"Airplane mode" does not disable GPS... is it safe to use GPS in flight?

Question

I noticed that even when my phone is in "airplane mode", I can still use the GPS.

Theoretically a GPS receiver just receives data, so I guess it would not jam anything.

On the contrary, on the iPhone airplane mode disables GPS, so it means that at least a few engineers at Apple are worried about people using GPS in flight.

Is it really OK to use my phone's GPS in plane, as a passenger of a typical commercial flight?
(when use of electronic device in airplane mode is allowed, obviously)

Answer 1

theoretically? GPS IS receive only. There's no way to talk to the GPS satellite network except using the big antennas of the network owners (the US DoD) and the communications are no doubt heavily encrypted.

Now, does that mean it's "safe" for aircraft electronics. Any receiving antenna does have an EM field around it, so if you believe the myth that those electronics are so sensitive and easy to jam that any EM field at all causes them to malfunction the answer is an obvious NO.
But that myth is just that, a myth. If they were so sensitive, that old walkman you were using in the 1980s during flights would have brought down that aircraft. Because not only do antennas have EM fields around them, any electrical device that's turned on has one (in fact any electrical cable with a current flowing through it has one, including the headphone cord for the in flight entertainment system).
And that GPS system in your cellphone uses a lot less power than that walkman, has a far weaker field surrounding it.

Of course it's theoretically possible that a strong enough EM field might interfere with the electronics in an aircraft, but then to you'd need just the right signal. Which would indicate you need intent (or incredible bad luck)!

Effectively the GPS in your cellphone is a lower powered version of the GPS receiver in the belly of the aircraft, which has its own antenna (which no doubt has a similar field around it, but of higher strength).

Answer 2

"It depends". As others have mentioned, GPS is receive only. In theory at least, when everything is operating normally. In practice, some GPS units can fail in a way that causes them to become an unintentional radiator of energy. GPS signals are VERY weak, so it doesn't take much to jam them. However, this should be an exceptionally rare event, and terribly unlikely to happen with integrated handheld devices. My personal GPS unit failure experience:

I was flying a light twin (Piper Twin Comanche) with a Garmin 430 WAAS onboard and an iPad with EFB/position tracking software. All of a sudden both of them lost GPS signal. Eventually through trial and error we found that turning off the onboard (certified) Garmin 430W restored the iPad, but not the other way around. Upon landing we gave the airplane over to the maintenance crew who discovered that the antenna cable to the Garmin was damaged. Apparently it uses an amplified antenna, which when the cable became damaged turned into an incidental transmitter and jammed the GPS signal around us!

Answer 3

The regulations on a plane used to forbid the use of any devices capable of sending radio signals - whether deliberately, or accidentally. Most radio receivers contain several "intermediate frequency" stages, where the incoming frequency is mixed with a "local oscillator" to produce lower frequencies which are easier on the electronics. These local oscillator frequencies have a tendency to "escape" from the receiver - but on GPS in particular, the amount of signal thus emitted is very small, and the frequencies are "mostly harmless".

A very helpful site on this topic is http://gpsinformation.net/airgps/gpsrfi.htm . The most important sentence (IMO) from this site is

The interference potential of handheld GPS receivers is minimal. However, no passenger should EVER operate a GPS receiver on board an aircraft except in strict accord with the directives of the flight crew.

It also has an extract of FAA regulation 91.21 . I found the original at the ecfr website (I added some emphasis):

§91.21 Portable electronic devices.
(a) Except as provided in paragraph (b) of this section, no person may operate, nor may any operator or pilot in command of an aircraft allow the operation of, any portable electronic device on any of the following U.S.-registered civil aircraft:

(1) Aircraft operated by a holder of an air carrier operating certificate or an operating certificate; or

(2) Any other aircraft while it is operated under IFR.

(b) Paragraph (a) of this section does not apply to—

(1) Portable voice recorders;

(2) Hearing aids;

(3) Heart pacemakers;

(4) Electric shavers; or

(5) Any other portable electronic device that the operator of the aircraft has determined will not cause interference with the navigation or communication system of the aircraft on which it is to be used.

(c) In the case of an aircraft operated by a holder of an air carrier operating certificate or an operating certificate, the determination required by paragraph (b)(5) of this section shall be made by that operator of the aircraft on which the particular device is to be used. In the case of other aircraft, the determination may be made by the pilot in command or other operator of the aircraft.

Notice - the FAA basically does not have the final say - they say "if the operator determines it is safe, then go ahead". Conversely, if the carrier doesn't say it's safe, you are actually breaking the law by doing this.

So you will need to check with the operator of the flight. For example, Delta explicitly allows the use of portable GPS from gate to gate:

You may use the following devices from gate to gate on Delta and Delta Connection flights:

• AM/FM or satellite radios
• digital and video cameras
• calculators
• Delta-installed equipment such as in-flight entertainment systems
• DVD players*
• e-readers
• electric shavers
• electronic/digital watches
• global positioning system (GPS) receivers
• handheld computer games
• headphones
• laptop computers*
• medical devices**
• noise reduction headphones
• portable media players*
• pagers
• smartphones and any device with cellular network service must be turned off or in airplane mode
• tablets and wireless keyboards or mouse

I was surprised to see wireless keyboards in this list. These are devices that are designed to transmit radio signals - admittedly Bluetooth is low power, but it shows that airlines have actually gone from a blanket "no way" to doing the measurements needed to prove that these things are safe. Either that, or they are reckless in an attempt to attract customers. Let's hope it's the former...

Answer 4

Generally GPS is a receive only service and does not transmit. Because of this it is considered safe to use in aircraft as long as the use of electronic devices in general is permitted.

However, from your question it seemed that the whole topic came up because the iPhone turns off the GPS when turning on flight mode. You concluded "it means that at least a few people at Apple are worried about people using GPS in flight.".

I am sure this is an incorrect conclusion.

The reason the GPS is turned off on an iPhone when you turn on flight mode is that the GPS and radio are handled by the same chip in some of the iPhones models and that both services are intrinsically linked on iOS devices. This is not the case for some other smart phones. Specifically:

The GSM iPhone 4 still used the Broadcom BCM4750 which could be turned on separately from the radio, but the CDMA version uses the Qualcomm MDM6600 which has Radio and GPS in one chip and is not discretely powered. The iPhone 4S uses the Qualcomm MDM6610 chip for both. I have not researched the 5 or 5S. For Apple its simply easier to turn off all the stuff on all devices rather than throw in multiple options for multiple devices. On top of that Apple uses assisted GPS (AGPS) which downloads satellite coordinates over the network and reduces the time to get a GPS fix from minutes to a few seconds. This is another reason for apple to turn off both together rather than cripple GPS performance without network.

I hope this clarifies things.

Answer 5

The other answer about the possibility of a failure of the GPS receiver chip are not wrong, but they miss a significant point in the use of GPS in smartphones : A-GPS (Assisted GPS)

In order to compute a position, a GPS receiver needs to know the distance between him and a set of satellites, and the exact position of said satellites.

The satellite precise positions (ephemeris) are supposed to be retrieved with the navigation message broadcasted by the satellites.

This message takes some time to be broadcasted (about 12.5 minutes). Remember the old GPS which required a long time to give the first fix ? This is where it comes from. Because it is inconvenient for the average smartphone user, the A-GPS has been developped. Instead of waiting to receive the navigation message on the slow GPS datalink, this message is transmitted by the traditional cell-phone network much faster. So the A-GPS requires to query this data on the cell-phone network, which means that the cellphone radio transmitter must be on and transmit, which in incompatible with the "airplane mode".

To make it short :

• A-GPS : fast for first postion, requires the cell-phone link to be active, hence to transmit. Incompatible with "airplane mode"
• "plain old GPS" : slow for first position, is receive only, compatible with "airplane mode"

Although it is not necessary, Apple apparently chose to disable the plain old GPS if the cell-phone link is off.

EDIT : I have just learned from the wikipedia page that some A-GPS device are not even capable of working in "plain old GPS" mode, the position computation being handled by a distant server access by the cellphone network

Answer 6

Having done this before while in flight (I was trying to see if the GPS would be disabled as I had heard commercial GPS systems are designed to not work after a certain velocity).

I can confirm the following:

1. It works, but its not as smooth or accurate as the in-flight display. There are many reasons for this - one is that due to the size of the screen and the distance covered (and possibly, the scale), each refresh interval has a large gap. In other words, your phone will refresh, and your pointer/pin on the map will jump around a lot.

2. It takes a long time to get a fix; I could only get 5 satellites (normally the receiver picks up 8+) - again, due to the speed of the aircraft and the fact that when you are not in airplane mode, other radios are used to approximate your position.

3. Is it harmful to the overall flight systems? Consider that on an average large airliner, not everyone remembers to turn off their devices, I think having a passive thing such as a GPS running - in the long term, would not be harmful to the electronics of the aircraft. I think an electric shaver that you can use freely would cause more problems.

Answer 7

Cellphones can affect airplane navigation equipment when not in airplane mode. This was shown in a 2006 episode of Myth Busters. In the link, please read the paragraph, this is true even though it says "Busted" on that page. This test was done in reasonably scientific manner and it shows that navigation equipment can be affected.

More recently however PED (Portable Electronic Devices) were deemed safe by the FAA for use during flight. This includes cellular phones, with the expectation is that phones will be set to airplane mode. Many devices, including the iPhone and iPad, with GPS functionality are included on this safe list.

GPS, for your cellphone is for the most part a receive only activity. Your cellphone does not have the power to transmit signals the 22,000+ miles it would need to get to a geostationary satellite in space. Nor does it need to. You can read how GPS works here. Any electromagnetic device turned on will transmit some interference, but there isn't much reason to believe that your phone would add much more interference running with GPS on, versus while it's off.

Interference coming from your GPS antenna should be coming from the frequency 1575.42 Mhz. This range isn't mentioned on the RF exposure information offered by Apple describing it's iPhones, presumedly because it is negligible.

It's reasonable to believe that having a GPS enabled phone should have negligible impact on a plane's navigation system.

Answer 8

The big question that is raised here is part of the great urban myth of do electronics actually play up with an aircraft's avionics.

My simple answer to your question is if the airline or Pilot in Command has asked you to shut off all electrical devices, there is no exception, it must be turned off.

My realistic answer - I've sat on the phone, flying around using my Bose Headset, and never had an issue. In Australia, there is no law against use of electronic devices except during refueling. Not sure about the rest of the world.

Answer 9

I would have added a comment to @JamesHam's answer but I don't have the rep.

Long story short, Your GPS on your phone is receive only. Yes your antenna will generate some field but I doubt it is much stronger when the phone is on than when it off. (However, antenna and interference were not my area of expertise.)

GPS signals travel at the speed of light. Even plane at top speed is still very close to stationary by comparison to the speed of light. Any issues you have with regards to GPS at speed have to do with your phone's hardware/software.

As far as the safety issue is concerned... It is a myth. Period.

While this is not my area of expertise a little simple logic goes a long way. Consider that your plane is awash in signals at a lot of frequencies while it is on the ground and at landing and takeoff (the most dangerous times). Signals from cell towers, FM and AM radio stations, GPS, DirecTv, lightning, etc. All of these sources bathe the airplane in EM waves across a slew of frequencies. Most bathe airplane with much more power than your phone can. Additionally, every wire with electricity in the airplane puts out an EM signal as well.

Another logic argument, there are approx 200+ people on every major flight in the US. If only 1% forget to turn off our phone that means there are at least 2 people on every flight with their phone's on the entire flight. EM interference happens over the course of less than a second so every hr of flight time represents at least 3600 discrete opportunities for EM interference per operational phone. How many flights crash every day?

Any modern aircraft is digital not analog. So interference due to EM can really only cause a flipped bit or a few. As far as a sweet spot in frequencies is concerned the most likely culprit for interference will come from other aircraft systems that sample and process data at the same frequency as the system being interfered with.

But so what? Every sensor man has ever created will create the occasional bad data point. Planes, satellites, etc all have to be robust to a few bad data points. So every computer that consumes sensor data on airplane has filters built in to it specifically to deal with a short series of bad data points.

Specifically the navigation system is usually blamed when the FAA or someone says EM interference brought that plane down. They then go about telling you how impossible it would be to prove it. So ... why do we believe them?

The navigation system in particular is built in such a way as to cope with missing and bad data. Typically the nav system uses some combination of multiple attitude, acceleration, velocity, and position sensors operating at different update frequencies. GPS is often only updated once a second while gyros and accelerators are updated sometimes 1000s of times a second. Then there is a propagator (or estimator) that calculates the aircraft's position, velocity, and attitude in between GPS updates using the faster sensor data. All of that sensor data is filtered so that the occasional garbage data point (which every sensor ever built generates) doesn't cause the aircraft to behave wildly, unpredictably, and sometimes fatally.

The nav system is often blamed because it is all software and sensors. The engine can't spin up or slow down fast enough for a few spurious data points to cause issues. Neither could the controls surfaces (flaps, ailerons, etc). A spurious blip in the radio or radar by themselves doesn't cause an aircraft to go down. So by deduction it can only be the nav system becoming confused by bad data. Except every modern nav system is inherently robust to noise on the sensor data and the larger the error is in the spurious noisy data the easier it gets filtered out.

Oh, and every single computer system on an aircraft is shielded against EM interference.

EM interference is a boogey man the FAA uses when they don't have an answer. It is reproduce-able in a lab in just the right setting. But the aircraft would never have gotten off the ground if it were sensitive enough to crash due to your cell phone or even 200 cell phones simultaneously in use on board.