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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.

Rodrigo de Azevedo
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Ruslan
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  • Cloud is an obstacle. Cloud plus the fuselage could be the difference between a good enough reception and none. – Antzi Aug 01 '16 at 12:22
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    @Antzi I'm fairly sure that clouds don't block GPS signals, do you have some reference for that? – Pondlife Aug 01 '16 at 12:40
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    @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. – mins Aug 01 '16 at 18:10

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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.

Quentin
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    This seems to be a reasonable explanation. – Ruslan Aug 01 '16 at 14:06
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    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? – Chris Woods Aug 01 '16 at 16:43
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    @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. – Quentin Aug 01 '16 at 20:59
  • 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 – TomMcW Aug 01 '16 at 23:04
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    @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. – Jan Hudec Aug 03 '16 at 19:33
  • The satlellites are constanly moving too, they are not stationary "GPS satellites fly in medium Earth orbit (MEO) at an altitude of approximately 20,200 km (12,550 miles). Each satellite circles the Earth twice a day. Expandable 24-Slot satellite constellation, as defined in the SPS Performance Standard." https://www.gps.gov/systems/gps/space/ – CrossRoads Apr 06 '19 at 15:02
  • If you get a good GPS APP you can see all the sats that are potentially known about, including those that are on other other GNSS (global navigation satellite service) services than GPS if your device supports it. The airplane body is a Faraday Cage, so is anything with a solid exterior metal skin. At the microwave frequencies GPS uses, even closely placed metalic dots can screen the signal that is why you can see into your microwave oven for instance without getting burned yourself. 1.9 GHz is not too far really from 2.4Ghz so the dots would only need to about 1/5 as close together. – Rowan Hawkins Dec 31 '21 at 13:26