Apriltag Relocalization with GPS?

[Evan Hertafeld]

Hi all!

We have the wonderful luxury of GPS in our environment, but want to leverage the precision of vision-based fiducial markers. My first thought was to rely on the PX4 EKF to fuse these, but the PX4 docs discourage using two position-based systems simultaneously.

Then I realized - can't we leave vision out of the EKF entirely, fly by GPS, and rely on the relocalization to provide the correct setpoints translated into the drone's GPS NED frame?

Whether this is feasible seems to depend on whether the voxl-vision system assumes that the drone's NED frame is the same as its own vision-based NED frame (in which case this wouldn't work out-of-the-box), or if it can pull in the drone's NED frame and relocalize based on this.

In short, my question is: Will Apriltag relocalization still work if I use GPS for the PX4 EKF, assuming there's a known tag in view?

[Cliff Wong]

Modules such as Precision Landing in PX4 perform a indirect correction using relative camera coordinates translated to the global frame upon identification if the "tag/marker". I suggest you take a look into the Precision Landing module of px4 as an example in solving your problem. Of course that is using the IR markers available out there. Now we have internally modified that module for april tags in the landing case as well that will be available shortly.

This method corrects the drone relative to actual ground truth (i..e the ARtag/IRTag), but does not correct the Global Frame position itself as its used as a relative offset to correct its setpoint position. That offset could be used to correct the global frame position itself (doesn't necessarily exist in PX4), but then again it would be better to transmit the actual GPS position of the tag to account for the curvature of the Earth, etc... and then calculate a proper offset.