Quoting Daniel O'Connor <dar...@dons.net.au>:
If you are using it for NTP then GPS+PPS over USB is quite adequate
(from personal experience).
Ian Lepore (RIP) who worked for Micro Semi and worked on FreeBSD did
a bunch of tests on a PPS over USB setup and found it more than
acceptable for keeping a PC in (good) time. Here's the thread:
https://lists.freebsd.org/pipermail/freebsd-arm/2019-August/020263.html
That ntpq snapshot is a little bit misleading, the latency added by
USB will vary depending on the offset between when the PPS happens and
the next USB poll. The USB polls are relatively stable in the short
term, depending on the host frequency driving USB. So the snapshot
will only tell you the offset between all the PPS sources in that
short timeframe, not how they wander over time.
Here's a measurement of that wandering from the USB device's perspective:
https://blog.dan.drown.org/content/images/2017/07/usb-cdc-latency-zoom3.png
As long as you are ok with your time having an offset between ~0ms and
~1ms, PPS over USB (USB fullspeed) is acceptable. There are plenty of
uses where that would be "good enough".
I have more info on my blog post: https://blog.dan.drown.org/pps-over-usb/
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