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:
- CSG Shop EMI / M8N / CGGBP.35.6.A.02 / 8cm ground plane
- CSG Shop XL / M8N / CGGP.35.3.A.02.
- DroTek / M8N / T0027
- Virtual Robotix / M8N / GCCP.25.4.A.04
- 3D Robotics / 6H / GP1575.25.4.A.02
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
Comments
I'm very happy about the result of our VR GPS 8 , the project is 1 year old now and we are very happy about the result on our drone technology about the emi we found the way to solve emi interference during this year and our customer normally use the potentiality of this new GNSS technology.
Actually we are workting to NEO8T revision of VR GPS 8 for support the application that need the raw data . It's available on request.
About bigger antenna we prefer mantain small form factor for fit on the small drone , but could be release with a special ground plane , we implement in standard pcb somo point where is possible to solder the ground plane with original power plane available on VR GPS 8
Respect of 3DR GPS we have a lot better result in autonomous fly like follow me and other similar application.
Thanks Thorsten for this test.
@Marcus,
unfortunately, I do not have a zubax. But from logs I saw from other users I assume it has a comparable performance to the DroTek with ground plane and the CSG EMI/shielded version. The zubax is shielded as well so there should be less interference.
@AKcopter,
I will look in the parameters and missing samples in the next test.
@Pbreed,
what you you mean with "setup for dynamic operation" and "averaging"? I all RTL tests I made so far the M8N is more accurate compared to the LEA 6H. But for sure we need a series of different tests.
@letiidronletii,
an older version of the one I liked actually (with the LEDs on the bottom) - not the XL version.
http://www.drotek.fr/shop/en/home/512-ublox-neo-m8-gps-hmc5983-comp...
Thanks Randy and Gary!
I am still wondering why we haven't seen such a test earlier.
@Thomas,
yes, I thought about the laptop being a source of EMI. But even if it has a negative influence it would show the benefit of a ground plane. I guess from this bench test we learn that a reasonably sized ground plane is important. As mentioned the next step is to compare the additional EMI shielding.
Cracking work Thorsten
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.
Do you plan to look at the Zubax GNSS system?
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.
I'd be interested to know where the point of diminishing returns is on ground plane size.
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