Hi @Jeffk!
There’s another discussion going on at the moment about flight recording, which will be well worth a read: Flight details missing from timeline
Airlines are quite a special case, with very different and constrained conditions. I go into detail in that other discussion, but the gist of it is that when you’re on the ground your phone determines your location by making use of three different methods: 1) cell tower triangulation, 2) wifi hotspot triangulation, 3) GPS/GNSS triangulation, but when you’re in an airplane only the last of those is possible - GPS/GNSS.
Ok, so, I do recommend reading the other discussion, but here’s one of my signature overly detailed technical explanations anyway:
Cell tower triangulation and wifi hotspot triangulation don’t need “line of sight” - they can see through walls. That’s how iPhones are capable of determining your location when you’re inside buildings, or in built up city areas with limited view of the sky. GPS/GNSS on the other hand requires “line of sight”, meaning that the phone needs to be able to visibly “see” a broad area of the sky, so that it can pick up signal from multiple satellites.
When you’re in an airplane, dependent solely on GPS/GNSS, and your only view of the sky is through a small window, your phone is pushed to its limits and is very likely to lose line of sight to satellites, thus losing track of your location completely. That means it’s not just going to get low accuracy, it’s going to get nothing at all.
Typically line of site to four or more satellites is necessary, and the more broadly distributed they are in the sky the better. Out your airplane window the phone is going to find it a challenge to maintain line of sight to four satellites, and will find it impossible to see four broadly distributed satellites (because the view out the window is in only one direction and within a constrained viewing area).