[ooterness]

Over local distances, all you need is a shared clock reference. As others have pointed out, the RTL-SDR dongle can do this with mods.

Over longer distances, this is an active area of research with many different approaches for various applications.

Two recent well-known examples are the "Event Horizon Telescope" (the network of radio telescopes that has been generating images of black holes) and optical frequency combs (a recent demo published in Nature achieved time-transfer accuracy of a few femtoseconds).

https://eventhorizontelescope.org/

https://www.nist.gov/topics/physics/optical-frequency-combs