[time-nuts] PPS offset between GPS receivers

Tom Van Baak tvb at LeapSecond.com
Tue Dec 4 05:18:25 UTC 2012

Hi Gabs, and welcome to the list.

Or, the 58534A is 2 us late compared to the SKG25A1. If you have a 'scope handy check the risetime of the signal at all points in the long chain from the 58534A to the GPIO.

Better yet, if you have a dual-channel 'scope or TI counter, compare the 1PPS's as close as possible to each GPS receiver. You can also use two channels to view the propagation delay along the signal path.

A third GPS receiver may help as well, or it could add a third variable.


> I'm using a Symmetricom 58534A GPS timing receiver and a GPS board with a
> SkyNav SKG25A1 module driving stratum 1 NTP servers.
> On one of the servers, the ppstest output while the 58534A is connected
> looks like:
> source 0 - assert 1354495734.000000102
> source 0 - assert 1354495735.000000040
> When I switch the PPS source to the SKG25A1, the ppstest output
> indicates the PPS of that receiver is about 2 us early compared to the 58534A:
> source 0 - assert 1354495740.999997923
> source 0 - assert 1354495741.999997905
> The setup looks like this:
> 58534A GPS -> 10 m CAT5 cable -> MC3486 RS422 receiver -> GPIO
> active antenna -> 3 m cable -> SKG25A1 GPS -> GPIO
> Both receivers were factory reset before the test.
> I'm planning on getting another GPS receiver to check which receiver
> is at fault. Has anyone found timing differences this large with their
> GPS receivers?

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