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

[Magnus Danielson]

Hi Erik,

On 2022-04-30 12:32, Erik Kaashoek wrote:
> 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

Well, there are many sources of bias both in hardware and firmware.

As mentioned already, delays of antennas and cables remains 
uncompensated. Seeing a difference of 100 ns is equivalent of 20 m of 
cable. If you know you have about the same length of cable, then that is 
not your culprit.

There is a peculiar effect in that the experienced delay in receiver 
becomes different depending on the PN code used, so per satellite. This 
should be lower. At the same time, considering that a single chip of the 
PN code is just shorter than 1 us, so maybe there is a narrow-band 
effect there. Still, that should not give such huge difference, so it is 
really currious.

There could be some peculiar issue on how state is set up as it locks 
up. Try restarting one of them a couple of times and see if the offset 
varries or is consistent.

Try swapping antenna cables to see if the offset follows the receiver or 
antenna/coax.

Try using another receiver in parallel.

Cheers,
Magnus