568 the velocity vector directions and lowers substantially the drag on the spacecraft which reduces the number  of propulsion burns required to maintain the SC in the desired altitude . In addition feathering also reduces  the solar array drive movement s resulting in less number of cycles and wear on the actuators . As can be  seen in Figure 16, the use of Profile K has so far resulted in savings of 14,000 cycles on the –Y actuator  and 23,000 cycles on the +Y one. On the other hand the reduction in propulsion maneuvers has already  resulted  in considerable savi ngs in propulsion fuel  to date which  will allow GPM to have a longer mission  life [5]. Currently the GPM Core SC is expected to be in service until 2035.  

Figure 17. On orbit Drag for Tracking Profiles G & K  

The behavior  of the actuator is monitored by  the Flight Operations Team at  the Missions Operations Center  located at GSFC . Temperatures, currents, voltages, and othe parameters are tracked and supervised to  ensure no anomalies occur, or should they occur, to generate the corresponding corrective action. The  performance of the +Y and – Y SADAs has so far been remarkable. As can be seen on  Figure 18,  Commanded vs. A ctual MAX and MIN differences are just 2 steps for both actuators . This is expecte d since  the command steps occur before the rotor motion. S o when the telemetry is sampled determines the  difference. A step difference of 6 or great er means that the motor missed commutation steps . The ±2 Min  & Max levels are consistent with ground testing and beginning of life which indicates the motors, encoder,  and drive electronics are all performing nominally . As shown also on Figure 18,  the motor current on both  +Y and –Y SADAs has consistently remained at approximately 300 mA . Because the motors are using a  voltage drive, the current being co nstant indicates that the motor back electromotive force and the  resistance are not changing. This telemetry shows that both SADAs have healthy windings and the  magnetics (motor constant) are nominal .  

The encoder consists of a metal disk with slots t hat block or allow light from a Light Emitting Diode (LED)  to hit a phototransistor . The 6 tracks include 3 tracks on the motor rotor side to indicate each step and a  once around index, and 3 on the harmonic drive gear output to indicate the output angle at unique locations .  The HEMI (Hemisphere) Encoder Track is dark for one- half of the range of motion, and light for the other  half. The voltage plots  shown on Figure 19 show how the maximum Voltage has remained steady at around  14.5 V for both the +Y and –Y SADAs  indicating that when the light is blocked, the phototransistor leakage  current is small and unchanged. The minimum voltage is also steady at around 0.2 V showing that the LED  is not dimming,  and the phototransistor current transfer ratios  are nominal . The LED driver current set point  has not needed to be changed due to aging or environmental degradation in the optoelectronic parts. Both  SADA motors are driven using a single drive card, the Mechanism Control Electronics (MCE), which  accepts the spacecraft bus voltage and a 1553 interface. The card sends an analog housekeeping packet  updated at 1 Hz that includes the secondary voltage rails, the LED currents for each motor, and several  thermistors on the card. The MCE also telemeters detailed status on the motor command steps, positions,  and commanded parameters at 10  Hz. The SADAs and the MCE telemetry continue to show consistent  performance from beginning of life to the present . As seen on Figure 19, t he –Y/+Y actuators  temperature  has also remai ned steady at an average of 14 and 11°C  respectively, indicating the thermal environment