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In general, it can also be desirable to have tho OFS centered at the center of rotation of the aircraft, so that any rotation (yaw) can be rendered as pure rotation. If this is not possible, offsets for the optical flow sensor can also be added but they must be known to centimeter level. If you do not add offsets - the drone will attempt to rotate around the optical flow sensor, which may not be the natural center of rotation of the drone. This increases other sensor errors (especially GPS), and can cause unpredictable behaviour.
When pairing the optical flow with a rangefinder, it may be desirable to mount them close to each other (so data is more similar), but this is not a hard requirement.
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