scope:

Description of Driving Event:

The transmit antenna on the Advanced Communications Technology Satellite (ACTS) has the capability to traverse in both elevation (north-south) and azimuth (east-west) directions in order to adjust the alignment between the transmit and receive antennas. The adjustment was to be accomplished by using a bi-axial drive assembly mounted between the transmit antenna and the spacecraft. The bi-axial drive assembly consists of two axial drive mechanisms mounted orthogonally. Each drive mechanism contains a stepper motor capable of rotating a minimum of 0.30 degrees in either direction, in steps of 0.0075 degrees +/- 0.0015.

The structural joint between the bi-axial drive assembly and the transmit antenna was designed to accommodate final adjustments for final positioning or alignment of the transmit antenna. The joint was held by friction with shims via four bolts through large holes to allow for adjusting or shifting of the transmit antenna for final RF alignments.

The mechanical joint between the bi-axial drive assembly and its mounting to the spacecraft allowed the transmit antenna to shift locations during launch aboard STS-51. The four bolts did not maintain the proper preload and the joint slipped during launch loading. This has reduced the available design adjustment range for the ACTS transmit antenna.