4 ms·
> Also, did you know that unassisted GPS takes minutes to establish a lock even in most good cases; it is only because of the tower-provided assistance that GPS
by iamaelephant 7y ago
> Also, did you know that unassisted GPS takes minutes to establish a lock even in most good cases; it is only because of the tower-provided assistance that GPS can resolve in less than a minute, ever.
Then how does every GPS device without a network connection work? I'm no GPS expert by my understanding is that GPS lock can be attained far more quickly if the device caches satellite position data and uses that to assist. That's why a Garmin watch may take 3 minutes to lock on first use (or after departing an international flight) but will subsequently lock in seconds after that.
- chipotle_coyote 7y agoI think the assertion about unassisted GPS taking minutes to establish a lock in good cases simply isn't correct; my car's in-dash navigation system will often take minutes to establish a satellite lock if it starts up in a garage, but if it starts up outside, it's often well under half a minute. (Ephemeris data is broadcast from satellites every 30 seconds.) While it's been a long, long time since I worked in the GPS field (I was a contract technical writer at Global Locate for a few months, a company that created one of the first A-GPS networks), it's my understanding that GPS receivers can cache almanac data (the basic orbital positions of GPS satellites) but not ephemeris data (more precise orbital information, including clock information, from individual satellites). To get a GPS lock, your GPS unit needs to get ephemeris data from three satellites. Assisted GPS provides initial ephemeris data over the cellular network so your phone or other cellular device can get it much faster.