[thoop]

Very interesting! Thanks for the great response.

Is there a better frequency available to amateur hardware that would give tolerance within more reasonable limits?

Without a shared external clock reference, i.e. over longer distances, how expensive does the hardware get if you want to be able to accurately measure the time/phase wandering to correct in software?

Just curious if it’s a limit of the low cost RTL-SDR or if it’s a harder problem than that (or both?).

[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