For Scout to auto-stabilize, it needs sensors to find its orientation in space. No one sensor can do this on its own. It takes several different sensors working together to calculate Scout’s orientation. The sensors used to calculate orientation are gyroscopes and accelerometers. More recently, magnetometers are being incorporated to calculate direction.
Conveniently, there are boards that incorporate all of these sensors together. These boards are called inertial measurement unit, or IMU. An IMU uses accelerometers and gyroscopes to measure and report the aircrafts velocity, orientation and gravitational forces on multiples axis.
Scout uses the ArduPilot Mega IMU shield. I chose this board because it was developed to work with the ArduPilot Mega board.