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RTK strange behaviour when hovering or stationary on ground

Dear all,

We have a couple of questions regarding the use of our Sirius Rover and base station.

  1. Is there some detailed documentation on how RTK works and how the rtcm messages are processed in the rover.

  2. We use the rover and base station in combination with a pixhawk4 running px4 and qgroundcontrol. What we see is that the gps coordinates when flying are within the lock-in radius of 0.6 meter that we use in our test. However before the flight when the drone is stationary on the ground, and when the drone is done with its mission and hovering, we see a lot of drift and the gps coordinates are often outside the 0.6m boundary. Why is this and is there something we can do to prevent this? For an example of this behaviour click here

  3. A. In the end we want to fly with a higher precision of about 0.2 meters. What are the things we can do to speed up the lockin, or is there a way to find the location of the base station within this accuracy with the Drotek software, and how can this be achieved?

  4. B. An alternative option for us would be to fly relative to the base station. Is this possible and if so what changes need to be made to set it up?

Best regards,

Tijmen

For a long time I was having good and bad RTK sessions. Finally, I saw that GPS parameters can be advanced.

On a good day, the difference in coordinates between two GPS’s close together is the same as their physical separation.

Tijmen,

Make sure you have a Fixed Fix and not a Floating Fix before you takeoff and that the copter is maintaining a Fixed Fix while hovering.

The absolute accuracy of the base station fix does not matter. The copter’s coordinates will always be relative to the base station fix sent in the RTCM3 1005 or 1006 record. In other words, the copter’s global accuracy will only be as accurate as the base station fix, but the relative accuracy will be 2 cm. The lat/lon of the copter will be in a pseudo coordinate system relative to the base station. In fact, once you have a RTK Fixed Fix, you can move the base station and the copter by the same offset and you will see no change in the copter’s lat/lon position. While flying, if you move the base station the copter will follow it while in loiter to maintain the same pseudo lat/lon.

Things that can speed up the fix:

  • Fly in an area clear of buildings and trees
  • 10 cm radius copper or aluminum Ground Plane for the antennas
  • Update rate from the ground station at 1 hz (not higher since the RTK processor might not be able to keep up.)
  • Try adding or removing constellations
  • Only use satellites with SNR > 40 (not sure if there is a filter in the ublox for this)
  • Try changing the satellite elevation threshold

The F9P is dramatically faster (10 second Fixed Fix) than the M8P BTW.