# Micro-Satellite Systems Design, Integration, and Flight
## 4 Attitude Control System (ACS) Development
### Duties of the CubeSat ACS  
The CubeSat deployer does not provide stable ejection when it releases the CubeSat from the International Space Station (ISS). The CubeSat's initial spin is unknown. However, the CubeSat deployer ensures that the CubeSat's angular speed will not exceed five degrees per second. It is crucial to reduce any remaining momentum, as it would make connecting  
Figure 27: Satellite height as a function of TinyGS transmission timestamps.  
\begin{table}
\begin{tabular}{c l l l} \hline \hline
**ID** & **Requirement** & **ID** & **Requirement** \\ \hline
1.0 & The system SHALL detumble to 10\% of the & & The Teensy flight computer SHALL be able to \\  & initial angular velocity in the x and y direction & & handle the ACS computations \\ \hline
2.0 & The z angular velocity SHALL spin stabilize & & & The system SHALL calibrate the IMU against \\  & within 10\% of omega\_final = [0 0 1] rad/s & & & hard iron offsets caused by internal electronics \\ \hline
2.1 & The system SHALL spin stabilize within 8 hours & & & The system SHALL calibrate the IMU against \\  & & soft iron offsets caused by actuators & \\ \hline
3.0 & The system SHALL point its z axis tangent to the & & & The system SHALL calibrate the IMU against \\  & Earth’s surface a majority of the time & & & temperature offset effects \\ \hline
4.0 & The controller SHALL not use more than 0.9 & & & \\  & watts (0.2 amps at 4.2–3.7 volts) & & & \\ \hline \hline \end{tabular}
\end{table}
Table 5: Requirement table for the CubeSat ACS subsystem.  
with the CubeSat difficult. Requirement 1.0 in Table 5 specifies that the leftover momentum shall be reduced by 10% of the leftover angular speed in the x and y directions. Figure 28 illustrates the CubeSat's ejection.  
At the same time, the CubeSat is made stable by spinning. When the CubeSat rotates around its largest axis of inertia, it can resist any kind of disturbance. Also, it ensures that the light sail will have the same stability after it comes out. A common example of spin stabilization is how a frisbee keeps its balance by spinning when it is thrown. The frisbee can stay stable around its largest axis even if there is wind or other interference. Requirement 2.0 states that the final spin should be 1 rad/s around the z axis, with a margin of 10%. Having this spin condition for both the CubeSat and the light sail will ensure stability. As a result, stability improves the reliability of the communication between the transceivers on the ground station and the satellite. Another advantage is that the steady rotation will provide active cooling for the CubeSat. Spin helps prevent any one side from being exposed to the sun for too long. Figure 29 shows how spin stabilization will be achieved for both the CubeSat and the light sail.  
Figure 28: CubeSat detumble after deployer ejection. The blue, red, and green arrows symbolizes the initial spin of the satellite along its three respective axes. Image Credit: [PERSON].  
Figure 29: (**a**) CubeSat and light sail spin stabilization; (**b**) CubeSat achieves pointing. Image Credit: [PERSON].  
The last essential part of the ACS is to orient the satellite. Table 5's requirement 3.0 aims to make it more likely that the \(z\) axis will point to space. A space-pointing \(z\)-axis ensures good radio signal strength to the ground and would also assist with the light sail deployment. Both the positive and negative Z sides of the CubeSat must be kept at right angles to Earth's surface. Figure 29 above shows the desired pointing of the CubeSat.