Why long links need help
Radio and microwaves travel in straight lines. The Earth is round, so a tower can only "see" a limited distance. Beyond that, the signal goes off into space or is blocked by the ground. To join far places we need a relay, or a path that guides the signal.
Satellite links
A communication satellite is a relay station in space. The ground station sends a microwave beam up (the uplink). The satellite amplifies it and sends it back at another frequency (the downlink). Different frequencies stop the strong outgoing signal from drowning the weak one coming in.
A geostationary satellite orbits about 36 000 km above the equator. It takes 24 hours for one turn, the same as Earth, so it seems to stay over one spot. A fixed dish can point at it. Its signals reach a huge area. The cost is delay: 36 000 km up and 36 000 km down, which is about 0.24 s one way.
Low satellites (a few hundred km up) give less delay but move fast, so many are needed.
Optical fibre
An optical fibre is a hair-thin glass thread with a core in the middle and a cladding around it. The cladding glass slows light a little more than the core does. Light going along the core hits the wall at a shallow angle, and every time it is reflected back in. This is total internal reflection: it happens when light moves from a denser material to a lighter one at an angle bigger than the critical angle. So the light zig-zags along the fibre and follows bends.
Data is sent as very fast on-off flashes of laser or LED light. A fibre loses little light, is not disturbed by electric noise, and carries far more data than a copper wire. Cables of fibre cross oceans on the sea floor. In glass, light travels at about 2 × 108 m/s.
Satellite or fibre?
- Satellite: reaches ships, planes and remote hills, one satellite covers a wide area, but delay is longer and bad weather can weaken the signal.
- Fibre: fast, huge capacity, low delay, but cables must be laid place by place.
Key formulas and definitions
- time = distance / speed
- Satellite delay (one way, geostationary) = 2 × 36 000 km / (3 × 10^8 m/s) ≈ 0.24 s
- Speed of light in glass ≈ 2 × 10^8 m/s
- Total internal reflection: light goes from denser to rarer, angle of incidence > critical angle
Worked examples
1. A signal goes up to a geostationary satellite 36 000 km away and comes down 36 000 km. How long does it take?
Distance = 72 000 km = 7.2 × 10^7 m. Time = 7.2 × 10^7 / (3 × 10^8) = 0.24 s.
2. A fibre cable is 6 000 km long. Light moves at 2 × 10^8 m/s in it. Find the time.
Distance = 6 × 10^6 m. Time = 6 × 10^6 / (2 × 10^8) = 0.03 s.
3. Why does a fibre need a cladding?
The cladding is a lighter material around the core. Light in the core reflects totally from it, so the light stays inside the core.
4. Why does a satellite use a different downlink frequency from the uplink?
The satellite sends a strong beam and receives a weak one. Using different frequencies stops the strong one from drowning the weak one.
Common mistakes
- Saying satellites "reflect" the signal like a mirror. A satellite receives, amplifies and re-sends it.
- Thinking light in a fibre goes straight. It zig-zags by total internal reflection.
- Forgetting that the signal travels up AND down: the distance is doubled.
- Mixing up core and cladding. The core is the middle; the cladding is the outer layer.