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Author Topic: Occasional crazy inaccuracy - serious problem  (Read 1123 times)

J2R

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Occasional crazy inaccuracy - serious problem
« on: May 23, 2013, 02:18:29 pm »

I got a GH625-XT a couple of weeks ago to replace a GH615B which had served me well for the last 6 years. In general I'm pleased with the new watch (I mean to write a fuller 'report' in another thread soon). There is one serious problem I've run into, though, which is potentially a show-stopper, and I'm surprised no-one else on here has been affected by it. Once in a while, when I get back from a run and upload the data to SportTracks, I find that the track is wildly inaccurate, all over the place - and by inaccurate, I mean maybe even hundreds of metres from where I was! It's happened 4 times now, and with the latest one just now, it actually records my start point as a point over 200 metres from my house. The rest of the time, on all the other runs, it's incredibly inaccurate.

Three of the times the problem has occurred with a run starting at my house, and the other time was after leaving another building, so in each case this has in effect been starting the watch from 'cold' having just turned the watch on, and getting a satellite fix afresh. It's not occurred at all when I've reset my watch while out running, so this may give a clue as to where the problem lies. On each occasion the watch has indicated that it has a proper satellite fix and is ready to go, so it's the not the case that I'm starting to run before the watch is ready.

Just to be clear, the problem is with the watch, not the data upload, because I've noticed a clearly inaccurate distance reading on the watch before doing the upload.

I thought (and hoped) that it might be a firmware issue because the watch had version 1.0 firmware on it when I got it, so I upgraded it to the latest firmware last week. But the problem has occurred again today with the latest firmware.

Could it be a fault with this particular watch? It doesn't seem likely to me given that 9 times out of 10 the accuracy is spot on - I would have thought it would be wrong all the time or not at all.

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USG_Tech12

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Re: Occasional crazy inaccuracy - serious problem
« Reply #1 on: May 29, 2013, 06:28:34 pm »

 Try waiting in till the device gets a good GPS fix before starting your run. It sounds like the watch is trying to establish a fix before you start. But by the time it fixes onto a satellite you've moved from your start position and your recording is from where the watch established a fix and not your actual start point. To better insure accurate logging of your data and faster lock time make sure you give the watch a clear unobstructed view of the open sky and time to get a strong fix on the GPS signal before starting your run. Remember, running through dense woods and between high buildings or mountain terrain can hinder the signal and lower the accurateness of your device. I hope taking these actions will help eliminate the issues your experiencing. Thank you.   
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J2R

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Re: Occasional crazy inaccuracy - serious problem
« Reply #2 on: May 30, 2013, 06:31:12 am »

Try waiting in till the device gets a good GPS fix before starting your run. It sounds like the watch is trying to establish a fix before you start. But by the time it fixes onto a satellite you've moved from your start position and your recording is from where the watch established a fix and not your actual start point. To better insure accurate logging of your data and faster lock time make sure you give the watch a clear unobstructed view of the open sky and time to get a strong fix on the GPS signal before starting your run. Remember, running through dense woods and between high buildings or mountain terrain can hinder the signal and lower the accurateness of your device. I hope taking these actions will help eliminate the issues your experiencing. Thank you.   

As I said in my original post, "On each occasion the watch has indicated that it has a proper satellite fix and is ready to go, so it's the not the case that I'm starting to run before the watch is ready". I am very well aware of the requirement to get a proper satellite fix before starting the run (I have been a user of GlobalSat GPS watches for 6 years, as I mention in the post). Clearly the fact that this problem occurs only after switching on the watch (and not while resetting the watch while out running, say) would suggest the possibility that it is to do with having an inadequate satellite fix - but in that case, it is clearly a fault that it is reporting that it is ready when it is not.
« Last Edit: May 31, 2013, 05:25:06 am by J2R »
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gerhard

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Re: Occasional crazy inaccuracy - serious problem
« Reply #3 on: May 30, 2013, 01:23:50 pm »

but it is in that case, it is clearly a fault that it is reporting that it is ready when it is not.

I agree that this could happen.

This is a little cumbersome to investigate. After starting, go to the GPS Info page to see number of sats and the approximate accuracy.

I seriously miss fields for no of sats and accuracy as fields. Competitors have that.
Even better would be to record the accuracy, to analyze it afterwards.
I did some training runs starting at separate watches. I do not recommend that running in terrain...
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J2R

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Re: Occasional crazy inaccuracy - serious problem
« Reply #4 on: May 31, 2013, 05:29:18 am »

If I know for certain that that is the problem, I can work around it by, for example, always leaving the watch outside for a couple of minutes before starting the run. It is, as gerhard says, though, very cumbersome to test, especially as it's a sporadic problem and doesn't occur most of the time.

Would it help if I sent someone (who?) the erroneous tracks in question, as exported to SportTracks?
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gerhard

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Re: Occasional crazy inaccuracy - serious problem
« Reply #5 on: June 02, 2013, 07:26:31 am »

Would it help if I sent someone (who?) the erroneous tracks in question, as exported to SportTracks?
I doubt it would be useful, there is no information to why there was inaccuracies after recording.

If just keeping the device outside for a while improves the problem indicates that the "issue" is reporting a fix too quickly.
Personally I prefer a quick sat fix, but then I want to determine myself if the fix is good (which is cumbersome now).
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J2R

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Re: Occasional crazy inaccuracy - serious problem
« Reply #6 on: June 03, 2013, 10:35:28 am »

Quote
I doubt it would be useful, there is no information to why there was inaccuracies after recording.

Does the information recorded by the device not contain details of how many satellites were used for determining each trackpoint? I haven't had a look at the 'raw' data, only what was exported to disk via a Python application which I used to use with my GH615B (one of yours?), so am not sure what is actually recorded.
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gerhard

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Re: Occasional crazy inaccuracy - serious problem
« Reply #7 on: June 03, 2013, 03:40:02 pm »

Quote
I doubt it would be useful, there is no information to why there was inaccuracies after recording.

Does the information recorded by the device not contain details of how many satellites were used for determining each trackpoint? I haven't had a look at the 'raw' data, only what was exported to disk via a Python application which I used to use with my GH615B (one of yours?), so am not sure what is actually recorded.
The recorded data has no accuracy information. I tried to use the speed data compared to GPS distance for this purpose, but no good results.

In addition, there is no documented protocol to get current status via USB.
So the only way to estimate accuracy is to take a photo of the sat status page...
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GS_TECH01

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Re: Occasional crazy inaccuracy - serious problem
« Reply #8 on: June 06, 2013, 05:02:14 am »

Dear J2R

Sorry for late reply
our engineer mention GPS fluttered sometimes that is normal Phenomenon

we can't duplicate your problem so far

I will check latest firmware for you and let you know tomorrow

Thank you and have a nice day :)
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