A mission needs an information path between spacecraft and Earth.

Communications carry commands and reports. Navigation estimates where the vehicle is and where it is going. Telemetry turns temperatures, pressures, voltages, positions, health indicators and other measurements into evidence the mission team can interpret.

• What can the spacecraft sense?
• What information can reach Earth?
• How accurately is position and motion known?
• What is delayed, missing or uncertain?
• Which measurements should change the mission decision?

This is the World Return channel: prediction meets measurement and the mission discovers whether its model still matches reality.