Re: [chrony-users] Synchronizing clock with GPS with PPS

[Ryan Govostes <rgovostes@xxxxxxxx>]

Thanks, I removed the offset and delay so the reference clock configuration is now:

	refclock SHM 0 refid GPS precision 1e-1
	refclock PPS /dev/pps0 refid PPS

My intention is to have GPS set the system date and time and then have the PPS signal keep it from drifting.

After applying this, I ran again and am now getting:

	MS Name/IP address         Stratum Poll Reach LastRx Last sample
	===============================================================================
	#x GPS                           0   4   377    16   +587us[ +587us] +/-  100ms
	#x PPS                           0   4   160    82   -128ms[ -128ms] +/-  759ns

The #x suggests that “time may be in error.” However I am still seeing gpsd work (monitored via cgps) and the PPS device still appears to be working (according to ppstest).

Furthermore timedatectl suggests that the system clock is not synchronized:

	$ timedatectl status
	Local time: Tue 2020-09-15 18:34:48 UTC
	Universal time: Tue 2020-09-15 18:34:48 UTC
	RTC time: n/a
	Time zone: Etc/UTC (UTC, +0000)
	System clock synchronized: no
	systemd-timesyncd.service active: yes
	RTC in local TZ: no

What appears to be the problem?

Ryan

> On Sep 15, 2020, at 12:47 PM, Bill Unruh <unruh@xxxxxxxxxxxxxx> wrote:
> 
> 
> 
> William G. Unruh __| Canadian Institute for|____ Tel: +1(604)822-3273
> Physics&Astronomy _|___ Advanced Research _|____ Fax: +1(604)822-5324
> UBC, Vancouver,BC _|_ Program in Cosmology |____ unruh@xxxxxxxxxxxxxx
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> 
> On Tue, 15 Sep 2020, Ryan Govostes wrote:
> 
>> Hi all,
>> 
>> I am setting up chronyd on an embedded Linux device to synchronize the system clock using a GPS module. The GPS device sends NMEA strings over the character device /dev/ttyAMA1 and I have also configured /dev/pps0, both of which appear to be working OK.
>> 
>> The system is running Ubuntu 18.04 and the latest package versions are chronyd 3.2 and gpsd 3.17.
>> 
>> I configured gpsd to listen to the serial device and then added these lines to my chrony.conf:
>> 
>> 	refclock SHM 0 refid GPS precision 1e-1 offset 0.9999 delay 0.2
> 
> Why those figures for ooffset? That is 1 sec offset. NMEA is not that bad.
> 
> 
>> 	refclock PPS /dev/pps0 refid PPS
>> 
>> When I run `chronyc sources` they both seem to be kind of working:
>> 
>> 	210 Number of sources = 2
>> 	MS Name/IP address         Stratum Poll Reach LastRx Last sample
>> 	===============================================================================
>> 	#- GPS                           0   4   377    12   +128ms[ +128ms] +/-  200ms
>> 	#* PPS                           0   4   377    12     +6ns[ +119ns] +/-  203ns
>> 
>> However it looks like the GPS source is “not combined”. Is this a degraded state, e.g., it is using one of these two sources?
> 
> Why would one want to combine GPS with PPS. PPS is good to the nanosecond
> level. GPS toabut 100 ms -- that is almost a  million times worse. It would be like combining your wristwatch with some clock which says "its
> spring so it must be April".
> 
>> 
>> Also, I am not sure why the LastRx from the PPS (or frankly either) ticks upwards so long—shouldn’t it constantly be receiving updates?
> 
> Yout tell it that POLL is 4 which means 16 seconds. So every 16 seconds that
> clock is read. The driver massages the input ( once a second) to get rid of
> obvious outliers but reports to chrony once every 16 seconds. Note it is a bad
> idea to reduce the poll even further. Then obvious ouliers are not thrown out,
> and the ability to determine the rate of the clock is degraded.
> 
>> 
>> I am just using the precision / offset / delay figures that several examples use. Is there documentation on calibrating these values?
> 
> Get rid of the offset and delay. The GPS is useless except for setting actual
> number of the seconds.
> 
>> 
>> Finally, I read that using Unix sockets rather that shared memory is preferable, but my chronyd does not seem to create those sockets.
> 
> Why is it better? Leave things as they are. With PPS your time will be
> accurate to microseconds just as things are now. Do you need any better time?
> If you do need time to nanoseconds, then you will really have to throw away
> your computer (its ability to read interrupts is only at the microsecond
> level) and begin compensating for propagation delays in your gps unit, and
> also the sawtooth offset on the ns level due to your gps receiver innards. But
> then, why would you want to know the time to 1 billionth of a second?