AP_GPS Undulation is opposite common convention causing incorrect ellipsoid height #29199
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After some experimentation and private discussion with Ryan it appears that (fortunately!) this issue does not affect the common use case of DroneCAN, UBX, or NMEA GPSes. When using these GPSes, whether through AP_Periph or a flight controller, the ellipsoid height is correct over MAVLink This makes a fix a lot more palatable for me personally and I do think it is important for users of other GPSes. |
tpwrules
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ArduPilot's internal definition of undulation is reversed in sign compared to the accepted convention and this creates confusion: ArduPilot#29199 . Instead of the internal definition, log the altitude above ellipsoid, which ArduPilot already consistently and correctly calculates and broadcasts over e.g. DroneCAN and MAVLink. Switching the quantity logged allows users to accommodate the problem in old logs (i.e. always flip GPA.Und sign to match the accepted convention) and reduces opportunity for future confusion. Future work will clean up or replace the internal representation, but this should be a complete fix for the issue from a user perspective.
tpwrules
added a commit
to tpwrules/ardupilot
that referenced
this issue
Apr 26, 2025
ArduPilot's internal definition of undulation is reversed in sign compared to the accepted convention and this creates confusion: ArduPilot#29199 . Instead of the internal definition, log the altitude above ellipsoid, which ArduPilot already consistently and correctly calculates and broadcasts over e.g. DroneCAN and MAVLink. Switching the quantity logged allows users to accommodate the problem in old logs (i.e. always flip GPA.Und sign to match the accepted convention) and reduces opportunity for future confusion. Future work will clean up or replace the internal representation, but this should be a complete fix for the issue from a user perspective.
rmackay9
pushed a commit
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Apr 29, 2025
ArduPilot's internal definition of undulation is reversed in sign compared to the accepted convention and this creates confusion: ArduPilot#29199 . Instead of the internal definition, log the altitude above ellipsoid, which ArduPilot already consistently and correctly calculates and broadcasts over e.g. DroneCAN and MAVLink. Switching the quantity logged allows users to accommodate the problem in old logs (i.e. always flip GPA.Und sign to match the accepted convention) and reduces opportunity for future confusion. Future work will clean up or replace the internal representation, but this should be a complete fix for the issue from a user perspective.
rmackay9
pushed a commit
that referenced
this issue
Apr 29, 2025
ArduPilot's internal definition of undulation is reversed in sign compared to the accepted convention and this creates confusion: #29199 . Instead of the internal definition, log the altitude above ellipsoid, which ArduPilot already consistently and correctly calculates and broadcasts over e.g. DroneCAN and MAVLink. Switching the quantity logged allows users to accommodate the problem in old logs (i.e. always flip GPA.Und sign to match the accepted convention) and reduces opportunity for future confusion. Future work will clean up or replace the internal representation, but this should be a complete fix for the issue from a user perspective.
rmackay9
pushed a commit
to rmackay9/rmackay9-ardupilot
that referenced
this issue
Apr 29, 2025
ArduPilot's internal definition of undulation is reversed in sign compared to the accepted convention and this creates confusion: ArduPilot#29199 . Instead of the internal definition, log the altitude above ellipsoid, which ArduPilot already consistently and correctly calculates and broadcasts over e.g. DroneCAN and MAVLink. Switching the quantity logged allows users to accommodate the problem in old logs (i.e. always flip GPA.Und sign to match the accepted convention) and reduces opportunity for future confusion. Future work will clean up or replace the internal representation, but this should be a complete fix for the issue from a user perspective.
rmackay9
pushed a commit
to rmackay9/rmackay9-ardupilot
that referenced
this issue
Apr 29, 2025
ArduPilot's internal definition of undulation is reversed in sign compared to the accepted convention and this creates confusion: ArduPilot#29199 . Instead of the internal definition, log the altitude above ellipsoid, which ArduPilot already consistently and correctly calculates and broadcasts over e.g. DroneCAN and MAVLink. Switching the quantity logged allows users to accommodate the problem in old logs (i.e. always flip GPA.Und sign to match the accepted convention) and reduces opportunity for future confusion. Future work will clean up or replace the internal representation, but this should be a complete fix for the issue from a user perspective.
rmackay9
pushed a commit
that referenced
this issue
Apr 29, 2025
ArduPilot's internal definition of undulation is reversed in sign compared to the accepted convention and this creates confusion: #29199 . Instead of the internal definition, log the altitude above ellipsoid, which ArduPilot already consistently and correctly calculates and broadcasts over e.g. DroneCAN and MAVLink. Switching the quantity logged allows users to accommodate the problem in old logs (i.e. always flip GPA.Und sign to match the accepted convention) and reduces opportunity for future confusion. Future work will clean up or replace the internal representation, but this should be a complete fix for the issue from a user perspective.
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@Ryanf55 we've got patches which address this issue - all done yet? |
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Solved and back ported to 4.6 |
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Bug report
Issue details
ArduPilot's idea of undulation is not following common convention. It's been this way since it was first implemented, and we have seen PR's come through to correct device drivers undulation, when in fact, ArduPilot was using the wrong definition.
See here for a report:
Ardupilot GPS Issue - PUBLIC3.pdf
If you use ellipsoid height for an algorithm, you may run into large offsets on some GPS's.
Version
Every version of ardupilot that publishes GPS_RAW_INT and dataflash logs to report UND, including
master.Platform
[x] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine
Airframe type
All
Hardware type
A few GPS's have been tested
Logs
N/A
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