Which way are there to get a robots position (x,y,z) from a measuring center in a radium of about 50-100 meters? It has to go through walls and trees and so on, and it may not by GPS.
To obtain a position in 3 axis you will need at least 4 'beacons' to transmit/recieve data. Using more beacons will increase accuracy.
Not sure how to construct any such beacons, look at www.howstuffworks.com/gps.htm to get any idea of the principles used and then it may be easier to design something for yourself.
You might consider a system that starts in a known position and that get updates but otherwise keeps track of changes in position - and calculates a new position based on direction of travel, velocity/time or possibly acceleration. I'll admit that this could get pretty complicated.
It's not good using the position updating system, because of the robots job. It can be pushed out of "the way" and then the system would be out of control.
I was thinking something like an ultrasonic system, but the disadvantages is the point, that sound would be bent around corners, and can't go through wall, so the exact position can be calculated.