Update to RTKLIB config file recommendations

I’ve just updated my “RTKLIB: Customizing the input configuration file” post from a few months ago with information on all of the new config parameters I have added to the demo5 code up through B26B.  I’ve also added more notes to some of the existing features based on my more recent experiences.

RTKLIB on a drone with u-blox M8T receivers

Drones are a popular application for RTKLIB and quite a few readers have shared their drone-collected data sets with me, usually with questions on how they can get better solutions. In many cases, the quality of this data has been fairly poor and it has been difficult to get satisfactory results. I was curious to understand why this environment tends to be so challenging since in theory a drone should have more open skies than just about any other application.

To do an experiment, I bought an inexpensive consumer drone from Amazon. I chose the X8C from Syma since it is beginner model and a little larger than some options. I figured the larger size should make it better able to carry some extra weight.

After a few practice flights to get the hang of flying it, I used some duct tape and double-sided foam adhesive to attach a u-blox antenna and 90 mm diameter ground plane to the top of the drone and a u-blox M8T receiver with my custom CHIP data logger underneath where the camera usually goes. I used the landing gear as a spool to wind the unnecessary five meters of antenna cable which was the heaviest part of the whole setup. From a weight perspective, the Emlid Reach units would have been a better choice, but I wanted to collect data from the Galileo constellation of satellites as well as GPS and GLONASS so I used my CSG receiver with the newer 3.0 firmware. I used a second CSG receiver mounted on top of my car as the base station.  Here’s a stock photo of the drone on the left and after my modifications on the right.

Although the drone was able to lift the extra weight fairly easily, it seemed to affect the stability of the flight control system and after a few minutes the prop motors would start to fight each other. At that point the drone would start to descend even at full throttle and the drone would land hard enough to usually bounce on its side or back. Still I was able to make a number of short flights which were adequate for testing purposes.

Here’s the observation data for the first set of flights, base station on the left and drone on the right. Red ticks are cycle-slips and gray ticks are half-cycle ambiguities. Ideally, the drone data would look as clean as the base but as you can see it is significantly worse and it turned out to be unusable for any sort of reliable position solution.  The regions without cycle-slips in the drone observations are the times in between flights in which the drone is sitting on the ground.

Clearly, while the drone is flying, something is interfering with the GPS receiver or antenna, most likely either EMI or mechanical vibration. I could have used a fancy test stand and RF sniffer to try and locate the source of interference but since this blog is focused on low-cost solutions I just used some duct tape, a steel bar, and the RTKLIB code instead.

I used two types of duct tape, both the polyester/fabric type that everyone calls duct tape, and also the metal foil type that is actually used to repair or install ducts. I first used the non-metal duct tape to securely attach the landing gear to the heavy steel bar. The steel bar was convenient because it was easy to attach but anything heavy enough to prevent the drone lifting off under full throttle would work fine.

I then started an instance of RTKNAVI on my laptop and connected it to the receiver on the drone.  The goal was to simulate a complete drone flight while the drone was sitting on the ground and at the same time watch the RTKNAVI observations to detect any degradation of the measurements.  I used a wireless connection but a USB cable would have worked too.

Unfortunately RTKNAVI won’t plot the observation data real-time, but by selecting the tiny “RTK Monitor” box in the bottom left corner of the main RTKNAVI screen, then choosing “Obs Data” from the menu, I was able to get a continuously updating listing of the observations.  Cycle-slips show up as non-zero values in the first column with the I heading. I chose a location outdoors with open enough skies that any degradation in the observation data would be obvious.

I first observed the cycle-slip column with the drone powered down to verify I wasn’t getting any cycle-slips on all but the lowest elevation satellites. I then continued to observe the cycle-slip column while sequencing through the steps required to fly the drone. I first powered on the drone, then powered on the transmitter, then issued the calibration/connection sequence, then turned on the throttle to low. So far, so good, no sign of cycle-slips. I then started moving the joysticks to issue steering commands which caused the motors to change speeds. All of a sudden I started getting cycle-slips, the more aggressive the steering commands, the more cycle-slips I saw. Aggressive changes in throttle also caused cycle-slips but full throttle with no adjustments or steering commands was fine.

Next I moved just the antenna, then just the receiver away from the drone while issuing steering commands. Moving the antenna away had no effect but moving the receiver away eliminated the cycle-slips.

At this point my guess was that the interference was coming from the relatively high power switching in the motor control circuits and that the antenna ground plane was shielding the antenna from this interference but nothing was shielding the receiver. To test this theory, I attached a layer of the metal duct tape to the bottom of the drone to act as a shield between the drone controller board and the receiver.  I then re-attached the receiver to the bottom of the drone and re-ran the experiment. This time there were almost no cycle-slips even with the most aggressive steering.

I then removed the steel bar and ran a second set of short flights with the layer of metal tape still in place. I was a little concerned that the new shield would interfere with commands sent from the transmitter to the drone so I first tested everything while still on the ground and then kept the drone fairly close during the flight. Fortunately I didn’t see any sign of commands not getting through.

The drone data looked much cleaner in this flight!  Unfortunately, this time the base data was no good with many simultaneous cycle-slips throughout the observation data. At this point I realized that I had placed the base station receiver directly on the top of the car when collecting the data which was very hot at the time. Usually I keep the receiver in the car to avoid this and only place the antenna on the roof. I have seen this kind of severe temperature effects cause simultaneous cycle-slips before but never to this extent. Again the data was completely unusable.

So, back out there again for a third round of flights. This time, everything looked much better. I still saw a few cycle-slips, especially when first applying the throttle at take-off, so my shielding was not perfect but a dramatic improvement over the first flight. The plots below show the results. The top two plots are position solutions for the z-axis. The top plot is with continuous ambiguity resolution and the middle plot is with fix-and-hold enabled. The bottom plot is the drone observation data.

I made three adjustments to the input configuration file from what I would normally use for my car based measurements.  First of all, since the drones have very open skies, I adjusted the minimum elevation angles from 15 degrees to 10 degrees.   Then, after plotting and observing the accelerations from an initial solution, I increased the vertical acceleration dynamics estimate (stats-prnaccelv) from 0.25 to 1.0.  Finally, because I was seeing slightly higher position variances in the initial solution than I usually do, I adjusted the position variance AR threshold (pos2-arthres1) from 0.004 to 0.1  Both of these last two changes would make sense if the level of vibration were higher in the drone than I am used to seeing, which is quite likely.

Each time the drone landed/crashed due to the unstable flight control system it would bounce to the side or upside-down and that is what is causing the cycle-slips and switch from fix to float at the end of each flight. As you can see though in every case I quickly get another fix after I put the drone upright again. The fixes are solid enough to hold through the entire flight even in continuous mode for all but one of the flights. With fix-and-hold enabled all flights maintained 100% fix rate. The data is as good as or better than similar experiments where I have mounted the rover on top of a car.

This is not surprising since the skies are more open in this experiment. Having over twenty satellites available for ambiguity resolution also helped. I used all the satellites (GPS/GLONASS/Galileo/SBAS) for ambiguity resolution and took advantage of the new feature in the demo5 b26 code that cycles through all the satellites and will throw a single one out if it is preventing a fix. This will automatically occur anytime the number of satellites available for ambiguity resolution is greater than the config parameter “pos2-mindropsats” which defaults to twenty.

I have added the raw data and the configuration file to the  sample data set section at rtkexplorer.com

I imagine different drones will have different issues and not all will be as easy to fix as this one, but the method described here or something similar should be helpful any time drone data is not looking as clean as the base station data.

The fix I chose was very easy to implement but a better fix would probably have been to wrap just the receiver in a shield rather than placing a shield between the control board and the receiver. This would protect the receiver better and avoid affecting commands sent from the transmitter.  In fact, based on these results, I suspect shielding the GPS receiver on a drone is always a good idea.

Zero baseline experiment

I’ve been busy with some consulting projects recently so it’s been a while since my last post but I’m finally caught up and had some time to write something.  I thought I would describe an experiment I did to both try out the “fixed” mode in RTKLIB and also provide some insight into the composition of the errors in the pseudorange and carrier phase measurements in the u-blox M8T receiver.

The “fixed” mode is an alternative to “static” or “kinematic” in which the exact rover location is specified as well as the base position and remains fixed.  The residual errors are then calculated  from the actual position rather than the measured position.  I describe it in a little more detail in this post.  It is intended to be used as a tool to characterize and analyze the residual errors in the pseudorange and carrier phase measurements.

The basic idea in this experiment was to connect two M8T receivers to a single antenna and then compare residuals between the two receivers.  I first looked at the solution using one receiver as base and the other as rover (the zero baseline case) and then compared solutions between each receiver and a local CORS reference station about 8 km away.

The M8T is typically setup to use an active antenna for which it provides power on the antenna input.  I was concerned about connecting the two antenna power feeds together, so to avoid this, I added a 47 pf capacitor in series in one of the antenna feeds to act as a DC block.  In the photo below, the capacitor is inside the metal tape wrapped around a male to male SMA adapter.  I cut the adapter in half, soldered the capacitor to each end, then wrapped it in metal tape as a shield.

The receivers are from CSG and each one is connected to a Next Thing CHIP single board computer, which logs the data and transmits it over wireless to my laptop.  They are very similar to the Raspberry Pi data loggers I described in a previous post, but the on-board wireless makes them more convenient to use.  At $9 each, they are also quite affordable, especially since they do not need micro SD cards like the Raspberry Pi Zeros.  They also have a built-in LiPo battery connector which can be convenient for providing power., although they can also be powered over the USB connectors.  They are also linux based, so setting them up is very similar to the instructions in my Raspberry Pi post.

I first looked at the zero baseline case where I used one receiver as base and the other as rover.  In this case the two receivers are seeing exactly the same signal from the single antenna.  Any error contributions from the satellites, atmosphere, or antenna should cancel and the only contributor to the residual errors should be from the receivers.

I collected about an hour of measurement data from my back patio.  It is next to the house and nearby trees so as usual, the data quality is only mediocre and will include both some multipath and signal attenuation.  I prefer to look at less than perfect data because that is where the challenges are, not in the perfect data sets collected in wide-open skies.

Here are the residuals for a high elevation, high signal strength GPS satellite.  Standard deviations are 0.24 meters for the pseudorange and 0.0008 meters for the carrier phase.

For a lower elevation GPS satellite with low and varying signal strength, the standard deviations increased to 0.46 meters for the pseudorange and 0.0017 meters for the carrier phase.  Notice how the residuals increase as the signal strength decreases as you would expect.

 

The GLONASS satellites had noticeably higher residuals.  Here is an example of a satellite with high elevation and reasonable signal strength.  The standard deviations were 1.02 meters for pseudorange and 0.0039 meters for carrier phase, more than twice the GPS residuals.

I’m not quite sure how relevant it is, but the ratio between the pseudorange residuals and carrier phase residuals in each case is roughly 300, the same value I have found works best for “eratio1”, the config file input parameter that specifies the ratio between the two.

RTKLIB also estimates the standard devations of the GLONASS satellites measurements at 1.5 times the standard deviations of the GPS satellites which is less than the difference I see in the example above.

However, my numbers are for only the receiver components of the measurement errors, I’m not sure exactly which components the RTKLIB config parameters are intended to include.

For the second experiment, I calculated solutions for both receivers relative to a CORS reference station about 8 km away.  In this case, I was curious to see how close the two solutions are as they will have common satellite, atmospheric, and antenna errors but will differ in their receiver errors.  The plot below shows the residuals for a GPS satellite from each solution plotted on top of each other.  As you can see the errors are quite a bit larger than before and the correlation between the two receivers is very high.  Based on the frequency of the errors, I suspect they are dominated by multipath which will vary roughly sinusoidally as the direct path and reflected path go in and out of phase with each other.

I found it quite impressive to see how repeatable the errors are between the two solutions.  It indicates, at least at this distance, that the errors from the receiver are small compared to the other errors in the system.

Again, the GLONASS results were not as good as the GPS results and include a DC shift in the carrier phase that I’m not sure exactly what the cause is.

I haven’t spent a lot of time trying to figure out how to best use the information in these plots but in particular I found the similarity between the two receiver solutions in the longer baseline experiment quite encouraging.  If the errors are dominated by multipath as I expect, then the baseline length isn’t that relevant and I would expect to see similar results with shorter baselines.  If that’s true, then it may be possible to derive information about the receiver’s environment from the multipath data.  People do this with more expensive dual frequency receivers to monitor things like tides and ground moisture content.  It would be interesting to see if it can be done with these low cost receivers.  Or maybe it already has been done …