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

unruh unruh at invalid.ca
Wed Feb 13 17:14:12 UTC 2013

On 2013-02-13, Ralph Aichinger <ralph at pangea.at> wrote:
> After having my Raspberry Pi with PPS running, with an
> offset and jitter value in the region of 1 to 5 microseconds,
> I am wondering: How precise is the PPS signal of typical
> consumer hardware (MTK chips, etc.)?

Sub microsecond to tens of nanoseconds. FAr more accurate than the
interrupt handling of any standard computer (which is in the low
microsecond realm)

> Has anybody hooked up the PPS output of different consumer
> GPSes to an oscilloscope and compared it against eachother  
> with a precision clock?

Sure they have. 

> Is the precision of the PPS pulse higher than what
> the computer/ntpd daemon can usefully handle, or is
> the PPS pulse the limiting factor?

The computer is the limit.
emember that in order to find your place on the earth, the gps chip
needs to do timing to the sub nanosecond regime. (1 meter is 3
nanoseconds) Most use a "zero crossing" algorithm which apparently puts
a tens of nanosecond sawtooth on the timing. Some then tell you how much
that sawtooth error was, after the fact. Remember than the velocity down
the wire from the device to your computer is about 5ns/meter.
But your computer is microseconds to handle the interrupt.

> /ralph

More information about the questions mailing list