In order to determine where you are you need to know where all of the satellites are. For a standalone receiver this involves downloading a almanac of the satellites from the signal, but GPS receivers have small antennas and the satellites don't blast out at tremendous power so the available bandwidth is very low. This means the effective bitrate of a GPS signal is only 50 bits per second so it takes twelve and a half minutes to transmit the entire list.
Cell phones get around this by downloading the almanac from the internet. Standalone receivers also keep the almanac in nonvolatile storage, but the almanacs eventually go stale if you leave the receiver off for too long.
surely you need to know where you are not, to know where you are. if the difference between where you are not and where you were, or vice versa, is correct, then you are being targeted by the missile.
Cell phones get around this by downloading the almanac from the internet. Standalone receivers also keep the almanac in nonvolatile storage, but the almanacs eventually go stale if you leave the receiver off for too long.