1ns is about the best you can do with the nearly 1 GHz carrier (as mentioned in a sibling comment).
> 2Way
Does that mean you won't get to know the time unless you let them spy on your physical location too? That's what the diagram implies: https://www.gps.gov/sites/default/files/2025-06/CGSICMeeting...
GNSS time is still cooler because you can have extremely accurate time and a reading of your position without the broadcasting satellites knowing you're there.
In this case it is only figuring out the distance from the other receivers for time synchronization and not positioning you on the globe.
A live event in a convention center for example. You might have a truck outside with GNSS, but it’s blocked inside by the building.
Even if you want a NIC with a stable oscillator or GPS inputs to act as a grandmaster, you can buy an E810 with the necessary hardware from eBay etc. for a few hundred or DIY something yourself much cheaper.
You can achieve microsecond accuracy with a lot of non-timing-specific networking hardware, but it's around as good as you get with modern NTP...
To get sub-microsecond, you need hardware that supports transparent/boundary clock and doesn't just 'say' it does, but actually does (vendors have stamped PTP support on things that definitely don't account for time correctly internally!).