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Recently, I flew on an Airbus A320. I turned my Lenovo A328 Android phone into airplane mode, and then used the default Maps app to watch my position using GPS. For most of the flight it worked normally. However, at the end of the flight, at about the time the plane started to descend so as to land, the GPS position marker suddenly jumped to an obviously wrong spot. After that, it ceased to update — at least until I got out of the plane.

Is it expected behavior of a phone GPS receiver during a flight? If so, what causes it?

It seems the plane does need GPS itself to be able to, e.g., send ADS-B data, so it doesn't seem likely that it jammed the signal. It also doesn't look like the clouds could be an obstacle for GPS signals, since after the landing the operation of my phone GPS receiver resumed (being under the clouds).

For reference, here's flightradar24 data about my flight.

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  • $\begingroup$ Cloud is an obstacle. Cloud plus the fuselage could be the difference between a good enough reception and none. $\endgroup$
    – Antzi
    Aug 1, 2016 at 12:22
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    $\begingroup$ @Antzi I'm fairly sure that clouds don't block GPS signals, do you have some reference for that? $\endgroup$
    – Pondlife
    Aug 1, 2016 at 12:40
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    $\begingroup$ @Pondlife: And you are right, even if this attenuation graph is for water vapor (which is invisible) and not water droplets. However leaves attenuate significantly the signals. $\endgroup$
    – mins
    Aug 1, 2016 at 18:10

1 Answer 1

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A GPS needs a line of sight to (at least) 4 satellites in order to compute a position. Given that you were able to get a position during the cruise, I assume that you were close to a window and had a clear view of a part of the sky.

During the cruise, the aircraft trajectory was close to a straight line (or changing slowly), so your receiver was able to keep a line of sight to the same set of satellites. But during the approach, the trajectory was much more dynamic (lots of turns), so your receiver didn't had a constant line of sight toward the same set of satellites and "lost" them. It takes time for a receiver to "lock" on the signal of a given satellite, so the receiver it couldn't compute a position from other satellites because they were not in sight long enough.

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    $\begingroup$ This seems to be a reasonable explanation. $\endgroup$
    – Ruslan
    Aug 1, 2016 at 14:06
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    $\begingroup$ Add to that the fact that larger planes seem to possess Faraday cage-like qualities; all commercial flights I've flown on recently only gave me very poor, borderline GPS reception. Pressing the phone against the inner window made a big enough difference that if I moved it half an inch away, I'd lose signal. High speed trains seem to exhibit similar characteristics. Phone GPS chips being deafened by local RF sources? $\endgroup$ Aug 1, 2016 at 16:43
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    $\begingroup$ @ChristopherWoods GPS chips being deafened by local RF sources ? I don't think so, if it were true then cell phone would also be jammed. $\endgroup$
    – Quentin
    Aug 1, 2016 at 20:59
  • $\begingroup$ I would also consider tha the fuselage will block all satellites except the ones near the horizon. As you start to descend those will disappear over the horizon $\endgroup$
    – TomMcW
    Aug 1, 2016 at 23:04
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    $\begingroup$ @ChristopherWoods, the GPS L1 band (the one interesting to civilian receivers) is 190 mm wavelength and that only barely passes the window, with a lot of diffraction and attenuation. There is no local noise on the band; it is really mainly the shielding. $\endgroup$
    – Jan Hudec
    Aug 3, 2016 at 19:33

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