GPS stability

[Jean-Francois]

Hello Robin and other QHY174GPS user,

I'm processing the different films of my expedition to the occultation of Betelgeuse.

I used 2 GPS cameras and 3 other non-GPS. I'm now looking in detail the time stability between the computer system time and the GPS time.
I have some surprises.

One camera/computer time was very stable, except for 2 events.
Here the region with the special time ... where the end of the frame is before the start of the frame.

GPS_Time_bug.png (51.62 KiB) Viewed 1867 times

Does somebody already remark the same ?
What could be the reason ?


Here the complete curve from the beginning to the end of the capture.

GPS_Time_stability_RC6.png (48.26 KiB) Viewed 1867 times

And for the other camera: without extreme event with 1 s difference, but with several "flat" sequences.

GPS_Time_stability_Newton.png (23.08 KiB) Viewed 1867 times

My setups were the following:
On the first EQ8 mount, 2 telescopes (RASA8 and Newton 254/1000) and 3 cameras (QHY268 on the RASA, QHY174GPS and QHY178 on the Newton with a beam-splitter cube). On the second EQ8 mount, 2 telescopes (GSO RC6 with QHY174GPS and GSO RC8 with Allied Vision Goldeye SWIR camera).

For the synchronisation of all the cameras, I prepared a photo flash ... it was in my hands switch-on ready to use.
But I looked up and saw a cloud band cross Orion ... so I was concentrated on the cloud and I completely forgot to use the flash for the synchronisation. So it means that I have more work now to synchronise the other 3 cameras with the GPS time.

Best regards and nice Christmas,
Jean-Francois

[admin]

Hi Jean-Francois,

from my discussions with the guys who use these cameras for the New Horizons program and related observations (I think they have about 100 rigs now for observing campaigns), they have to do postprocessing of the data to deal with consistency and look for glitches, so I suspect you are not alone in finding this sort of thing.

I noted that both your glitches occur roughly as the second ticks over to the next, and I wonder if that is significant. With the QHY GPS, the start/end frame times are read back in two parts - the time to the second from the GPS and the offset from that timepoint in microseconds. SharpCap adds the two parts to get the start/end times (and I just checked the code to make sure I wasn't adding the start second and the end microseconds value, but all is well there). I wonder if under some conditions the camera reports the wrong second, leading to the glitches you are seeing? If you assume a 1s error then the figures drop back into line with the rest.

For the second camera, how long had it been running for? I have noticed that the camera time will jump a bit, but does seem to settle. I had assumed (no real evidence) that the jumps were happening as the GPS system got more information to refine its calculation. We are used to AGPS on phones that locks instantly these days, but the cameras are more basic and have to wait for the almanac and then the ephemeris to download over the GPS signal. Maybe with incomplete information it is possible to get a 'lock' but a less accurate one?

cheers (and Merry Christmas too!),

Robin

[Jean-Francois]

Hello Robin,

I compare the system time (computer) with the GPS of the camera from the Log file.

For some case, we see more the variation of the computer time.
But, what could be the reason that a computer time will jump +/- 1 second ?

Both cameras were connected in the afternoon before the occultation. Also at least 6 hours before the event.
Each camera was connected to a different computer. One with Windows 10 other with Windows 11.

Here for both cameras with a longer time:

GPS_Time_stability_90minutes_Newton.png (51.67 KiB) Viewed 1806 times

GPS_Time_stability_30minutes_RC6.png (27.56 KiB) Viewed 1806 times

I will do some additional tests later ... when the wind will stop.
I need to put the GPS antenna outside and so open the window, but I can not open the window on the West side with the strong wind.

Regards,
Jean-Francois

[admin]

Hi Jean-Francois,

a tricky one, since there are lots of potential differences between one system and the other (different windows version, different camera, etc). I would run each camera on a single system and see if they behave the same (if not - camera firmware versions, etc). Then run both on the other system and see what you get.

I just about get a GPS signal with the antenna inside on the windowsill, but very poor, takes ages to sync and often drops out

cheers,

Robin