[Al Skierkiewicz]

Chitu,
There is really nothing that will be accurate over any terrain for any great distance. All of the methods will have some built in error that accumulates over time and distance. Even RADAR based sensors will lose track of the reference object from time to time. What kind of distance is your application aimed at? 100 yards, 100 miles? Over short distances, reading wheel rotations is a good choice. It is simple, easy to decode and the errors are relatively small. If you read several wheels and correlate the data you would be able to compensate for turns and any one wheel slipping. Over hilly terrain they won't give you straight line distance traveled, though.