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.
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:
- 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.
- 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.
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.