Solar Array Drive (SAD) refers to the system that positions the solar panels on the satellite to efficiently convert s the sun light into electrical energy. This system is comprised of the Solar Array Drive Assembly (SADA) and the Solar Array Drive Electronics (SADE).

The SADA includes the stepper motor, drive gear assembly, Barring And Power Transfer Assembly (BAPTA) and the mounting hardware required to mount the solar panel to the body of the satellite. The solar panel is mounted to the SADA and allowed to move by means of the barrings in the BAPTA. It also includes a slip ring assembly used to transfer the power generated by the solar panel to the EPS bus. The drive gear assembly is installed to smooth the step movement of the motor and provide a steady constant travel through 360 degrees of rotation. Stepper motors are known for their ability to provide high torque and controlled movement, this makes them ideal for this application. Rather than mounting two motors in the assembly, to increase the reliability these stepper motors are designed with redundant coils, this effectively reduces weight and provides improved redundancy.

Each SADA is controlled by independent Solar Array Drive Electronics (SADE) unit that provides the interface between it and the EPS subsystem or flight computer. It is comprised of all of the circuitry required to generate and shape the pulses needed to drive the stepper motors. Telemetry monitoring sensors collect data such as current, temperature, position, speed, rotational direction of travel, step counts, and status indicating movement or stopped for the SADA.

In operation the rotation of the solar panel is started when the SADE is enabled and step pulses are received, movement stops between steps or when it has been disabled. Control of the direction of travel, can be commanded to rotate in either forward or reverse direction to allow for pointing position optimization. The SADE is normally configured to output a consistent train of pulses at the required intervals to maintain movement through the 360 degree rotation called Normal Mode. With the panel rotating less than 15 degrees each hour it is easily commanded at less than 5 times a second allowing for rest intervals between movement. Speed and torque can be adjusted by increasing or decreasing the rate of pulses per second (pps) sent to the motor. If the motor is normally driven at 5 pps then driving it at 25 pps will increase the speed by a factor of 5. This system is typically design for the solar panel to complete a full 360 degree rotation in less than 15 minutes at the highest pps command rate. When commanding the movement of the SADA, commands are processed by the SADE, stored and then pulses are sent at a 5 pps or 25 pps rate until the complete commanded step count is reached. At completion the SADE must be commanded back to the Normal Mode or transitions back automatically.