Attitude Determination & Control

Introduction

Attitude Determination & Control System (ADCS) is responsible for determining and controlling AcubeSAT’s attitude and orbit at any point during the mission in a safe and orderly fashion. 

The main ADCS operation modes include after-launch stabilizing (detumbling mode) and achieving nadir-pointing (nominal mode) while in orbit. Our nanosatellite shall be able to maintain the desired orientation ensuring continuous data transmission to the ground station via the patch directional antenna. 

The design phase has been successfully concluded with the Critical Design Review submission in January 2021.

Sensors

Controlling the satellite’s attitude presupposes knowledge of its current state. To determine the state, we acquire the essential information from the satellite’s environment through the following sensors.

  • Gyroscope
  • Two magnetometers
  • Fine sun sensor
  • Six Coarse sun sensors

These sensors are deemed sufficient to cover our mission’s needs, as we can fully define our orientation and angular velocity. We have chosen the above sensor configuration, due to its cost-efficiency and reliability, in accordance with the general cubesat-based philosophy.

Determination Strategies & Algorithms

To correctly interpret the acquired sensor measurements and accurately estimate AcubeSAT’s state, we need to develop a sensor fusion algorithm. To that end, an Extended Kalman Filter has been developed combining gyroscope, magnetometer, coarse and fine sun sensor measurements while also taking into account the dynamic and kinematic environmental model of space. However, the Kalman Filter requires an initial state estimation, thus we developed an SVD algorithm solving Wahba’s problem, combining both sun sensor and magnetometer measurements.

Actuators

Τo control the satellite’s attitude we are using actuators that produce the desired torque by exploiting the unique space environment.

  • Three Magnetorquers (1 per axis)
  • Reaction Wheel

Each magnetorquer creates a magnetic dipole that interfaces with the Earth’s magnetic field and produces the required torque. The reaction wheel, when accelerated, causes the spacecraft to counter-rotate, due to the law of conservation of angular momentum. We opted for magnetic actuation, because it is a low-cost and efficient and efficient way to control small satellites, which when combined with a reaction wheel, constitutes a robust solution to satellite control. 

Control Strategies & Algorithms

To correctly control the satellite’s attitude at any given time during orbit, we are developing controllers which calculate the optimal torque to be applied by the actuators. For each satellite operational mode, we have developed different controller designs, as presented in the figure below.

Operational Mode Controller
Detumbling B-dot
Nominal PD (quaternion-based)

The calculated required torque is then translated to the desired voltage in order to correctly command our actuators.

Orbit Propagation Strategies & Algorithms​

An on-board orbit propagator is used to determine AcubeSAT’s position and velocity at any point during the mission. The propagator’s on-board orbital elements will be initialized using TLE files.

Robotics

To describe the satellite’s attitude, we initially defined the relevant frames using rotation matrices and Euler angles. Due to singularities that are inherently present in Euler angles, a special type of 4-dimensional hyper-complex number system, named quaternions, is utilized instead.

Simulations

All of the previously described subjects have been developed and simulated by our team. The simulations are currently based on the MATLAB software and are openly accessible.