3.2 Mission Components

The goal of the overall spacecraft mission engineering process (from the last chapter) is to define a mission concept such that the following components are defined [New SMAD]:

1. Subject

2. Payload

3. Spacecraft Bus

4. Ground Station

5. Mission Operations

6. Command, Control, and Communications Architecture

7. Orbit

8. Launch Segment

This course will center around the design of the spacecraft bus but will briefly discuss how the other components affect the spacecraft bus design.

Subject and Payload

The mission centers around the subject, which includes physical phenomena or objects, that the spacecraft needs to observe, discover, or manipulate; these subjects could include gravitational waves [GRACE], broadband internet frequency waves [Starlink], asteroids [OSIRIS-REx], or neutrons [Neutron-1]. We will assume that you discussed this topic matter outside the scope of this course, like in EPET 301: Space Science Instrumentation, or with your fellow scientists/technologists who are your customers. You may also find compelling science or technology missions in NASA’s decadal surveys, strategic plans and roadmaps, and the taxonomy report. The subject drives the orbit requirement if the spacecraft must be proximal to the subject source to detect the subject (like OSIRIS-REx, GRACE, and Neutron-1). The payload is the hardware or software that detects, measures, or interacts with the subject. If the subject is what, the payload is the how. There can exist different levels of payloads if we widen the scope of the system. For example, the payload on the Neutron-1 mission is a neutron detector but if we consider the rocket that launches Neutron-1 to space, the rocket’s payload is the entire Neutron-1 satellite. We will strictly focus on the payload within the spacecraft bus for which an entire section will be dedicated because the subject and payload drive the spacecraft bus design.

Artemis Kit Specific

Ground Segment

The ground segment consists of all the components that stay on the ground: ground station, mission operations, and command, control, and communications architecture.

A ground station is a “terrestrial radio station designed for extraplanetary telecommunication with spacecraft”. These ground stations communicate and control spacecraft by receiving dim signals from the spacecraft and transmitting powerful signals to the spacecraft. Large ground stations are commonly characterized by their parabolic dish, which is an antenna that offers high directivity. These large dishes, like NASA’s Deep Space Network, may also detect sensitive radio signals from astronomical radio sources; although today, the state-of-the-art astronomical discoveries are made with specific dishes that are significantly larger than dishes necessary for spacecraft communications.

Small ground stations typically consist of an array of dipole antennas, the most widely used class of antennas in everyday life. These dipole antenna arrays are omnidirectional, require less construction, and are more theoretically simple to understand, which are characteristics that make them more approachable for university teams. Both types of antennas may either be fixed or rotate to track a satellite; the larger directional antennas are less likely to dynamically track signals due to the complexity of the dynamic system to support such an immense mass. The construction of ground stations is outside the scope of this course but you are welcome to reach out to HSFL to support your mission, schedule access through AWS Ground Station services, or participate in the SatNOGS, an open-source global network of satellite ground stations.

Mission Operations

The mission operations team can be separate specialists in a big program, like the NASA Curiosity rover operators that had to be specifically trained or can be the satellite design team for small projects. If you’re a student on a small team, you will most likely be one of the mission operators; who better to operate the satellite than one of the engineers who created the satellite? You will have the distinct advantage of knowing precisely the capabilities of the spacecraft without having an engineer to consult as that engineer is in your head! The mission operations software allows the operators to monitor the state of health of the spacecraft and relevant mission parameters, whilst also commanding the spacecraft from a computer. NASA has created Open MCT, a next-generation Web-based mission operations data visualization framework for desktop and mobile.

Artemis Kit Specific

Command, Control, and Communication Architecture

The command, control, and communication architecture is the interface between mission operations, your ground station, and the spacecraft. The architecture includes the telecommunications link between the spacecraft and the ground station, then the wires or wireless connections between the ground station and the mission control center. Ideally, the signal path is as uninterrupted as possible to provide real-time information and reaction with your spacecraft, critical for high priority missions, but for low priority missions, communications can be delayed. For example, you may rely on amateur radio or ham radio enthusiasts to pick up your signals and report them back to you. There is no guarantee that anyone radio operator will be listening or that they will send the packets to you if they pick them up.

Artemis Kit Specific

A critical decision in the communication architecture is the communication frequency and subsequent RF licensing to use that frequency band [CSLI Chapter 9]. The Artemis CubeSat kit can only get an amateur or experimental license and we will certainly attempt to guide you through the documentation to gain FCC licensing [FCC Guidance and Spectrum Guidance]. This step is infamously difficult and bottlenecked; the government is attempting to make this process easier from the top level down and we’re going to try to meet them in the middle by educating you from the bottom up.

Orbit and Launch

The orbit is the path of the spacecraft during its mission with respect to planetary bodies and astronomical references. A section is dedicated to describing various orbits and the subsequent space environment so this paragraph will serve as a brief, high-level overview. The orbit dictates the space environment that the spacecraft must survive and the launch vehicle that the spacecraft must interface with. Orbits’ characteristics vary the dominant physical phenomena in that space environment, which impose different technical requirements on the spacecraft subsystems to achieve the mission objective. The space environment may even affect the mission operations as there are time delays or blackouts in communication. The distance or delta-V the launch vehicle must provide to the spacecraft to achieve the desired orbit affects the size of the rocket and thus the size of the fairing on top of the rocket that the spacecraft must fit into.

Artemis Kit Specific

The 1U CubeSats are typically “integrated into dispensers on the ground, transported to the ISS in a pressurized cargo vessel (e.g., SpaceX Dragon, Orbital ATK’s Cygnus, etc.), and hand-carried onto the ISS from the cargo vessel. Astronauts aboard the ISS are responsible for deploying the CubeSats from the ISS typically 1–3 months after arrival” [CSLI Chapter 3.5].

Artemis Kit Specific

If you choose to launch outside of the CSLI ecosystem, like UNP, you can find other launch providers with their own environment testing standards that ideally are less rigorous than CSLI standards so that you do not have to redo all the environmental testing we did for you.

Spacecraft Bus

Upon setting out on a preliminary design, the suggested mass and power budgets to initially allocate for a non-propulsive spacecraft are as follows. A systems engineer typically keeps track of these budgets, along with budgets for pointing and alignment for ADCS, propellant (for a propulsive spacecraft), downlink and uplink for communications, and data usage for command and data handling. These budgets will be described in-depth in their respective subsystems.

CubeSat Design Specification Rev. 14