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

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.

DroTek with the additional ground plane:

XY scatter:

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

Views: 22764

Comment by Arshish Maneckji on January 29, 2016 at 3:39am

Hi,

I have the Drotek M8n XL Module with the ground plane. My Mag offsets have been in the 50's for x & Y and around 155 for Z.

After reading this article I decided to solder a wire from the ground of the GPS to the ground plane. I also made another change where  I drilled a hole in round aluminium the base plate of the GPS Mast onto which the Drotek Ground plane is fastened using alluminium screws.

Very strangely I started getting offsets in excess of 600 on all 3 axis immediately after making this change. Assuming I had damaged the GPS module while soldering, I plugged another GPS in. (I had a spare) Once again I got similar offsets in excess of 500.

This was all without the Main battery plugged in. Just powered using the mini USB cable so EMI shoukd not be an issue

Totally perplexed, I came to the conclusion that I had somehow magnetised the GPS mast baseplate during the drilling process. I unfastened the GPS leaving the wires connected and performed another calibration but this time by rotating only the GPS module in my hand. (Quad was stationary). The offsets I got were fantastic; all below 20.

What I have realed is the closer the actual GPS module is to the Ground plane; worse the offsets. Initially there was a gap of about 5mm between them and I was getting the horrible offsets. I have now increased the gap to about 20MM and I am back to 50's for the x & y axis and below 200 for the z axis.

Any advise on what I could do to fix this. The Ground plane which is supposed to help the GPS is making the offsets worse...

Comment by Arshish Maneckji on January 29, 2016 at 3:43am

Apologies for the typos - I cannot seem to find an Edit Comment button to correct them :-(


T3
Comment by Thorsten on February 16, 2016 at 12:00pm

Hi Arshish,

that sounds strange! Can you post a picture?

Are you using an additional ground plane? Is so what king of material is it?

Comment by Arshish Maneckji on February 16, 2016 at 12:43pm
Hi Thorsten. Thank you for replying. Yes its an additional ground plane. I dont know the material but its the one that Drotekbsells qith the gps unit. You have to aolder it tohether.

Pictures will follow.
Comment by Arshish Maneckji on February 16, 2016 at 12:51pm
Link to pictures. As you can see the scatter x y is not that great. Nowhere near as good as what you get with the drotek M8N and ground plane.

https://plus.google.com/102379878843287353744/posts/R3GSQtFWHdv

Developer
Comment by Philip on March 21, 2016 at 2:30pm

@Thorsten
really nice write-up! For Pixhawk2 we did similar testing, and found that a 6cm Ground-plane on a 25mm patch was nice, but the comments from most users was just about how big it was :) 

I will refer people to this writeup to show them why!

Thanks again

Phil

Comment by Artem Bulashev on March 28, 2016 at 7:37pm
Thorsten, I have come across a post of yours almost a year ago where you have said to revive new drotek unit and a new Zubax unit with baro on board.

I'm now in process of finding a best gps for the $, and Drotek new m8n+5983 (xxl) looks great for 65€... But I would love the external baro+compas+gps combo of Zubax gnss2!

Have you had any test results or experiences with them?

Kind a feel like it's better to take brotex with m8n+compas and add baro to it... didn't find anyone do that though...
Comment by Artem Bulashev on May 22, 2016 at 3:41pm

Come on people :) there are only 3 best options now

1. CSG neo-M8N Triple Band & EMI & Double SAW and LNA + HMC5983 (pro version) = 120$

2. CSG neo-M8N Triple Band & EMI + HMC5983 (XXL version) = 100$

3. DroTek neo-M8N Triple Band & EMI + HMC5983 (XXL version) = 51$ (they exclude TAX for shipping aboard)

My conclusion: DroTek XXL clearly wins over CSG XXL on paper - any real test comparison, because CSG IMU did extremely well - will this new drotek do as well?

AND THE BIG ONE - does CSG Pro worth 2.5 times more cash??? 

Comment by Pro Backup on July 24, 2016 at 1:16pm

@Torsten On May 12, 2015 you wrote:

What was strange was that with the DroTek the signal strength of some satellites suddenly dropped for some ms. I am not sure what happened. Maybe a problem with the FTDI cable.

Did you ever figure out why the signal strength for some satellites was suddenly dropping for multiple minutes with the DroTek u-blox M8N based GPS modules?

Comment by Artem Bulashev on July 25, 2016 at 8:01am

Hi, I don't think any one replies to this thread any more. . . One thing I figured on my own is that new Drotek m8n Triple Band & EMI + HMC5983 (XXL version) is as good as CSG  XXL, but coasts twice less - ultimate price/performance winner 

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