[ntp:questions] How exact are the PPS pulses of common GPSses?

Gabs Ricalde gsricalde at gmail.com
Thu Feb 14 23:10:10 UTC 2013

Hi everyone,

On Fri, Feb 15, 2013 at 1:06 AM, unruh <unruh at invalid.ca> wrote:
> To get an estimate you could output a pulse on one of RPi's GPIO pins as
> soon as it gets the PPS and begins to process the input, and then compare them
> on a high bandwidth digital scope. Certainly I would not be surprized at
> 1usec, but 10 would be surprizing.
I did this on a router using a custom GPIO PPS driver which asserts
another GPIO port after receiving the PPS edge, the PPS echo returned
after 0.5 us. This is a polling driver, interrupt driven GPIOs would
probably have a higher latency.

On Fri, Feb 15, 2013 at 1:03 AM, unruh <unruh at invalid.ca> wrote:
> The evidence from the people who do radio astronomy ( who use gps to
> provide the time base for long baseline interferometry I believe) is
> that it is good to better than that. I cannot point you to the relevant
> documents unfortunately.
> Now it is true that this depends on the receiver, and I recall tests
> that the more recent Motorola timing receivers at the time had an
> unexplained 30ns difference from the older ones. But that is ns, not us.
> Computers are good to us.
Found some resources about VLBI timing and GPS PPS comparisons here:

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