[chrony-users] Application requires frequency accuracy but not time accu

Hello everyone,

Last night I successfully installed Chrony and GPSD on a Raspberry Pi 3B with the great assistance of many discussions I found on the Internet. I discovered a gentleman who appears to have an application similar to mine. I've looked over his chrony.conf and looked up the commands and options in the Chrony docs:

driftfile /var/lib/chrony/drift

makestep 1 10

keyfile /etc/chrony/chrony.keys

commandkey 1

allow

refclock SHM 0 refid GPS precision 1e-1 offset 0.9999
        delay 0.2

I think it would be helpful to describe my application:

I use the RPi + Adafruit MotorHAT to control a stepper motor on my telescope's Right Ascension axis. This is the axis that compensates for the Earth's rotation. The 50-year old German Equatorial Mount (GEM) has a polar axis that I precisely align with the Earth's axis. The motor advances one step when I tell it to. The step rate is 32.8 Hz or one step every 30.49 milliseconds. My Python software is not at all interested in absolute time. It is interested in time spans. Here is a snippet of my code from the stepper thread:

while True:

     self.stepper.oneStep()
     target = self.tracking.next()
     sec_sleep = target - time.time()
     if sec_sleep < 0:
          print "Fallen behind!"
     else:
          time.sleep(sec_sleep)

This solution has worked very well for two years until, like an idiot, I blew up the Pi with a screwdriver. The replacement Pi's system clock runs a bit slower. I lose about 25 arc-seconds every 30 minutes. Furthermore I have always had issues with star tracking deviations as the ambient temperature fluctuates. I would like to solve these issues.

I purchased a uBlox GPS dongle. From what I see it works perfectly and like I mentioned I now have Chrony set the time after a reboot or power up. The Chrony docs are extensive. I came across "corrtimeratio" and it got me thinking about what I am asking Chrony to do for my application.

I calculate that I can achieve excellent star tracking when the GPS is sampled every one to two minutes. But like I said I am not really interested in time accuracy just frequency accuracy? Is this possible with Chrony? If so can you suggest a minimally complete chrony.conf file?

Thank you very much for your time and assistance.

Brian Morgan