FG and RTK GPS

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FG and RTK GPS

Postby Richard Sands » Fri Feb 26, 2010 6:21 pm

I put this on the POB forum but am interetsed in users comments of other (or similar) GPS's and FG.

yesterday I was RTK'ing with 8 sats, PDOP 1.8± and I dropped it on a point and collected it about 10 times in Field Genius, then looked for a nearby RM and after about 5 minutes dropped the pole back on my previous mark and repeated. Only to find this set of observations was about 65 mm from the mean of the previous set. (Bubble OK so not centering issue)
I've never been happy with FG and its handling of the 2650. At times with 8 sats and very low PDOP and clear skies it just wont fix. And so at those times I fiddle with settings only to find it instantly fixes.
I had been running in a fairly loose config - Hor 30mm, vert 60mm, PDOP 3.5 and 5 sats for a 'fix'. (That for me seems to work well most of the time)
But yesterday I changed the setting to 15/30mm Pdop 3, 6 sats and it was then I noticed the 65mm difference.
It made me wonder just how well FG handled the processing of the satellites.
I would have thought a 'sloppy config' properly attended to in terms of watching what the results are should give equal results to a 'tight config'.
Not sure if my reasoning is accurate or not.
yesterday the results at the time, as showing in FG, were 8-15mm Hor and 20+mm vert which I was more than happy to run with. This was the same for my 'sloppy config'.
Is this typical of all GPS software?
Richard Sands
 
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Re: FG and RTK GPS

Postby Jason Poitras » Tue Mar 09, 2010 7:46 am

Hi Richard,

Data collector software is just that a collector, it does not compute GPS positions, that is the job of your receiver.

The filters that you define in your FieldGenius profile are there to help filter out measurements made by the receiver. If you have sloppy or loose filters defined, then you can expect to get sloppy or poor results. The important thing to note is that this might not happen all the time which is part of the headaches with using RTK GPS. You can sometimes set loose filters but still get good results, but in those situations where you either have poor satellite geometry, or multipath, the less accurate measurements are going to make it through your loose filters and produce poor results.

In practice if you have important measurements to make you need to use "tight" filters in your profile. This will mean that sometimes you will have to wait longer to make your measurements, but you will end up with more accurate results. Also I recommend that on important shots you validate them by either re-observing at a later date and time, or by other means.
Jason Poitras
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Re: FG and RTK GPS

Postby Richard Sands » Tue Mar 16, 2010 12:03 am

Jason I was of the understanding that loose configurations or tight configurations, if the displayed RMS's are showing say 8mm hor, 15mm vert then thats what is to be expected. (well not quite - I am not saying the GPS is reading to that accuracy - rather the expected results are of equal quality)
But I gather you are suggesting this ain't necessarily so.
I could then select whether to save or refuse a collection of GPS observations based on the stated RMS's. I found this an easier and more painless way to get FG to continue to work.

Following up I did another days repeat of same marks and this time I set the base up in another location and just logged points with very tight settings. I found it at times just refused to fix (usual low PDOP and plenty sats) and everytime this happened if I went into the settings and clicked on the Active Tolerance setting it started logging. All I did was to select Tolerance mode 1 each time and close and it was a way.

Its this sort of behaviour that makes me wonder if FG goes to sleep on the job and just won't wake up without a nudge.
Richard Sands
 
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Joined: Mon Feb 14, 2005 1:10 pm
Location: Tasmania


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