Gravitational Droop in Side Mounted Alt-Azimuth Instruments

Abstract

Surveying instrument such as theodolites and tracking interferometers are designed to be operated in an upright position. There is typically an alidade with a single azimuth bearing and a laser or telescope trunnion having two bearing for the elevation axis. When operated in an upright position the center of gravity (CG) is over the azimuth bearing and is essentially a compressive load. In a non-upright position the CG shifts off the bearing. Once the CG is not over the azimuth bearing the loads change to tension and shear loads. Depending on the bearing preload during its construction the bearing can "open up" and cause imperfection in the "vertical" axis through the instrument, in what herein is called "gravitational droop". This paper analyses gravitational droop which is a function of gravitational direction and azimuth angle, creating bias error in the measured space.