When the FAA conceived NextGen as the long-awaited modernization of the U.S. air traffic control system, it hooked its progeny to the Global Positioning System. Proven in nearly two decades of service, phenomenally accurate, and rapidly becoming the preferred medium for air and surface navigation, GPS was the logical choice as the centerpiece for a 21st century ATC system. Furthermore, its positioning accuracy to within a few feet (or, with dual-frequency receivers and augmentation, a couple of centimeters) enabled Automatic Dependent Surveillance, whereby aircraft could report their own locations irrespective of radar. And like GPS, ADS already had been proven in oceanic operations as FANS 1/A, delivering positions to control centers in near real time via data links to communications satellites that then downlinked to the centers.

To form the armature of the NextGen architecture, FAA chose a new variation on Automatic Dependent Surveillance, ADS-B (for “broadcast” as opposed to the FANS ADS-C, where the suffix refers to a virtual “contract” forged between aircraft operator and controller). With ADS-B, the aircraft’s position could be data-linked either to ground stations and thence relayed via landline to ATC centers (or after March 2019, to Iridium communications satellites through the Aireon space-based service) that, like the ADS-C FANS system, downlink to the control centers. 

Thanks to the broadcast function, aircraft could maintain their own separation while controllers monitored aircraft movements on digital displays.

ADS-B’s effectiveness and potential were proven in the FAA’s Capstone project, a trial conducted in Alaska between 1999 and 2006. It significantly reduced accidents among charter and bush operators and regional airlines operating in some of the most hazardous weather and terrain in North America. Based on that success, early in NextGen planning, ADS-B was put forward as key to a comprehensive, cost-effective technology that would initially supplement radar and ultimately reduce it to second-tier or backup surveillance.

One of the strongest arguments in favor of ADS-B as a replacement for radar and the legacy navaids network was economics. First, it is vastly less expensive than radar. The small ground stations that accept ADS-B transponder signals — roughly the size of cellphone repeaters and consisting of antenna, receiver, target processor and telecom links to ATC facilities — can be installed almost anywhere. And unlike radar, the units feature no moving parts, and thus require considerably less maintenance and electric power to operate them. Secondly, ADS-B saves money for operators since the tighter procedures and point-to-point routing it supports reduce flight times and thus, fuel burn, CO₂ emissions and wear on airframes and engines.

“When you look at a ground-based system, it looks like a road map,” Ric Peri, vice president, government and industry affairs at the Aircraft Electronics Association (AEA), observed. “Typically, you would fly point to point on ground-based transmitters [e.g., VORs]. With GPS, though, you can fly the most efficient route, and in fact, the FAA will allow you 360 of them.” Either ATC chooses the most efficient route or, in some cases, permits self-routing and is only required to maintain separation of aircraft. “Given that the shortest distance between two points is a straight line, not having to go through intermediate waypoints saves time, fuel and money.”

The natural evolution from FANS to a domestic surveillance system substituted the inexpensive ground stations for the communications satellite reporting function, and in its planning for NextGen, the FAA envisioned an array of these VHF-band transceivers covering the U.S. and its territories. The agency outsourced the design, construction, deployment and operation of the ground-based ADS-B network to the private sector.

In competitive bidding, Exelis Corp. (formerly ITT) won the 18-year, $1.8 billion contract in 2007. It was subsequently purchased by Harris Corp., which in turn selected subcontractors Thales, Selex and AT&T to manufacture radios for the ground stations and provide connectivity between the stations and ATC facilities. Under the contract, the FAA does not own the ADS-B network. It leases it as a turnkey operation that Harris runs from a small control center in Herndon, Virginia.

Legacy Navaids

When the ADS-B deployment was completed in 2013, 650 ground stations had been installed across the U.S. and in Guam and the Virgin Islands, and even aboard 13 drilling rigs in the northern Gulf of Mexico to provide ADS-B coverage to the helicopter fleet servicing the oil industry.

Fully tested that year, the network went on line, capable of providing ADS-B Out positioning data streams to the control center and ADS-B In information to equipped aircraft. It is estimated that as a surveillance tool, ADS-B is as much as 20% more cost-effective than radar. In the initial planning. there was an assumption that once the ADS-B network was up and running and aircraft equipage mandated, the FAA could begin to decommission its expensive legacy radar and navaid network. But there were considerations that had to be taken into account.

First, as the AEA’s Peri points out, by making its new Air Traffic Management (ATM) system entirely dependent on GPS, the FAA had essentially “hitched its future” to a navigation system it doesn’t control. After all, GPS was conceived, developed, paid for, and its satellites launched by the U.S. Air Force under the auspices of the Department of Defense (DOD) and approval of Congress. Federal legislators appropriated the funding on the condition that the system also be available to civilian users — the FAA among the many. FAA is one of those users, however, the satellite constellation remains firmly under the control of DOD.

And as noted in “GPS Vulnerabilities” (BCA, March 2019), the 21-satellite constellation rotating 12,500 sm above Earth is unprotected, as are its signals to the ground, and thus vulnerable to tampering. Certain national entities have routinely “spoofed” GPS signals in the vicinity of U.S. or its treaty members’ military maneuvers. And while there is a federal law prohibiting interference with GPS signals, small jammers are sold on the so-called “dark web” to people interested in blocking GPS tracking services. Often these jamming efforts take place on or near airports, with the result of disrupting operations.

Understandably, the DOD reserves the right to compromise portions of GPS coverage for military training or for hacking tests to learn how the system could be attacked.

Speaking on GPS vulnerability, John McGraw, vice president, regulatory affairs at the National Air Transportation Association (NATA), pointed out, “There have been occasions of local outages, relatively small scale, that were quickly resolved. But what would happen if you had a widespread outage? Say, half the U.S. How would it be managed and what would it take to backstop that? From an aviation perspective, you would go ‘procedural.’”

A GPS Backup

Given NextGen’s GPS dependency, in order to protect aviation operations, a backup nav system seems imperative. During the early years of GPS development and implementation, there was talk of preserving the Loran-C network, a low-frequency nav system, to serve in standby. But it was expensive to maintain and its usage was declining, so in the end it was decommissioned and dismantled.

The FAA “knew it needed a backup for the [GPS] system,” Peri said. But with no state-of-the-art option available, the agency had no choice but to rely on the existing analog ground navaid system — or a portion of it — for backup. In December 2011, the FAA announced in the Federal Register a “notice of proposed policy” on its dual strategy of reducing the legacy VOR system to a “Minimum Operational Network” (MON) that would supplement Performance Based Navigation (PBN, i.e., ADS-B) and come to the fore should GPS go down.

Given no technological alternative or the funds to invest, the FAA’s choice of the MON was the most logical option available to ensure safety in the National Airspace System. “If you had a complete failure of ADS, ATC would revert to the ground-based system,” Peri explained. “Losing GPS completely would create a challenge, hence the hybrid system. This means they will retain ground-based systems for the high-density infrastructure — radar, VOR — but will transition to a primary RNP, the preferred method, and still have the analog approaches on the books as a backup. The FAA philosophy is that we will keep the last generation to supplement the next generation, should it fail.”

According to an FAA document, the following criteria were employed to tailor the existing 1,300-unit VOR network into the MON:

• Retain VORs to perform ILS, localizer or VOR approaches supporting MON-designated airports “at suitable destinations” within 100 nm of any location within the continental U.S. (CONUS). Selected approaches would not require ADF, DME, radar or GPS.
• Retain VORs to support international oceanic arrival routes.
• Retain VORs to provide coverage at and above 5,000 ft. AGL.
• Retain most VORs in the Western U.S. mountainous areas, specifically those anchoring Victor airways through high-elevation terrain.
• Retain VORs required for military use.
• Retain co-located DME/TACAN systems.

Phase 1 of the VOR reduction program began in 2016 with 311 units targeted out of 896 total across the U.S., including Alaska, Hawaii and possessions. Thus the completed MON would consist of 585 legacy VORs plus a handful of new ones added in some high-density traffic
locations.

According to the FAA’s Malcolm Andrews, enterprise services director, Program Management Organization, by midsummer of this year, 69 removals out 74 targeted had been completed and the FAA was looking at around 300 total decommissions. “That will still leave about 600 in the NAS for predominantly general aviation operators,” Andrews told BCA, adding, “allowing them places to land within 100 mi. of where they are.” In addition, Andrews claimed the FAA was “improving and adding about 125 new ones to the DME network. We continue to maintain and keep ILSes — no changes there right now.” Phase 1 of the MON is scheduled to be completed by the end of this year; Phase 2 will extend to 2025, completing the takedown.

Thirty Down a Year

Steve Brown, vice president for operations at the NBAA, said that for each of the approximately 30 VORs targeted each year for decommissioning, there is a public notice for commentary. Brown should know: In his previous career, he was FAA associate administrator for air traffic.

“There is a lot attached to these facilities,” he explained, “en route airways, instrument approach procedures, and so forth, and this is what we comment on. [The FAA] won’t shut one down until there is a procedure in place for the function the VOR performs, for example, a VOR approach. These are navaids in specific locations that affect specific airports. In almost every case, the replacement will be GPS — which may already exist as an overlay of the VOR procedure. New GPS procedures are being produced every week, a continuous process. The new services — largely satellite-based — will go in and be commissioned prior to the VOR being decommissioned.”

Another former FAA executive is the NATA’s McGraw, who managed the Flight Technology and Procedures Div. for seven years. During that stint, he led the planning effort for PBN, which is GPS dependent. “When we started pushing out GPS procedures,” he reminisced, “the performance was good, and it didn’t require the cost and overhead required to maintain the old ground system that defined the existing procedures, so it was very attractive. There were even things you could do with satellite navigation that you couldn’t do with the ground network.”

With accurate and cost-effective GPS procedures in place, the FAA began decommissioning NDBs at some airports where the 1940s-era facilities anchored approach and landing procedures. “Congress said they had to sell the components by auction for $5,” McGraw continued, “and several airports bought them and lobbied Congress to maintain them. So, the FAA said, ‘If you need it, justify it.’”

This gambit resulted in the FAA providing GPS approaches at every runway where there had been an NDB procedure. Doing so not only improved the technology, but the new equipment didn’t need nearly the maintenance required by the old NDBs.

“With RNAV and RNP, you no longer need airways and en route VORs to establish them,” McGraw continued. “So, what do we need to remain in place? Maybe DME and the terminal VORs, perhaps even adding them while decommissioning the en route ones. So, we’d wind up with a hybrid solution. ILSes will be available for many years because the vertical component is still more accurate than with GPS, even with WAAS.”

In the event of a GPS outage, an FAA statement provided by the Southwest Region communications office declares, “Air carrier aircraft will revert from GPS to DME area navigation [RNAV] and continue to the destination to fly an ILS approach.” But “air carrier aircraft” only? What about business aircraft, many of which are better equipped than the average airliner, we posited to the FAA’s Andrews. Indeed, he answered, business jets would be included in this category.

Moreover, the NextGen DME program will add approximately 125 new units to support continued RNAV operations during GPS outages.

On the other hand, general aviation aircraft not equipped with DME RNAV avionics will revert to “conventional navigation” using the VOR MON and proceed to one of the airports within 100 nm of their positions to execute either ILS or VOR published approaches.

“When you get into the general aviation airspace,” the AEA’s Peri said, “you will see the loss of the infrastructure that will not be replaced when it ends its useful life. The FAA is not rebuilding the towers; they are letting them go by attrition. We are already seeing ground-based approaches being replaced with GPS approaches. It was slow at first and is now being ramped up as the analog facilities are reaching the end of their useful life and not being replaced.”

Safety, Resiliency and Radar

And what about radar, given that ADS-B’s strong suit is that it’s a surveillance tool that can establish aircraft position even in places where ground-based radar may be obstructed or beyond reach?

“We have an extensive radar net and will retain the crux of that,” David Gray, the FAA’s deputy director for surveillance services, said. “We always expected that we would have to maintain safety and resiliency. We are removing 32 radars, a mix of the non-cooperative and cooperative ones, or full sites. Overall, maybe another 60 or 70 [will be decommissioned] over the next five years. This leaves us with a resilient network to ensure safety.”

Added McGraw, “The plan is not to eliminate radar but to take advantage of ADS-B improvements such as a better latency and higher accuracy. So, radar will be retained in the terminal areas. ADS-B was required to have coverage in all areas where we have coverage now with radar — and in fact, better coverage in terrain, expanded coverage where we didn’t have it before.”

On its timeline, the FAA is looking at 2030 to reevaluate the legacy system, Peri claimed. “They want to get about 10 years of good RNAV data to figure out what to do with the analog backup system — what works and what doesn’t. It’s all about RNAV, but they need time to measure it and decide what to do in terms of a GPS backup. There is talk of a hybrid navigator that could switch between GPS or Loran. But in the meantime, we’ll have the legacy aids for at least 10 more years. Until the FAA has a reliable backup to the satellite system, we will continue to have the ground-based system as a backstop. And since the DOD controls the satellites, the FAA has to control the backup, and right now there is nothing else out there on the shortlist.”

But the FAA’s Gray affirms that the agency will not get rid of any infrastructure if doing so compromises safety. “And that safety has to continue even in the case of a GPS outage,” he said. “Aviation is only a small piece of GPS, which is a multibillion-dollar industry. So, this is why we have a robust backup. We use it because of the benefits it offers, primarily accuracy. We have techniques from a surveillance standpoint to ensure that the data that gets to controllers is accurate, and we check it in real time to ensure it is of the highest quality. We check every report once a second for every aircraft that is using ADS-B.”

And, Gray claimed, while the DOD reserves the right to periodically conduct tests on the GPS network, the FAA is informed “well in advance” to prepare for them. Same for radiation blasts from the ionosphere. “Safety will not be compromised; we may be a little less efficient operating on the MON, but safety will be contained. Operations continue during the COVID-19 pandemic. Deployment of equipment at some of our facilities has been slowed due to the pandemic but not greatly. We’ve adapted quickly.”

So, what does the future hold for a GPS backup that would incorporate 21st century technology? “eLoran has been discussed for years,” McGraw speculated. “There’s been a lot of talk but no funding. Other technologies are being looked at, as well, to provide a reliable position source and act as a backup or even a future replacement. There’s a continual study of what we will do next and protect what we have currently. [See “GPS Tech Backup Study” sidebar.]

“Enhanced vision systems? Enough nav to get you into the vicinity of an airport and the enhanced viz to get you on the ground? There was a proposal to use cell tower signals, hybrid systems that pull together any and all signals you might have. If you have a local vulnerability, it can be easily managed and resolved — we can do that — but the large-scale disruptions would take something pretty big to create it, something really horrific. ‘Other government agencies’ are looking at that, as well!”