GPS Backup Strategies

General Aviation is increasingly dependent on GPS. So much so, in fact, that the FAA is getting rid of many VORs. But, what if the satellites stop working?


In the October 2015 issue of IFR, I noted that the VOR reduction plan included a provision for something called the Minimum Operational Network, or MON. The FAA says that MON is specifically aimed “at lower-end GA IFR aircraft in case of a GPS outage.” I added that MON is a transitional strategy to get to something called APNT, or Alternate Positioning and Timing. Let’s dig into this and explore the GPS backup support you can expect in the future as we transition from terrestrial to satellite navigation.

Where GPS Stands Today

Statistics show that both GPS and WAAS have been extremely reliable. The last GPS outage due to an ionospheric sunstorm was in 2006. Today there is not just one GPS signal, but three, and there will be a fourth in 2017. The latter two signals are in the highly-protected Aeronautical Radio Navigation Services band. These signals are more resistant to interference and jamming than before.

Ironically, the biggest interferer with GPS is our own military. When they conduct GPS testing, the area affected can be huge. There are several hundred local outages annually, mostly near military bases when military operations are underway. The chance of a complete system failure is virtually zero because basic GPS has a long-term uptime of essentially 100%. WAAS has made the system even more robust, giving you—if you needed one—yet another reason to upgrade to WAAS.

The anticipated scenario of a GPS failure is short-term and local. The MON is designed to help pilots fly to either a MON airport or an area where GPS and/or radar coverage is available. The nightmare scenario is a catastrophic loss of both GPS and ATC. Then the MON would be the only way to recover airborne GA aircraft.

It’s Partly About VORs

The Minimum Operational Network is a scaled-down network, from 967 VORs today to about 650 by 2025. Only FAA owned-and-operated VORs are affected. If a VOR is discontinued, DME and TACAN will remain if the site is so equipped. This means one-third fewer portals to communicate with FSS by transmitting on, say, 122.1 and listening to the specialist talking over the VOR signal. (Got ADS-B In?) However, the FAA is also engaged in an ongoing assessment of FSS communications.

The MON is solely a backup. Its purpose is to help ensure safe GA aircraft operations until GPS service is restored. So what capabilities will MON offer, and how will we use it?

The MON’s capabilities are summarized below. Note that the standard service volume for a MON VOR will be a planned 154 nm diameter cylinder (77 nm radius). The cylinder begins at 5000 feet AGL and extends to FL450. The larger radius is still undergoing tests and will be slightly smaller at 4000 and 3000 feet AGL. Most navigation will be VOR-to-VOR and new airways might be created between MON VORs. The FAA knows about current route utilization and factors it into MON planning.

VORs on death row show that MON support in the Western Service Area will remain strong with only about 15 VORs to be discontinued, the least of any area in the U.S. VORs outside the contiguous United States (CONUS) will escape the chopping block as will those needed for international arrival airways from the Atlantic, Pacific, the Caribbean and the core 30 airport megaplexes.

Only about 200 airports will be charted as MON airports. MON airports are qualified by location because they are suitable destinations for aircraft within a 100 nm radius. (See the map.) Each must offer a GPS-independent approach, which currently include around 150 ILS and localizer approach airports and 50 offering VOR approaches. None require DME since most of GA uses GPS in lieu of DME.

Speaking of DME

Area navigation (or RNAV), simply put, allows direct navigation from any point to any other point. GPS is but one form of area nav, just like inertial navigation (through inertial reference units, IRUs), rho-theta navigation (VOR-DME a la King KNS-80) and even rho-rho (DME-DME), all of which are now the province of sophisticated flight management systems (FMS) that consequently provide built-in GPS-independent navigation.

In the enroute environment, the backup objective for dual-DME aircraft is to provide DME/DME area navigation throughout the CONUS at FL240 and above. No VORs required. In the central and eastern U.S., coverage will extend down to FL180. Some larger airports will have coverage down to about 2000 feet AGL. The FAA will have to add or upgrade about 110 DME stations to assure enroute DME RNAV service.

In the terminal environment, seamless DME/DME coverage down to about 5000 feet AGL to support RNAV procedures at big metroplex airports for those aircraft lacking inertial navigation is planned. This will call for about 25 additional DMEs.

Though not directly related to the MON, the FAA lists about 1200 “critical DMEs.” A critical DME is a one that, “when not available, results in navigation service insufficient for DME/DME/IRU operations along all or portions of a specific route or procedure.” Being ground-based, these DMEs could be crucial to RNAV Departure Procedures as well as STARs in the absence of GPS-derived distance.

In Reality

If the FAA expects to use the MON until 2025, the remaining 650-odd VOR systems will have to be replaced and recapitalized. This is an unavoidable expense and surely concerns FAA bean counters, but 87 percent of VORs are already beyond their useful service lives. Many have not had their magnetic variations adjusted since the mid-60s, if ever. Yet it’s MON or no backup at all. Do you want to walk a GPS high-wire without a MON net beneath you?

Enroute MON coverage above 5000 feet AGL blankets the entire U.S. Coverage for approaches and landings excludes only a few small areas as seen on the map. VOR coverage is assured from at least one VOR within the planned 77 nm, and you’re never more than 100 nm from a MON airport with a ground-based approach. MON VORs and airports will likely be marked on your enroute chart. Plan your route as usual. Check the nearest MON airport(s) along your route to assure a non-GPS approach is not out of service. Check GPS NOTAMs, too.

Should GPS fail aloft, do a local RAIM check. Check the system status page in your navigator to see if you are receiving the necessary satellites. Call ATC to find out whether it’s you or you’re really in GPS darkness. If the latter, let ATC vector you out of the affected area. Alternatively, request diversion to the nearest MON airport via vectors or your trusty VOR. With a planned 154 nm service diameter, you might need only one VOR to reach a MON airport 100 nm away. Fly the altitude ATC suggests or if no ATC, no lower than a published MEA or OROCA. Expect a crowded airport. Close your flight plan, too!

Operational Issues

Currently a simple black M in the center of the airport symbol is the leading contender for identifying MON airports on enroute charts. Another suggestion is that MON airports should only be identified in the A/FD, a perfect place to bury them. Sometimes you wonder what the FAA is thinking.

In coming years, expect some VOR airway segments to be deleted and replaced by RNAV low-altitude T-routes and flight-level Q-routes as part of the GPS migration.

Another operational question is whether MON airports could influence flight planning and filing of alternate airports under IFR. The latter seems implausible and excessively conservative. Were we forced to select a MON airport as an alternate, we’d be limited to about 200 airports as far as 100 nm away.

Virtually every instrument chart and other documentation will have some changes due to VOR discontinuances. The FAA’s intent is to “change once,” meaning that if they change an approach, they will remove all VORs that will eventually be removed during the redesign phase.

Life After MON: APNT

According to the timeline, MON’s lifetime expires in 2025. Most likely it will continue beyond then because everything is more complex and time-consuming than it looks, and timelines tend to be optimistic. Regardless of when, after MON there’s APNT.

GPS provides three essential functions that, taken together, permit us to traverse airspace as we do. These are Positioning, Navigation and Timing, or PNT. Let’s define each one and see how they work together.

Positioning is the ability to accurately and precisely determine your location. It includes orientation—which way you’re facing. Sometimes positioning includes a third dimension, altitude. GPS positions you on a map, most commonly the World Geodetic System 1984 or WGS84.

Navigation goes beyond positioning. It offers you the ability to specify a desired position. Navigation then finds a direction to that position and applies course corrections, orientation and speed calculations to get you there.

Timing is the ability to acquire and maintain accurate, precise time from a standard (for GPS this is UTC), anywhere in the world. For this, each GPS satellite contains a super-accurate atomic clock, because both your GPS navigator’s positioning and navigation abilities depend on top-quality time.

VOR alone provides none of these elements. You dial in a radial that will take you to a VOR. That alone cannot determine your position. All you know is the orientation you require to go to that VOR. You need a second VOR or DME in order to find your position. With either, you can navigate to your destination. No timing can be derived from either VOR or DME signals.

PNT is essential to providing NextGen services. Performance-based navigation and ADS-B both enable what’s called Trajectory-Based Operations (essentially optimal routing), RNAV, and Required Navigation Performance (see IFR, August 2011).

APNT Options

If GPS goes down, PNT goes with it. If we could devise an alternate source of PNT, GPS would keep working and we might not even notice the difference. That’s called Alternate PNT, or APNT. As the timeline shows, APNT is the ultimate in GPS satellite backup.

APNT uses existing NextGen infrastructure, almost seamlessly. Importantly, APNT allows controllers to land aircraft safely, with no significant increase in pilot or controller workload. Routing of aircraft continues normally, including dispatch of aircraft to, from or through GPS “dark territory.”

For now, the FAA settled on something called “NextGen DME.” This calls for reconfiguring the DME network, deploying enroute and terminal DMEs as earlier noted, and could reduce the number of DME stations by up to 30 percent. This option does not offer PNT.

Another option includes something called pseudo-satellites (pseudolites)—ground stations that behave like satellites. They’d form a ground-based PNT network. This could actually be superior and more robust to normally weak GPS signals. A pseudolite’s signal can be as strong as the power available to it. That strong signal would be more interference resistant and hence harder to jam.

Current longer-term options both employ DME. Enhanced DME offers PNT such that RNAV and RNP operations can continue. The highest-performer of all is a hybrid of DME and ADS-B. It offers positioning for ADS-B, and can support all commercial and especially GA operations. Don’t expect to see this one for a long time.

Hopefully, this information has helped relax those of you who see GPS as a giant single point of failure. It isn’t a single point of failure, and the FAA is taking definitive action to assure that it won’t become one.

Fred Simonds is an active G1000-certified CFII in Florida. See his web page at


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