Real time Differential correction requires a receiver designed for use with a real time differential service. Often different service providers have their own receiver, or they supply an auxiliary unit which will feed the Differential correction into the receiver. Real time Differential services broadcast an extra signal, which allows a GPS receiver to cancel out many of the errors inherent in GPS signals. Some of these services use their own satellites, and some have ground stations sprinkled around the covered area.
Some services combine the two.
Unlike GPS itself, Real time Differential services do not necessarily provide a global blanket. See the Differential Service Coverage Maps Page for some service coverages. Real time Differential provides some significant gains in accuracy, with sub-meter accuracy available. Because of this accuracy, some issues come to light. HDOP is an acronym for Horizontal Dilution of Precision .