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On a flight across Europe, I took my iPhone out to see if the GPS can find my position. I was sitting away from a window, so reception would have been bad. I wouldn't have been surprised to get no result at all, but the GPS did show roughly the location where I was, showing a circle with a radius of 100 km as the error!

How can that be? If the GPS did't receive enough satellites, I would have thought it would have no idea at all about my position, and with enough satellites received it would know exactly where I am, so how can it know my position with a 100 km error?

PS. Moving my phone to the window made it display information with tiny error bounds, and the position moving on a map at the right speed - impossible to say whether the information was correct from 10,000 meters height. And moving further to the aisle it would display that there was no reception at all. But in between it did show very roughly where I was, with huge error bounds (the software draws where it thinks you are, and a circle around it based on the estimated precision).

PS. Found an explanation, but don't know if it is correct: Even at a height of 10,000 meters, an iPhone can register signals from cell towers. It knows where these cell towers are. And knowing that there is a cell tower with a really week signal means it can determine its location with a huge error.

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  • $\begingroup$ Just partially related to your question, as far as I know most airlines are still wary of fitting GPS as the primary navigation aid. $\endgroup$ – user108787 May 23 '16 at 21:04
  • $\begingroup$ I don't know if it applies when using a GPS while in flight, but many airplane trackers intentionally introduce substantial error into the displayed position to stop people from waiting under the predicted flight path. $\endgroup$ – CoilKid May 23 '16 at 21:21
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    $\begingroup$ See en.wikipedia.org/wiki/Dilution_of_precision_(GPS), but really this seems more like an iPhone software question, not a physics one. A poor DOP should fail to show a solution, not one with 100km error. $\endgroup$ – BowlOfRed May 23 '16 at 21:32
  • $\begingroup$ @count_to_10: GPS is not a guaranteed service (the US government reserves the right to deteriorate the position signal or to turn it off without as much as a notice and it will not take any legal responsibility for the consequences), so legally an airline can not use GPS as their critical or even as their primary navigation system. $\endgroup$ – CuriousOne May 23 '16 at 21:37
  • $\begingroup$ A GPS receiver requires visibility of a large section of the sky, which is simply not the case when you are sitting in a metal tube. That the GPS works, at all, is already close to miraculous. $\endgroup$ – CuriousOne May 23 '16 at 21:39
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There is a hint of it here (https://travel.stackexchange.com/questions/8861/do-you-have-a-gps-signal-on-board-of-a-plane?newreg=bc0d5655b2a14ee5a21e77fe52978b3e)

Unfortunately the plane body does an excellent job of blocking GPS signal. In general if you're in a window seat you will be able to get a signal by holding the GPS near the window (or, for example, on a tray table) - but if you're elsewhere on the plane it's very unlikely you'll get a signal.

The exception to this may be the new Boeing 787, which is made primarily of Carbon-fiber reinforced polymer (CFRP). Not only does CFRP allow for larger windows (which would allow a greater visibility of the sky, and thus more GPS satellites), but it's likely that the GPS signal would pass through the aircraft shell better than on conventional planes.

Keep in mind that not all airlines allow passengers to use GPS receivers in flight (they are technically "radio receivers" which are often not allowed). Be sure to check with the specific airline and/or the flight crew to confirm that you are allowed use one.

BJ Raval added

I have tried getting the GPS signal on a 747 on a transcontinental flight from a window seat and was able to get it throughout the journey. This was late 90’s and GPS receivers were first generation consumer receivers.

Also please note that GPS receivers are unreliable over 60,000 feet of elevation and commercial flights are at around 35,000 feet.

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