Author: Alexander Doknjas

The Attitude Determination and Control System (ADCS) is what allows ORCASat to point in a specific direction while in space. This system is repsonsible for determining the spacecraft attitude (orientation in space), and if the spacecraft is not pointing towards Earth, the ADCS will then menuevre the spacecraft such that it is. It is a requirement of the ORCASat mission that the spacecraft will be pointing it’s payload towards Earth. Otherwise — if it is pointing away— the light emitted by the mission payload to calibrate telescopes won’t be visible!

The ADCS is also is also the only component of the ORCASat spacecraft which is not designed, assembled, and tested in-house at the University of Victoria. Our ADCS is a 3-axis stabilised, Y-Momentum system purchased from South African aerospace company CubeSpace. It uses a single momentum wheel in the Y-axis (hence the name) to reject disturbances on the other spacecraft axes. You can think of it as spinning a top/gyroscope on a table. If you tap or disturb the top, its angular momentum will reject the disturbance and keep spinning around its axis.

This video from the ESA shows how we can use a spinning momentum wheel to keep the spacecraft stable in space.

The momentum wheel is only one of the actuators on the spacecraft. It also has magnetorquers; electromagnets which apply a torque on the spacecraft when the magnetic field they generate reacts with Earth’s magnetic field. They are used to point the spacecraft in a specific direction while the momentum wheel helps to keep it stabilized and remain pointing in that direction.

Image 1: The Attitude Determination and Control System

In order to know where to point to the spacecraft, we first have to know where the spacecraft is currently pointing. We do this by using a sun sensor, a specialized camera that looks for the brightest object in space (our sun) and determines where it is with respect to the spacecraft, known as the sun vector.

The ADCS also includes a magnetometer, which measures the Earth’s magnetic field to help determine where it is. Lastly, it includes angular rate sensors to know how fast it is tumbling or spinning. We also have a Global Navigation Satellite System (GNSS) receiver on board (basically a fancy GPS) that lets us know where the spacecraft is positioned above Earth.

All of these sensors working in conjunction provide useful data to determine where the spacecraft is and where it is pointed. The ADCS uses the infomration gathered about its orientation to then automatically make adjustments to correct its orientation if it is not pointing towards Earth.

The ADCS is the single most expensive component in ORCASat. We have implemented measures to protect the ADCS from being damaged or compromising its performance while we use it in integration testing with other spacecraft systems. Currently, it is sitting in an enclosed fibreglass box, inside of our cleanroom at UVic. We avoid touching it at all costs. The ADCS might only cost $30,000 CAD, but if we break it, we don’t have money to replace it. So, the cost of breaking it would be the total project cost to this point. In the office we have a saying; it’s not a $30,000 ADCS, it’s a $1 million ADCS. That usually gets the attention of co-op students.