Lesson 05 · 4 min · 6 things to do
Why everyone wants the same orbits
Match an orbit to the job it is uniquely good at.
A television satellite appears to hang motionless above one point on Earth. How?
- Yes.It is moving at over 3 km/s. It only looks still because the ground below is turning at the same angular rate — which is why a dish can be bolted in place and never moved again.
- Not quite.It uses small nudges to stay on station, and the position is maintained by the orbit, not by fighting gravity.
- Not quite.Gravity at that height is about 3% of the surface value — weak, and doing all the work.
There is no best orbit. Each altitude buys one thing and gives up another, and the good ones are crowded because the trade is so clear.
Match the job to the orbit that suits it.
Detailed imaging of the ground.
Continuous television broadcast to one region.
Internet with low delay.
Weather pictures of a whole hemisphere at once.
A crewed station that can be resupplied cheaply.
Yes.Low orbit is close, cheap and fast-moving, so it sees detail and is never over one place for long. High orbit sees a third of the planet at once and never moves relative to the ground. Almost every satellite design starts from that trade.Slide the altitude and see what the orbit buys.
550 km20,000 km36,000 kmSignal delay, there and back4 msFrom 36,000 km, for comparison240 ms550 kmCheap to reach, tiny signal delay, sees fine detail — and passes overhead in minutes, so you need hundreds of them.
Needed for global coverage24 satellitesNeeded from 550 km1,500 satellites20,000 kmTwelve-hour orbit. A few dozen cover the whole planet continuously — this is how satellite navigation works.
Needed for near-global coverage3 satellitesNeeded from 550 km1,500 satellites36,000 kmStays over one spot forever. Three cover nearly the whole planet — and every signal takes a quarter of a second.
Why did anyone build the thousand-satellite version when three would do?
- Yes.Light takes about a quarter of a second to go up and back at that height. For television nobody notices; for a video call or a game it is the difference between working and not.
- Not quite.Each launch is cheaper and you need hundreds of them, replaced every few years. The reason is the delay, not the bill.
- Not quite.The geostationary belt is genuinely congested, and it is a constraint on new slots rather than the reason for low-orbit constellations.
Move the control to see what changes.
Why is the geostationary belt a single ring rather than a shell?
- Yes.Any tilt makes the satellite trace a figure of eight in the sky, and any other altitude makes it drift east or west. One altitude, one plane — which is why slots there are allocated by treaty.
- Not quite.It is harder from most launch sites, which have to change plane to reach it.
- Not quite.Interference limits how close two satellites can sit in the ring. It does not create the ring.
A signal to a satellite 36,000 km up and back travels 72,000 km. Light covers 300,000 km per second. How many milliseconds is that round trip?
msYes.72,000 ÷ 300,000 of a second. A quarter of a second, imposed by physics, that no better equipment can remove — which is the whole case for low-orbit constellations.A sun-synchronous orbit passes over each place at the same local time every day. What is that useful for?
- Yes.Change detection needs everything else held constant. Photographing a forest at 10:30 every morning makes a difference between two images meaningful rather than a trick of the light.
- Not quite.A helpful side effect for some designs. The orbit exists for the consistency of the view.
- Not quite.It moves constantly over the ground. What repeats is the local time of the pass, not the place.
Lesson complete
Every orbit buys one thing and gives up another, and the delay is set by physics.
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