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: 23122

Comment by Artem Bulashev on July 25, 2016 at 8:02am
Comment by Pro Backup on July 28, 2016 at 1:39pm

I was wondering what the improvement in signal strength (C/N0) would be when adding a (copper) ground plane to a Chinese GY-NEO6MV2 with a GPS-001 active GPS antenna.

For the test setup I ordered 19x19cm 0,6mm copper plate from Aduis. A standard test cable and clip was used to attach the copper plate to the ground pin of the FTDI. I used u-center 8.21 software its "sky view" to measure the average relative signal strength.

The test location is bad in terms of signal: a metal - there was magnetic attraction between the flag post and the GPS-001 antenna - flag post in front of a window at 4 till 5 meters above street level. Many buildings are blocking the signal. 41 out of 216 sky section have no GPS space vehicle (SV) view at all. There is also a canal in the street and a lot of parked cars and some trees.

The test duration was at least 12 hours (LEO).

The first test was without copper plate. There were 14 patches out of 216 with an average GPS satelite signal strength above 45 dBi:

The second test was with active GPS-001 antenna and with the 19x19cm copper plate. There were 29 patches out of 216 with an average GPS satellite signal strength above 45 dBi.

In a third test I used a 20x20cm copper plate and I was sloppy in positioning the GPS-001 in the center of the copper plate. Result: only 7 patches out of 216 with an average GPS satellite signal strength above 45 dBi. I do need to do a re-run, where the antenna is more precisely positioned in the center of the copper plate.

Comment by Pro Backup on July 28, 2016 at 2:04pm

Comment appended to the above test. The Neo 6M deviation doesn't improve at all. It still drifts 27 meters to the left and 25 meters to the right in this static position (relative to earth).


MR60
Comment by Hugues on July 29, 2016 at 3:16am

@Pro backup,

To see improvements using a ground plane, the ground plane should ideally be the one (prolongating) of the ceramic antenna of your GPS. As this is physically impossible, the best approximation is to connect the copper plate to the ground of the ceramic antenna of your GPS.

Comment by Pro Backup on July 30, 2016 at 9:58am

@Hugues, thank you, it is good to know that its best to mount the ceramic patch antenna onto the ground plane itself.

The repeated result with the 20×20cm 0,6mm copper plate is: 37 out of 216 patches are on average above 45 dB (C/N0):

The static position drift deviation range now stays within 16,3m:

Comment by Michael Smorto on September 17, 2016 at 8:21pm

Hi all,

Don't know if this is still active but I am just getting into the nuts and bolts of GPS using the Drotek  Neo-M8N referenced in this series of posts.  I decided to run a test on the unit while indoors (by a window) and was still getting reasonable very reasonable HDOP and PDOP (less than 1 and 2 respectively) but when I compare the position against google maps it is showing a 20m delta.  Not sure what is off the M8N or Goggle Maps. 

I collected the position using NMEA strings with the NEMA.exe and NEMAStats.exe programs referenced earlier in the series of posts.  Here are the respect plots for Average Coodinates from the M8N and True Coordinates (as compared to Google Maps.  Hope someone can explain what I am seeing and what I can do to correct the deltas.  I am also looking at using RTKLib as a last resort.

Comment by Artem Bulashev on November 26, 2016 at 6:14am

JPerin, why Drotek uses 35.3 antenna on their latest GPS, instead of taoglas 35.6.a.02 antenna (originally used on XL version with 80mm ground)? And also XL used to be with 80mm plate, now that is XXL and XL is with 50mm - I would appreciate if you could tell us why so?

Comment by JPerin on November 26, 2016 at 7:23am

We use the same antenna on XL and XXL version since a long time. These antennas come from the same manufacturer that also provides taoglass.

Comment by Artem Bulashev on November 26, 2016 at 7:59am

 JPerin, I understand that, but the question is more about Why 3mm thick instead of 6mm thick - Initially you have used 6mm thick one, and CSG have gone from 3mm -> 6mm thick one (you have made an opposite decision).

I'm interested in creating a all in one "GPS shoot out" test, which would evaluate all major GPS unites and draw some conclusions on what allows to make a better gps unit. That is why I'm interested in your perspective now regarding the antenna change (from higher gain 6mm to lower  gain 3mm). May be Radiation patterns is better on smaller ground planes?

BTW, would you be interested in comprehensive static and active testing of different GPS units (from no name china/ebay to your new models and more expensive CSG?). It is in the design stage now and your product seem like the best in Price/Performance ratio. 

Comment by JPerin on November 26, 2016 at 1:26pm

We have tested several antennas and we found no improvement with the 6mm. So why pay more? :-)

Our GPS modules are used by all Ardupilot and PX4 team since many years. If you are looking for the best price/accuracy ratio, don't hesitate. 

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