How a satellite works

Power, pointing, propulsion, talking to Earth — and what finally kills a satellite.

1. What are a satellite's bus and payload? — The payload is the part that does the satellite's job, such as a camera or radio; the bus is everything that keeps the payload alive and pointed.
2. Where does a satellite get its power? — Almost every Earth-orbiting satellite turns sunlight into electricity with solar panels and stores some of it in batteries for when it is in Earth's shadow.
3. What happens to a satellite in Earth's shadow? — In Earth's shadow its solar panels produce nothing, so the satellite runs on batteries charged in sunlight until it comes out again.
4. How does a satellite know which way it is pointing? — Sensors measure the satellite's orientation against the Sun, Earth and stars, and wheels, magnets or thrusters turn it to face the right way.
5. How do reaction wheels, gyros and star trackers work? — Star trackers and gyros measure which way a satellite faces; reaction wheels spin one way to turn the satellite the other way, with no fuel used.
6. What do a satellite's thrusters do? — Thrusters are small rocket engines that raise a satellite's orbit, keep it in its slot, turn it and finally move it out of the way at end of life.
7. What is the difference between chemical and electric propulsion? — Chemical thrusters burn propellant for a strong, short push; electric thrusters use electricity to fling out gas very fast, giving a tiny push but using far less propellant.
8. How does a satellite stay at the right temperature? — Blankets, special paints, radiators, heat pipes and small heaters balance the Sun's heat against the cold of space so that every part stays within its safe range.
9. How does a satellite talk to Earth? — By radio: antennas on the satellite and on the ground send and receive signals that carry the satellite's data down and commands up.
10. What are satellite frequency bands like L, S, C, Ku and Ka? — They are named slices of the radio spectrum; lower bands pass through rain easily, higher bands carry more data but fade in heavy rain.
11. What are telemetry and telecommand? — Telemetry is the stream of health readings a satellite sends down; telecommands are the instructions engineers send up to control it.
12. What is a ground station? — A ground station is an antenna site on Earth that tracks satellites, receives their data and sends them commands.
13. Why do navigation satellites like NavIC carry atomic clocks? — Your receiver works out distance from how long each satellite's signal took to arrive, so the satellites must keep time to billionths of a second — only atomic clocks can.
14. How long does a satellite last? — Most are designed for a set number of years — roughly 5 to 15 — but the real end comes when propellant, power or a key part runs out.
15. What kills a satellite? — Satellites die when a vital part fails, when propellant or power runs out, or when the launch or an early manoeuvre goes wrong.
16. What is the difference between launch mass and dry mass? — Launch mass is the satellite fully fuelled at lift-off; dry mass is the satellite without its propellant — often less than half the launch mass.
17. What are ISRO's satellite buses I-1K, I-2K and I-3K? — They are ISRO's standard satellite platforms, named for their mass class — about 1,000, 2,000 and 3,000 kg — onto which different payloads are fitted.
18. What Is the International Space Station (ISS)? — The International Space Station (ISS) is a large crewed spacecraft orbiting Earth at about 408 km.