> and my phone has no problems finding GPS and I have navigation instructions pulled up in about 5 seconds.
That's because your phone isn't actually using GPS ( e.g. Navstar or Glonass ) at that point, it's using cell triangulation or wifi-based location.
For example the fastest possible first-fix with Navstar from a warm-start, using cached prior-location and ephemerides, is 30 seconds[0]. From a cold ( position-unknown ) start, such as a car nav-system powered-down in a parking-lot for an extended period, the first-fix will take longer than 12 minutes. There is no technological way around that, it is an artifact of the system architecture.
[0] and that's for a top-end receiver than can sync to four satellites 'simultaneously' using time-division.
How often do cars change position while powered down? That is, why not always (even on "cold start") use the last known position as a starting guess? How do phones get GPS-quality location data even immediately after being rebooted?
Annectodatally, I remember that some years ago my phone used to take 5-10 minutes to find GPS whenever I asked for directions. Now adays, it's instantaneous.
The software has improved dramatically, using some combination of previously saved location, WIFI triangulation, and dead reconning to figure out a near accurate location to within 50 feet so long as you're in a city.
Car navigation systems can do better. They're not even 'pure' GPS services, they have access to mobile cell, they do online updates for maps, and their accelerometers are far better than phones. The software though is simply worse than what Google can provide.
Phones do more than use the towers for location information, they use the data connection to download information about the satellite positions, shortcutting a 12.5 minute step:
I wonder if the average car system really has the sensors for high quality inertial navigation. If they don't, the navigation won't work very well anyway until it has a decent GPS fix.
A warm start is actually typically faster than 30 seconds, averaging closer to 20 seconds. Additionally, almost all fixes using smart phones are hot starts. This is because smartphones use assisted GPS, they download almanac and ephemeris data not from the GPS network itself (where they are broadcast every 12.5 minutes and 30 seconds, respectively) but over a data network, in seconds. Similarly, smartphones have a pretty good idea of where they are already, because they keep track of their positions historically and they use cell tower triangulation as a secondary location system. From a hot start a GPS receiver could get a fix in only a handful of seconds, potentially.
I have some GPS receivers that would seem to contradict this. Is there something special about the way that cars receive GPS data?
My bluetooth GPS, connected to a non-cellular iPad tends to get lock in under 2-3 minutes, even when it gets turned off, tossed in checked luggage, and flown halfway across the world.
That's because your phone isn't actually using GPS ( e.g. Navstar or Glonass ) at that point, it's using cell triangulation or wifi-based location.
For example the fastest possible first-fix with Navstar from a warm-start, using cached prior-location and ephemerides, is 30 seconds[0]. From a cold ( position-unknown ) start, such as a car nav-system powered-down in a parking-lot for an extended period, the first-fix will take longer than 12 minutes. There is no technological way around that, it is an artifact of the system architecture.
[0] and that's for a top-end receiver than can sync to four satellites 'simultaneously' using time-division.