Re: Help needed with USB GPS device

[Bryan Christianson]

Hi Gavin

> On 3/03/2024, at 11:11, Gavin Davenport <gavdav@gavdav.net> wrote:
> 
> I'm now using this approach in chrony
> refclock SHM 0 refid GPS precision 1e-1 offset 0.9999 delay 0.2
> refclock SHM 1 refid PPS precision 1e-7
> 
> And chrony can see the GPS source as follows:
> MS Name/IP address         Stratum Poll Reach LastRx Last sample
> ===============================================================================
> #x GPS                           0   4   377    18   -964ms[ -964ms] +/-  
> 200ms
> #? PPS                           0   4     0     -     +0ns[   +0ns] +/-    
> 0ns
> ^+ 2001:8b0:0:23::205            2   6    77    24    +23us[  +75us] +/-   
> 19ms
> ^- 2001:7b8:3:2b::4              2   6    77    25  +3106us[+3158us] +/-  
> 334ms
> ^- 2001:7b8:3:2b::5              2   6    77    24  +2865us[+2917us] +/-   
> 53ms
> ^- 2001:418:3ff::1:53            2   6   377    21  -1315us[-1263us] +/-   
> 96ms
> ^+ 2a05:b400:c::123:63           2   6   377    19   -296us[ -245us] +/- 
> 6816us
> ^* 131.111.8.63                  2   6   377    18   -437us[ -386us] +/- 
> 7137us
> ^+ 2a05:d01c:9d2:d200::be00:5    2   6   377    17   -370us[ -370us] +/- 
> 5314us
> 

You have specified offset 0.9999 delay 0.2 in your GPS refclock. This is why 
you are seeing the ~900ms offset in the chronyc sources output.
These attributes are used when you have a valid pps source, but are totally 
skewing the results in your case.

I would comment out the other ntp sources in the chronyd configuration and 
remove the offset/delay attributes from the GPS source. This will then give you 
some indication of how well your GPS receiver is working as an NTP refclock.

Please note - this is more a chrony configuration issue rather than gpsd. 
Please see https://chrony-project.org/documentation.html
gpsd is doing the best it can with your hardware.

Bryan Christianson
bryan@whatroute.net