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u-blox M8N - ground planes, antennas and positional accuracy

Posted by Thorsten on May 10, 2015 at 1:25pm

3689649924?profile=original

Hi all,

following my last post on positional accuracy, the discussion about ground planes and shielding as well as the general debate about the M8N, I started a series of tests comparing different M8N modules. This is the first part where I focus on comparing the modules in a bench test to compare positional accuracy.


The setup

I tested the following boards with settings optimized for Ardupilot:

Additionally, I tested the DroTek / M8N / T0027 with an "external" 9cm ground plane.

Since all modules produce low HDOP/PDOP values - around 0.7 and 1.3 respectively - the comparison focusses on positional accuracy/stability. Therefore, all GPS boards were plugged in for 10min before recording. Then I recorded their positions for 10-15min using u-center. Scatter plots of the position errors are used to compare the boards.
As a reference I recorded the GSG EMI in parallel when testing the others.
The image above shows the setup on the roof and the image below a closeup of test rig.

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DroTek with the additional ground plane:

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XY scatter:

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Results

  • It is obvious that the CSG EMI and the DroTek with external ground plane outperform all other boards. 
  • The DroTek and the VR show comparable accuracy.
  • The CSG XL shows better performance compared to the DroTek and the VR but is not as good as the CSG EMI and the DroTek with external ground.
  • The 3DR 6H shows a much more scattered distribution.

Discussion

  • The board design (electronics) does not seem to have any influence.
  • Larger patch antennas result in higher accuracies.
  • A larger ground plane results in higher accuracies.
  • The ground plane seems to have a higher influence compared to the antenna.

u-blox provides a diagram (page 19) showing the effect of the size of the ground plane for patch antennas. Unfortunately, the ublox document only lists 18mm and 25mm antennas. For 25mm antennas 7cm for the ground plane seem to be sufficient. For the 35mm it should be larger. 

Remark
The results presented are only from one test. So there is for sure uncertainty. However, I made similar test the past days with comparable results. The 3DR 6H performed better in previous tests but not as good as the M8Ns, which performed not as good as in the results presented above (except the CSG EMI which showed similar results - I have not tested the DroTek with additional plane in previous test).


The next step is to compare the CSG EMI, the DroTek with and without the additional ground plane and the 3DR on a copter to compare the influence of the ground plane as well as of the shielding.

Cheers,
Thorsten

Tags: 3d robotics, 3dr, 6h, bitmapping, csg shop, drotek, m8n, virtual robotix, accuracy, antenna
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Comments

  • T3
    Thorsten January 13, 2016 at 5:24am

    Hi all,

    some new interesting numbers on positional accuracy: http://diydrones.com/profiles/blogs/gps-positional-accuracy-iii

    Thorsten

  • CSG November 16, 2015 at 11:28am

    New PRO version avail3702126078?profile=original

  • CSG August 17, 2015 at 12:07pm

    Now avail NEO-M8N and NEO-M8T with HMC5983 magnetometr on the board

    3702073126?profile=original

  • MR60
    Hugues July 7, 2015 at 8:01am

    I made magnetometer tests and found out something weird. The good thing is that my M8T was not faulty.

    By chance I did my bench tests on a Pixhawk that had arduplane (3.3) installed on it. While I was upgrading my X8 with Arducopter on it (3.2.1). And I could not at first understand why I could not see the HUD moving when rotating the magnetometer on the X8, but I could do it with a 3DR external mag+gps on the test bench one. I tus logically first suspected a faulty Drotek GPS+mag...But that was not it.

    What happens is that Arduplane and arducopter do not process the same way as shown in this video. I do not know if this is a known behavior, or a bug or even a MP difference in the HUD display between Arduplane and Arducopter:

  • JPerin July 6, 2015 at 2:13am

    I have just tested the GPS with our Dropix and you need to configure the compass orientation on ROTATION_ROLL_180. That works fine! 

  • JPerin July 5, 2015 at 1:40pm

    I will try and let you know tomorrow. 

  • Jesus A July 5, 2015 at 12:05pm

    Can you check if your mx, my, mz, mx2, my2 and mz2 are moving in the status page whithin Mission Planner?

  • MR60
    Hugues July 5, 2015 at 12:03pm

    (I meant pictures on page 7)

  • MR60
    Hugues July 5, 2015 at 10:24am

    Jesus,

    On mine (see pictures on page 8) there is a white arrow. Logically this arrow should point forward. The cable connector (there is only one) is on a side at 90 degrees from the arrow direction clockwise. Then the magnetometer chip is on top of the PCB, while on 3DR it is on the bottom.

    So my deduction is to use roll_180.

    But my issue it that direction does not move on the heads up display of mission planner when Drotek compass is moved. (it works fine when I do the same test with the 3DR module). Why ?

  • Jesus A July 5, 2015 at 10:09am
    Thorsten.

    I have a drotek XL and i took my time until I found the good orientation .
    If I remember well the orientation was yaw_90 when the module is installed with cable connectors pointing forward.

    Anyway take into account that yaw is calculated by mixing gyros and compass. If you move only the compass without the pixhawk, it will take its time for APM to point where the compass says.

    Regards
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