SharpCap adjustment of System Clock using QHY174-GPS

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andyho
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Joined: Wed Apr 07, 2021 7:12 pm

SharpCap adjustment of System Clock using QHY174-GPS

#1

Post by andyho »

After the QHY174M-GPS gets a good lock on the signal, SharpCap offers to sync the computer's System Clock with GPS. Even after stabilizing for 20 minutes or longer, SharpCap continues to report a difference of 0.1 - 0.2 second between GPS and the System Clock.

What's puzzling is that the computer's Windows Clock is already synced to a GPS time server on my home network using Meinberg NTP. Meinberg NTP is very good at syncing time, and usually keeps the Windows Clock synced to within +/- 10ms of UTC.

Are the System Clock and the Windows Clock the same thing? If so, I don't understand why SharpCap and Meinberg NTP would show a difference of 0.1 - 0.2 second, when both are synced to GPS.

I am putting this forth as anecdotal data. I have not collected hard numbers yet to show how the difference between SharpCap and Meinberg changes as a function of time.

Andy Howell
Gainesville, FL
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Re: SharpCap adjustment of System Clock using QHY174-GPS

#2

Post by admin »

Hi,

I think this is just a reflection of the lag between the frame being captured on the camera and delivered to SharpCap. This lag includes the time taken to transfer the frame over USB and the any time taken by the QHY SDK processing the frame before handing it to SharpCap. SharpCap makes no attempt to take account of this lag when setting the system time (since it will vary based on all sorts of things including PC speed, camera parameters, etc), so some offset will remain.

The goal of this feature was to give a way to correctly set the system time in the field (think observing programs in wild places like Africa with no network connection) to a sufficient accuracy for mount pointing, etc to work. Exact correspondence with the GPS time isn't required, but it is nice to have a consistent offset between the GPS and System times - that means that any frames with scrambled GPS data can be dealt with by assuming a constant offset based on adjacent frames that have time stamps from both sources.

cheers,

Robin
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