[time-nuts] Re: Phase coherence with 2x GPSDO

[Lux, Jim]

On 3/7/22 9:48 AM, Krishna Makhija wrote:
> Thanks Mattia. What did you use for your Layer 1? I need to place one of
> the SDRs on a drone and one on the ground so a fiber or LAN cable is out. I
> could use WLAN but can you get sub-nanosecond performance over wi-fi? My
> initial guess would be no but I am not certain.

Not over WiFi - one could create a wireless reference link - you need to 
accurately measure the light time delay.  Typically, this would be done 
(at least how we do it in the space world) by sending a carrier out, 
having the remote unit turn that carrier around with a known 
frequency/phase ratio, and record it on the ground. That gives you an 
accurate "round trip" delay (you have to measure the delay through the 
remote unit ahead of time).  An accuracy of centimeters or mm is 
certainly possible if you're careful (stable oscillator on ground with 
low enough ADEV/phase noise for the round trip time, good understanding 
of phase center movement on both antennas).


But, that's post processing.  And if you're going to post process, you'd 
probably do better just recording raw GNSS observables and your local 
clock, then post processing on the ground. There's very well established 
ways to do this (as in "feed your data file to someone, and they send 
back the analysis")

There are ways to get accurate time correlation at both ends of the link 
(which would be needed if you were going to do something like phased 
array *transmitting*) - the same coherent turnaround technique, but 
starting at the remote end, for instance.

Just to say "what's possible" - The round trip time delay from Earth to 
Jupiter and back is regularly measured with an ADEV of ~1E-14 over tau 
of 1000-10,000 seconds.  That's about 1E-11 seconds (0.01 ns) uncertainty.