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

[Erik Kaashoek]

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.