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
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