What is ADCS?

The Attitude Determination & Control Subsystem (ADCS) is concerned with any sensors that allow the spacecraft to determine its position and orientation (jointly referred to as "pose"), as well as any actuators that can operate to modify these states.
For MARCO POLO, attitude determination and control are particularly important to ensure that the spacecraft is optimally oriented for each imaging and communication opportunity. Additionally, accurate satellite sub-point knowledge is required to tag each image for transmission and use later.

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Attitude (and Position) Determination Components

The Tyvak Intrepid platform provides many of the required sensors for attitude determination. Sun sensors and magnetometers allow coarse determination of the attitude of the spacecraft based on knowledge of the position of the sun and the Earth's magnetic field (well known in LEO). Additionally, the inclusion of an Inertial Measurement Unit (IMU) on the Intrepid platform, which consists of 3-axis accelerometers and gyroscopes, allows fine refinement of attitude estimates through strap-down integration. The use of these components, included in the Intrepid platform, allows significant cost savings over more costly methods of attitude determination, such as star trackers.

For position determination, a GPS sensor is added to the spacecraft. Since the GPS constellation is positioned at a much higher altitude, GPS position estimation operates in LEO in a similar manner to its operation on the Earth's surface.

CubeSat Kit GPSRM 1 GPS Receiver Module

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Attitude (and Position) Control Components

In order to ensure that the spacecraft attitude is well-known for accurate imaging, fine attitude control is required. The Tyvak Intrepid platform includes magnetic torque rods (or magnetorquers) embedded in the structure, which provide coarse attitude control. Magnetorquers leverage the Earth's magnetic field, so at any given moment, 3 orthogonal magnetorquers can only achieve 2-axis control, unable to provide control torque around the local magnetic field vector due to the cross-product nature of this phenomenon [2]. Over the course of an orbit, a pseudo-3-axis control can be established since the magnetic field orientation varies, but 3-axis controlled precise pointing control is not guaranteed for any instant in time [3].

Therefore, to supplement the attitude control system and provide finer pointing performance, a single fixed momentum wheel is also added. The wheel is oriented such that it can provide some torque in any of the three body axes to augment the magnetorquers, allowing full 3-axis control. Additionally, the magnetorquers serve to provide momentum wheel de-spin capability.

Additionally, a propulsion module is required for the constellation deployment phasing stage. The amount of thrust required is very small, just enough to change the orbit periods and allow the satellites to drift before returning to a circular orbit. A small velocity-change cold gas thruster was selected to minimize cost and toxicity or storage risks. This module is specifically designed for cubesat constellation deployment [4].

(Above) CubeWheel Small Momentum Wheel

(Below) Aerojet MP120 CHAMPS (MP110 image unavaiable)

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References -
[1] - CubeSat Kit GPSRM 1 GPS Receiver Module - cubesatkit.com
[2] - "Magnetorquer." Wikipedia - https://en.wikipedia.org/wiki/Magnetorquer
[3] - "Design and Analysis of Fully Magnetic Control for Picosatellite Stabilization" D. V. Guerrant. MSc Thesis submitted to California Polytechnic University, 2005
[4] - MPS-110 Cold Gas Propulsion System, Aerojet Rocketdyne - https://www.rocket.com/cubesat/mps-110