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
Just out of curiosity, why would anyone desire a 24 hr test? To many variables would affect results, namely atmospherics, space weather, satellite movement, etc...
NOTE: GPS satellites are NOT geosynchronous or geostationary.
GPS satellites orbit in MEO -- Middle Earth Orbit (as opposed to LEO -- Low Earth Orbit where the Space Shuttle and Space Station fly, or GEO -- geosynchronous orbit, where telecommunication satellites fly.)
LEO = 90 minutes per orbit
MEO = 12 hours per orbit
GEO = 24 hours per orbit
It takes roughly 12 hours, (two orbits per day) for GPS satellites to orbit the Earth. So in my humble opinion shorter time is satisfactory, say 2 hrs.
What is the average time any GPS would be powered should be the real test.
Fixed wing aircraft have the longest time in the air, so it makes sense to establish tests that would benefit those folks who fly for hours and in turn will also provide data to any other drone hobbyist.
For a true test of all your modules, I would recommend running them all at the same time and for the same period of time. This will ensure you get a fair sampling of satellite data due to all the events that go on overhead.
As for follow on tests... I feel that the best way to sample EMI is to use one drone and test all GPS modules on it under exact conditions. This process will be far harder to due as constellation and atmospherics change and wil require you to collect shorter samples with the drone powered up.
Look forward to seeing more of your test results. You got a jump on an idea I was thinking on doing as soon as more GPS modules arrive.
It is good to see some actual tests, but as others have mentioned you really need more data. Satellite availability and multipath conditions can change over 15 minute periods. For your test I would be especially concerned about changing multipath conditions. A good start would be a 24 hour recording session from each unit or perhaps ten of these shorter sessions so individual variations will tend to average out. It takes time, but there is really no way around that for a good comparison.
You may want to make sure all M8N receivers are set up the same. Perhaps you did this, but I'd start by resetting each to the default parameters and then entering the changes you listed.
Since you are really testing antenna setups a couple wavelengths of separation or more between antennas is a good idea. Tallysman, for example, recommends at least 0.5 meters. Your test rig might be fine, but it's something to consider.
@Lockhart,
You are right the length of time for data collection is too short for a statistically sound comparison. I was thinking about how to test it and decided to try it under normal conditions. With normal I mean: you go somewhere with your copter, setup the GCS and the camera, plug in the battery and wait until EKF is ready and start the mission. So the period between pugging in the battery and takeoff is often relatively short. What I learned from this test is not do so, but wait for at least 5 min to give the GPS some time to "warm up". The data from the first minutes are much more scattered. This is why I decided to give the GPSs 10-15 min to warm up and 10-15 to record the data.
The second scatter plot of the CSG EMI version was from a parallel test running 100 minutes. So there is a difference for sure. But the versions without a proper ground plane show higher positional uncertainties within 10 min only. I would love to see a full comparison as well. But I simply have to time to do so.
Anyway, the major results of this test are: use a GPS with a proper ground plane and give it time to warm up before flying a mission.
JoeBob,
I had a look at the z axis as well, but not in detail. What I have seen so far is that the larger ground plane also has a positive effect here. In one case the standard deviation of the altitude was 57cm for the round CSG one and 230cm for a normal M8N. But I was analyzing the kml files only.
@Jani,
you find the setting on page 1 in the comments.
Many thanks
Are,
sounds like a good idea!
The Ardupilot master branch now allows to record RXM raw u-blox messages (https://github.com/tridge/ardupilot/commit/18f79deacc8b74917f0440a1...).
Even if it is not used in "a tightly coupled GNSS/INS kalman filter" it can be used in post-processing for example for image georeferencing.
Hi Nampilot,
thanks for the clarification! I am wondering why both have the same effect.
I had a look at the mnfr recommendations for ground plane sizes and they are all around 70mm - 80mm for the 25mm patch antennas.
Regarding the reduction of EMI I'll make some further tests, especially with the CSG which has an additional EMI shield below - same as the zubax. I'll compare it to the DroTek setup.
@ Nikola & Thomas - and the rest -
I have discussed some of the issues re 'ground planes' and related effects in another posting on DIYd, so will try to be brief here -
I believe the function of the 'ground plane' is being misunderstood, or rather misinterpreted here, and needs separating into the actual antenna ground plane, and the R (EMI) shielding ground plane.
In Thorsten's images above, all the top image planes are in fact a ground plane that is PART of the antenna ( the Ceramic patch antenna). The lower image plane ( on the Drotek additional plane image) the ground plan is not a part of the antenna at all.
The patch antenna requires a ground plane against which it 'works' The actual radiator patch element on the upper surface of the ceramic patch has a size that is determined by the frequency of operation, the dielectric constant of the ceramic base, the height of the ceramic base, and therefore the distance of said element to the ground plane below, and the size of the ground plane.. ALL these factors determine the operating frequency, the radiation pattern, antenna impedance and antenna efficiency.
The radiating element is in fact 2 horizontally spaced dipoles, in phase, and the radiation is 'focused' away from the ground plane in a manner related to the distance from the ground plane, and related RF fringe currents between the edges of the patch element and the ground plane.
If the ground plane size is to small, the fringe currents are affected, as is the radiation pattern, the antenna impedance, and its efficiency.
All reputable patch antenna suppliers will give in the datasheet the required ground plane size for the ceramic patch to be mounted onto. This should not be reduced! Doing so will result in the above effects.
What is the optimum ground plane size? An infinite plane is optimum - most of the radiated energy is away from the plane, with no back lobes. No EMI reaches the antenna from below either. What is good enough? A 1 wavelength plane is very effective and will serve in 99% of needs. A 1/2 wave (mechanical length) plane should really be the smallest and will give good results in radiation pattern, as well as EMI shielding, good enough in most cases...
However, again, GPS patch antenna element suppliers try to fit in with the worlds miniaturisation and the wearables world cannot accommodate such size planes. So they design for a compromise, the smallest plane giving a 'usable' gain and radiation pattern ( also normally published in the antenna datasheet). When the plane is designed in this manner, you cannot simply change its size, either smaller or larger, to try 'improve' the radiation pattern, or EMI shielding, etc - you will alter the feed impedance drastically, and so the return loss and efficiency.
I presented on the other posting return loss measurements on a patch element on a 'large' ground plane, and showed measurements as the plane is reduced in size to the recommended size - the large plane (1 wavelength) gave SWR reading way above 4:1, quite unacceptable. You must use the size recommended by the mnfr for the antenna to work as advertised.
Also, using '3 wires 1/4 wave long' will not work as a patch antenna ground plane surface.. The ground plane, as explained, is not simply a reflector. The capacitance between it and the radiating element is paramount - 3 wires do not form a fringe current surface at all...The 3 wires work fine in a 1/4 wave ground plane vertical ( 1/2 a dipole radiator..), but the function is entirely different.
The true ground plane, as part of the antenna structure, is therefore a fixed , defined element, and is unfortunately constrained by the patch antenna element designer/manufacturer. It is located directly under, touching, the ceramic patch element. The radiating element feed pin passes through this plane to the RF connection of the GPS or LNA. This ground plane must be homogeneous and continuous on the surface touching the patch element. The underside of this plane normally contains all the electronics ( GPS module, etc).
This brings us to the 'other' ground plane of image 2 - the Drotek additional plane image- This ground plane is separated by some distance below the actual GPS antenna ( remember, the module IS in fact the antenna - the module ground plane is a fundamental part of the patch antenna) and has little effect, if any , on the antenna impedance, matching, etc. So, if this plane is spaced at least 10 to 15mm below the patch antenna, its size can be increased 'almost' with impunity. If spaced 20mm or more, its size will have not effect on antenna matching worth considering. However, the benefits ( as seen from Thorstens results) can be positive.
This plane will reduce back lobes considerably, which means reduced EMI from the electronics below, and it will 'focus' more of the antenna radiation upwards, albeit slight, esp at a 20mm or more spacing. The closer the plane is to the actual antenna, the better it will focus, but that begins to affect the antenna matching as well.
The main benefit is from the reduction of pickup of EMI from the rear.
Incidentally, this shielding plane must also be electrically connected to the ground connection of the GPS module- a single short wire will do.
So , to be clear, these two 'ground planes' are not the same and serve very different functions. As in my other postings, I recommend that before you purchase any GPS module, see what the part number of the patch element is, obtain its datasheet ( google..) and see what the size ground plane is recommended by the mnfr - if the module you are interested in differs by more than say 10 or 15%, buy something else...And once you have made your (sound) choice, if you experience EMI, add a larger shield plane below, as Thorston did..or shield your electronics.
The Nampilot.
- It would be interesting to see what the mnfr recommended ground plane size is for the patch element on the DroTek GPS...
@Roberto,
Will the VR GPS 8T-revision be able to feed raw pseudoranges to VrBrain/Pixhawk? In that case it should be possible to make a tightly coupled GNSS/INS kalman filter. That would really make a (big) difference in performance for Ardupilot.
In addition to a tightly coupled GNSS/INS kalman filter in apm, it could receive corrections from a NTIP/RTCM service (via 3G-dongle/3DRadio) bringing the precision down to 10-20cm level.
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