When a GPS receiver is switched on, it needs to gather data from at least three or four GPS satellites before it can report its position. This usually takes about a minute, but it might take longer if the receiver is far from where it was last used or it's been off for a very long time.
Before a GPS receiver can use a satellite, it first needs to "find" the satellite's signal and then wait to receive data describing the satellite's orbital position (ephemeris data).
Finding a satellite involves isolating its signals from background noise by listening for a precisely timed pattern (unique to each satellite). If a particular satellite's pattern cannot be found, it could be that the satellite is not nearby or its signals are blocked by an obstruction or interference.
After your receiver finds a new signal, it needs to listen for 30 seconds to receive current ephemeris data (which describes exactly where the satellite is located). If the signal is lost during this time, the process for that particular satellite needs to start again.
Synchronizing the Internal Clock
Your GPS receiver's clock needs to be synchronized with the clocks in GPS satellites in order to accurately measure how long signals are taking to reach it.
Once your receiver has acquired signals from 3 satellites, it may be able to report a position if its internal clock is still synchronized or you're still at about the same elevation where the receiver was last used. In many cases, however, it needs to acquire a 4th satellite to accurately synchronize its internal clock.
Saving Time by Predicting which Satellites are Nearby
A GPS receiver can often save time by predicting which satellites are overhead. It does this by calculating satellite orbits using the time from its internal clock and almanac data it's collected from GPS satellites in the past.
This trick works if you're somewhere near the location where the receiver was last used. If not, your receiver will eventually figure out that you've moved and start searching for other satellites.
Saving Time by using Recent Ephemeris Data
If your receiver was used recently (within the last couple hours), it might still have current ephemeris data for some satellites. If so, these satellites can be used in position calculations as soon as their signals are found. (The receiver does not need to listen for 30 seconds to collect orbital data.)
Wow! - That's it in a nutshell (leaving out a lot of detail). Let's review some key points.