FAA and MITRE Collaborate on Navigating NextGen to New CapabilitiesNovember 2014
Topics: Airports, Air Navigation, Air Traffic Management, Aviation Administration
The Very High Frequency (VHF) Omni Directional Range (VOR) system became the navigation backbone of the National Airspace System (NAS) in the 1950s. The system uses ground-based stations to transmit radio signals so aircraft can determine the angle at which they're approaching or departing a VOR. Routes guide aircraft from one VOR to another.
In recent years, however, Performance-Based Navigation (PBN) systems such as GPS and Distance Measuring Equipment have gradually begun replacing VOR use. PBN enables precise navigation and more direct routes and is a centerpiece of NextGen—the next-generation air transportation system.
Because of the decreased reliance on VORs, the FAA sought MITRE's help in determining how many VORs—and which ones—could be decommissioned while maintaining satisfactory levels of safety and service in the NAS. Although the plan will save money in the long run by reducing future acquisition and maintenance costs, its principal benefit will be to transition the NAS away from a route-based and ground-based system to a PBN system.
A Change in Aviation Philosophy
The FAA and MITRE developed a plan to draw down VORs to a minimal operational network, or MON. The MON will provide a capability for VOR-equipped aircraft to navigate to an airport and land safely under instrument flight rules in the event of an unplanned but localized GPS outage.
The combined project team designed the MON largely to support small general aviation aircraft. Airliners and other large aircraft are almost all equipped with Distance Measuring Equipment, which they can use to navigate using PBN systems in the event of a GPS outage. But general aviation aircraft typically don't carry this advanced equipment. They need a back-up system in case of a GPS outage.
The team faced an additional complication when designing the MON: nearly every route, instrument approach procedure, airspace design, and FAA interfacility agreement uses the VOR system. The entire NAS is based on VORs, so the implementation of the MON requires a fundamental philosophy change. It will require a move from defining nearly everything on VOR distance and radials toward using latitude and longitude fixes.
Identifying "Safe Landing" Approaches
The FAA currently owns and operates 971 VORs in the NAS. MITRE's analysis has identified approximately 500 of these devices the agency can decommission. To come up with that number, and identify which VORs can be removed, the FAA/MITRE VOR MON team examined existing VORs based on a number of criteria.
First and foremost, a system must enable aircraft to land safely without GPS. To do so, an airport must have an instrument approach that doesn't require GPS. These airports, with "safe-landing approaches," could have an Instrument Landing System (ILS) approach or a VOR approach. ILS and VOR approaches require at least one VOR for the aircraft to descend and safely align with the final approach course.
MITRE identified all VORs required to conduct these "safe-landing" approaches. Each approach requires at least one VOR, but some approaches require two or more. The criterion was that there must be a safe-landing approach within 100 nautical miles of any point in the continental United States.
In a sparsely populated area, there might be only one safe-landing approach available, so the FAA would retain VOR(s) to support that approach. However, in more densely populated areas with multiple approaches, some can be safely eliminated. Instrument Landing System approaches give the lowest minimums for poor weather, so they were used whenever possible. The criteria gave preference to airports with Instrument Landing System approaches that required the least number of VOR stations.
If no safe-landing Instrument Landing System approach was available, then the team chose an appropriate VOR approach. Figure 1 shows an example of airports with safe-landing approaches that might be retained for the MON. Green circles show the area within 100 nautical miles of an Instrument Landing System approach and red circles show the area within 100 nautical miles of a VOR approach. In some areas, there is no safe-landing approach today; those areas are white.
Guaranteeing En Route Coverage for General Aviation Aircraft
Next, the team wanted to ensure that aircraft could navigate from their position to a safe-landing approach in the event of a GPS outage. Since virtually all general aviation aircraft can fly at 5,000 feet above ground level, the team decided to retain sufficient VORs to provide en route coverage anywhere in the NAS when aircraft flew at 5,000 feet or more above ground level. The VORs selected to support safe landings without GPS provided most of the coverage needed to achieve this objective. Any gaps were filled with other VORs identified for retention.
The ability to fly at 5,000 feet above ground level isn't reasonable in the western U.S. due to high terrain. The team recommended retaining nearly all of the VORs in western mountainous regions, including retaining the associated low-altitude (below 18,000 feet) air routes. Similarly, the FAA plans to retain all VORs in Alaska. Figure 2 shows the coverage of an example set of VORs at 5,000 feet above ground level across the continental U.S. The area outlined in green is the Western Mountain Region.
In 2012 and 2013, MITRE staff visited the FAA Service Centers in Seattle, Fort Worth, and Atlanta to build consensus around the MON retention criteria and the lists of sites to retain or discontinue.
The FAA and MITRE are now working with the general aviation community and the Department of Defense to achieve their support for the VOR MON, and the FAA has formed a national working group to support this effort. Also, at the request of the FAA, the RTCA's Tactical Operations Committee formed a subgroup to review the VOR MON concept. In September 2014, the committee approved the concept as well as the educational and outreach programs required for successful implementation. (RTCA is an association that develops consensus among aviation stakeholders worldwide.) The DoD has also provided feedback on the VOR MON sites they require.
Integrating Feedback from Aviation Stakeholders Is Crucial
During 2014, MITRE has been integrating the feedback from RTCA and DoD to form a final VOR MON list of sites. Once this list is finalized, work can begin on developing a schedule to remove the non-MON VORs from FAA routes and procedures.
Removing the selected VORs isn't just a matter of flipping a switch, however. Before a VOR can be eliminated, all of the routes, procedures (approaches, departures, and arrivals), and references to it must be either deleted or revised. As an example, Figure 3 shows the routes in the USA before and after the MON is fully implemented. One can see that the route system will require significant revision as a result of the VOR MON.
The VOR MON team must also determine the most efficient process for decommissioning the selected VORs. The team is working on a sequence of VOR removal that minimizes the cost to the FAA for procedure redesign.
It may seem at first that removing VORs would be simple. In reality, the process is very intricate, involving an array of considerations, thoughtful criteria, and stakeholder input to make sure the VOR MON keeps general aviation users flying safely and transitions the NAS from a route-based ground infrastructure to a PBN NextGen structure.
—by Marlis McCollum