Not necessarily. Even without GPS, commercial airliners have a good idea of where they are using their INSes (inertial navigation systems). That technology well predates GPS receivers being common in aircraft. Most of the Boeing and Airbus aircraft you fly, for example, have 3 INSes and probably 2 GPS receivers. All of the data feeds into the computers to average to the position the plane thinks it's actually at. When the GPS signal is reliable, that may be used as the actual position and can be used to keep the INS drift minimized, so if the GPS signal is lost (or the receivers malfunction), the INS takes over as the primary source of position data. This is all further supplemented and verified with radio navigation aids -- as the planes fly along their routes, the airplane computers automatically tune in to various radio navigation aids. In short, even for the newest big commercial jets, functional GPS is very much optional and positional data is quite good without it.
As for the moving maps on the seat-back displays: I guess it all depends on the data source they use. If they have their own GPS receiver for the plane's "entertainment" system, that doesn't get the computed current position from the real navigation systems, then the map would presumably stop working or make up (e.g. interpolate) data from when it last had good position data.
But now that I think about it... I've only ever seen those maps draw the route as the great-circle path from the departure airport to the destination airport. So they're fictional in that regard -- whether they're interpolating the position on the great-circle path as a percentage of the actual track distance flown and remaining from the real navigation data, or doing something even more fictional, I don't know.
> But now that I think about it... I've only ever seen those maps draw the route as the great-circle path from the departure airport to the destination airport. So they're fictional in that regard -- whether they're interpolating the position on the great-circle path as a percentage of the actual track distance flown and remaining from the real navigation data, or doing something even more fictional, I don't know.
Very interesting. Thanks for the detailed answer. Next time I'll make sure to record the position information independently as well. For some reason I had just taken their great-circle path at its face value and never questioned it. In hindsight, it was all too perfect. :)
It varies between airlines - off the top of my head I couldn’t tell you which does what, but some are completely accurate, others are essentially a great circle progress bar - and they always turn it off for the interesting bits around takeoff and landing.
As for the moving maps on the seat-back displays: I guess it all depends on the data source they use. If they have their own GPS receiver for the plane's "entertainment" system, that doesn't get the computed current position from the real navigation systems, then the map would presumably stop working or make up (e.g. interpolate) data from when it last had good position data.
But now that I think about it... I've only ever seen those maps draw the route as the great-circle path from the departure airport to the destination airport. So they're fictional in that regard -- whether they're interpolating the position on the great-circle path as a percentage of the actual track distance flown and remaining from the real navigation data, or doing something even more fictional, I don't know.