Conical scanning approach for Sun pointing on the CYGNSS microsatellite

The small scales of area, volume, and power of small spacecraft, such as NASA's 25-kg Cyclone Global Navigation Satellite System (CYGNSS) satellites, constrain the number of independent subsystems that they can support. Consequently, small satellites often require novel approaches to execute the same mission functions that a larger satellite can easily perform with familiar sensor, actuator and algorithm options. In the case of CYGNSS, the spacecraft must execute a Sun acquisition and pointing phase but the actuator suite does not include 2-axis sun sensors or rate gyros; two measurements that seem like obvious inclusions for the Sun acquisition task. Instead, during Sun acquisition, the CYGNSS attitude control system uses a limited actuator and sensor set consisting of three magnetic torque rods, a three-axis magnetometer, and Sun incidence-angle measurements from three solar panel faces. This paper describes the sensing and control algorithms implemented in CYGNSS flight software to acquire and maintain Sun pointing with the available measurements and actuators. The Sun pointing algorithm uses a conical scanning approach based on traditional RF pointing and target-tracking systems, which consists of two key control loops: (1) a rate loop, which initiates a body spin about the solar-array face axis, and (2) a slower angle controller that tracks the array power gradients measured over the course of the fast spin. A slew toward the peak power eventually drives the solar panel face normal to spin in a cone centered about the Sun vector. The Sun acquisition process has a large convergence basin, is stable in the Lyapunov sense, and demonstrates excellent performance behavior in simulation.