Why GPS Doesn’t Work

In the post 9/11 world, security is clashing with utility and aviation is on the front lines, with consistent and reliable GPS signals at the center of the conflict.


Dark clouds are looming on the horizon of our otherwise exciting GPS future. With increasing frequency, the military is jamming or spoofing GPS over huge swaths of airspace. A typical GPS NOTAM for Albuquerque center covers a radius of 237 NM at 10,000 feet, 207 NM at 400 feet and 165 NM at 50 feet daily for most of a week. Pilots in the Southwest have become accustomed to these NOTAMs, but they’re becoming more frequent and expanding to other areas of the country.

Jamming is bad enough, but there’s been little talk of spoofing. GPS spoofing occurs when the GPS signal is replicated in a manner that provides incorrect location information. For example, at the Geneva Motor Show, a spoofing attack reported show-car locations, not in Geneva, but in Buckingham, England, in the year 2036.

While that might seem cute, imagine flying the RNAV approach into Angel Fire, New Mexico. Five miles either side of center line on the initial segment from the west puts you in the mountains.

GPS spoofing is insidious and doesn’t provide user clues like jamming does. We just don’t know. A report by RTCA found that GPS interference can affect most every system of a modern aircraft; ADS-B, GPS, TAWS, AHRS, communications, PFD/MFD, and ELT.

The War on Terror

National Security Presidential Directive 39 ordered the military to train in settings as realistic as possible. Managing GPS in a war theater, either through interference or spoofing, is critical. The Center for Advance Defense reported “GNSS spoofing activities in the Russian Federation, its occupied territories, and its overseas military facilities are much larger in scope, more diverse in geography, and longer in duration than any public reporting suggests to date.”

According to a National Defense article, Russia and China already assume that any signals from space would be blocked during a military offensive.

Before 2000 the military reduced GPS accuracy with Selective Availability (SA). Selective Availability would reduce the accuracy of the civilian signal significantly. In March 2000, President Clinton issued a Presidential Directive bringing the end of SA to “make GPS more responsive to civil and commercial users worldwide.” Interestingly, the same statement noted, “we have demonstrated the capability to selectively deny GPS signals on a regional basis when our national security is threatened. This regional approach to denying navigation services is consistent with the 1996 plan to discontinue the degradation of civil and commercial GPS service globally through the SA technique.”

Where does it leave us?

So … What’s the Backup?

When the FAA developed its Performance Based Navigation (PBN) and NextGen strategy, they recognized that “with both navigation and surveillance through ADS-B being derived from GPS, an opportunity for common mode failure is created. An aircraft can lose navigation and the FAA can lose surveillance that would normally compensate for loss of navigation. The ability to disrupt air transportation by interfering with GPS becomes more likely as aviation grows more dependent on GPS as its major source of flight information.”

One alternative to GPS is DME/DME or inertial positioning, but those systems are expensive and primarily the domain of transport aircraft.

Discussions about eLoran as a backup to GPS pop up and disappear like a prairie dog in North Dakota. As an update to the old LORAN-C, eLoran includes UTC synchronization, differential corrections like WAAS, and more data. It promises to allow instrument approaches down to non-precision minimums. Want another box in the panel?

Status Quo

With no additional capability few pilots can justify equipping with an RNAV alternate to GPS. Additionally, there isn’t any support where it really matters—the approach phase. Other than an RNAV approach specifically requiring GPS, there is only RNP with a whole new can of worms. Even if a magical genie made an alternative method of RNAV operational overnight, the performance probably wouldn’t allow a plug-n-play with GPS and there would be no charts to use!

Bottom line is that there is no real backup to GPS. Any discussion about backups is about triage—getting airplanes on the ground. It’s like flying non-radar in the U.S.—possible but it’s a total mess because no-one is proficient.

With the current trajectory of GPS-based RNAV, aviation is going to run into similar problems. Losing GPS navigation, and the ATC surveillance it’s predicated on, could leave both controllers and pilots quickly trying to dust off old skills.

A growing number of airports are like Angel Fire, NM—served only by RNAV(GPS) approaches, a significant distance from a VOR or airway, and frequently subject to GPS interference testing. The AIM notes that both “UNRELIABLE and MAY NOT BE AVAILABLE are advisories” only and that if GPS is available pilots may continue to use it. This appears to be a cavalier approach since the system can’t predict intentional interference events.

Before issuing a clearance for an RNAV (GPS) approach ATC must advise if GPS NOTAMs are in effect. But the RTCA found in a 2018 report that GPS “interference may be done at different power levels, terrain can have a big influence, and that interference may start and stop at any time during the approved window. In this case, a pilot who decides to fly an approach may find his signal abruptly lost or degraded.” At an airport like Angel Fire, surrounded by terrain and below radar coverage, the only option might be to climb to the 14,400-foot MSA. In my little Cessna, I’d be half way across the Texas panhandle—or embedded in a mountain—before I could reach that altitude.


“Stop Buzzer”

A tool for pilots within an area of intentional DoD GPS interference is the phrase “stop buzzer.” Stop buzzer is an ATC term used to stop electronic attack jamming activity that’s being brought into the pilot lexicon. If experiencing GPS anomalies within an area of NOTAMed activity a pilot can tell ATC to “stop buzzer” and they will coordinate with the DoD to stop the GPS jamming activities. The request could take some time since it’s routed through ARTCC to the DoD.

Ironically, in an FAA pamphlet touting the advantages of WAAS to airport operators (undated, but still available online), the agency stated “if for some reason the WAAS service becomes unavailable, all GPS or WAAS-equipped aircraft can revert to the LNAV decision altitude and land safely using GPS-only, which is available nearly 100% of the time.” We’ve learned this just isn’t the case.

It isn’t the worst of times, as Dickens would say. The aviation community is increasingly aware of the challenges caused by intentional GPS interference and spoofing. As more events occur, additional pressure will be placed on the government and DoD to balance national security requirements with those of civil aviation. When flying into areas only served by GPS and there are GPS interference NOTAMs, caution is prudent. Regulations haven’t caught up to the opaque threats presented by intentional GPS interference. Have a plan in your pocket for when GPS goes haywire, including an alternate that uses terrestrial navigation aids, although one isn’t required for most. And keep that snazzy, cool-sounding lingo at hand: Stop Buzzer!

Jordan Miller spends as much time flying in China as in the U.S., and both countries are starting to look eerily similar to him. If only U.S. controllers would start separating traffic by course offsets…


  1. The solution is simple, leave the VOR system more intact, it served for years, was reliable, and given it’s distributed nature, is resistant to attack. No additional effort required to develope a “New box in the panel”.
    A non-precision VOR approach is better than no approach.
    Of course, that would require the FAA to admit a GPS only based navigation system was a mistake, so…

  2. A ground based LORAN system would be cheaper to maintain and easier to monitor than the aging VOR system, while still allowing direct-to navigation similar to the GPS system. The VOR system is going to be increasingly obsolete and expensive to maintain. I don’t see it coming back.

  3. Eliminating the VOR system in the USA may have been a premature idea, and as Ed states above.
    It does remain a viable backup option if supported financially.
    I have flown the approach into Angel Fire, and it is spectacular on a typical New Mexico VFR day. Not so much in IMC.
    Until this issue is fixed, I only see the GPS outage problem getting worse.

  4. GPS jamming and spoofing could have major implications for pilots flying aircraft that have glass Attitude Indicators (such as Dynon G3) or autopilots (such as Garmin GFC-500) that require GPS data for operation. Imagine you are in IMC, have a vacuum failure, and your vacuum gyro-based primary Attitude Indicator fails. Will your glass backup Attitude Indicator give reliable (or any) information if GPS is being jammed or spoofed? You can tell ATC to “Stop Buzzer”, but if you are already in an inverted flat spin or spiral dive you will probably be dead by the time that the DOD restores the GPS system to normal operation.

  5. Good warning. It’ll take a while, but better GPS receivers are on the way that use L5 signals for acquisition and tracking.L5 is a modern signal coming grin the newsy GNSS satellites. It offers much better reassurance to jamming, is a stronger signal, and is more accurate. In addition, it promised access to more than one GNSS constellations (more satellites to see and the ability for comparing accuracy between constellations fur accuracy) probably 2-3 years away from on commercial products. Check out: OneNav.ai

    • And it won’t save us from a constellation of other issues. Unless the signal is anchored to a known point on terra firma it can AND WILL be most certainly unreliable under foreseeable circumstances. There is absolutely no doubt that our most likely adversaries in any serious conflict will pull out all stops to disrupt civil as well as military GPS (or other) electromagnetic signal important to navigation as well as all forms of digital communication. This is truly “no Duh! territory. The satellite system is nifty, but inherently a poor bet it a crunch.

  6. We had this very discussion back in at least 1996 when I worked for Mitre Corporation. At the time, not only was GPS under consideration to supplant ground-based systems for surveillance and navigation, it was also being looked at as a time source for increasing communication capacity by using time-division-multiple-access methods to allow frequency sharing by multiple aircraft and controllers. So…put one bad actor with a GPS jammer out there, and 1) Planes don’t know where they are, 2) ATC doesn’t know where the planes are, and 3) They can’t talk to each other, either.

    Not ideal. I should note that this was all being discussed before Loran-C was decommissioned. and the notion that a robust GPS backup was A Really Good Idea was known to the FAA. With no particular knowledge of what was going on behind the scenes over on Independence Avenue, I suspect there may have been some interest in eliminating any possible GPS competition. The cost of upgrading Loran was relatively small, yet instead of doing that it was attacked with vigor and taken out of the system altogether.

    We didn’t end up putting all the eggs in the same basket, but we came pretty close. I think we would have been well served by maintaining upgraded Loran capability as a backup.

  7. With the recent addition of Galileo and BeiDou global coverage capabilities as well previously available GLONASS, there are other alternatives to GPS for space based GNSS (Global Navigation Satellite Systems). In fact, even today’s cell phones, as well as other products, make use of Galileo and GLONASS (example is Garmin’s GLO GPS and GLONASS receiver) even though owners of these devices may not have read that in their instruction manuals. So perhaps it is time for Avidyne, Garmin, etc. to consider adding these satellite systems to their product lines. Perhaps an even better idea is to require the military to do their GPS interference testing in the Washington, D.C. area !!!!

  8. I have kept the LORAN-C navigator removed from my airplane after the Obama administration mandated shut-down. I know it won’t work with e-LORAN but I keep it as a reminder of government ineptitude and short-sightedness.


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