[time-nuts] Designing an embedded precision GPS time

[Denny Page]

Depends upon the results you are trying to achieve. Using Linux pretty much guarantees that your server clock will be off by 6-10us, with substantial variance. Even with a good nic that supports hardware timestamping, the variance will increase substantially as you go off box (spread spectrum is a big annoyance!). If you don’t have hardware timestamping, the base error will increase by another 10-100us, and the variance will simply go through the roof. Any load on the system whatsoever will quickly drive further degradation throughout. This is why people generally talk about NTP having a “typical" accuracy of 1ms and a standard deviation over 100us. For casual use, this is fine for most people.

The LeoNTP units operate in a completely different world. Leo advertises accuracy of under 1us, which matches the general performance of PTP. In my testing, the units actually do a bit better than that. Using hardware timestamps on the client, I generally see less than +-100ns, with a standard deviation of around 35ns. And the performance remains constant under load. I am not able to do heavy load testing, but Leo has described the heavy load performance earlier in the thread. Basically the units are capable of operating at 100Mb wire speed. As I said, a completely different world.

Your mileage may vary.

Denny



> On Oct 31, 2017, at 17:11, Attila Kinali <attila at kinali.ch> wrote:
> 
> Basically, all you have to do is use an SBC that runs linux and has
> a GPIO with an interrupt to act as a PPS input. Attach a GPS receiver
> and you are almost done.