feet above mean sea level) Hgh Ceiling of Coverage Cylinder (100's of feet above mean sea level). A value of 999 indicates that the sector has no ceiling.9 Class A airspace extends up to FL 600 (60K MSL). The top altitudes for commercial jets would be in the low FL 400's. High performance business jets will go all the way up to FL 600.

3. Terminal Airspace Redesign The 1998 CAASD study showed where expected traffic growth would create the need for terminal and transition airspace redesign up until the year 2015. The terminal airspace restructuring would typically involve sector splitting requiring additional circuits. The following table gives the total number of circuits needed in each region based on the 1998 study.

9 Class A airspace extends up to flight level (FL) 600 (60 thousand feet above mean sea level [AMSL]). The top altitudes for commercial jets are slightly above FL400. High performance business jets will go all the way up to FL600.

Circuit 5and for Terminal Area Redesign Through 2015

Region Number of Circuits AEA 2 ACE 1 AGL 2 ANM 1 ASO 4 ASW 3 AWP 13 Total 26

The OEP gives specific locations for terminal airspace redesign through 2007. Redesigns are scheduled to occur in the following areas: Potomac (Washington, D.C.), Los Angeles basin, Northern California, Houston, Great Lakes corridor, St. Louis and the New York, New Jersey, and Philadelphia metro region. These anticipated redesigns were not “forced” to occur in Prospector as were the en route sector splits. Area navigation or RNAV is a procedure change that could be utilized around airports where the airspace structure is a key limiting factor to traffic. RNAV is an umbrella term that encompasses any procedure or operation that utilizes a path/route for navigation by the aircraft. Development and implementation of RNAV routes at multiple airports is anticipated in the OEP report. If RNAV alleviates some of the congestion in terminal area sectors, it could serve to postpone sector splitting. However, there are no performance or circuit demand estimates on which to base a change in the DPAT forecast. Thus, this analysis put the DPAT generated circuit demand analysis into the programmatic demand forecast. 4. High Altitude Sectors These are described in the FAA document Strategic Vision for the Provision of Air Traffic Services, March 2002 (Version 1.1), prepared by ATP-400/ASD-103. Three new sectors were identified through 2010. This estimate was doubled for the 20-year forecast. The document states: There are three high altitude facilities - East, Mid-West and West. In this structure, major traffic flows are captured intra-facility rather than across multiple facilities and boundaries. This allows for the span of control to tactically manage these flows in a single facility, improving the allocation of airspace along the flows and providing an organizational and operational structure that will allow

each flight to establish its preferred path to the transition point for arrival. These facilities will initially be established at flight level 350 and above to reduce the inherent problems due to legacy aircraft equipage. Over time this base altitude may be lowered to reflect the changing demographics of the fleet.

ZSE

ZMP ZBW ZLC

ZAU ZOB ZOA ZDV ZNY ZID ZKC ZDC ZLA

ZAB ZME ZTL ZFW

ZJX ZHU

ZMA

Figure 4. High Altitude Sector Concept Diagram

5. AWOS/ASOS/ATIS The table below shows the estimated growth in AWOS/ASOS/ATIS transmitters in each FAA region.

AWOS Installation Forecast by Region

Year New Cum New AAL ACE AEA AGL ANE ANM ASO ASW AWP 2001 35 35 1 3 5 8 1 4 5 6 3 2002 40 75 2 6 10 17 3 8 11 12 6 2003 60 135 4 11 17 31 5 14 19 22 10 2004 60 195 6 16 25 45 8 21 28 31 15 2005 60 255 8 21 33 59 10 27 37 41 20 200665320102641751234465225 200770390123250911541566330 2008 75 465 14 38 60 108 18 49 67 75 36 2009 80 545 16 44 70 127 21 58 78 88 42 2010 85 630 19 51 81 147 24 67 91 102 49 2011 90 720 21 59 93 168 28 76 104 116 55 2012 95 815 24 67 105 190 31 86 117 131 63 2013 100 915 27 75 118 213 35 97 132 148 70 2014 105 1020 30 83 132 237 39 108 147 164 79 2015 110 1130 34 92 146 263 44 120 163 182 87 The column “Cum New” is a cumulative count by year of projected AWOS installations.

6. Chokepoints Detailed airspace redesign to reduce traffic complexity has been made by CAASD for airspace over Chicago, Boston, Indianapolis, Minneapolis, New York, and Cleveland. Thirty-one new sectors to deal with chokepoints have been identified by location and shape. Prospector inputs for these sectors were created. Because they are near term and since these sectors will create circuit demand at high (and ultra-high) altitude, it was important that they appear in the Prospector results. Therefore, they were the first assignments made in the Prospector run before any other circuit demand was considered. There is more involved in airspace redesign than just to alleviate chokepoints. Eventually, some high altitude sector redesign might reduce the total count of sectors. The OEP points to several changes that may involve additional new sectors. The cloning process in Prospector anticipates additional high altitude sector splitting throughout the United States. Again in 1998, CAASD simulations showed that 54 sectors in total by 2010 would require splitting to alleviate excessive congestion. The programmatic forecast calls for 54 sector splits that include the 31 high-altitude sectors needed to alleviate chokepoints. The 10-year chokepoint estimate was doubled to produce the 20-year estimate.

Glossary

A/G air/ground ACE Aviation Capacity Enhancement ADS-B Automatic Dependence Surveillance Broadcast AL Alaska Region AM amplitude modulation AMSL above mean sea level ASOS Automated Surface Observing System ASR Office of Spectrum Management and Policy ATC air traffic control ATIS Automatic Terminal Information Service ATS Air Traffic Services AWOS Automated Weather Observing System

CAASD Center for Advanced Aviation System Development CE Central Region CPDLC Controller/Pilot Data Link Communications

DPAT Detailed Policy Assessment Tool

EA Eastern Region

FAA Federal Aviation Administration FL flight level

GL Great Lakes Region GMF Government Master File

IOC initial operational capability kft thousand feet kHz kilohertz

LAAS Local Area Augmentation System

MHz megahertz

NARC NEXCOM Aviation Rulemaking Committee NAS National Airspace System NE New England Region

NEXCOM Next-Generation Air/Ground Radio Communications System NM Northwest Mountain Region

OEP Operational Evolution Plan

RVSM Reduced Vertical Separation Minima

SO Southern Region SV service volume SW Southwest Region

TBD to be determined TDMA time-division multiple access

VDL3 VHF Digital Link Mode 3 VHF Very High Frequency VOR VHF Omnidirectional Range

WAAS Wide Area Augmentation System WP Western Pacific Region