Comparing the u-blox F9P to the Comnav K803 receiver

In my last post I compared the internal u-blox F9P RTK solution to an RTKLIB demo5 RTK solution for a couple of cases, one with easy and one with moderately challenging conditions. In this post I will extend that comparison to some more difficult conditions, specifically under more dense tree foliage. To make things more interesting I will add a pair of ComNav K803 eval boards to the experiment.

The ComNav K803 is a relatively new module from Comnav that supports triple frequencies, L1, L2 , and L5. It is more expensive than the u-blox F9P at about $1000, but should have higher performance. ComNav generously gave me two of these boards for evaluation purposes.

As far as user interface goes, they are very similar to the older K501G or K708 ComNav boards and I was able to configure the K803 boards using RTKLIB command files very similar to what I previously described for the K708 in this post.

Here is a photo of the ComNav board on the left and the Ardusimple F9P board on the right. The ComNav board is inside a plastic case which is a nice feature. It is supposed to work with just the USB port for power and communication but I did not find this to be reliable and had to fall back to using the RS232 port with USB adapter for communication which is why the photo shows two cables for the K803. ComNav is aware of the issue and hopefully they will sort this out relatively soon.

In my previous experiment, I used a nearby CORS station for the base which limited the solution to GPS L1/L2, GLONASS L1/L2, and Galileo L1 and also extended the baseline to several kilometers . To take full advantage of both receivers available constellations and bands in this experiment, and to minimize the baseline, I used local base receivers for this experiment.

I used two Harxon geodetic GPS-500 antennas, one on the roof as base and one for the rover which was located on a tripod in an area underneath some moderate tree foliage. Both antennas were fed to an antenna splitter and then to an F9P receiver and a ComNav receiver. I tested three locations, each one further under the trees and so more challenging than the previous one. To avoid too much complexity in the experiment, I did not run a real-time RTKLIB solution but logged the F9P and ComNav raw data for post-processing.

Like in the previous experiment, I disconnected the antenna and then reconnected it multiple times to measure the time to first fix (TTFF) for both receivers. The plot below shows only the fixed epoch results for the first two locations. The F9P results are in green, and the ComNav in blue. The constant offset between the two is not real, I accidently configured the ComNav receiver to use a self-surveyed approximate position for the base location instead of an independently determined precise position.

Fix epochs for u-blox F9P (green) and ComNav K308 (blue) for location 1 and 2

In the first and least challenging rover location, both receivers performed well. The average ComNav TTFF was slightly faster, but both were below 20 seconds.

In the second location, both receivers were were still providing usable results but the ComNav receiver started to become noticeably better than the F9P. Average TTFF was 54 seconds for the F9P and 12 seconds for the ComNav receiver. In addition, the F9P had a couple of short false fixes and intermittent float after first fix that were not present in the ComNav data.

Here is the data for the third and most challenging location. Again, the F9P results are green and the ComNav results are blue.

Fix epochs for u-blox F9P (green) and ComNav K308 (blue) for location 3

In this case the differences were quite significant. The ComNav reliably found the same location on each re-acquire but more than 50% of the F9P fix epochs were false fixes, sometimes in error by several meters.

I also ran the RTKLIB demo5 solution with the same config settings as in the previous post. The results are below for the first two locations. The RTKLIB fixed epochs are in green and the ComNav fixed epochs are in blue. The performance at the first (least challenging) location was similar between RTKLIB and the F9P internal solution but the RTKLIB solution struggled with the second location and for the third location did not find any fixes except for a single short false fix. At least in these conditions it looks like there is still some room for improvement in the RTKLIB solution. However, it is also true that it did not get as much false fixes as the F9P solution which is important as well.

Fix epochs for u-blox F9P RTKLIB solution(green) and ComNav K308 (blue) for location 1 and 2

As in my other experiments, this is just a quick comparison of relative performance and is not intended to be any type of in-depth analysis.

To summarize the results, The u-blox F9P is a very capable dual-frequency receiver for easy to moderately challenging conditions, but for the most challenging conditions you will still get better performance from a more expensive receiver.