Assisted GPS

The main advantage of Assisted GPS (AGPS) for the iPhone is that it will allow the iPhone’s GPS to operate in weaker signal conditions and determine positioning quicker than most standalone GPS receivers. AGPS also helps save battery power by handing off some computational duties to an assistance server and by allowing iPhone users to turn GPS off and on with less inconvenience.

How does it work? From a cold boot-up, a regular (non-assisted) GPS receiver requires a strong signal in order to search and discover satellite signals, decode information relating to satellite positioning, and then calculate its position. This process can sometimes take up to several minutes depending on initial signal strength. This startup time is why GPS users don’t like to turn off their GPS receivers. However, an always-on connection is a major problem for battery life in a cell phone.

AGPS discovers its position with the help of GPS equipment at cell phone towers, which maintain a constant strong connection to GPS satellites.  The cell phone towers use approximate positioning of the iPhone, similar to the first-gen iPhone’s My Location, to send pre-decoded ephemeris satellite positioning and clock information to the iPhone. The result is that the iPhone will discover its position quicker, and thus be more conducive to having its GPS turned off and on to save battery.

Helpful Links:

Assisted GPS at Wikipedia.