Robot Localization: Different input sampling rates & acceleration in motion model

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Since we have different sampling rates (eg. we receive 100 IMU messages and 1 VO message in the same time window) the question arised whether this problem is handled within the package or not.

Sure. The package makes no assumptions about the data rates of the various sensors. All it sees is a queue of measurements that it has to fuse. For you, most of those will be quantities measured by the IMU, with a VO message thrown in once in a while.

We ask ourselves why the X and Y -Velocities are simply calculated by multiplying the X/Y acceleration times the delta without influence of any angle

The filter assumes that both accelerations and velocities are reported in the same body frame (the coordinate frame rigidly affixed to the robot's centroid). So linear acceleration just needs to be integrated to get velocity. If your IMU is mounted sideways, for example, the data obviously needs to be rotated to match the body frame. But all of that happens outside the kinematic model, before the data is fused.

The velocity is rotated when it is integrated into position, however, as is the acceleration.