Quaternions are a 4-parameter representation of attitude just as yaw, pitch, and roll are a 3-parameter representation. Yaw, pitch, and roll based attitude filters encounter problems commonly referred to as "gimbal lock" at certain orientations (e.g. +/- 90 pitch angle). We use a quaternion based attitude filter because it is continuous over a full 360 degree range of motion such that there are no limitations on the angles it can compute. However, the VN-100 has a built-in capability to output yaw, pitch, and roll angles from the VN-100 (the sensor automatically converts from quaternions and outputs yaw, pitch, and roll angles if these are preferred).