[time-nuts] Re: What time difference to expect from two clocks using internal GPS receivers?

[André Balsa]

Hi Erik,
Since each Chinese GPS receiver module has its own Xtal oscillator driving
a very low power 32-bit processor usually at around 48MHz, and we can
expect these oscillators to have a slightly different frequency, let's say
the difference is 500ppm, then I would imagine there would be a
correspondingly significant phase difference in the PPS pulses of these two
Chinese GPS receivers.
I would also imagine if you took two Neo-M9N modules and adjusted their PPS
with the reported quantization error, the PPS phase difference would be
much, much smaller.
As usual, you get what you pay for.
But then, depending on the use case, the Chinese GPS receiver modules may
be perfectly adequate, it all depends on your specific needs.

On Sat, Apr 30, 2022 at 9:32 PM Erik Kaashoek <erik at kaashoek.com> wrote:

> Some more info
> The two GPS do keep their phase stable vs a Rb within +/-10 ns. But the
> absolute time difference of their PPS pulses  was, after a cold start,
> stable within +/- 20ns but  the average value could be up to 100ns and
> differed after every cold start.
> The two GPS antenna cables had a length difference of 1 meter, but that
> should cater for only 5 ns (?) One module is connected to the antenna with
> only a C, the other has a 1 GHz CLC high pass filter between antenna and
> module
> Erik
>
>
>
> Op za 30 apr. 2022 om 12:32 schreef Erik Kaashoek <erik at kaashoek.com>:
>
> > The PPS jitter of a cheap Chinese GPS module was measured at about +/-
> > 10 ns.
> > But the phase of the PPS compared to a Rb varied substantial more.
> > To verify if this was possibly due to ionospheric or atmospheric
> > conditions the time difference between the PPS of two identical modules
> > using two identical rooftop antenna was measured. Both only used the GPS
> > constellation.
> > This showed difference of up to 100 ns. Switching to GPS+GLN did not
> > make a visible difference.
> > It was tried to set both GPS modules into fixed position mode but the
> > reported position still kept moving a bit (within 3 m) and the fixed
> > mode did not have a visible impact on the time difference variations.
> > Is a time difference of up to 100 ns to be expected when using two GPS
> > receivers or is this difference possibly due to bad application or
> > performance of the cheap Chinese GPS modules
> > Erik.
> >
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