This website conveys the impression a rover on the Martian surface necessarily navigates by dead-reckoning. Stereo images are used to determine how far a rover has travelled; an advancement over older technology which may have merely counted wheel revolution.
On Earth's surface, accurate navigation is made easier by the aid of satellite navigation systems such as GPS. A system such as GPS is apparently a bunch of satellites in orbit queried by a dedicated receiver to determine the position of the receiver itself.
Pure speculation on my part - feel free to shoot the thought down. I guess each question in the list below could be a separate question in itself ... let me know if that is a better way to go about it
How many satellites would be needed in orbit around Mars to be used as a navigation aid? My guess is the smaller sphere may translate to a lesser number than required for Earth.
Are the rovers on Mars capable of communicating with the satellites in orbit?
Can the 5 satellites currently in orbit around Mars be made into a jury-rigged system to provide a satellite based navigation system for the rovers?