Test on Taxi - GPS on #110

-- The satellites transmit both 'almanac data' (every 30 sec) and 'ephemeris data' (continuous) to the ground.
-- For locking satellites, the receiver needs the almanac data.
-- Therefore if you're moving, it's very easy to miss the signal because of those buildings, and the roof of your van.
-- Once a particular satellite is locked, the receiver could re-use it easily, i.e. re-acquire the locking. However, since those satellites are moving fast (7000miles/hr), it'll disappear after 6 hrs at most (goes to the other side of the earth) and the watch needs to find another sat.
-- Garmin 110 already equipped with SiRF IV chipset which is one of the best in the industry la (though not army-grade).
For mobile phone, usually it also uses 'A-GPS' (Assisted GPS). The almanac data is also supplied by the ground network (the phone network) and it actually tells the device "hey, for current time and location, you can use satellites X, Y and Z, etc."

Thus the device no need to widely search the sky.
For location-based apps, i.e. 愛風check-in, depends on the situation, they could utilize GPS, AGPS, mobile network(cellular bases location) and wi-fi (access point location) to determine your approximate location.

But one thing for GPS -- line of sight. If you can't see the sky (even under a bridge), no GPS signal could be used. So for running HK SCM, we have 3 tunnels la. For the watch it could only draw a straight line from the entrance to the exit.