Communication satellites, or comsats, act like relay stations in space for television, phone, internet, weather data, and emergency communications. A ground antenna sends an uplink signal to the satellite, and the satellite sends a downlink signal back to another region of Earth. This makes it possible to connect places separated by oceans, mountains, or damaged ground networks.
Satellites are especially useful when direct cables or line-of-sight radio links are impractical.
Key Facts
- Geostationary orbit altitude is about 35,786 km above Earth's equator.
- A geostationary satellite has orbital period T = 24 hours, matching Earth's rotation.
- Signal travel time for one 35,786 km path is t = d/c, so t ≈ 0.119 s for d = 35,786 km.
- A complete ground-to-satellite-to-ground relay often has a minimum delay of about 0.24 s, not counting processing time.
- Photon frequency and wavelength are related by c = fλ.
- Received signal strength decreases with distance, following an inverse-square pattern: intensity ∝ 1/r^2.
Vocabulary
- Communication satellite
- A spacecraft that receives radio or microwave signals from one location and retransmits them to another location on Earth.
- Geostationary orbit
- A circular orbit above the equator where a satellite appears to stay fixed over one point on Earth.
- Uplink
- The signal path from a ground station or user terminal up to a satellite.
- Downlink
- The signal path from a satellite down to a ground station, receiver dish, ship, aircraft, or user terminal.
- Transponder
- An onboard satellite device that receives a signal, shifts or amplifies it, and retransmits it back toward Earth.
Common Mistakes to Avoid
- Thinking a geostationary satellite is motionless in space, which is wrong because it is orbiting Earth once every sidereal day while staying above the same equatorial longitude.
- Forgetting signal delay, which is wrong because radio waves travel at the speed of light but still need noticeable time to cross tens of thousands of kilometers.
- Drawing the satellite directly above any city on Earth, which is wrong because geostationary satellites must orbit above the equator, not above high-latitude locations.
- Assuming one satellite can cover the entire planet, which is wrong because Earth blocks the far side and polar regions are difficult to serve from geostationary orbit.
Practice Questions
- 1 A geostationary satellite is 35,786 km above Earth. Estimate the one-way signal travel time from a ground station straight up to the satellite using c = 3.00 x 10^8 m/s.
- 2 A TV signal uses a frequency of 12.0 GHz. Find its wavelength using c = fλ and c = 3.00 x 10^8 m/s.
- 3 Explain why geostationary orbit is useful for satellite television dishes, and describe one limitation of using that orbit for global communication.