What is a CubeSat?

What is a CubeSat?

If you’re here, it’s likely you’ve heard of cube satellites, or CubeSats for short. But what goes into a CubeSat? What distinguishes it from larger satellites that you see, like the James Webb telescope or the ones that make up the GPS system?

Essentially, it all comes down to size. A CubeSat is basically a baby satellite, usually between 1 and 5 kilograms (though sometimes larger), designed and built to conform to the CubeSat standard. First published in 1999 by a group of researchers at CalTech, the standard defines a whole host of things from the size and shape of a CubeSat to the mass, electrical characteristics, and even provides standards for included propulsion systems and materials that should make up the structure (the complete, most recent version can be found here).

The reason for defining all these things is to encourage compatibility and standardization. When more satellites have similar requirements, it is easier to build components such as computers, radios, and batteries that can be used by multiple different satellites. This leads to more prolific mass-manufacturing of components, bringing costs down for organizations that may want to build and launch a satellite but are unable to afford the non-recurring engineering costs required to develop a system from scratch. In addition, it simplifies launching the satellites, since they all have similar size and mass. This allows for standardized deployment mechanisms and loading configurations on rockets, further bringing costs down.

What Makes Up a CubeSat?

There are 5 standard sizes for CubeSats set by the CalTech standard, each defined in terms of the amount of volume and mass they take up. Choosing the correct size is one of the first and most important things your team will need to do as CubeSat engineers. It will likely come down to 2 things, the engineering requirements of the payload(s) you are planning to fly and your team’s budget, though other factors may come into play as well. The table below shows the typical CubeSat configurations covered by the CubeSat standard.

CubeSat TypeVolumeMassConfiguration
1U10 cm x 10 cm x 10 cm1.33 kgVery Common or Easy Difficulty
1.5U15 cm x 10 cm x 10 cm2 kg
2U20 cm x 10 cm x 10 cm2.66 kgUncommon or Moderate Difficulty
3U30 cm x 10 cm x 10 cm4 kgRare or Very Difficult
6U30 cm x 20 cm x 10 cm8 kg
Table 1: Sizes of CubeSats defined by the CubeSat standard

Sometimes, you’ll hear of even larger configurations like 12U or 18U. These typically involve the stacking of multiple 6U volumes together. Smaller configurations exist that are a fraction of a unit, like 1/2U or 1/3U. We won’t focus too much on these in this article, though many of the principles you learn on Crash Cube can be applied directly to larger and smaller satellites as well.

Like all satellites, a CubeSat is a system that can be broken down into many smaller subsystems, each with a specific purpose. When discussing subsystems, it’s useful to use a hierarchical structure to examine what each is describing. This structure doesn’t follow any hard and fast rules and may differ depending on where you learn it. For the sake of consistency on this site, however, we’ll try to stick to it as often as possible.

\small
System \rightarrow Subsystem \rightarrow Unit \rightarrow Subassembly \rightarrow Part/Component
\\
CubeSat \rightarrow ADCS \rightarrow Reaction \, Wheel \rightarrow Electronics \, Board \rightarrow Capacitor

A part (or component) is the smallest piece of a CubeSat. These will typically be the raw materials you purchase from a manufacturer, such as a capacitor that goes on a board, a piece of the metal structure, an integrated circuit chip, or a screw that’s used to attach pieces of structure together.

A subassembly is the next level up. It is an agglomeration of parts that forms a recognizable, self-contained part of the satellite. A subassembly will typically serve a specific function as part of a larger unit, and often can be built and tested separately from the rest of the unit. The electronics board is a subassembly of a reaction wheel, along with the wheel itself, the motor, and the mounting bracket.

A unit is one stage up from a subassembly. It forms a recognizable part of a subsystem that can be built and tested independently of the subsystem itself. An example of a unit would be a computer, reaction wheel, or radio receiver. Sometimes a unit will not have subassemblies. For example, while a reaction wheel may have several subassemblies, a radio receiver may just be a single PCB populated with components.

A subsystem is a collection of different units that serve a role within the satellite. The various subsystems are defined below in the next section. Each of them will have different requirements placed upon them (and therefore be designed differently and composed of different units) based on the mission. An example of a subsystem is the Attitude Determination and Control Subsystem (ADCS), which can be made up of reaction wheels, magnetometers, magnetorquers, star trackers, sun sensors, and more (or less!).

The system is the highest level subdesignation that’s part of a satellite mission. This would be the CubeSat itself in most cases. The ground station is an example of another system that’s part of a satellite mission.

CubeSat Subsystems

Despite its small size, a CubeSat contains many different subsystems that each do something important to ensure that its mission can be completed, and often it’s impossible for one person to be completely familiar with the inner workings of all of them. A team of engineers is therefore needed, but to make sure that all the subsystems function together properly, one (or more) people typically take on the role of a systems engineer. The systems engineer’s role is to define concrete requirements for each subsystem and the system as a whole to ensure that they work seamlessly together to accomplish the mission.

Each subsystem will also typicaly have a lead engineer, who reports to the systems engineering team. Below is a list of the most common subsystems that make up most CubeSats.

ADCS, the Attitude Determination and Control System

For a lot of satellites, the objective is to point a device (like a telescope or other sensor) at something worth observing. The Attitude Determination and Control Subsystem is where this happens. Attitude is the orientation of the spacecraft, and therefore which way it is pointing (but not concerned about the translation of the spacecraft). For this subsystem, we need to measure (determination) how the spacecraft is oriented using a variety of different sensors and feed these results into actuators that change (control) the orientation. The complexity of the ADCS hardware and software varies greatly by mission; some require redundant parts and high precision sensors, while others might forego an ADCS entirely. It depends on what the payload and mission objectives are for the spacecraft.

Communication

For any satellite, it’s important to be able to tell it what to do, and to be able to send any data it collects back to the Earth. The Communication subsystem is all the components directly related to this goal, including receivers, transmitters, transceivers, cables and waveguides, and antennas. Some CubeSats may even use optical communication systems, which would include lasers, receiving diodes, and optics. It’s important to note that if your payload is a communication system, that will typically be classified as the payload subsystem.

Computer

The brain at the centre of your satellite is a computer. Data collection and processing from the payload can occur here, which is then stored in onboard memory or passed to the communication system for transmission. Housekeeping telemetry, which lets satellite operators know the health of the satellite, is also collected by the computer from each subsystem and their components. The computer must also act on any commands sent by operators, such as to take a picture or restart all systems.

Payload

Payloads come in several forms, but they are there to serve the main purpose of the satellite. Whether the objective is to measure greenhouse gases in the Earth’s atmosphere or relay communications to a remote receiver, the payload defines whatever hardware and software is needed to meet these objectives. Examples of payloads include telescopes for Earth observation missions, communication systems for satellite-based internet and communications, platforms for onboard biological and chemical experiments, and possibly any other device that needs flight testing.

Power

The power subsystem is all the components related to electricity generation, conversion, storage, and distribution. Most CubeSats generate power using photovoltaic cells (solar panels) attached to the body, though some will also use larger deployable panels to generate more power. Power is converted between voltages by using voltage converters and stored in a battery. Switches are used to turn on and off various subsystems, units, and components based on the needs of the mission, and wires carry electrical current from the power subsystem to the other subsystems.

Propulsion

Many CubeSats will not use propulsion systems, but for some specialized missions they are necessary. Propulsion is used to adjust the orbit of the CubeSat. It comes in several varieties including solid, liquid bipropellant, liquid monopropellant, cold gas, and electrical. Cold gas and, to a lesser extent, electrical propulsion systems are far and away the most common on CubeSats due to simplicity as well as safety and regulatory concerns.

Software

The software subsystem includes all the code that is written to control computers and microcontrollers located throughout the satellite. Typically this will primarily consist of the code for the main onboard computer, but depending on how your team is distributed may also include microcontroller code for other subsystems and/or firmware for FPGAs (Field-Programmable Gate Arrays) if you use them.

Structure

Holding the spacecraft together is the structure. This structure protects all other subsystems from the vibrations of launch and provides the precise positioning to ensure everything fits and is oriented properly. The structure also serves as a primary conduit for heat transfer and as the common ground for all electrical systems. Mechanical (i.e. moving) components also fall within the structure subsystem, and can include deployable antennas, solar arrays, and payloads.

Thermal

The Earth orbit environment can be challenging for temperature control. In the vacuum of space, no convection heat transfer is available to cool the spacecraft’s electronics, so engineers must manipulate conduction and radiation to ensure the system survives. By understanding and selecting the conductivity, absorptivity, and emissivity of the spacecraft, thermal engineers can regulate the spacecraft temperature. In some cases, passive and active thermal control systems are required, however this is typically reserved for specialty payloads and extreme orbits.

References

  1. The CubeSat Program, CalPoly SLO. CubeSat Design Specification Rev. 13, California Polytechnic State University, San Luis Obispo, CA, USA. Feb 20, 2014. Accessed on: Feb 03, 2020. [Online]. Available: https://www.cubesat.org/s/cds_rev13_final2.pdf
  2. The CubeSat Program, CalPoly SLO. 6U CubeSat Design Specification Rev. 1.0, California Polytechnic State University, San Luis Obispo, CA, USA. June 7, 2018. Accessed on: Feb 03, 2020. [Online]. Available: https://www.cubesat.org/s/6U_CDS_2018-06-07_rev_10.pdf
  3. Pelton J., Finkleman D. (2020) Overview of Small Satellite Technology and Systems Design. In: Pelton J. (eds) Handbook of Small Satellites. Springer, Cham. https://doi-org.myaccess.library.utoronto.ca/10.1007/978-3-030-20707-6_7-1

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Ian Bennett
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Thomas Sears
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