What do you want to do here? What kind of distances in what scenario?
Tracking devices with radio is really touchy. Judging distance by amplitude of signal is very vague if you don't have line-of-sight or there are reflecting surfaces present. Antenna orientation will change the received intensity too. Not that there aren't tracking collars for wildlife which were used effectively but it was tricky to track down.
The hot thing nowadays is using GPS receivers to determine an exact position and transmit that. The receiver has his own GPS position so it can calculate the difference easily.