FAA's plan for the retention of legacy navaids in the future.
FAA will decommission 497 VORs by 2020. Remaining VORs will form the Minimum Operational Network or MON. Decisions made today concerning the ground based legacy navaids being kept will determine the equipment we need to fly in tomorrow's airspace.
One requirement is the MON will have the same VOR coverage as exists today. Western US Mountainous Area (WUSMA) shown in green will not lose any VORs as part of the MON drawdown.
APNT: What are we talking about here?
APNT coverage as a backup to GPS is defined by 3 zones. It is desired that whatever methods used for APNT provide service in all 3 zones. Zone 1 is aligned with Class A airspace over the CONUS. Zone 2 is from 5000 feet AGL to FL180. Zone 3 consists of a conical section of a flat constant altitude surface at 1000 feet height above an airport from the Airport Reference Point (geographical center of the airport) out to 10 nm. Coverage for Zone 1 will be met 100% by SSR and non GPS RNAV. Achieving coverage for Zones 2 and 3 throughout the NAS will require several systems.
RNP: Required Navigation Performance
RNP is the concept that any approved means of navigation using GPS, legacy navaids or other systems such as baro-aiding or INS (inertial navigation system) include the requirement for aircraft onboard performance monitoring and alerting.
A critical characteristic of RNP operations is the ability of the aircraft navigation system to monitor the navigation performance it achieves and to inform the crew if the requirement is not met during an operation. This onboard monitoring and alerting capability enhances the pilot's situation awareness and can enable closer route spacing without intervention by air traffic control. Pilots often express RNP limits as full deflection of the CDI when in the enroute, terminal or approach phases of flight. However this is only one aspect of RNP, the other being the critical monitoring and alerting.
All means of fixing a point in space is considered RNAV regardless of the method. The air carrier fleet and certain high end corporate operators will meet RNAV RNP using DME-DME ranging with less reliance on GPS and WAAS for enroute and terminal operations. The reduction of the VOR network in the CONUS from the current 967 to less than 500 for the MON bears some explanation as to how this will be done.
Legacy navaids and the MON
The cost to replace all of the VORs that will remain as part of the MON is estimated to be $1 billion. There is no current funding to replace these VORs that will remain with newer units. The observation has been made that the FAA will have 497 systems worth of spare parts to keep the MON operating for a while. The Department of Defense network of TACANs will remain and is not part of the MON drawdown. The western mountain area will not lose any VORs. All systems decommissioned will be in non-mountainous areas.
Additionally, there will be the same coverage with the MON as exists today although with only a single VOR within reception range in some cases.
FAA will also extend the nominal low altitude service volume of the remaining VORs from 40 nm to 77 nm as part of the MON.
Many of the site locations of the decommissioned VORs will remain in use for other communications chores. An important consideration for the MON is an increase in DME coverage in the NAS. Because most of the air carrier fleet is and will remain using DME and other non-GPS RNAV to meet any RNP requirements with either ILS or GPS for the approach phase of flight only, a robust DME network is part of the APNT solution. Special cases, such as VOT signals and test and evaluation research in VHF navigation, will be provided as part of the MON or funded by state, local or private funds.
Safe Harbor concept
Recognizing the core 30 airports as critical to the NAS is nice but this leaves out a lot of flight operations that rarely or never fly into these destinations. The Safe Harbor concept is a plan to ensure every aircraft in the NAS is within 100 nm of an airport with an IFR arrival requiring only VOR. Much of the country will have Safe Harbor airports more closely spaced than the 100 nm limit.
However the FAA may even install new VOR transmitters in sparsely populated parts of the country where the 100 nm goal cannot be met with existing VORs. Considering the single VOR coverage that will be in place in parts of the country and a 77 nm service volume up to 14,500 ft, enroute and terminal secondary surveillance radar (SSR) will be part of this concept to get aircraft on the ground if there is no GPS signal.
This reliance on SSR as part of the APNT is one reason there is a requirement to retain mode C or mode S transponders once Automatic Dependent Surveillance and Broadcast becomes the primary means to track, separate and sequence traffic by ATC. ADS-B is GPS dependent.
SSR using onboard transponders forms one part of the APNT backup plan. Note that the 1283 existing Category 1 ILSs are part of the Safe Harbor concept—even if flown by a single VOR as a localizer-only approach.
Questions have been asked about not replacing the remaining VORs with newer units in light of the fact that this MON may become a lifeline for IFR traffic if the GPS signal is lost. FAA intends to increase the monitoring and maintenance of the MON VORs in light of the fact that there will be only single coverage in some areas. Outer markers associated with existing ILS systems will also remain as part of the system.
Note that the Safe Harbor concept does not require an aircraft to have DME capability or more than a single VOR receiver. It might be a busy time in the cockpit, but it will get the aircraft, crew, and passengers on the ground if GPS is lost.
APNT and Multilateration (MLAT)
Multilateration (MLAT) is a method to accurately locate an aircraft by measuring the Time Difference of Arrival (TDOA) of a signal from several transceiver sites. The principle of TDOA has been in use in various forms since the 1930s. Essentially several ground based transceivers plus passive receive-only sites interrogate the aircraft's transponder and receive the reply.
Measuring the time difference between the interrogation and reply, the ground station knows the aircraft is somewhere along a curved Line Of Position (LOP). The point where these LOPs intersect is the aircraft's position. MLAT has been in use in western Colorado since 2009 and Juneau since 2010. MLAT transceivers are in place around the target airports and supplement the SSR at locations or altitudes below which radar was ineffective.