I 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

I 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.