I thought the cell-tower positioning is simply "in addition to" the GPS location provided by Satellites and that just improves the accuracy.
There are several different types of Assisted GPS that cellphones can use.
From the wiki article:
"Assistance falls into two categories:
1.Information used to more quickly acquire satellites
It can supply orbital data or almanac for the GPS satellites to the GPS receiver, enabling the GPS receiver to lock to the satellites more rapidly in some cases.
The network can provide precise time.
2.Calculation of position by the server using information from the GPS receiver
The device captures a snapshot of the GPS signal, with approximate time, for the server to later process into a position.
The assistance server has a good satellite signal, and plentiful computation power, so it can compare fragmentary signals relayed to it
Accurate, surveyed coordinates for the cell site towers allow better knowledge of local ionospheric conditions and other conditions affecting the GPS signal than the GPS receiver alone, enabling more precise calculation of position. (See also Wide Area Augmentation System)
As an additional benefit, in some A-GPS device implementations, known as "MS-Assisted," the amount of CPU and programming required for a GPS receiver is reduced by offloading most of the work onto the assistance server.
A typical A-GPS-enabled receiver will use a data connection (Internet or other) to contact the assistance server for aGPS information. If it also has functioning autonomous GPS, it may use standalone GPS, which is sometimes slower on time to first fix, but does not depend on the network, and therefore can work beyond network range, and without incurring data usage fees.[3] Some aGPS devices do not have the option of falling back to standalone or autonomous GPS.
Many mobile phones combine A-GPS and other location services including Wi-Fi Positioning System and cell-site triangulation and sometimes a hybrid positioning system.[4]
High Sensitivity GPS is an allied technology that addresses some of these issues in a way that does not require additional infrastructure. However, unlike some forms of A-GPS, high-sensitivity GPS cannot provide a fix instantaneously when the GPS receiver has been off for some time.
"
http://en.wikipedia.org/wiki/Assisted_GPS
