Approaching ILS in NextGen
Embry-Riddle Aeronautical University

Abstract
Currently the Federal Aviation Administration (FAA) and the aviation industry are working towards a revolutionary revitalization of how air traffic will be controlled in National Airspace System (NAS) in the future. The goal of the effort behind the Next Generation Air Transportation System (NextGen) is to provide for safer and more efficient operations in the NAS. The backbone of this modernization will rely on celestial based systems such as the Global Position System (GPS) to provide the primary source of all navigation information during all phases of flight from taxiing, take-off, enroute, approach, and landing. It is the final two phases of flight, approach and landing, that require the greatest level of precision and continuity, and which will witness the largest change; the eventual elimination of the Instrument Landing System (ILS). Today, ILS offers the most accurate method of safely guiding pilots into a landing in low to almost no visibility conditions. NextGen plans for the elimination of this system in favor of a combination of GPS and Automatic Dependent Surveillance-Broadcast (ADS-B), and eventually ILS would not even be available for redundancy purposes. For such a critical phase of flight not only is an ILS currently the most accurate system, it should also be available for the foreseeable future to mitigate the lapses in the available GPS and INS technologies.

Keywords: Instrument Landing System (ILS), Next Generation Air Transportation System (NextGen), Air Traffic Control (ATC), Global Positioning System (GPS), Wide Area Augmentation System (WAAS), Local Area Augmentation System (LAAS)

Approaching ILS in NextGen It is a transformative time for the aviation industry; great strides in the evolution of technology are providing an opening for revolutionary change in how aircraft transit the firmament. The need for a change has never been clearer, the Federal Aviation Administration (FAA) forecasts that the demand for air travel will increase by 71 percent over current levels by the year 2027 (JPDO, 2010). Such a demand, as well as the need to increase safety, security, and efficiency, could not be met by the current “human-centric” method of air traffic control (ATC) and technology (JPDO, 2007). To meet the air travel needs of the near future and those of the year 2027, the FAA and the aviation industry have developed and initiated a program known as the Next Generation Air Transportation System, usually referred to as NextGen The goal of NextGen is to modernize the National Airspace System (NAS) by increasing its capacity and efficiency while also improving security, safety, and environmental impact. To accomplish these goals the FAA will leverage current and emerging technologies in conjunction with new airborne and traffic management procedures (FAA, 2010). Among both the current and emerging technologies to be utilized in NextGen, the systems are primarily centered on putting more performance and traffic management responsibilities into airborne avionics, which is a significant shift from the current system of ground based ATC. This shift is a necessity due to the practical impossibility of scaling up the current ATC system and its personnel to address the anticipated doubling of traffic in the NAS in the coming decades (FAA, 2010). Among the systems that will be the foundation of NextGen are the Global Positioning System (GPS) in Earth orbit, ground based Wide Area Augmentation System (WAAS) and Ground-Based Augmentation System (GBAS), and Automatic Dependent Surveillance-Broadcast (ADS-B) aboard aircraft (JPDO, 2007). These systems, and others, will be used to replace terrestrial based radio navigation systems such as Very High Frequency Omnidirectional Range (VOR) and landing approach systems such as the Instrument Landing Systems (ILS). Although there are significant economic gains to the elimination of such ground based radio beacons, there is a concern about the pace and scope of the elimination of these navigation beacons, particularly in light of the dismantling of ILS installations which guard over a crucial phase of flight. ILS provides an accurate depiction of an aircraft’s position as it approaches a runway, allowing an aircrew to safely guide their craft as it descends closer to the ground (Higdon, 2011).
Due to the needs of today’s air traffic, and the demands of the future, it is necessary to begin a long needed revitalization of how the NAS is controlled. However, eliminating critical safety systems such as ILS simply because of the longevity of the system is not a safe tactic. Although future systems offered by NextGen offer some advantages over the current method of precision approach, there are drawbacks to each of these systems in the near future. Furthermore, in the effort to gain savings in the current system, there has been a push to eliminate ILS installations at a quicker pace than initially proposed, without the necessary technological maturation and deployment of the replacing technology (NTSB, 2008). Lastly, as ILS installations are reduced, there will be decreased availability of any fallback systems in the case of any failure in NextGen systems.
Instrument Landing System To understand the role that ILS has had, and may have, in the NAS, it is necessary to first understand its role in making flying safer and more reliable as the NAS developed, as well as the benefits and limitations of an ILS. An instrument landing system (ILS) is a ground based system that utilizes a combination of radio signals that depict course and glideslope information on cockpit displays, marker beacons, and airfield lighting arrays which assist pilots to perform safe landings in conditions of low-visibility. ILS traces its roots back to the earliest days of aviation. The system, which was developed as a combination of two prevailing navigation systems at the time, radio beacons and lighted airways, was first tested in 1929. The testing and development of the system continued into the 1930s and was first used for navigation in 1938 by a Pennsylvania-Central Airlines Boeing 247-D during a snowstorm as its sole manner of navigation and landing guidance. By 1941 the Civil Aeronautics Administration (CAA), the forerunner to the FAA, authorized its installation at six airports (Komons, p. 110). Following World War II, the pace of installation and development quickened, eventually becoming the primary system for precision low visibility approaches.
Figure 1: ILS Localizer Signal Pattern. (Source: MacDonald, 2011)
Figure 1: ILS Localizer Signal Pattern. (Source: MacDonald, 2011) ILS directional information is transmitted from ground equipment that transmits by discreet radio frequency received on an aircraft’s navigation radio, which is then interpreted and projected to flight crew by cockpit instruments. The ground equipment is comprised of two directional transmitters; the primary component is the localizer which provides lateral guidance, the other component is the glideslope which provides vertical guidance (DOT, p. 523). As depicted in Figure 1, the localizer transmitter is aligned with the centerline and located at the opposite end of the runway from the approach it is directing. For example, if a runway is designated as 9 on one end and 27 on the other, the ILS localizer transmitter directing the approach to runway 27 will be located at the 9 end. In reference to Figure 2 below, the glideslope is transmitted from a separate location from the localizer; typically the glideslope transmitter is offset from the runway centerline by 400 to 600 feet, and located 750 to 1,250 feet down the runway from the threshold. Using the previous example, the location of the glideslope transmitter would be offset from the threshold of runway 27 (MacDonald, 2011). Other components of the system include two to three marker beacons – inner, middle, and outer – which are low-powered transmitters radiating an elliptical beam upward from the ground along the approach path to indicate the progression of the aircraft along the approach path. At some locations, due to terrain limitations, an outer marker may not be available (DOT, p. 524). Other elements of the system include specific runway lighting to assist the flight crew with identifying the runway environment.
Figure 2: Glideslope Signal Pattern. (Source: MacDonald, 2011)
Figure 2: Glideslope Signal Pattern. (Source: MacDonald, 2011) Although a majority of the time the systems offer a precise measurement and depiction of the flight path of an approach, there are limitations to the installations. On a technological level, the radio transmissions are susceptible to errors inherent to such devices; the signals can be blocked by air field buildings, topography, and the localizer can be interrupted by aircraft taking off over the transmitter. Also, without significant investments in additional equipment to form complex approach paths, ILS is limited to straight-in approaches, which inhibits the use of more efficient routes without the diligence and direct control by ATC specialists providing vectoring guidance. Of more concern for the system are the costs imposed. Since each runway needs to have specific installation, for an airfield to have full ILS coverage, the operators must install separate systems for each runway direction; meaning that one runway would require two systems to enable operations at each end of a runway. The financial impact to operators and the FAA of these multiple installations is added to by the continuing operational costs. Those costs include, but are not limited to, the power needed for operation, certification requirements, and continuing maintenance (MacDonald, 2011). These costs have been estimated by the FAA to be more than 110 million dollars a year, due significantly to the over 80 percent of the 967 VORs that are past their economic service life. These costs are also forecasted to continue to rise as replacement parts for these legacy systems are “becoming increasingly difficult to obtain” (FAA, 2012). A fundamental requirement of any future system is the need for it to improve not only the accuracy of the current ILS, but to significantly reduce the economic impact of the labor and equipment costs of ILS equipment.
A NextGen Approach A key goal of NextGen is to reduce many of the financial burdens caused by legacy ATC system such as ILS. To accomplish this, the NextGen system proposes the use of a combination of terrestrial, celestial, and aircraft based systems to provide enroute and approach navigation. Although the NextGen systems would still require infrastructure at airfields, it would be a less substantial presence on an airfield, with a corresponding reduced demand for maintenance than the existing system. The purpose of new ground-based infrastructure that would replace the legacy systems is to provide the accuracy needed for navigation and approaches, as well as correct for the inherent and potential problems of the Global Positioning System (GPS). Since a discussion of the intricacies of the functioning of the NextGen system is beyond the scope of this paper, the following is a brief overview of the components of the system that are intended to replace legacy ILS installations. The thirty-one actively broadcasting and two spare orbiting satellites of the Global Positioning System (GPS) are the primary components of the navigation and positional data for the NextGen system. As has already begun, in the new system signals from the GPS are the basis by which aircraft establish their position through all phases of flight. Furthermore, the system’s signals are used to establish navigational information. However, as accurate as the signals from the GPS are, those signals are not considered accurate or reliable enough for precision approaches. These issues are due to errors caused by the distance of approximately 12,600 to 16,500 miles that the signals have to travel from the satellites; a delay error that can result in discrepancies of as much as 50 meters (Higdon, 2011). Significant impacts on the reliability of the system can also be attributed to unintentional outages, or intentional outage initiated by the United States government for testing, or national security issues (FAA, 2011). Other errors can also be introduced into the system, including effects from the atmosphere, solar radiation, time-keeping offsets, and frequency interference (Bar-Sever & Kuang, p. 2).
Figure 3: WAAS. (Source: USCG, 2006)
Figure 3: WAAS. (Source: USCG, 2006) To compensate for these errors and to provide the greater accuracy required for precision approaches, NextGen will utilize a combination of new systems to augment the GPS. The first system developed by the FAA to augment the GPS is Wide Area Augmentation System (WAAS), which has been operational since 2003 for general aviation use, and approved for commercial passenger carrying operations since 2009 (Alaska Airlines, 2010). WAAS, through a combination of satellites, ground stations, and user-units aboard aircraft, provides a correction to GPS signals to increase the accuracy and reliability of the system. As depicted in Figure 3, the system monitors GPS signals for small variations, and when there are deviations a correction signal is transmitted to WASS satellites which then transmit the signals back to Earth. WAAS-enabled GPS receivers use the Deviation Correction (DC) value to improve the accuracy of the GPS data (Nolan, p. 87).
Since WAAS provides corrections for the entire NAS, the FAA and the aviation industry has determined that there is a need to provide a localized system at airfields to provide quicker and more accurate corrections to aircraft on approach. To meet this need to supplement WAAS the FAA has begun development of a program known as the local area augmentation system (LAAS), which consists of ground-transmitters known as the ground-based augmentation system (GBAS) that measures system signal errors in “one local geographic area” (p. 88). As shown in Figure 4, GBAS local reference receivers will be placed around an airport at surveyed locations. Figure 4: LAAS: Local Area Augmentation System. (Source: Xiao, 2008)
Figure 4: LAAS: Local Area Augmentation System. (Source: Xiao, 2008)

The GBAS compares its known surveyed location with the position determined by signals from the GPS. The results of the comparison are used to formulate a location specific correction message that is then transmitted to receivers via data link; a user’s GPS receiver then uses this data to correct the GPS signals that are being received. All of the receivers around an airport would be able to utilize the same VHF frequencies set aside for ILS, through the use of a technology used by cell phone operators known as Time Division Multiple Access (TDMA). By utilizing GBAS in a local area the hope is to mitigate almost all accuracy errors inherent in the coverage of the GPS and WAAS; which would allow aircraft to perform the same range of low to no-visibility approaches that are now flown, or to even improve the type of approaches flown (FAA, 2011). Improvement is the impetus behind these changes. Improving the accuracy and flexibility of approach is a key facet of LAAS. Through the use of GBAS the FAA forecasts benefits to capacity and efficiency. These benefits would be realized by the ability to use the LAAS to support complex, rather than straight-in, approaches and terminal procedures; flight paths through airspace could be better designed to de-conflict aircraft, as well, obstacle clearance requirements would be altered due to better control of ground paths. Airports would only have to rely on one system, since a single LAAS can support multiple approaches. For example Chicago O’Hare International has twelve ILS approaches to its twelve runway ends, and a single LAAS could replace all the ILS on the field. A further benefit at an airfield would be the ability for ATC to alter approaches or procedures without infrastructure changes. Further efficiency gains could be achieved since ATC controllers’ workload would be reduced through less voice communications and vectoring requirements. Additional efficiency would be realized by the ability to limit an aircraft’s operating time in the terminal area by the use of multiple descent profiles (Xiao, 2008). In the future, by utilizing LAAS to augment and refine the accuracy of the GPS, WAAS, and an aircraft’s own avionics, it is believed that aircraft will be able to land in almost any reduced visibility condition at airfields that are able to support a greater amount of air traffic than has ever been possible before in history of aviation.
The Future May Come Too Soon That ability to efficiently handle the increased volume of air traffic of the future is also still in the future. As is the case with many government programs, NextGen, like its predecessor NAS revitalization programs, has been marked by delays. These delays have included issues with funding, scope, and legislature. Another detriment has been the technological requirements for the development of the new airspace system. Aspects of NextGen rely on the use of new or emerging technologies; this reliance has at times affected the implementation of the program due to disruption in the project’s various timelines along with budget overruns (Higdon, 2011). One aspect in the development of NextGen that has been affected by the lack of a clear understanding of the technological issues involved has been in the approach to landing phase of a flight. As originally conceived, the WAAS would meet the requirements of providing the precision and accuracy needed for conducting safe low visibility approaches at all airports. As the system was tested and approved, it was discovered that WAAS was not capable of regularly providing the accuracy needed for more than the basic form of instrument approaches, also known as a Category I approach. The higher level approaches, requiring increasingly higher accuracy is Category II and III (NTSB, p. 6). In response, the FAA began the development of the LAAS program. While this was the correct and seemingly only approach for the FAA to take to meet their targeted efficiency and safety goals, the need for an additional program with more infrastructures has required a significant increase in projected budget and deployment schedule. A contributing aspect of the increased budget and time line was the decision during the early proposal for the airspace program that would become NextGen, to base budget projections on the then untested technology of WAAS (USAF, 2011).
Figure 5: Federally owned and operated VORs, now and after MON. (Source: AOPA, 2012)
Figure 5: Federally owned and operated VORs, now and after MON. (Source: AOPA, 2012) However, the delays caused by the need to develop LAAS have not been passed on to all parts of the revitalization effort; in fact some parts of the timeline have been accelerated, most significantly the decommissioning of navigational beacons. In attempt to make up for budget shortfalls due to delays and unanticipated development needs such as LAAS, the FAA has moved up the timeframe for decommissioning VORs and ILS installations, a process referred to as developing the Minimum Operational Network (MON) of VORs and ILS. MON is the FAA’s program for shutting down approximately five hundred VOR and ILS transmitters. The remaining roughly 400 transmitters would provide a backup source in the case of a disruption in the GPS or support networks (FAA, 2011). Significant numbers of the beacons that remain will be located in the western United States to provide augmented service where LAAS is unlikely to deploy in great numbers, and natural features such as mountain ranges can produce signal disruptions at lower altitudes. Figure 5 indicates the location of FAA navigational beacons that will be maintained – green – and those to be divested – in red; the magenta line outlines the Western United States Mountainous Area (AOPA, 2012). In an effort to gain momentum and to begin to be able to realize some budget savings, the FAA has pressed forward with the decommissioning effort in hope to meet its goal of reaching MON on time in 2020 (Xiao, 2008). This includes the effort to begin the removal of ILS installations at some locations. Even though the MON process has begun, LAAS and the GBAS installations that form the system are still in the testing and certification phase of development and have only been approved for installation at six airports (FAA, 2011). Discussion The need for a radical revitalization of the NAS of the United States to meet near term air traffic demands as well the exponential projected future growth is evident. As the last couple of decades have shown, the NAS has experienced significant issues in handling existing capacity, much less the 71 percent growth in capacity that is forecasted to occur by 2027 (JPDO, 2010). Now with a mandate, budget, framework, and technology, the FAA is in the early days of re-making the NAS to meet air traffic demand through the NextGen program. However, in the eagerness to finally implement the long needed changes, the FAA may be moving too fast, which may negatively impact the safety and efficiency that they are seeking. Of all the phases and weather conditions that a flight will experience, an aircraft approaching to land in low visibility is a critical phase, and potentially the most dangerous moment of the flight. In this critical moment it is important for the flight crew to have the most precise and accurate navigational information possible. For decades that precise and accurate information has been reliably transmitted by ILS. With NextGen, GPS, augmented with ground and airborne systems will provide that critical navigational data. However, those systems, LAAS and its GBAS installations, have yet to successfully prove that they are worthy or superior successors to the ILS. To date the only system to be considered a successor, WAAS, has proven to be inadequate, although a major portion of the original NextGen proposal was predicated on the belief that the then unproven capabilities of WAAS would be up to the task. Although exploring the unknowns of emerging technology such as WAAS is not in itself a fallacy, it is necessary to develop technologies that will be utilized for decades to come. It is a risk to safety and efficiency when a legacy system such as ILS is dismantled for unproven technology. After years of starts and stops in attempts to modernize the NAS, it is understandable that the FAA and the aviation industry would want to maintain momentum on a program such as NextGen. For the FAA there is also a desire to alleviate budget pressures by reducing the older maintenance heavy systems such as VORs and ILSs earlier than originally proposed. However, the replacement of navigational beacons such as ILS is proceeding on a schedule that was established when WAAS was presumed to be the only system needed to replace ILS approaches (Higdon, 2011). Now it would be more opportune for ILS and VOR transmitters to remain operational until the NextGen systems of the future have had the chance to mature. This would allow the opportunity to discover the drawbacks and difficulties of the emerging technology without the pressure of testing a new system, while keeping pace with the replacement of the legacy precision approach systems. Otherwise unforeseen issues, as was found in WAAS, may cause unwanted budgetary and deployment timeline disruptions, which could have a larger detrimental effect of the entire airspace reform effort. In the coming years, after NextGen has matured, the goal should be to maintain a robust yet redundant system to provide a safety net for the NAS. It could be a much needed redundancy to a satellite system that has already proven vulnerable to unplanned outages; aside from national security related events, two WAAS satellites have had various technological issues that have caused losses of signal and orbit degradation that has affected signal coverage. Each event was unexpected and caused disruptions in service (FAA, 2010 April). These incidents and others could pose a significant safety hazard for aircraft inflight. Of lesser concern is an interruption in navigation signals that would naturally cause a great disruption in air service. According to the FAA’s proposal for navigation services, under a NextGen system it would maintain a minimum operational network of navigational and ILS beacons for the case of emergencies such as a complete failure of the satellite based system (FAA, 2011). Under the MON biases of a backup system there would still be significant safety and operational concerns since only a few air facilities would only have limited runway capacity to support low visibility operations (AOPA, 2012). A tenable solution for the future would be to pre-position deployable ILS to medium to large airports. The systems which are being developed through a United States Air Force contract would provide the ability to quickly respond to NextGen outages. A deployable ILS would have several benefits; the system would not require a large fixed footprint for an installation, also any maintenance the system may need could be completed in location away from the active airfield. Along with a reduction in maintenance costs, a single deployable system would be capable of serving the emergency needs of one airport (USAF, 2010). Although there is not yet enough data about deployment times for these systems, what is known is that it would be possible to place a system on a pre-surveyed mark with guidance frequencies and requirements pre-installed (ANPC, 2008). A deployable ILS could meet the need of maintaining a backup system to support NextGen systems, as well as maintaining the ability to meet the needs of the new airspace system without a serious disruption in traffic or creating greater hazards. Such a system may still be a few years off from being ready for use, but it would be a reasonable consideration to offer a safety net for the NAS. That is the approach that the FAA and the aviation industry should be taking towards the modernization of the airspace of the United States. In the desire to finally take hold of the opportunity to create the airspace navigation and control system for the future has given NextGen advocates a bias towards legacy systems like ILS. This bias appears to have clouded the perspectives of policy makers, as they seem to forget that a majority of the technology to be relied upon is still new. As the difficulties with WAAS have shown, there are still a number of unknowns with emerging technology. The lure of system and financial efficiency cannot be so strong that the basic concept of maintaining the reliability of the NAS can be forgotten. As bonfires once gave way to lighted beacons, which were replaced by radio courses that were refined into VORs and ILS, new technology has always evolved to make flying from one destination to another safer and quicker. Aviation history also shows that the old systems did not disappear quickly in favor of the new and better system, for a while both coexisted to have a fallback. While relying on a system that beams its signals from the heavens is the future of aviation, the legacy of safety and reliability of an ILS should not be forgotten.

References
Advanced Navigation and Positioning Corporation (ANPC). (2011). Instrument landing system. Retrieved from http://www.anpc.com/wp-content/uploads/2011
Alaska Airlines. (2010). Horizon makes aviation history with first WAAS flight. AlaskaWorld. Retrieved from http://splash.alaskasworld.com/newsroom/QXnews/
AOPA. (2012, January). A minimum operational network (MON) of VORs. ePilot. Retrieved from http://download.aopa.org/epilot/2012/120112VOR-MON-White-Paper.pdf
Bar-Sever, Y., & Kuang, D. (2004, November). New empirically derived solar radiation pressure model for Global Positioning System satellites. IPN Progress Report 42-159. Retrieved from http://ipnpr.jpl.nasa.gov/progress_report/42-159/
Department of Transportation (DOT). (2011). 2012 FAR/AIM. New Castle, WA: Aviation Supplies & Academics, Inc.
Federal Aviation Administration (FAA). (2010, March). Fact sheet – NextGen Goal: Performance-based navigation. Retrieved from http://www.faa.gov/news/fact_sheets/
Federal Aviation Administration (FAA). (2010, April). WAAS Intelsat GEO satellite ceases broadcast. Navigation Services – WAAS- News. Retrieved from http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/waas/news/
Federal Aviation Administration (FAA). (2010, November). Navigation at the crossroads. Stanford 2010 PNT Symposium. Retrieved from http://scpnt.stanford.edu/pnt/PNT10/
Federal Aviation Administration (FAA). (2011). Proposed provision of navigation services for the Next Generation Air Transportation System (NextGen) transition to Performance- Based Navigation (PBN) (Docket 76 FR 77939). Washington, D.C: Federal Register. Higdon, D. (2011). NextGen goes live. Avbuyer. Retrieved from http://www.avbuyer.com/articles/
Joint Planning and Development Office (JPDO). (2007, June). Concept of operations for the Next Generation Air Transportation System: Version 2.0. Retrieved from http://www.jpdo.gov/library/
Joint Planning and Development Office (JPDO). (2010, September). NextGen Avionics Roadmap: Version 1.2. Retrieved from http://www.jpdo.gov/library/
Komons, N. (1989). Bonfires to beacons. Washington, D.C.: Smithsonian Institution Press.
MacDonald, A. F. (2011). From the ground up: Navigation systems – level 3 the instrument landing system (29th ed.). Retrieved from http://www.allstar.fiu.edu/aero/ils.htm
National Transportation Safety Board (NTSB). (2008). Wide-Area Augmentation System performance analysis report: Report #26. NTSB/WAAS T&E Team. Retrieved from http://www.ntsb.tc.faa.gov/reports
Nolan, M. S. (2011). Fundamentals of air traffic control (5th ed.). Clifton Park, NY: Delmar.
United States Air Force (USAF). (2010, March). Trimmed-Down Instrument Land System for Air Force. Defensetalk. Retrieved from http://www.defencetalk.com/trimmed-down-instrument-landing-system-for-air-force-24649/
United States Air Force (USAF). (2011). Project justification: Air traffic control systems. Research, development, test & evaluation, Air Force Operational Systems Development. Retrieved from http://www.dtic.mil/descriptivesum/Y2012/AirForce/
United States Coast Guard (USCG). (2006). FAA WAAS status update. Retrieved from http://www.navcen.uscg/cgsic
Xiao, X. (2008, November). ILS replacement. ACI World Safety Seminar. Beijing, China. Retrieved from http://airports.org/aci/acisafety