Assisted GPS

Assisted GPS is a type of GPS used in cell phones. It has advantages over many ordinary GPS units in that it can work in poor signal conditions that ordinary GPS doesn't work, such when there are obstructions such as tall buildings, trees, and being indoors. This make the cellphone useful for calling E911 in many cases where many ordinary GPS units could loose a signal. Another advantage is that with assisted GPS an Assistance Server is used. The Assistance Server locates the cell phone through what cell site on the cellular network it is connected to at the time. The Assistance Server has such good satellite signal and computation power that it can compare fragmentary signals relayed to it by cell phones, with the satellite signal it receives directly, thereby the cell phone's position is known. The Assistance Server can supply orbital data for the GPS satellites to the cell phone. Some ordinary GPS units may not get to download the almanac and ephemeris information until a clear signal is received continuously for up to one minute. Which means the GPS won't function if this information needs to be updated. Another advantage of using the Assistance Server is it can have better knowledge of ionospheric conditions and other errors affecting the GPS signal, thereby enabling more precise calculation of position. The accuracy of assisted GPS is between 5-10m. Perhaps the latest technology is even more precise. The assistance server can also offload most of the work of CPU and programming required for a GPS Phone.