143 it was later found that meeting the specified requirement of no greater than 2 seconds ramping timing to  move the high-inertia Solar Array was extremely difficult.  Satisfying the Accel/Decel profile requirements while maintaining the necessary micro stepping resolution, and meeting the requirement to always finish motions at the cardinal step was very challenging. This requirement also presented complic ations in FPGA implementation of multiple microstep resolutions and rates. Although the problem was re solved for 90% of possible combinations of microstepping resolutions and step rates, the function ended up disabled as LRO’s FSW employs its own  acceleration and deceleration algorithms. In LRO’s case, hard-coded Accel/Decel profiles did not match with control algorithms and were not useful  as a tool to reduce SC disturbance.   In addition, LRO’s system utilized micro-stepping based on cardinal steps rather than pure micro-stepping; in the LRO system, the mi cro-stepping was incorporated within the cardinal step and was  required to stop at each cardinal step. Implementing Accel/Decel profiles in this system used a large  percentage of FPGA capabilities, and made design alterations extremely difficult.  Based on the difficulties encountered, and the ability to provide acceleration/dec eleration algorithms via  FSW, it may not be practical to hard code Accel/Decel profiles into the GCE.  Constant Current vs. Constant Voltage Drive

Although the use of a constant current mo tor drive in the system does offer some benefits, it is not clear if  these benefits outweigh the complexities they introduce  into the system, especially while meeting current  (and torque) requirements at Hot and Cold conditions with high set points.   Comparatively, the a constant vo ltage stepper motor drive delivers better performance at a given set  point; the power delivered to the motor in a “Cold”  environment is greater than that in a “Hot”  environment, providing more balanced torque margins at various ambient conditions.  The constant current drive also introd uces complications in accurate moni toring of the actuator current in  a pulse-width modulation scheme (as discussed earlier), due to the fact that the sense resistor does not measure the re-circulating ("freewheelin g") current flowing in the motor.   Taking these considerations into account, a consta nt voltage drive may be more appropriate for use in  similar conditions.  Two Encoder Current Options

LRO’s GCE design specification required two set points for encoder LED excitation. Although this was a proven and uncomplicated circuit implementation, it appe ars to be excessive requirement for a mission of  LRO’s duration and radiation environment, especially wi thout including the capability to measure possible  read-head performance degradation. Additional command and control functions required significant analysis and testing during GCE development and SC te sting. This requirement should be re-visited for  similar applications.   HGAS and SAS Flight Operation and Performance 

Both the HGAS and SAS subsystems were  shown to perform exceptionally  well from deployment through  slewing and tracking operations at various operational currents and stepping rates.   No Position Errors

No position errors were seen in any mode for either the SAS or HGAS gimbals from launch throughout  the commissioning phase. From HGAS and SAS deploy ment, through the first month and a half of  operation, the gimbals were operated at the nominal micro-step and current levels determined to provide  required torque margins, satisfy GEVS Gold Compli ance, and provide reliable operation in safe hold  mode. For the SAS, nominal micro-step resolution is 16 micro-steps per cardinal step at a Y-axis current  NASA/CP-2010-216272