A-GPS

Assisted Global Positioning System (A-GPS) is a satellite-based positioning system, which is generally the most accurate of the geo location methods available on the phone. The accuracy afforded by A-GPS, however, comes with a power penalty. It uses significantly more power than the other two sensing technologies. Because of this, it should be used only when an application needs a high level of accuracy.

GPS uses radio signals from satellites and does not perform as well in dense urban areas. In poor signal conditions, signals may suffer multipath errors; signals bounce off buildings or are weakened by passing through tree cover. Later in the chapter, you see how to reduce the effect of multipath errors by applying a movement threshold to signal readings.

A-GPS is an extension to GPS. It improves GPS’s startup performance by using an assistance server to supply satellite orbital information to the device over the network. This is faster than acquiring the information from the satellite itself, which can also prove difficult in low signal conditions.