## 1 Introduction  
The generation of electric power on board a spacecraft is fundamental to guarantee the survival of the space system. Whenever a satellite is powered by exploiting the electricity generated from the solar arrays, the capability to orient them to the Sun is a vital requirement for the system design. Solar array orientation can be achieved with a solar array drive assembly (SADA), with a proper rotation of the spacecraft, or with a combination of these two. However, the SADA cannot fully orient the solar panel in three-dimensional space, and many spacecraft are even not equipped with these orientation devices. Thus, the solar arrays pointing to the Sun are commonly demanded to the attitude determination and control subsystem (ADCS) of the spacecraft, which rotates the satellite body in a way to generate the proper electric power with the available solar cells. Given the relevance of this ability for the maintenance of the spacecraft health status, the ADCS should be capable to orient the solar panels to the Sun even after multiple failures have occurred on-board. Hence, the most dramatic safe mode-or survival mode-should guarantee a minimum power generation to support the basic functions of the space system. For these reasons, the availability of a robust and fault-tolerant ADCS, with an emergency Sun pointing mode, is crucial for the resilience of the entire system with respect to unexpected circumstances. The paper presents an attitude determination and control subsystem design for solar arrays pointing to the Sun, with a minimal set of sensors and actuators. The sensors are limited to the solar arrays and to the angular rates sensing devices, and the actuators are required to provide a torque only along two orthogonal axes. The proposed ADCS exploits the current/voltage sensing capacity of the electrical power subsystem to measure the Sun angle with respect to the arrays normal. In fact, this research work considers a spacecraft with the solar arrays orthogonal to one direction in a body reference frame. As a consequence, the complete Sun direction unit vector cannot be estimated from the electric measurements. This assumption is motivated by the fact that common spacecraft have the Solar arrays deployed along a single axis of the spacecraft, with all the solar cells lying in a single plane. Even if this is not always the case, and additional secondary solar cells may be pointed in other directions, the larger primary solar panels area commonly has a unique normal direction. The determination and control algorithms compute the angle of the Sun with respect to the solar arrays normal and command the actuators to rotate the spacecraft in a way that this angle is minimized, having the Sun perpendicular to the solar arrays. The rotation of the spacecraft around the direction orthogonal to the solar panels is not controlled, being not relevant for electric power generation. The control algorithm is based on sequenced pulsed rotation commands modulating a proportional-derivative (PD) control scheme. The angular rate measurements are needed to have a proper derivative control implementation. The proportional control direction is unknown from the solar arrays data, and the algorithm is capable to find the necessary rotation until the correct Sun pointing is achieved. If the Sun is behind the solar arrays, a constant rotation is commanded until the Sun is in the visibility of the solar cells, producing the electric power that is needed to drive the proposed ADCS. The constant rotation is not commanded if the spacecraft is in eclipse, when the spacecraft is controlled to maintain null angular velocity. The methods to detect the eclipse condition are briefly described in the paper as well. The outcome of the research work is the sequenced and pulsed modulation algorithm, superimposed to a classic PD control, to implement an ADCS capable to orient the spacecraft solar arrays to the Sun by exploiting an incomplete Sun direction estimation.