Just a little more info on range finding with radio, you are timing the speed of light that is just under 300,000,000 m/s. This is fast most likely faster than you will clock your micro controller at, so you will have to find another way to measure it.
Also your carrier will need to be faster than the smallest unit you are going to measure say around 400 mhz because you don't want your carrier slower than what you want to measure. It can be done because you can buy laser range finders off the shelf and they use speed of light to calculate distance. You could possibly use a PLL to clock to some analogue storage, maybe use a 400 mhz > clock to pump up a cap during the timing window then read the cap voltage?
If line of sight is not a concern then you may want to look at ultra sonic transceivers as the speed of sound is a lot slower and easier to clock.