ma3 absolute analog encoders, from usdigital
http://usdigital.com/products/ma3/
we've used them on all of our swerves, and started using them instead of pots for other functions like arm position and such, they've been very reliable for us.
111 has used them and has noticed some static issues as a warning though.
this year we had 4 on our bot, 1 on each side of the drive, and 1 on each sides bogey wheel. no issues with them