The JCMT is an alt-az telescope supported by a central bearing and 4-wheels
rolling around a broad 14-segment track.
If the track was flat and horizontal, the telescope pointing would be
dependent only upon 7 parameters describing errors in the construction and
alignment of the antenna.
In reality, however, the track is irregular in profile, and as the antenna
moves over it the 4 supporting wheels will each move up or down. The two
wheels on each side are part of an A-frame, whose apex is one end of the
elevation axis. Motion of the wheels over the irregularities in the track
thus causes the A-frame to rock, forward or backward, in the plane
of the A-frame. (That this is only part of the story is revealed by
the new inclinometer system, as described later).
This causes the elevation axis to be moved in the (aero-)nautical directions
of roll (the right end of the elevation axis rises w.r.t. the left, or
vice versa), yaw (the right end moves forwards w.r.t. the left, or
vice versa), and pitch (in the same sense as a rotation of the
elevation axis). These effects combine to move the telescope beam in azimuth
and elevation.
The inclinometry project aims to correct the pointing of the JCMT for the
effects of the track profile.