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Several sanity check questions for you. Frankly it hard to say
because once you have translated to the vehicle frame of reference
it is hard to tell if there was a projection error there, or if an
angle of arrival error (antenna or environment caused) in one or
more of the devices. Also, if your kinematic path involves enough
lateral stress to tilt the vehicle, or the ground is tilted, you
will need to account for that as well (but as you are only looking
at one vehicle and presumably the roof did not deflect you can
ignore that for now). <br>
I find a better way to do this comparison is to just plot the
overall 3D or 2D distance between each set of rovers point streams
first (before any frame of reference translation is made), and
ignoring time issues. If you see odd repeating patterns there, look
for antenna nulls and the phases shift that come with them. [And if
you are going crazy, try the whole test with one shared antenna to
confirm that your three rovers do in fact track the same to some
limit with the same input signal] When you plot the rover paths do
you see (per each loop on the track) concentric circles? Do these
circles get corrupted when you see the sign change in you other
plot? What was the antenna separation distance (one hopes
>>19 cm), and is this observable from these circles? Always a
good idea to snap a photo of your roof setups.<br>
I presume that during the ten loops, that RTKLIB was more of less in
a fixed mode all the time? Your line about removing the initial
data concerns me that you are not fixed or understanding how to
check for that. Some folks come to rest on the test track close to
a known point for a few minute to "rest and re-lock" between laps.
If these things check out, I suspect you are using low cost mag puck
antenna and that every time you place it the effective phase center
changes. We have a circle track we do much the same runs with, and
there one tactical grade type of antenna fails at one part of the
loop for such reason, so its not just a cost issue. If we presume
your Novatel choke ring ant is ideal (reasonable) what are you using
for the three rovers - that normally where the compromising issues
come in. <br>
Regards, DC Kelley<br>
<div class="moz-cite-prefix">On 4/13/2015 8:49 AM, Simon Trény
<pre wrap="">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 : <a class="moz-txt-link-freetext" href="http://postimg.org/image/ploa4ttpt/">http://postimg.org/image/ploa4ttpt/</a> <a class="moz-txt-link-rfc2396E" href="http://postimg.org/image/ploa4ttpt/"><http://postimg.org/image/ploa4ttpt/></a>
- Differences between rovers 1 and 3 : <a class="moz-txt-link-freetext" href="http://postimg.org/image/hu7k69pkh/">http://postimg.org/image/hu7k69pkh/</a> <a class="moz-txt-link-rfc2396E" href="http://postimg.org/image/hu7k69pkh/"><http://postimg.org/image/hu7k69pkh/></a>
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!
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