Assisted GPS, A-GPS, AGPS

Your cellphone probably has both Global Positioning System (GPS) and Assisted GPS built into it. GPS draws its information from the web of satellites above 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 in smartphones and other mobile devices that are connected to a cellular network. Assisted GPS improves the location performance in two ways:

• A-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 may be impeded by tall buildings, and they don't penetrate building interiors well. Assisted GPS uses proximity to cellular towers to calculate position when GPS signals are not available.

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. The process can take from 30 seconds to a couple of minutes before your device can acquire 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 doesn't 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 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 or assisted GPS that is not accessible to users at all.