Rovers RTK system and Calibration

Hi, friends,
Hope you all good.
We are trying to develop an agriculture rover based on ardupilot. I have some queries about the rover:

  1. I am using HERE 3 GPS for the compass and GPS. In copter, I know how to calibrate the compass, but in rover how can I calibrate the compass(rover weight above 100kg)?
  2. How to set the rover heading?
  3. If I am using Here+ RTK, does the RTK base antenna has to be fixed, or is it not needed?

I really appreciate any help you can provide.

Google is your friend:

Let alone Here+ for RTK, need an F9P or equivalent.