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On Instruments: The GPS backup

New life for old VORs and ILSs

Make no mistake: We live in a world guided by GPS. And it’s not just the flying world. It’s the new standard, for everything from Google Maps to missiles.
On Instruments
The green areas of the proposed MON coverage map show where 130 airports will have ILS and/or localizer approaches. The 66 airports served by VOR approaches are in red. In western mountainous regions, changes will be slight, and most existing VORs will be retained.

For pilots, that’s a very good thing. GPS offers unmatched advantages. Like direct-to navigation, and the extreme precision offered by signals augmented by the Wide Area Augmentation System (WAAS), which allows LP (localizer performance), LNAV/VNAV (lateral navigation/vertical navigation), and LPV (localizer performance with vertical guidance) RNAV GPS instrument approaches. LP and LNAV-only GPS approaches—those with lateral-only guidance—routinely let us descend to decision altitudes 400 to 500 feet above runway elevations in visibility conditions as low as one-half or one mile. LNAV/VNAV approaches add vertical guidance that can take us lower than that—usually 250- to 300-foot decision heights. And LPV approaches let us descend to decision heights as low as 200 to 250 feet in visibilities as low as one-half mile—minimums that match those of ILS approaches.

Because GPS navigation depends on satellites, its signals can provide more airports with more—and more accurate—arrival, approach, and departure procedures. Ground-based navaids that power VHF (very high frequency)-based VORs and ILSs, and low-frequency-based nondirectional beacon (NDB) procedures, are subject to siting effects (for example, nearby structures, power lines, or terrain) that affect their accuracy.

Of course, replacing VOR and ILS approaches with the RNAV GPS variety was the FAA’s whole idea back in the early 1990s, when the first iterations of GPS approaches were implemented. Today, there are 4,078 LPV approaches serving 1,960 airports, of which 1,191 had never had ILS capability. Another 730 LP approaches serve 535 airports, 432 of which don’t have ILS approaches.

But as WAAS-enabled RNAV GPS approaches grow, ILS approaches hold steady at around 1,550. For now. As for the existing 967-strong VOR stations and their associated approaches, as of 2020 the plan is to cut their number to 589. And NDBs? The last I heard, some 490 had been summarily shut down in 2005. Let’s just say that NDB approaches continue to drop like flies, with no end in sight. You’ll look long and hard through your ForeFlight or Garmin Pilot before you find one. I hear cheering, and understand.


For all its advantages, GPS does have downsides. Signal coverage can be compromised, and outages periodically occur. That’s why preflight checks of GPS coverage are so important. As a creature of the Department of Defense, GPS availability and accuracy can be denied or altered for security and other reasons. So, a move to an all-GPS navigation system means a system that’s susceptible to single-point failures.

All good plans have backups, and the FAA’s answer to transitioning to a GPS future is to preserve a minimum operational network (MON) of certain existing VORs and ILSs. Should GPS go down, the MON will leave us with a system that lets you navigate using VOR to an airport 100 nautical miles away that has a non-GPS (i.e., VOR, localizer, or ILS) approach. (This assumes you’ll be flying that en route VOR-guidance segment at or above 5,000 feet agl.)

The operational nature of the MON system seems to be a work in progress. An FAA document from January 2012 stated that the MON wouldn’t be an “efficient or usable navigation network for VOR-only aircraft,” that “VOR-based navigation using the MON would likely be circuitous, and not all airports will have instrument approaches that will be useable by VOR-only aircraft,” and that “the primary purpose of the MON is to support safe landing of IFR aircraft during a GPS outage.” So, under MON, direct routings may not be possible, and not all MON airports will have VOR approaches. But this will only affect VOR-only airplanes, something that’s hard to imagine these days.

Can you land at a non-MON-designated airport where there are GPS outages? Yes. But those airports may require radar vectoring, DME, or—here it comes—ADF.

MON skillsets

Let’s imagine how flying in a MON situation might go, worst-case. Let’s say a huge electromagnetic solar storm knocks out the GPS satellite constellation. This would be top news, so the word would get out quickly over the general media, but FDC notams would issue the official guidelines for operations in the MON. ATC operations, flight planning, and standard routings would all be affected. A sudden switch to MON has the potential to be every bit as disruptive as, say, the airspace measures taken after the air traffic control strike of 1981, or 9/11. If you think about it long enough, you’ll realize that a widespread GPS outage could be a lot more than an inconvenience.

Pilots who’ve let their VOR interception and tracking skills lapse could be challenged as they knock off the rust. For some, it may have been a long time since the last position fix was determined by centering VOR needles with “From” indications. Moving maps—both panel-mounted and on iPads—would be of limited use without “own-ship” depictions. Those long-ignored time-speed-distance dead reckoning skills might get an uncomfortable workout.

And ADS-B-Out and TCAS capabilities could be compromised.

It’s enough to make you want to review VOR procedures, and maybe even—the horror!—NDB tracking if you’re planning on flying in foreign airspace. Pilots having bearing pointers or remote magnetic indicators (RMIs) would be at an advantage since these can be called into action relatively quickly. If you have these navigation tools, have you used them lately?

The FAA’s MON plans are still evolving. Initially, the idea was to implement MON by 2020, but the timetable seems to have slipped. The decisions as to which ILSs and VORs will be decommissioned have yet to occur. The word is that under MON, the plan is to have a mere 130 ILS approaches. Feedback from user groups, such as AOPA, the Department of Defense, and other stakeholders is ongoing.

Time marches on. The MON may not come about until 2025, or later. Regardless, be prepared to work in a system with fewer VORs and ILSs. These navaids have been around since the 1940s, and while their useful lives may have been exceeded, they’ll transition to backup roles. They’ve done a great job over the years, but get ready to say goodbye to a slew of them.

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Thomas A. Horne

Thomas A. Horne

AOPA Pilot Editor at Large
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.

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