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

[Felipe G. Nievinski]

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>
> To: 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>
> 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/>
> - Differences between rovers 1 and 3 : 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
> Subject: Re: [FOSS-GPS] RTKLib - Constant offset between UBlox NEO-M8T
>         receivers
> Message-ID: <552BF189.9040606 at ITSware.net>
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