QHY1274M-GPS LAT data discrepancies

Christopher Erickson
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QHY1274M-GPS LAT data discrepancies

Post by Christopher Erickson » Mon Jan 15, 2018 10:23 pm

It appears that the GPS LAT data displayed in SharpCap Pro 3.0.4074.0 is consistently wrong.

HP Envy-17, Win10 Home. 2T SSD, 16G RAM, USB3

Compared to four other GPS sources (Samsung Note4, Samsung Tab3, Garmin ForeTrex 601 and Google Maps) the GPS LAT data displayed in SharpCap is off.

Google Maps - 19.945nnn, -155.7817nnn
174M-GPS - 19.937nnn, -155.7817nnn

The error starts in the second LAT digit after the decimal place and seems fairly consistent in direction and magnitude.

My hope is that this is a simple software math or numerical conversion error and can be easily fixed.

As it stands right now, I have to record GPS data for occultation expeditions from my smartphone as I can't trust the data from SharpCap.

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Re: QHY1274M-GPS LAT data discrepancies

Post by admin » Mon Jan 15, 2018 10:51 pm

Hi,

I get accurate latitude data pretty much all of the time - within 1/1000 of a degree (usually less) of being correct. My longitude is correct most of the time to the same accuracy, but sometimes jumps by 0.03 degrees and then jumps back again. I was testing on SharpCap 3.1, which has a newer QHY SDK, so may be worth trying that.

So, I don't think there is a simple error for me to fix in the code in SharpCap that interprets the data from the camera - I think it's more likely to be a problem that QHY need to fix. Save some frames to PNG format (or FITS) - the GPS data is embedded in the first 44 bytes of the frame - this is the QHY documentation for the format:

Code: Select all

Sequence Number is a hardware counter of the frames. It will start from zero and each frame it will add one. It will start from the cmos camera start working
 
0  Sequence Number MSB
1  Sequence Number
2  Sequence Number
3  Sequence Number LSB
 
4 temporary Sequence Number   (Normally no use)
 
5 Image Width  MSB
6 Image Width LSB
 
7 Image Height MSB
8 Image Height LSB
 
Latitude is the current latitude report by GPS.  
9    latitude MSB
10  latitude
11  latitude
12  latitude LSB
 
Latitude is the current longitude report by GPS. 
13  longitude MSB
14  longitude 
15  longitude 
16  longitude LSB
 
17 Start_Flag 
 
Shutter start time  (JS)
18 Start Second MSB 
19 Start Second
20 Start Second 
21 Start Second LSB
 
22 Start micro second MSB
23 Start micro second 
24 Start micro second LSB
 
25 End flag
 
Shutter end time  (JS)
26 End Second MSB 
27 End Second
28 End Second
29 End Second LSB
 
30 End micro second MSB
31 End micro second
32 End micro second LSB
 
33 now flag:  this can be used for the GPS statu indicator   bit[7..4] is the 
 
The now time is the time that of the vertical sync of the CMOS sensor. It does not the precision time of shutter open or close
34 now second MSB
35 now second
36 now second
37 now second LSB
 
38 now micro second MSB
39 now micro second 
40 now micro second LSB
The Lat/Long are stored as integers and then the digits in those integers are used as follows

SDDDMMFFFF

S=Sign - W or S if this is 1, E or N if zero
DDD = Degrees E/W or N/S
MM = Minutes
FFFF = Fraction of a minute (1/10000 th)

so 1002350500 is 2 degrees, 35.05 minutes east as a longitude.

I'd suggest grabbing a few screenshots, extracting the data and checking by hand if you can and then reporting the issue to QHY if it you get results that agree with the SharpCap numbers - if you do discover that SharpCap is handling the data wrong then please let me know.

cheers,

Robin

SteveInNZ
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Re: QHY1274M-GPS LAT data discrepancies

Post by SteveInNZ » Tue Jan 16, 2018 8:04 am

This seems to be a commonly described problem on the Occultation mailing lists.

My latitude reported by SharpCap is around 1-2km out. It's also quite unstable.

Using your description of the long/lat format, I tried it with QHY's TimeREC program and the decoded position reported is rock solid and is <2m from my antenna position.
The latitude value in TimeREC is 136547047 which is -36d 54.7047m = -36.9117. SharpCap is reporting -36.90075.
Google Earth says 36.9117

Hope that helps.

Also - If you turn GPS Off or minimize it, it drops down to the lower left corner and blocks the status report. There doesn't seem to be any way of removing it.

Steve.

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Re: QHY1274M-GPS LAT data discrepancies

Post by admin » Tue Jan 16, 2018 8:46 pm

Hi Steve,

sorry, the pattern I gave above was for LONG not LAT - I thought they were both the same, but they aren't.

The pattern for LAT is

SDDMMFFFFF

but I was using

SDDMMXFFFF

My home location is at 51deg XX.995 min North, so the fact that I'd lost the 1/10ths of a minute and brought the rest of it up by a factor of 10 didn't show very clearly, sorry.

this will be in the next SharpCap 3.1 update.

cheers,

Robin

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Re: QHY1274M-GPS LAT data discrepancies

Post by SteveInNZ » Tue Jan 16, 2018 9:16 pm

Thanks Robin. That's nice to get that one solved.

I still get rubbish GPS data if the image is set to 16 bit. I haven't seen anyone else mention it so I assume it's specific to my early camera but then I don't know if people are using 16bit mode for timing and they may not have noticed.
Could you confirm that you get good data with 16 bit images and perhaps Chris could too (as I think his camera is relatively new).
Now that it knows where it is, the "when it is" becomes more useful.

Steve.

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Re: QHY1274M-GPS LAT data discrepancies

Post by admin » Tue Jan 16, 2018 9:29 pm

Hi Steve,

as far as I know the GPS output is not supported in 16 bit mode (certainly mine has never worked). I have mentioned this to QHY, but it has never been fixed.

The latitude fix is in build 3.1.4944, which you can get from the beta downloads page - please give it a try and let me know if it is OK for you.

cheers,

Robin

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Re: QHY1274M-GPS LAT data discrepancies

Post by SteveInNZ » Tue Jan 16, 2018 10:16 pm

Direct hit. Thanks.

There's still the little niggle thing of the minimized GPS window sitting on top of the frame progress information if the main screen is maximised.
Thanks for the 16bit information.

Steve.

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Re: QHY1274M-GPS LAT data discrepancies

Post by admin » Tue Jan 16, 2018 10:21 pm

Hi Steve

good news on the accuracy :)

The minimized window is one of those things that I haven't got around to changing because I haven't managed to think of a better approach that remains simple (easy to code) yet.

* Stopping you from minimizing it would be more annoying
* Hiding the window or closing it would require a way to bring it back, and it's not obvious where that would fit in the UI
* Having it so that it could go behind SharpCap made it easy to loose it

I wonder if I could put a button to re-show it on the GPS on/off control - that might be OK.

cheers,

Robin

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Re: QHY1274M-GPS LAT data discrepancies

Post by SteveInNZ » Tue Jan 16, 2018 11:43 pm

I would have thought "GPS ON" would have the window visible and "GPS OFF" would close the window completely. Maybe there's a circumstance I haven't thought of where you would want the window present & minimized but the GPS Off.

Part of the issue for me appears to be the way the windows are configured (by Windows). If I minimize the GPS window on my Win7 machine there is enough of the title bar to allow me to position it on the screen. On my Win10 laptop, the minimised bar just has the maximise and close buttons and no room for me to grab and move it. Sorry for my lack of Windows vocabulary. That's probably something that I can change somewhere.

Steve.

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Re: QHY1274M-GPS LAT data discrepancies

Post by SteveInNZ » Tue Jan 16, 2018 11:51 pm

Scrap that. I've found how I can move that bar around (right-click / Move).
Learning Windows - One astronomy application at a time. :-)

Steve.

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