Good data from Doug Weibel today, who was testing the uBlox GPS vs the onboard absolute pressure sensor on ArduPilot Mega with his NexStar Mini EP. In general the uBlox is performing great. And we love the onboard datalogging on APM!
Doug explains the modest data diversion between the two sensors:
"You can see that in the area from roughly 1600 to 2500, there is greater difference between the two altitude measurements. This coincides with when I let my son fly the plane and took it out of fly by wire mode and put it in manual. He was busy trying to get the little NexStar Mini EP to do some aerobatics. Since he was spending a lot of time in very high bank and pitch angles I would guess that the uBlox performance was suffering and the baro altitude was more correct."
Comments
I have the same question that Peter Hollands asked:
What settings are you using for the uBlox?
The reason I ask the question is that I recently got around to testing a couple of uBloxs, and was disappointed in their performance, especially compared to excellent performance of the EM406 that I have measured during the past few years. The EM406 consistently achieves better than 1 meter accuracy shortly after initialization when I have taken it on rides and walks along well known paths. Lately I polished up altitude control in MatrixPilot, and was able to achieve autonomous landings, where the altitude accuracy must have been better than 5 meters in order for the plane to have landed where it did.
By the way, we have WAAS in our neighborhood, that is what I attribute the 1 meter accuracy to.
I like the EM406, but I don't want to be a "Ludite", so a couple of weeks ago, I decided I better catch up with everyone else. MatrixPilot supports both the EM406 and the uBlox, so I connected up a uBlox to the UAV DevBoard running MatrixPilot and took it out for a flight in my sailplane. Based on all the claims I've heard, I expected fantastic performance, so I was very surprised when I analyzed the telemetry from my first test flight. After the plane landed, the uBlox was reporting altitude values that ranged from 10 meters below the ground to 50 meters above!
So, I did some ground tests with two different uBloxs, one that I bought from the diydrones store, and one that I bought from SparkFun, with similar results: the reported locations had huge errors in them shortly after power up. As expected, the errors declined with time, but they never approached anywhere near the accuracy of the EM406.
So, I am puzzled, and look to the community for help.
Here are two tracks from the ground testing that I did with both an EM406 and a uBlox, at one of my favorite flying fields. In both cases, I waited 2 minutes after power up, and then took the GPS along the edge of a road, at an altitude of about 1 meter above the ground.
First, here is the lateral track of the EM406:
Here is a view of the altitude reports from the EM406:
Here is the lateral track of the uBlox:
Here is the view of the altitude reports from the uBlox.
I expect that the problem is that I did not have the correct settings for the uBlox. I am pretty sure that I had WAAS enabled, and had selected the uBlox dynamic model that allowed for 4 g acceleration. But perhaps those are the wrong settings to use, or maybe my uBlox is not programmed correctly.
In any case, could either of you tell me what settings I should be using for the uBlox?
Best regards,
Bill
Peter
http://www.sparkfun.com/commerce/product_info.php?products_id=177
Good call on the units. I'm pretty sure he's using Meters, and now it makes more sense to me that the Ublox is about where I expected it to be then considering its in meters. With that being the case, GPS is off by like 60 feet at times. To me that's what I would expect from gps and is why I would back it up with more sensors. Baro/accel etc. But good enough is in the eye of the beholder!
Can we draw any conclusions from these tests other than comfort that two indirect methods of measurement agree reasonably well?
Peter
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