How to send pitch and roll values to Pixhawk using MAVROS

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Hi, I am implementing an anti collision system on a quadcopter. I have a sensor connected to my Odroid running MAVROS which is connected via serial connection to the Pixahawk with APM stack. I am able to connect to the Pixahawk, change modes and get all the information. Now when the sensor detects an obstacle I want the quad to move back.

• How do I send Pitch and Roll values.
• How to override the RC inputs when the object is detected

Thanks