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

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

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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

  • MR60
    Hugues July 5, 2015 at 8:28am

    A bit more  troubleshooting info : when turning just the external compass, MP does not see move the compass. When moving the pixhawk board, MP moves the compass. When using a 3DR external GPS+compass, moving the external compass is ok.

  • MR60
    Hugues July 5, 2015 at 8:10am

    Succeeded to get compass readings again but can't figure out what the compass orientation is on that M8T+compass module. compass_orient 0 or 180° ? trying both does not change anything which is weird. I had once a pre arm check inconsistent compass which would indicate a bad compass_orient value but now I can't seem to reproduce this error message whatever the value of compass_orient is set to. Really disturbing. If Drotek reads this thread, could you confirm what the compass_orient should be when the board is mounted normally (as shown on pictures above, the front of the drone corresponding to the white arrow on the M8T module) ?

  • JPerin July 5, 2015 at 7:56am

    I'm not in our facility but I know you need to change the orientation in MP. 

    If you think there is a problem send us back it. Thanks

  • MR60
    Hugues July 5, 2015 at 6:47am

    Issue encountered with M8T+Compass , the external compass was first detected, calibration was done but North direction was totally incorrect. First thought was about a wrong GPS+compass board orientation. Double checked everything, cables, etc. Then Pixhawk does not detect the external compass anymore. No matter what, reboots, recalibration : it sees only its internal compass, not the one on the M8T.

    My current diagnostic is a faulty compass on this Drotek XL M8T+Compass board. I will ask for a Drotek RMA.

  • MR60
    Hugues July 5, 2015 at 2:27am

    Just did a quick bench test of the Drotek M8T+Compass XL module. I have to say I'm quite satisfied to say the least : the test is done on the ground floor inside my thick brick wall house, in the middle of a city where environment is not ideal. I got a fix in less than a minute with acquisition of 10 satellites and a hDOP  of 0,9.

    I did this quick graph (5 minutes) with U-Center showing the average position, and the maximum deviations:

    3702041451?profile=originalThis looks promising.

    I also printed a new mount for this big beast. It is huge versus the classical LEA6:

    New 3D printed XL M8T+Compass mount :

    3702041502?profile=original

    Relative size of Drotek M8T+compass versus the 3DR LEA6:

    3702041519?profile=original3702041474?profile=original

  • JPerin July 3, 2015 at 9:28am

    Good news!

    Thanks for your feedback. We will improve the cable asap.

  • MR60
    Hugues July 3, 2015 at 4:18am

    Just received my GPS from Drotek this morning !Hourray. This XXL version is very impressive and I noticed a few practical design improvements from Drotek:

    -One single connector instead of two for the GPS and compass

    -Very large passive antenna 35x35 versus 25x25 for the regular M8N

    -of course the large pcb plate that will probably shield much better from the drone's electronics interferences

    -Drotek increased the length pf the provided cable versus the regular M8N to about 30 cm which is perfect length for a setup of the GPS on a mast , faraway from EMI sources.

    On the "still to improve" side:

    -the provided cable is made of awful rigid plastic; would be better with soft silicon (for clean install and vibrations transmission).

    Will mount it this week-end on a quad for a quick test

  • T3
    Thorsten July 2, 2015 at 1:51pm

    Hi Linus and all,

    I just opened a new discussion related to interpreting the UBX messages and checking for interference problems. It seems there is little information available.

    http://diydrones.com/forum/topics/gps-receiver-condition-and-noise-...

    Cheers,

    Thorsten

  • JPerin July 2, 2015 at 1:27pm

    Hi Hugues,

    Can you contact me in MP please. I will check and find a solution quickly.

    Usually all order of our customers arrive without problem. Thanks in advance and sorry for the annoyance.

  • T3
    Thorsten July 2, 2015 at 12:18pm

    Hi Hugues,

    Jerome from DroTek has been very responsive so far. But I know there have been some technical problems with the XXL versions. So maybe it is related. 

    Unfortunately, I am a little behind my schedule...

This reply was deleted.