GPS can use post processing but one must return to base-station, down load data and apply the correction this takes time. To get immediate accurate locations real-time GPS can be used. The correction is broadcast by radio to the rover unit continuously and the corrections are made in real time . Currently has 17-18 units in orbit. No differential correction is possible.
Some units have compass and other features.
50 meters at best. Recreational Grade Units. Generally are hand-held and designed to store only a limited number of points. Generally cannot down-load data to a computer. Some can display maps in the background.