We are seeing that the drone starts to oscillate about 2-3 feet. I have contacted ArduCopter on this issue because I initially thought it was a Firmware GPS accuracy issue, bit they pointed me to you.
Here is their response from one of the developers
One of the issues is the marvelmind FW. It does not provide the RAW distances to the beacons so that we can feed them in our EKF. So until marvelmind fixes this (they know the problem) we can not do anything about it. And please change the title of your post to "Copter indoor Marvelmind accuracy"
Once they fix it, ping me and I will update the Marvelmind device driver.
Can you help me to fix this?