I'm studying the Technical Report of VINS-Mono: A Robust and Versatile Monocular Visual-Inertial State Estimator available at: https://github.com/HKUST-Aerial-Robotics/VINS-Mono/blob/master/support_files/paper/tro_technical_report.pdf
In section V, where Loop Closure is discussed is writen: "The IMU measurements render roll and pitch angle fully observable, so the accumulated drift only occurs in four degrees-of-freedom (x, y, z and yaw angle). To avoid importing spurious information, we directly optimize pose graph on these four degrees-of-freedom."
I'm having trouble to understand this, in my conception, imu noise should propagate in all degrees of freedom. This might have something to do with the gravity vector?
Thank You All