How to pick up a suitable CubeSat ADCS (Attitude Determination and Control System) components or an integrated system for your mission?
by Thomas Yen
In the world of engineering, we know there is no “best” solution, but only the “most suitable” solution. That’s why people used to build their own ADCS by customize space components or sometimes even develop their own attitude determination & control components.
However, as the space-agency-driven time has passed, the commercial players are now aiming for COTS (Commercial-Off-The-Shelf) attitude determination and control components. Even an integrated ADCS solution is available right now for shortening its mission development time and costs.
Therefore, we need to understand the specifications, pros & cons of these components & systems for a better comparison. Here are the suggested steps you could follow:
- Analyzing your mission requirements, then determining the pointing accuracy and slew rate of your ADCS.
- Analyzing your system requirements, then determining the volume, weight, and power consumption limitation of your ADCS.
- Analyzing your budget and schedule, then determining your affordable pricing range and lead time.
- Pick up a suitable ADCS component or system for your mission.
Part A. Analyzing your mission requirements, then determining the pointing accuracy and slew rate of your ADCS.
Pointing accuracy specification is usually determined by your functional payloads, which are often cameras (remote sensing satellites) or antennas (communication satellites). The former often requires a pointing accuracy of up to several hundred arcseconds [a]. The latter usually requires a ~1 degree pointing if an S-band or X-band antenna is adopted. However, if there is only a UHF and VHF antenna equipped on your satellite, rough pointing up to 10 degrees may be acceptable. (These antennas are often designed to be omnidirectional. However, communication quality might be influenced if satellite attitude is drifting)
[a] 1 arcsecond = 1/3600 degrees
Please noted that if there are optical payloads such as a nadir-facing camera or star trackers onboard, your ADCS will have to avoid them from letting the sun gets into their field of view (FOV).
Besides, solar panels, especially the deployable ones, should be pointed to the sun when extracting optimal power generating performance. When a deployable solar panel is adopted, a pointing accuracy for it to the sun is suggested to be better than +/- 10 degrees. As above, these are factors that one should be aware of while determining the pointing accuracy requirement of their mission. Let’s then talk about the slew rate.
Most of the time, we don’t want the satellite to have a body angular velocity larger than 5 degrees per second. (Relative to the earth-centered, earth-fixed frame) In some definitions, when the satellite rotates faster than this number, we call it tumbling. Therefore, your choice of ADCS slew rate should often be less than 5 degrees per second. Then, you may determine your required slew rate by checking how many specific locations your payload is aimed to point at in a given time.
Sometimes, integrated ADCS, reaction wheel, or magnetorquer suppliers may not specify their slew rate directly. You may calculate it via the moment of inertia (MOI) of your satellite and the maximum torque from the ADCS.
Part B. Analyzing your system requirements, then determining the volume, weight, and power consumption of your ADCS.
The above three factors are straightforward for understanding. The volume and weight limitation of the whole CubeSat could be referred to your launch provider (usually more accurate). If you haven’t chosen a launch slot yet, the CubeSat organization defines the CubeSat standard: (check figure B-1 for an example).
Figure B-1. Dimensional limits of a 3U+ CubeSat
Keep in mind that the supplier of your ADCS components or system will provide you with power consumption information such as “average power,” “maximum power,” “peak power,” “nominal power,” and “rated power.” These terms may refer to different definitions or testing conditions, so you got to get clear of its spec and confirm that your EPS (electrical power system) can afford such output, especially the transient output current.
Then, keep in mind that the ADCS has to be turned on 24 hours per day unless you planned to let your satellite enters into a sleep mode that even solar panel pointing is not required. Therefore, you have to estimate the power budget of your EPS to check if it can support enough current into your ADCS continuously.
Part C. Analyzing your budget and schedule, then determining your affordable pricing range and lead time.
Usually, CubeSat standard components (not only ADCS components but also other components for other sub-systems) have a lead time of 4 weeks. (some might be longer, of course). However, the integration time for you to build, program the algorithms, and test the system, could take more than two years. The exact time will depend on your team’s experience and the complexity of the ADCS you are trying to build.
The lead time of an integrated ADCS ranges from 6 months to sometimes even two years. Therefore, if you don’t plan to build your own ADCS, order your integrated ADCS as soon as possible after kicking off the execution of your CubeSat mission.
Figure C-1. Comparing different integrated ADCS products on commercial satellite component website like satsearch.co
Part D. Pick up a suitable ADCS component or system for your mission.
Usually, the pointing accuracy of an ADCS is determined by two factors: 1) attitude control accuracy and 2) pointing knowledge. The former is determined by the types and specs of the attitude actuator, and the latter is determined by the types and specs of attitude sensors. Let’s break them down for discussion.
There are two types of typical architecture for the attitude actuating system: 1) the type with direct-actuator only and 2) the type with both direct and in-direct actuators. The direct attitude actuator often refers to thrusters and magnetorques [b]; The in-direct actuator often refers to angular momentum exchanging devices such as reaction wheels, control moment gyro, or reaction sphere. The ADCS equipped with both actuators can usually achieve a control accuracy up to several hundred arcseconds level. In contrast, the direct actuator-only architecture can only reach +/- 5 degrees as its optimal control accuracy.
[b] Magnetorquer’s actuating principle is based on the interaction of coil magnetic field and earth’s magnetiic field. Therefore, only when your satelliite is orbiting on Low earth Orbit (LEO) or planets that has magnetic field, this kind of device might work.
However, these in-direct actuators can’t be used without the direct actuators because they usually have “saturation” issues. When the rotor can’t accelerate anymore, then these angular momentum exchanging devices can’t exert torque. Such a case requires a direct actuator to “de-saturate” the rotors by lowing down its speed without interfering spacecraft’s attitude too much.
The attitude sensors could also be separated into two different categories, image-based sensors such as earth sensors or star tracker and non-image-based sensors such as gyroscope, magnetometer, sun sensor, etc. The former has better accuracy up to arcsecond level but a lower update rate, usually around 1 Hz. As a result, these image-based sensors may fail to provide reliable information during high slew rate operations. The latter has worse accuracy. These non-image-based sensors have an accuracy of around 0.1 degrees under a very healthy calibration. However, a much higher update rate could be achieved. The exact number depends on your analog-to-digital-converter (ADC) on board, but hundred hertz of the update rate is often achievable.
By having the above knowledge, I believe you can choose your own ADCS components (if you want to build your own system) or systems (purchasing an integrated solution) comfortably!