I am trying to design a system to measure the distance between two RF transceivers with as small of a resolution as possible. Time-of-flight measurement is nice and easy to do, but it has a poor resolution with low frequency timers. I am thinking of using a low-cost mcu that will probably have a clock rate of ~30Mhz. So the best possible resolution I could get would be (1/3x10^7)*c=~10m/s. I would like to get much better resolution than that. I did some searching around and found some info on RF interferometry. Basically, you create a wave and split it in two. The first wave goes out and does it's thing and then comes back. The second one stays where it is. You then compare the phases of the two. Depending on the wavelength, you can determine the distance that the first wave traveled by the amount the phase shifted compared to the second wave. Obviously, this will only be accurate if the distance traveled is less than one wavelength. By combining time-of-flight measurements with phase difference measurements, could you not get a much higher resolution? It seems to me that distance measurement with interferometry is almost exclusively done with lasers or other forms of light, which is only good for very small distance measurements. Why is this? I must be missing something.