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

[Erik Kaashoek]

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.
>