Satellite constellations are groups of spacecraft designed to work together instead of acting as single isolated satellites. In Low Earth Orbit, or LEO, satellites travel only a few hundred to about 2000 km above Earth, so signals can make the round trip much faster than signals to geostationary orbit. Large constellations such as Starlink use thousands of small satellites so that users in many locations can see at least one satellite above the horizon.
This matters because it can bring broadband internet to ships, aircraft, remote regions, and disaster areas where cables and cell towers are limited.
Key Facts
- LEO altitude is about 160 km to 2000 km above Earth's surface.
- Orbital speed in circular orbit is v = sqrt(GM/r), where r is distance from Earth's center.
- Orbital period is T = 2πsqrt(r^3/GM).
- Signal travel time is t = d/c, where c = 3.00 x 10^8 m/s.
- A satellite footprint is the area on Earth's surface that can communicate with the satellite above a minimum elevation angle.
- More orbital planes and more satellites per plane increase coverage and reduce gaps between satellite passes.
Vocabulary
- Satellite constellation
- A satellite constellation is a coordinated group of satellites arranged in multiple orbits to provide continuous service over a large region.
- Low Earth Orbit
- Low Earth Orbit is the region close to Earth where satellites orbit with short periods, usually between about 160 km and 2000 km in altitude.
- Phased-array antenna
- A phased-array antenna electronically steers its radio beam by changing the timing of signals from many small antenna elements.
- Inter-satellite link
- An inter-satellite link is a communication connection, often using lasers, that passes data directly from one satellite to another.
- Ground station
- A ground station is an Earth-based antenna site that connects satellites to the wider internet and network control systems.
Common Mistakes to Avoid
- Assuming one LEO satellite can cover the whole planet is wrong because each satellite only sees a limited footprint below it.
- Treating LEO satellites as stationary in the sky is wrong because they move rapidly and may cross the sky in only a few minutes.
- Ignoring latency when comparing orbits is wrong because a longer signal path increases travel time even if the data rate is high.
- Thinking laser links replace all ground stations is wrong because data still needs gateways to enter and leave the terrestrial internet unless both endpoints are served through the space network.
Practice Questions
- 1 A LEO satellite is at an altitude of 550 km. Using Earth's radius as 6370 km, find the orbital radius measured from Earth's center.
- 2 A radio signal travels 1200 km from a user terminal to a satellite and back down to a ground station over another 1200 km. Estimate the one-way propagation time using c = 3.00 x 10^8 m/s.
- 3 Explain why adding inter-satellite laser links can improve internet service over oceans and polar regions where ground stations are sparse.