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

Simon Trény simon.treny at gmail.com
Mon Apr 13 08:49:09 PDT 2015

```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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