We know that the GPS receiver works with three or more satellite communication, and confirm the distance, the receiver goals is calculated by the three-dimensional coordinates. However, the actual positioning accuracy is related with the number of satellites receiving, satellite azimuth, signal strength and other related.
In addition, GPS performance has a time to first fix (TTFF), this parameter depends largely on the sensitivity of the receiver and satellites status. Under the same equipment conditions, the receiver performance in the open area is much better than in high-rises in downtown. it can be said that it is a unfavorable environment among the buildings, in such weak signal status, the GPS tracker may take several minutes to analyze and calculate signal.
A-GPS technology came into being, it’s full name (Assisted GPS).
A-GPS is mandated by the US Federal Communications Commission (FCC) to provide positioning of cellular phone driven emergency callers on E911 project. A-GPS is intended or improve TTFF, or make the positioning operation TTFF become possible when it does not improve
When a GPS signal is not reliable, A-GPS devices can be obtained additional information through the mobile network infrastructure time synchronization, Doppler, frequency, etc., and get the location of the mobile device by three times a few measurements. By communicating with a plurality of base stations, calculate position can be implemented in a mobile device or network server. This provides a more reliable way to obtain location information.
Construction based on user-level A-GPS does not need to add control plane mobile network special hardware, and this method is universal for different mobile standards. So long as a dedicated A-GPS chip can be achieved at low cost A-GPS positioning
Unfortunately, the A-GPS positioning network transmission must be achieved through multiple (up to six one-way transmission), which is undoubtedly require a great deal of resources for operators. Under China’s current 2G network air resources scarce, it is unrealistic to carry out A-GPS. Thus, 3G is the promotion of good soil for A-GPS.