How does GPS on an airplane report a 100 km error? 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. 
 A: 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.

