Velocity without gps

Hi everyone, i’m using gazebo-sitl and python.

I have a problem. Let me give an example, i send roll (rc 1 1800) for 1 second in alt_hold mode and then i set roll angle to 0. In this case, the drone has an acceleration to the right but roll angle is 0. How can i measure or get this data? Actually I want to write a pid controller to set the acceleration to zero on the roll axis. I don’t have an idea data format, it may a raw data and i need to use a math formula or can i get this data directly?

Thanks