What is A-GPS?

A-GPS, or Assisted GPS, is a technology that allows GPS receivers to use additional information from cellular base stations in addition to GPS satellites to provide the location. It is particularly useful when mobile devices cannot establish a reliable connection with GPS satellites or at locations where satellite signals cannot penetrate through. A-GPS is generally implemented in mobile phones.

GPS devices communicate with 4 or more GPS satellites to determine location with accuracy. A-GPS heps provide accurate location data even when a reliable connection cannot be made with the required number of GPS satellites. Along with better coverage area, A-GPS also improves the start-up time, which is the time required by the satellites and GPS receiver to establish a reliable connection. So while the GPS receiver is connecting to the satellites to establish a connection, A-GPS position data can be used in the interim. Though it is to be noted that GPS data from satellites is usually more accurate than A-GPS based location data. However, at the same time, in many applications some location information with less accuracy is more use useful than no location information. This is where A-GPS comes in.

To further improve the location accuracy and coverage area, mobile phones combine A-GPS with other location-based systems such as Wi-Fi positioning, cell-site triangulation and can also use information from nearby Bluetooth devices.

A-GPS has two modes of operation: Mobile Station Assisted (MSA) and Mobile Station Based (MSB).

Mobile Station Assisted (MSA)

In the MSA mode of A-GPS, the position calculation is done by A-GPS server using the information from mobile device and the positioning information is sent to the mobile device. The process of calculating position using MSA mode A-GPS can be understood using the following three steps:

1. The mobile device receives reference time, acquisition help and other optional data information from mobile network provider (or A-GPS server).
2. The mobile device sends back the incomplete GPS data to mobile network provider (or A-GPS server).
3. A-GPS server calculates the position of the mobile device and provides the location information to the mobile device.

Mobile Station Based (MSB)

In the MSB mode of A-GPS, the position calculation is done by the mobile device using assistance data from A-GPS server. The mobile device receives assistance data, such as time, ephemeris and other data, from the A-GPS server. Using this data, the mobile device calculates the GPS satellite position.