A variation of Continuous-wave radar from wikipedia:
"Ranging can be implemented, however, through a technique known as "chirping", or frequency modulated continuous-wave radar. In this system the signal is not a continuous fixed frequency, but varies up and down over a fixed period of time. By comparing the frequency of the received signal to the one currently being sent, the difference in frequency can be accurately measured, and from that the time-of-flight can be calculated."
Because frequency is one measurement that can be made very accurately (directly related to your time base accuracy, think cesium beam oscillator) this method might be able to get the resolution you are looking for. Won't be cheap but would be proven practical tech.
Lefty