Add RTK in ArduSub

I have worked with QGroundControl, Rover and RTK GPS. Now i am trying to utilise RTK gps feature in ArduSub. If i connect Pixhawk running ardusub with QGC, No RTK base station is detected.
How can i enable RTK in ArduSub with QGC ?
Can anyone help me.

For ArduSub, better to ask at or