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: 18265

Comment by Jesus A on May 10, 2015 at 1:32pm

Thanks for the test Thorsten.

I wonder why 3DR still sticks to the LEA6.

BTW, I have read on the forums about little spikes on the M8N setups with pixhawks. Have you experience them?


T3
Comment by Thorsten on May 10, 2015 at 1:45pm

The "problem" with the M8N is that it is more sensitive to interference. So the setup has to be checked more carefully. I guess that's the reason why 3DR is hesitating. I saw heavy interference with the bluetooth module. 

I also found these spikes. They result in visible twitches on some copters. They result from missing samples. But there is no solution/idea so far. It might have to do with the GPS settings. But I need to make further tests.

Apart from that my copters have flown the M8N for 200+ hours without any problem.

Comment by Joe Breznai on May 10, 2015 at 1:58pm

Nice to see some tests like this. Great job.

I was wondering if the ground plane you added to the DroTek was actually "grounded" to a common ground somewhere, or was simply a round copper clad PC board placed beneath the unit? 

Also can you share your "settings optimized for Ardupilot", or is that in your other posts?  Looking forward to the flight tests.


T3
Comment by Thorsten on May 10, 2015 at 2:19pm

Joe, thanks!

Yes, the ground plane was soldered.

These are the settings:

PRT
Protocol out UBX
Baudrate 38400

RATE
Measurement Period 200 ms
Measurement Frequency 5 Hz

NAV5
Dynamic Model Airborn 4G
Fix Mode 3 - Auto 2D/3D
Min SV Elevation 10 Degree
DGPS Timeout 120 sec

SBAS
Subsystem Enabled
PRN Codes EGNOS (EUROPE)
120, 124, 126, 131

Comment by Nikola Rabchevsky on May 10, 2015 at 4:58pm

I'd be interested to know where the point of diminishing returns is on ground plane size.


Developer
Comment by Randy on May 10, 2015 at 5:11pm

Really great analysis Thorsten.  These kinds of objective tests really help, I've always heard the M8 is "better" but I haven't seen analysis before actually showing it. It's also interesting to differentiate the different manufacturers.

Comment by Marcus Wright on May 10, 2015 at 8:17pm

Do you plan to look at the Zubax GNSS system?

Comment by AKcopter on May 10, 2015 at 8:53pm

Yup! the Zubax Module would be interesting...also what I want to know is if during your tests have you tried modifying the Update rate parameters of the GPS module to see if the problem of missing samples aka "INAVERR"can be solved

Comment by Pbreed on May 10, 2015 at 10:57pm

How the GPS is setup will make a differnce. A GPS set to be staionary will do differnt filtering than a device designed to be on a dynamic platofrm.  One wonders if the 3DR is poorer becayuse its setup for dynamic operation, where the others are averaging and would not work nearly as well on a dynamic platform?

The only real test would be to run the recievers back and forth on a several meter track where you recorded the position and the trak position individually.

Comment by letiidronletii on May 10, 2015 at 11:24pm

Which Drotek module have you used. They also have XL version is this what you used. 

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