Assisted GPS is an improved version of conventional GPS and it has revolutionized mobile navigation systems across the globe. The A-GPS system relies upon an assistance server to accurately resolve the location of a device. A-GPS system is commonly used in mobile phones to provide real time navigation services. The Global Positioning System works on the principle of trilateration to determine the coordinates of a point on earth. In this principle a satellite is considered to be at the center of a sphere and the mathematical treatment of three such spheres/satellites gives the intersections of the spheres which are used to determine the coordinates. By including one more sphere/satellite in the analysis the point can be located more precisely. The distance between the point on earth and the satellite is computed by transmitting a packet of data tagged with the sent time and by subtracting this from the time the packet was received, we get the time of transit. Assuming the packet traveled at the speed of light, we can estimate the distance of the satellite which serves as the radius of the sphere.
The standalone GPS system is susceptible to atmospheric conditions and multipath errors which tend to prolong the time to first fix. This is not appropriate for real time navigation and emergency applications. A-GPS offers a solution to this problem with an architecture that makes use of data connectivity to an Assistance server located at a place with strong signal connectivity with the satellites. There are two possibilities here.
- The assistance server may transmit orbital parameters and atomic time information to the mobile phone via the network. The GPS receiver in the mobile phone correlates this data with the fragmentary data received directly from the satellites thereby increasing its sensitivity and resulting in a quicker time to first fix.
- The mobile phone with limited processing capabilities can delegate the processing function to the more sophisticated assistance server by sending the weak fragmentary data to the processing server that has good satellite signal and receive the position information from the server through the network.
A-GPS is a feasible option to provide navigational services in an urban environment with tall buildings and high radio interference. On the flipside the service provider will charge the subscriber for network data traffic using GPRS/3G.
