A guide to selecting an ADCS for a Cube Satellite mission
by Thomas Yen
October 2021, a total of 5 ADCS suppliers around the globe have gathered together and shared their insights about this topic; Tensor Tech is one of them. This blog will summarize some of the points that I have talked about in this webinar. To watch the video clip, welcome to visit the youtube link posted below.
This guide is written from an attitude determination and control subsystem (ADCS) supplier’s point of view. It aims to provide essential information for those system engineers seeking to define the specification of their ADCS or trying to build one.
Figure 1. Objectives of (System) Engineers
There is a mission goal for every satellite mission. Once the mission coordinator has defined such a goal, the system engineers can define the system requirement for this particular satellite or satellite constellation. People usually define the system and subsystem requirements in their system design review (SDR) process. For preparing the SDR, it is crucial to be aware of the trade-offs and limitations of the subsystems.
First of all, the characteristics and the targeted performance of the payload define almost everything. It includes the data rate (downlink and uplink budget), pointing requirement, and power consumption. These three criteria are connected to the subsystem requirement for the communication subsystem, ADCS subsystem, and the power subsystem.
Usually, we define the data rate first so that the type of antenna can then be chosen. The antennas often have a half-power beamwidth (HPBW) specification unless it is an omnidirectional one. It leads us to be aware of the pointing requirement for the communication subsystem. To define the pointing requirement, we furtherly need to know the absolute pointing error specification and the relative pointing error specification of the payload. (the latter usually only exists for remote sensing satellites) Lastly, after knowing the power consumption for each subsystem on the satellite bus and the payload, the specification for the power subsystem can be defined. Iteration may be required when we find some subsystems cannot satisfy the requirements made by the previous subsystems. 
Figure 2. How to define the pointing requirement for a CubeSat mission?
If your payload requires pointing information from the ADCS, define the pointing knowledge carefully to make sure it can help you reach the goal. Otherwise, the process for defining your ADCS requirement will be pretty straightforward. If it is a communication satellite, all you need to consider is the antennas’ smallest HPBW. Deployable solar panels add another pointing requirement for your ADCS if you do have one.
A remote sensing satellite usually considers the relative pointing error (RPE) and absolute (APE). People often define a certain period to measure the drift of a satellite’s attitude. This drift is the so-called “jitter,” or the RPE.  The RPE defines by the Modulation Transfer Function (MTF) budget provided for satellite jitter. The APE of the ADCS of a remote sensing satellite is often defined by the field of view and the allowable budget that the image can lose around the targeted area.
Figure 3. Methods of building ADCS
After defining the specification for the ADCS subsystem, let’s realize one! As figure 3 shows. Here are four approaches that people often use. The first one is to build components and integrate them by yourself, which is the most time-consuming way. There were not many Commercial Off-The-Self (COTS) products available in the past days, making method 4 the only way.
The second approach is to purchase components from vendors and build your own ADCS. The greatest challenge is to develop the algorithms, calibrate, and test the ADCS. Therefore, companies or institutions have to hire some experienced engineers to do so. This is the most popular approach these days. However, the development cost could be high because of labor, equipment, and time.
Luckily, the so-called “integrated ADCS” has been commercially available these years. These integrated products include all ADCS components and algorithms altogether for their customers. All customers have to do is decide the correct attitude command to input into the ADCS.
In the last approach, people can always find a contractor for their satellite mission. These companies sometimes develop it into a “satellite-as-a-service” business model. Method 4 is usually the most costly way, and the customer might not be able to retain the know-how of building their satellite.
Figure 4. Things engineers should keep in mind while adopting method 2
Although four approaches are being proposed in the previous articles, We will focus on methods 2 and 3, the most popular ones.
If people are adopting method 2, several things should be kept in mind. While installing the components altogether, system errors happen because of the misalignment on the reference frames. Make sure to ask the component supplier the method of doing such a calibration. Furthermore, to debug and verify the performance of the ADCS, equipment must be built, and software has to be developed for simulating the space environment.
Method 3 is an emerging way of preparing an ADCS for a customer’s mission these days. It saves development time and labor cost, but the user cannot take control of the system configuration and get to know how the internal control loops work. People should think about it while making the system design, especially when the time is limited and there is no experienced ADCS engineer on the crew capable of integrating your one.
Figure 5. Pros and Cons of adopting method 3