Gimbal lock

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Gimbal lock occurs when the axes of two of the three gimbals needed to compensate for rotations in three dimensional space are driven to the same direction.

For example, assume a level sensing platform on an aircraft flying due north has its three gimbal axes mutually at right angles, i.e., Roll, Pitch and Yaw angles each zero. If the aircraft pitches up 90 degrees, the plane's and platform's Roll axes become parallel to the Yaw axis, and changes about Yaw can no longer be compensated for. This problem may be overcome by use of a fourth gimbal, driven so as to maintain a large angle between Roll and Yaw gimbal axes.

A similar situation occurs with the use of the Azimuth angle (rotation clockwise from North) to define a direction. This works everywhere on earth except at the poles (latitude 90° north or south). Azimuth becomes meaningless there because the poles are singularity points, where all directions in terms of Azimuth are South (or North). In other words, if you were standing on the north pole, no matter what direction you turn your body you will always be facing south.