When the article refers to an IMU, they’re referring to a combination of sensors; most commonly on Earth that’d be a 3-axis gyro, a 3-axis accelerometer, and a 3-axis magnetometer (compass). The problem with just using that is accumulated error and drift. On super small aircraft like this one, we usually use MEMS parts instead of big spinning physical gyros, and they don’t have great long-term performance.
MEMS Gyros measure angular rate, not absolute angle; to compute an actual angle, you’re taking the integral of the rate from t=0 to now. Any small errors in the measurements add up quickly to give you completely nonsensical results. For drones-on-Earth, we use a variation of the Kalman filter to combine short-term and long-term measurements. As an example, an accelerometer requires a double-integral to turn into position, so errors accumulate very quickly, but we can correct those errors using GPS. The accelerometer and its integrals give us really quick acceleration, velocity, and position updates (at, say 500hz), and then the GPS is used to correct the long-term position and velocity (at, say 5hz).
MEMS Gyros measure angular rate, not absolute angle; to compute an actual angle, you’re taking the integral of the rate from t=0 to now. Any small errors in the measurements add up quickly to give you completely nonsensical results. For drones-on-Earth, we use a variation of the Kalman filter to combine short-term and long-term measurements. As an example, an accelerometer requires a double-integral to turn into position, so errors accumulate very quickly, but we can correct those errors using GPS. The accelerometer and its integrals give us really quick acceleration, velocity, and position updates (at, say 500hz), and then the GPS is used to correct the long-term position and velocity (at, say 5hz).