Assisted GPS, A-GPS, AGPS

Your smartphone likely has functionality for both the Global Positioning System (GPS) and Assisted GPS (A-GPS) built into it. GPS draws its information from the constellation of satellites orbiting the Earth. Assisted GPS, also known as A-GPS or AGPS, draws its information from local cell towers and enhances the performance of standard GPS on mobile devices connected to a cellular network. Assisted GPS improves location performance in two ways:

• Assisted GPS helps the phone obtain a faster Time to First Fix (TTFF), which significantly improves startup performance. Assisted GPS acquires and stores information about the location of satellites using the cellular network, so the information does not need to be downloaded by satellite.
• Assisted GPS positions a phone or mobile device when GPS signals are weak or not available. GPS satellite signals don't penetrate wall very well and may be impeded by tall buildings. Assisted GPS uses proximity to cellular towers to calculate position when GPS signals are weak or unavailable.

A GPS system needs to make satellite connections and find the orbit and clock data before it knows its location. This is the period known as the Time to First Fix (TTFF). The process can take anywhere from 30 seconds to a couple of minutes before your device acquires a signal—exactly how long depends on the surroundings and amount of interference. It is easier to acquire a signal in wide-open areas than in a city with tall buildings.

When your device uses Assisted GPS, the time to signal acquisition is faster. Your phone pulls information about the location of satellites from the nearest cellular tower, which saves time. As a result, you:

• Get faster signal acquisition
• Save battery life
• Get location information even when GPS can't get a signal

By itself, Assisted GPS does not position the mobile device as closely as GPS, but working together, the two cover all the bases. All modern phones have an A-GPS chip in them, but not all phones use it. When you are looking for a new smartphone, ask if it has a full, autonomous Assisted GPS that is user-accessible. This is the best configuration for users, although only some phones support it. Some phones may offer only limited A-GPS.