Attitude Determination & Control System (ADCS) is responsible for stabilizing AcubeSAT after launch (detumbling mode) and achieving active nadir-pointing (nominal mode) during the mission. AcubeSAT shall be able to maintain this specific orientation ensuring that the patch antenna faces the earth. This is a crucial requirement set by COMMS, in order to be able to communicate with the ground stations on earth and transfer several images captured by the Scientific Payload’s microscope.

Sensors:

  • gyroscope and magnetometer integrated in an IMU
  • fine sun sensor
  • GPS

The measurements from the magnetometer and the sun sensor will be used to solve ‘Wahba’s Problem’ using SVD algorithm. To achieve better precision and to estimate AcubeSAT’s orientation an Unscented Kalman Filter will be used. GPS data are used to initialize orbital elements, which are needed for the sgp4 orbit propagator.

Actuators:

  • 3 magnetorquers (1 per axis)
  • Spin-Torquer

Magnetic actuation is an inexpensive and efficient way to control small satellites, while Spin-Torquer is used to provide robustness to the system. The torques needed to achieve the desired orientation are calculated via a B-dot (detumbling) and a PD (nominal) Controller during each phase of the mission. The Spin-Torquer’s angular momentum will be defined to match our pointing error requirements.

Main Requirements:

    • Orientation Error from Nadir: less than 15 degrees/axis. (COMMS)
    • Orientation’s Estimation Error: less than 3 degrees/axis. (ADCS)
    • Detumbling Time: less than 6 orbits. (ADCS)
    • Ability to detumble initial angular rates: up to π/2 rad/s. (ADCS)
    • Commanded Magnetic Dipole Moment: less than 0.2 A*m^2. (Hardware)
    • Slew Maneuver: less than TBD rad/s. (Hardware) 

  • Power Consumption: less than 1 Wh/orbit. (EPS)