HGAG Program Discussion   The HGAG program was a cooperative effort  between Jet Propulsion Laboratory (JPL) and Ball  Aerospace & Technologies Corp. (BATC). The over all design architecture and initial drive train  component sizing for the gimbal were established at JPL. The design was then turned over to BATC who  was responsible for the final detail design, produc tion, and test of the HGAG. A number of design  components (motors, harmonic drives, potentiometers, RF rotary joints and flex cables) were specified  early in the design process by JPL and they were delivered to BAT C as customer supplied components  for use in the HGAG design. 

HGAG Design Overview and Driving Requirements   The gimbal consists of two drive mechanisms mounted at 90 degrees to one another. The High Gain  Antenna is mounted at the end of the elevation drive and both t he elevation drive and antenna are  mounted in a cantilevered fashion on the azimuth driv e. This configuration accommodates the confined  packaging requirements for launch on the rover while providing the necessary field of view and rotation  angles for the HGA. The position of each drive is monitored using motor encoder information. In addition,  potentiometers are incorporated into each drive to pr ovide gross position determi nation in the case of a  power cycle condition during the mission. Figure 2 shows the HGAG with the HGA attached. 

Figure 2.  HGAG w ith HGA attached 

The assembly requires a locking mechanism to pr event drive rotation under launch and landing loads.  This was accomplished by incorporating a single pyrotechnic pin puller, with redundant NASA Standard  Initiator (NSI) charges, to lock both the azimuth and elevation drives. Once on Mars, the pin puller is fired  and the drives are freed to rotate away from their stowed position. During deploy ment, the azimuth drive  moves through a spring-loaded gate mechanism that closes  once the drive has passed. After it is through  the gate, the azimuth drive can no longer return to the original stowed position, protecting against  potential interference between the HGA and other ro ver components during elevation drive rotation. 

Because of the somewhat large angles of rotation (280 ° azimuth and 234 ° elevation), and the need to  pass power and signal electronics over both the azimut h and elevation drives, two twist capsules were  developed to house the flex cables. In addition, t he RF signal from the antenna also needs to pass  through the elevation and azimuth driv es while meeting a specified RF signal loss budget. Both the flex  cable twist capsule and the rotating RF system will be described in this paper.  

The Mars operating environment for the gimbal asse mbly provided specific challenges during design and  test of the hardware. The dusty Martian envir onment requires special attention to mechanism  contamination control, the therma l environment requires specific exte rnal surface treatments and heaters,  and stringent planetary protection guide lines dictated high bake-out temper atures prior to launch that  drove material and design choices. Drive testing was performed under expected operating temperatures,  atmospheric pressures, and loads. Table 1 summarizes the significant driving requirements for the HGAG  design.  238