Same shᴉt, different day.

I went to the neighbouring field that the farmer isn’t using because they are digging for gravel in 90% of it, still leaving a sizeable 10% right next to my house; again tried GPS tracking, and again overshat (overshotted? overshooted? overshot?) significantly.  She started 13 meters away from the orange frisbee based on the GPS position of both, correlating nicely with the video.  She’s facing almost exactly away from the target as passed from the autopilot and logged by the motion process:

 GPS TARGET 13m 173o

The target is in a NNE direction by gut feel and confirmed by the GPS log stats:

GPS tracking

GPS tracking

GPS thought it had travelled about 5.3 meters when in fact this was more than 16 meters, based both on the video and on the GPS itself showing 33 samples at 1Hz with ‘H’ programmed to fly at 0.5 m/s.

Now what’s interesting here are the spacing between the dots on the graph. Since each dot is one second apart, this give the speed the GPS thought it was moving:

GPS speed

GPS speed

As already mentioned, ‘H’ is flying at a constant and stable 0.5m/s, confirmed by the video, but GPS ‘speed’ climbed and has not yet reached the stable 0.5m/s.

As a result of all the above, I finally I have a clue of why ‘H’ keeps overshooting her GPS target: it’s like the NEO-M8T algorithm uses some form of low pass filter, which gives brilliant accuracy for a stable location (i.e. the waypoints and at takeoff), but when moving, each new reading is fused with historic readings, causing significant lag.

The NEO-M8T has a vast amount of config parameters accessible via its u-center app.  Time for me to explore the options.

P.S. A post to the u-blox forum quickly yielded the solution: the UBX-CFG-NAV5 can be set via the u-center app (or other means, I’m sure).  By default, the NEO-M8T algorithm uses a “stationary” model, but there are many other options:



I’ll try “portable” model first based on their descriptions of the different models in their NEO-M8T spec, section 7, followed by “pedestrian” and finally “Automotive”.

P.P.S. The “portable” model worked perfectly as shown by the next post.  There was one additional config change to make which is in the “CFG” section: changes need to be saved in all possible options so the update to “portable” survived after reboot.

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