"I've nailed the problem down to these issues:
Fault of APM:
* ArduCopter does not honour the manual serial port settings. It is somehow hard-coded to always use 38K baud even in manual mode/auto config disabled. When configured as a NEMA compass…"
"1) When talking about GPS settings especially uBlox rate more often means the MCU speed/refresh rate. When SBAS is enabled the max is 4Hz, without it's 5-10Hz depending on the exact model. This is documented in the configuration whitepaper from…"
"Driver integrated LED panels are now generally available and cheap. It would be a good time for somebody to help APM catch-up to with CleanFlight/Naze32 and it's support for programmable RGB LED support (including the status/location relative…"
"The connection cable provided with the GPS is straight, no cross-over of TX/RX (just the whole connector is up/down), but as the motherboard of the GPS (I opened it) is not labelled we don't know that the RX goes to RX as Cala suggests. The only…"
"...so this one defaults to 56k baud out the box, and must be set to 9k to get a reliable flash to the new 3.01 firmware for Galileo support and other improvements. So expect to get hold of a CP2102 USB serial adapter and make-up a cable to connect…"
"By the way this GPS unit is very good. It will run up to 10Hz in GPS mode and 3Hz in SBAS-DGPS mode. The only issue is that APM is unable to detect this device at speeds higher then 38k baud, so best leave the update RATE at 500ms (2Hz), The problem…"
"Tested in two different locations around Germany 100s of Km apart, both with flat surroundings and on the side of a wide open valley and at different times of the day (noon and evening). The same behaviour exists regardless of location, delays up to…"
"Maybe you can inject IMES messages, pretend to be a differential source/use the u-Blox AssistNow extension that they used to implement CellLocate.
That is, rather than trying to extend the GPS from the flight controller side and fight with EKF, take…"
"I have tracked this problem down to the M8N GPS (and probably other new more accurate GPS units) settings both in ArduCopter and the U-Center/GPS itself.
First the HDOP values seem to be mostly caused/affected by changing the various EKF filter and…"
"I think I fixed the problem as follows:
1) Increased the INS_GYRO_FILTER and INS_ACCEL_FILTER parameters to 48.
2) My U-Blox M8N (mini PixFalcon version) already had the latest firmware 3.01 with Galileo enabled, but not all the other satellites and…"
"I have some more information on this. I managed to isolate the problem and now the error makes sense. The GPS is actually fine. But what is not fine is the accelerometers are waving up and down and also the compass, seen when you click the EKF text…"
"I can confirm this. Me and my friend both have two new PixFalcon quads we built and suffer the same problems. Both originally and after being completely rebuilt with freshly soldered connections, so I'm sure we are not suffering an installation…"