6.4 Typical Requirements and Design Considerations

For the spaceflight mission, the Communications system must be designed to support the data uplink and downlink needs of the payload and spacecraft bus. Requirements include technical specifications for the:

• Above all, the communications system as a whole must have a link budget that closes and, optimally, has a positive margin.
  • The main drivers in a link budget are power available for transmission, antenna gain (geometry and mass), the temperature of components, and orbit (losses). These parameters will be expanded upon in the section about link budgets.
  • Losses from the system come from the temperature of the components. The thermal subsystem may need to adhere to requirements imposed by the communications subsystem.

• Data rate between the spacecraft and ground station in downlinking payload data and uplinking mission operation commands or software. This metric is very much like the CDH requirement of throughput but instead of the payload to flight computer interface, the data rate is the spacecraft to ground station interface.
• Contact time with ground stations (and data rate) dictates the total data a spacecraft is able to downlink. Contact time depends on the number and location of ground stations, and the spacecraft’s orbit.
  • Given that the spacecraft can communicate with the ground, the payload generates a certain amount of data that is necessary to downlink to fulfill the mission. This higher-level requirement contributes to the data rate and contact time requirements.

• Directionality (omnidirectional vs directional) of the antenna dictates the attitude determination and control system’s obligation to point the spacecraft during communications. This pointing maneuver also affects the way we carry out mission operations.
  

• Sufficient noise or loss in signals is a requirement that could be imposed by the principal investigator. The communications specialist then needs to accommodate this noise or loss in the coding and modulation design.

Constraints in designing the communications system include:

• Radiofrequency allocated for spacecraft transmission is controlled and licensed by the FCC. This licensing process can be quite restrictive and tedious, which drives the design of the radio technology and the schedule.

Required Documents

• The time between deployment and radio frequency transmission is externally required by launch providers.
  • The NanoRacks External CubeSat Deployer IDD specifies in the Deployment Switches:
  • 4.1.4-5) The CubeSat deployment switches shall reset the payload to the pre-launch state if cycled at any time within the first 30 minutes after the switches close (including but not limited to radiofrequency transmission and deployable system timers).
• The number of ground stations you have access to and the level of access you have. Unless you built your own ground station, you will most likely need to utilize someone else’s ground station or network. Working with others has constraints that depend on their availability, their cost, and your relationship with the controlling agency.
  • International Ground Stations (IGS) Network operated by our US and International Cooperator (IC) ground station network
  • Amazon Web Services Ground Station under Amazon
  • SatNOGS, an open Source global network of satellite ground stations, owned and operated by the community

During manufacturing and assembly, you as the communications specialist need to handle multiple components, ranging from the communications board, antenna, amplifiers, radios, etc. You will likely work with the power systems specialist as the communications system is power-hungry. The handling of the communications systems follows best practices of electrical power system components.

During testing, the coding and modulation software must be loaded and tested with the hardware. To verify end-to-end communications between the spacecraft radio and your receiver, a proper distance is placed between the radio and receiver, and the losses that would occur between space and ground are simulated by attenuators, attached to either end [NASA MAVEN]. The signals are monitored on a computer to see 1) if signals are picked up and 2) the amount of loss in the received signals. Integrating this code into the general spacecraft software will involve the command and data handling specialist.

During transport and handling, the flight computer is off and self-contained within the satellite. There is no requirement the communications specialist needs to adhere to from this phase.

From the time of delivery through on-orbit deployment, the communications specialist must ensure orbit tracking and ground station software is ready to go. Once the spacecraft is deployed, the communications specialist should update TLE, orbital elements, into their ground station software so that the directional ground station can slew toward the direction of the spacecraft accurately. For omnidirectional or directional ground stations, updating the TLEs will inform mission operators of when the spacecraft is passing overhead within a communicable range.

Artemis Kit Specific

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