Understanding of RC topics over mavros

Hi all,

I am trying to sned a RC message using offboard board for a 8 channel RC signal. however, this was not possible. Hence I have setup a SITL with keyboard based joystick for PX4 to check the reading over the RC topic in mavros over the topic /mavros/rc/out

I am reading the values over the mavros and the values looks like this :

[900, 900, temp0, 1750, Throttle , temp1,  temp2, PITCH, 1750,0, 0, 0, 0, 0, 0, 0 ]

These values were taken in stabilised mode.

On the keyboard when I change Roll or YAW, I see temp0, temp1, and temp2 varying. PITCH is mapped to pitch on keyboard and Throttle to Throttle.

My questions are as follows:

  1. Could anybody tell me how to convert this 16 channel RC input into 8 channel RC input?
  2. Why there are 3 values mapped to ROLL and YAW?
  3. What do other constant values 900 and 1750 suggest?

Thanks in Advance