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.

Current service volume for VORs. Note that up to 14,500 feet the SV is only 40 nm, line of sight. This will increase to 77 nm with the MON.

To the flightcrew MLAT positioning simply means ATC can track and separate traffic in areas previously out of radar coverage. MLAT is one contender to ensure the coverage of non GPS based APNT is part of the NAS in the future.

Today the main system used for ATC tracking is SSR. The backup system is primary radar. Primary radar is the traditional "skin paint."

Once ADS-B comes into effect, currently scheduled in the CONUS after January 1, 2020, the backup tracking system to ADS-B will become SSR. Primary radar systems will continue to be retained where they are currently used for aviation safety, weather and security purposes.

The study for a different, lower-cost backup system is still ongoing. WAM is the term for MLAT over a large area (Wide Area Multilateration) and could be that backup surveillance system. More tantalizing is the idea that aircraft with ADS-B In capability could receive their own WAM or MLAT derived position real time via Traffic Information Services-Broadcast (TIS-B).

This concept is new. There are yet unresolved issues of authentication for the positioning signal as well as how this might be integrated into the aircraft's navigation system. Still, the idea has possibilities as an alternate to GPS if signals are lost.


Development of alternate position, navigation and timing navigation methods as a backup to GPS is an ongoing process. The final methods to achieve reliable, low cost alternate means may change by 2020. In the 1950s and early 1960s the airspace changed from low frequency beacons and radio range to primarily VOR and ILS.

The change to ADS-B in 2020 will be no less dramatic. Perhaps much more is at stake simply based on the large increase in traffic volume jet travel brought as well as the requirement to fit more aircraft into the same airspace.

GPS has its weaknesses. Whatever method we use to create a solid backup to this single navigation source will determine what we fly with in the future.

Bill Gunn is the compliance manager for the State of Texas Aviation Division. He lectures nationally for a private aviation advocacy group and flies for work and pleasure.


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