Streaming SIMSTATE over SR2 via TCP connection

I’m trying to simulate an optical range finder given the simulated state of the quadcopter.
To do this, I am trying to retrieve the simulated state using mavlink.
I am connecting to the SITL over TCP on serial port 2.
I set SR2_EXTRA1 to 10 Hz and I expected to receive a SIMSTATE message, however none are arriving.
Are there any other parameters I need to change?