A GPS receiver must be able to get a lock on 4 satellites to be able to solve for a position.
The first lock it gets allows the receiver to obtain the almanac information and thus what other satellites it should listen for.
Common systems that use the control segment to improve accuracy are WAAS and DGPS.
The satellites continuously send data down to earth over dedicated RF frequencies.To combat these errors, a couple of different assistants have been created. This method uses wireless (ground-based) networks to help relay between the satellite and the receiver when the GPS signal is weak or not able to be picked up. The first is to provide the receiver with the proper almanac data and the precise time.The second utilizes the higher computing power and good satellite signal of the ground base to interpret the broken or fragmented information the receiver is receiving to provide a more accurate position reading to the receiver.DGPS requires a specific type of GPS receiver and gets centimeter accuracy.DGPS units are also expensive and tend to be larger because they require an additional antenna.