[FOSS-GPS] RTKLib - Constant offset between UBlox NEO-M8T receivers

Tue Apr 14 03:08:52 PDT 2015

Thanks Felipe for your answer!

The results I got were already generated with post-processing (in "forward" mode only as I wanted to simulate the same conditions as with live processing).
I'll take a look at the "Moving baseline" option, it indeed seems it can fit my need. But just to be sure, is there any reason I'll get more accurate relative positions with "moving baseline" rather than with kinematic? To my understanding, the "moving baseline" is the same mode as "kinematic", the only difference is that the resulting positions are relative to the moving base rather than absolute.

Cheers,
Simon

> Le 13 avr. 2015 à 22:18, Felipe G. Nievinski <fgnievinski at gmail.com> a écrit :
> 
> Just to second Kelley's suggestions: look at distances, as it's truly frame invariant.  You can't assume the vehicle attitude is perfectly known.  Also, when you get down to the cm level, antenna phase center offsets will start to matter.  I'd further suggest you compare post-processed kinematic (PPK) to your RTK results.  And while at PPK, try the "moving base" option, in which you process one of the rovers w.r.t. your stationary base then use that one rover as a moving base.
> -F.
>  
> Message: 1
> Date: Mon, 13 Apr 2015 17:49:09 +0200
> From: Simon Tr?ny <simon.treny at gmail.com <mailto:simon.treny at gmail.com>>
> To: foss-gps at lists.osgeo.org <mailto:foss-gps at lists.osgeo.org>
> Subject: [FOSS-GPS] RTKLib - Constant offset between UBlox NEO-M8T
>         receivers
> Message-ID: <72E829A9-49DB-410D-B600-31DB7D59B528 at gmail.com <mailto:72E829A9-49DB-410D-B600-31DB7D59B528 at gmail.com>>
> Content-Type: text/plain; charset="us-ascii"
> 
> Hello all,
> 
> I'm using RTKLib in Kinematic mode to obtain the relative positions between several rovers (they are all UBlox NEO-M8T at 1Hz with GPS+GLONASS). I use an additional NEO-M8T at 1Hz with a Novatel GPS-701-GG antenna as the base station.
> 
> To evaluate whether or not the relative positions are good or not, I've fixed 3 rovers on the roof of a car and I drove for 15 minutes on a looping race-track. Then, I take the positions outputted by RTKLib, I project them in the car's local coordinate-system (X and Y) and I plot the X (normal to the trajectory) and Y (tangential) differences between each rover. As the rovers are fixed in the car's local coordinate-system, the differences should be ideally constant over time.
> 
> Here are the results I get (yellow curve is the tangential difference, and red curve is the normal difference) :
> - Differences between rovers 1 and 2 : http://postimg.org/image/ploa4ttpt/ <http://postimg.org/image/ploa4ttpt/> <http://postimg.org/image/ploa4ttpt/ <http://postimg.org/image/ploa4ttpt/>>
> - Differences between rovers 1 and 3 : http://postimg.org/image/hu7k69pkh/ <http://postimg.org/image/hu7k69pkh/> <http://postimg.org/image/hu7k69pkh/ <http://postimg.org/image/hu7k69pkh/>>
> 
> As you can see, rovers 1 and 2 are quite well positioned relatively to each other: the yellow plot varies between -20cm and +20cm (54% below 10cm).
> But with rovers 1 and 3, the tangential difference varies between -50cm and +50cm (only 12% below 10cm). I get approximately the same plot between rovers 2 and 3.
> 
> The sinusoidal aspect of these plots seems to indicate that the error comes from a constant absolute offset of rover 3. Each time a half lap is completed, the sign of the difference changes.
> 
> Is there any reason that could explain why I get better results between rover 1 and rover 2, rather than between rover 1 and 3? If I redo the same run, results may vary, sometimes all the results seems good between all rovers, sometimes it may be another rover than #3 that performs badly.
> I've also noticed that sometimes, if I regenerate the positions by removing the first minutes of the run, the results may improve greatly .
> 
> Also, is there any settings that may be more appropriate than the default ones if I'm more interested by the relative positions of the rovers than by their absolute positions?
> 
> Thanks by advance for your help, and also thanks for all the hard work put into RTKLib!
> 
> Best regards,
> Simon
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> ------------------------------
> 
> Message: 2
> Date: Mon, 13 Apr 2015 09:40:41 -0700
> From: David Kelley <DavidKelley at ITSware.net>
> To: foss-gps at lists.osgeo.org <mailto:foss-gps at lists.osgeo.org>
> Subject: Re: [FOSS-GPS] RTKLib - Constant offset between UBlox NEO-M8T
>         receivers
> Message-ID: <552BF189.9040606 at ITSware.net>
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