Many phone GPS's work near instantly when you have an internet connection, but if you use them in a forest with no internet, you might well be waiting up to 10 minutes to get a position.
Thats because the forest blocks many of the GPS satellites, and the remaining small number of signals take an awfully long time to collect information about the satellite orbits (which is normally downloaded from the internet with AGPS).
If you didn't know this, you would probably not wait 10 minutes for it to work - you'd just assume it wasn't working.
That fact is made worse by the fact that on android, if you switch away from the map app and then then go back again, this 10 minute process entirely restarts because the GPS gets switched off and forgets any fragments of information it had already collected!
Technically 12.5 minutes :). Here's a bit of fun (to me) info about the GPS constellations and the wild speed of it:
- data transmitted at 50 bits/s
- data frames are 1500 bits long, divided into 5 subframes
- Subframes 1-3 are satellite-specific and subframe 3 contains the high-precision orbit information for the sat you're receiving from. Subframes 4 and 5 contain the almanac that gives you a list of coarse orbit info for the entire constellation.
- The almanac consists of 25 pages of data. Each sequential frame has one page of the almanac.
30 second frames * 25 pages = 12.5 minutes to download the entire almanac once you have a lock on a given satellite. All of the satellites broadcast the almanac at the same time, but they're time synchronized so all of them are broadcasting page 1 at the same time.
Thats because the forest blocks many of the GPS satellites, and the remaining small number of signals take an awfully long time to collect information about the satellite orbits (which is normally downloaded from the internet with AGPS).
If you didn't know this, you would probably not wait 10 minutes for it to work - you'd just assume it wasn't working.
That fact is made worse by the fact that on android, if you switch away from the map app and then then go back again, this 10 minute process entirely restarts because the GPS gets switched off and forgets any fragments of information it had already collected!