6.3 General Design Process

Given the payload and housekeeping data necessary to downlink from the satellite, the COMMS specialist must define a communication architecture with many free parameters. This process follows:

1. Choose a radio frequency. This frequency dictates the maximum bandwidth available and depends on the class of the mission. Many CubeSat missions apply for amateur or experimental class frequencies.

2. Choose a modulation scheme. The modulation scheme determines the required signal-to-noise ratio.

3. Choose coding algorithms, which affect not only the signal-to-noise ratio but also the data rate.

4. Analyze the available contact time the satellite will get using the available ground stations, orbit, and constellation design if relevant. Contact time is directly proportional to how quickly spacecraft can downlink data and frequently mission operators can issue commands.

After defining multiple options for each aspect of the communication architecture, for each alternative:

1. Compute the required data rate from all sources (depending on orbit)
2. Use the link budget equation to size the antenna/transmitter so we have enough margin over the signal-to-noise ratio. This ensures all alternatives meet the requirements

Once alternative architectures are defined, use other metrics such as total subsystem cost or risk to select one alternative. Note: Often, this is an iterative process, and we may change our requirements depending on feasibility.