Spacecrafts use different networks like TDRSS, DSN, ... to accomplish uplink/downlink. But how does it know which one to use during the mission? At what point does it find TDRSS more suitable than DSN (Brazil)? How does it know which TDRSS satellite to point its antennas toward?
In the event that a spacecraft has multiple communication systems, it can chose which one to use by a few ways.
- Test receiver by each network, and use the best one. This is pretty much what your cell phone will do when connected to both wifi and 4G/5G. Figure out which one is best, and use that one.
- Simultaneous communicate via both- Used for really mission critical stuff, primarily for human systems at critical moments.
- Time boxed- Use one system at one time, another at another time. Can also apply to location, use one system over one part of the globe and another system elsewhere.
- Condition based- If your pointing is great, use the high gain antenna. If it is not quite so great, use the low or medium gain antenna system.