Air traffic controllers monitor and direct traffic in a designated volume of airspace called a sector. Each sector requires a separate channel assignment for controllers to communicate with aircraft flying in that sector. As the amount of air traffic grows, the need for additional sectors and channel assignments also increases. FAA’s present air-ground communications system operates in a worldwide, very high frequency (VHF) band reserved for safety communications within the 118 to 137 megahertz (MHz) range. Within this range of frequencies, FAA currently has 524 channels available for air traffic services. During the past four decades, FAA has primarily been able to meet the increased need for more channel capacity within this band by periodically reducing the space between channels (a process known as channel splitting). For example, in 1966, reducing the space between channels from 100 kHz to 50 kHz doubled the number of channels. The last channel split in 1977, from 50 kHz to 25 kHz, again doubled the number of channels available. Each time FAA reduced this space, owners of aircraft needed to purchase new radios to receive the benefits of the increased number of channels. FAA can use or assign its 524 channels several times around the country (as long as channel assignments are separated geographically to preclude frequency interference). Through channel reuse, FAA can make up to 14,000 channel assignments nationwide. While aviation literature often refers to channel and channel assignments as frequency and frequency assignments, throughout this report, we use the terms channel and channel assignments. Because the growth in air traffic during the past decade has created a need for more communications channels since the 1977 split, FAA has been increasingly concerned that the demand for channels would exceed their availability, which would cause frequency congestion. FAA first explored this issue at length at a 1990 International Civil Aviation Organization (ICAO) conference, at which the ICAO member countries addressed increasing congestion in the air traffic control communications band and the especially acute problem in the U. S. and Western Europe. Over the next 5 years, ICAO evaluated different solutions that were proposed by the conference’s participants. While the Western European countries proposed further channel splitting to increase capacity, FAA proposed a totally new air-ground communications system. FAA’s proposed technology, known as VDL-3, would be based on a new integrated digital voice and data communications technology, which would assign segments of a channel to users in milliseconds of time, thereby allowing both voice and data to travel over the same channels using one of the available time slots. Under the current system, each channel is used exclusively and continuously for voice, so the air traffic controller can communicate at all times with the aircraft. This new technology could provide up to a fourfold increase in capacity without channel splitting, thus meeting the demand for new voice channels. VDL-3 digitizes a person’s voice and sends it as encoded bits of information, which is reassembled by the receiver. Moreover, this technology could provide real-time data link on-board communications of air traffic control messages and events. Although ICAO adopted FAA’s proposed digital air-ground communications system VDL-3 in 1995 as its model for worldwide implementation, it also approved standards allowing Western Europe, which was then experiencing severe frequency congestion, to further reduce the spacing between channels from 25 kHz to 8.33 kHz. While this action tripled the number of channels available for assignment, it also resulted in the need for aircraft flying in Western Europe to install new radios that are capable of communicating over the 8.33 kHz channels. ICAO intended that this reduction would be an interim measure until 2004, when FAA estimated that the technology it had proposed would be operational. However, FAA did not pursue developing VDL-3 in 1995, in part, because its existing communications system still had available capacity to meet near-term communications needs, and because the agency’s need to modernize its air traffic control system became an urgent priority. In 1998, FAA resumed developing VDL-3; however, the agency is not expected to implement this technology until 2009. Figure 1 depicts how channel splitting has increased channel capacity since 1966 and how FAA’s proposed use of VDL-3 will further increase channel capacity. FAA has identified 23 measures to improve its existing voice communications system. While FAA and the U. S. aviation industry generally believe that implementing all these measures would add several years to the useful life of the existing system, they believe it would not meet aviation’s future voice communications needs beyond 2009. Because increases in air traffic create the need for more channel assignments, the events of September 11, which have resulted in slower than expected increases, might delay by a year or two when FAA starts to encounter problems systemwide in providing new channel assignments. Agency and industry representatives agree that it is not possible to precisely predict when the existing system with its planned improvements will no longer meet aviation’s needs. As a result, FAA plans to annually assess whether this system will be capable of meeting the projected need for more channel assignments for at least 5 years into the future. FAA plans to release the first of these annual assessments in September 2002. While the focus of FAA’s efforts has been to meet aviation’s need for voice communications through 2009, FAA recognizes that its data communications needs are evolving. The agency expects to increase its use of data communications to help alleviate voice congestion and to help controllers and pilots accurately exchange more information. Because FAA’s current system cannot do this, it has been leasing data link services from ARINC. However, even with the planned improvements, this service will not be able to meet FAA’s projected need for more data communications. As FAA relies more on data communications, this leased system will not be able to meet the agency’s need to prioritize those messages that must be delivered expeditiously. Recognizing that accurately projecting the growth in aviation’s need for data link communications beyond 15 years would be difficult, FAA is designing a system to provide a sevenfold increase in capacity to meet future needs. During the 1990s, several of FAA’s studies found that, historically, increases in air traffic were closely related to the growing need to assign more channels for voice communications (see fig. 2). In its most recent study about the growing need for more channel assignments for voice communications, FAA found that this need had grown annually, on average, about 4 percent (about 300 new channel assignments) since 1974 (see fig. 3). This growth paralleled the increase in domestic air travel during that time frame. Despite the recent downturn in air traffic resulting from a recession and the September 11 terrorist attacks, FAA expects it to resume its historical 4-percent annual growth within a year or two. Currently, FAA’s voice communications system is limited to a maximum of 14,000 channel assignments. Because increases in air traffic require more new channel assignments, FAA expects that providing them in some metropolitan areas will become increasingly difficult. If the system is left unchanged, FAA has concluded that, as early as 2005, it could no longer fully support aviation’s need for voice communications and that in such high traffic metropolitan areas as New York, Chicago, and Los Angeles the need for additional assignments could be evident sooner. Because FAA has delayed NEXCOM’s implementation until 2009, the agency’s 23 planned improvement measures are designed to add approximately 2,600 additional channel assignments for voice communications. (See table 1.) FAA has classified these initiatives, which involve a variety of technical, regulatory, and administrative changes, according to how soon it expects to implement them. However, FAA recognizes that there is no guarantee that all of these measures can be implemented because some of them largely depend on gaining agreement from other entities, such as other federal agencies and the aviation community, and some may involve international coordination. FAA also recognizes that the exact degree of improvement resulting from the totality of these measures cannot be precisely projected and actual test results could show less gain than anticipated. Many of these initiatives involve reallocating channels being used for purposes other than air traffic services and increasing FAA’s flexibility to use already assigned channels. For example, FAA is reviewing its policy for assigning channels to such special events as air shows to determine if fewer channels could be assigned to them so that channels could be used for other purposes. While it is not possible to predict exactly when FAA’s existing voice communications system will run out of available channel assignments, agency and aviation representatives concur that, without the 23 improvement measures, the system will be strained to provide enough channel assignments. According to a MITRE Corporation study completed in 2000, even if the need for more channel assignments for voice communications were to grow at 2 percent per year (instead of FAA’s projected growth of 4 percent per year), by 2005 or sooner, it would be difficult for FAA to meet the need for air traffic communications in major metropolitan areas. MITRE also projected that the shortage of available channel assignments would become a nationwide problem by 2015 or sooner. In 2000, FAA first encountered a shortage problem when it had to reassign a channel from one location to another that FAA viewed as a higher priority in the Cleveland area. Figure 4 shows MITRE’s analysis of how the projected demand for more voice communications capacity will intensify if FAA does nothing to improve this system. Currently, FAA is leasing ARINC’s Aircraft Communications Addressing and Reporting System (ACARS) to provide data link communications that are not time critical, such as forwarding clearances to pilots prior to takeoff. Because this analog system is also reaching its capacity to handle data link communications, FAA plans to use ARINC’s new digital data communications system, known as Very High Frequency Digital Link Mode 2 (VDL-2) until 2009. By then, FAA expects to use its VDL-3 system, which is being developed to integrate voice and data communications, to meet aviation’s needs for about 1,800 channel assignments for data communications over the next 15 years and to prioritize messages that must be delivered expeditiously, which VDL-2 cannot provide.