Gimbal Configurations
A gimbal is a two or more axis mount. The configuration may be altitude over azimuth (Alt-Az), altitude over altitude (Alt-Alt), equatorial, or a combination of the three.
Examining the three configurations leads to the conclusion that they are really the same. Only the orientation of the gimbal referred to the fixed mounting surface (typically the earth's surface) is different.
Alt-Az:
This configuration consists of a structure where the azimuth axis is attached to the fixed reference point and the altitude (or elevation) axis is attached to the rotating part of the azimuth structure.
One rotation is about a vertical line (an azimuth rotation) and the second motion is about a horizontal line (the elevation rotation).
This configuration is used for surveying, landscape camera mounts, gun mounts, and is the typical way a person uses their eyes to look at terrestrial scenes.
Alt-Alt:
This configuration consists of a structure that rotates about a horizontal axis sometimes called the "X" or "Major" axis and, attached to the major axis, is another rotation stage whose axis of rotation is perpendicular to the major axis.
This axis is sometimes called the "Y" axis or "Minor" axis. Usually the axes are located North-South (for the major axis) and East-West (for the minor axis), but the azimuth alignment can be at any position. Rotation in either axis moves the payload in an up-down motion.
The typical use for the Alt-Alt configuration is for tracking artificial satellites.
Equatorial:
This configuration is a special alignment case of the previous gimbals where the major (or azimuth) axis is inclined at an angle from the horizontal equal to the site latitude and the azimuth alignment is true North.
This configuration places the major axis of rotation parallel to the rotation axis of the earth.
The typical use for the equatorial configuration is for astronomical telescopes because a simple rotation about the "major" axis (called the "polar" axis) counteracts the earth's rotation.