Lesson overview
Communicating with waves: How does a phone call reach the other side of the world?
Say “hello” into a microphone in London and send it to Cape Town three ways at once: along an undersea cable, through low-orbit satellites and through a geostationary satellite, and time each one. Let an hour pass and aim two dishes on a roof, one at each kind of satellite. Supplement: match four devices to the waves they use, then send the word along the cable as an analogue and as a digital signal, and switch off a repeater.
Suggested sequence
- Lesson 1 · Signals around the world. Communication with satellites by microwaves: low-orbit satellites, which move across the sky, and geostationary satellites, which stay above one place; how long a signal takes by each route; Supplement: why mobile phones and wi-fi use microwaves, Bluetooth radio waves and optical fibres visible light or infrared. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Analogue and digital signals. Supplement: analogue and digital signals; a sound sent as an analogue or a digital signal; why noise adds up in an analogue signal and is removed when a digital signal is regenerated; the benefits of digital signals. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Signals around the world
- Know that communication with artificial satellites is mainly by microwaves: some satellite phones use low orbit artificial satellites; some satellite phones and direct broadcast satellite television use geostationary satellites (3.3.5)
- Supplement: Know that many important systems of communications rely on electromagnetic radiation: mobile phones and wireless internet use microwaves, which can penetrate some walls and only require a short aerial; Bluetooth uses radio waves, which pass through walls but are weakened on doing so; optical fibres (visible light or infrared) are used for cable television and high-speed broadband, because glass is transparent to visible light and some infrared, and these can carry high rates of data (3.3.7)
Part 2: Analogue and digital signals
- Supplement: Know the difference between a digital and analogue signal (3.3.8)
- Supplement: Know that a sound can be transmitted as a digital or analogue signal (3.3.9)
- Supplement: Explain the benefits of digital signalling including increased rate of transmission of data and increased range due to accurate signal regeneration (3.3.10)
Misconceptions to listen for
- “Sound waves, because the satellite passes on the voice.” — Sound needs a material to travel through, and space is a vacuum. The satellite receives and sends microwaves that carry the signal.
- “Visible light, because light travels fastest.” — All electromagnetic waves travel at the same speed in space. Heavy cloud blocks visible light, but microwaves pass through it.
- “Radio waves, because radio is used for broadcasting.” — Long radio waves are used for broadcasting across the land: from a dish of this size they would spread out widely. Satellite links are mainly by microwaves, which the dish can aim at the one satellite.
- “Satellite P is geostationary.” — P is just above the Earth. A geostationary satellite is about 36 000 km up, far above the equator, like Q.
- “Satellite Q is a low-orbit satellite.” — Q is high above the equator. Low-orbit satellites are only a few hundred kilometres up, like P.
- “A low-orbit satellite takes a whole day to go round.” — A low-orbit satellite goes round in about an hour and a half. Only a geostationary satellite takes one day.
- “A geostationary satellite crosses the sky in a few minutes.” — A geostationary satellite never moves in the sky. A low-orbit satellite crosses it in a few minutes.
- “The fixed dish points at the low-orbit satellite, because it is nearer.” — A low-orbit satellite crosses the sky in a few minutes, out of the beam of a fixed dish. The dish points at the geostationary satellite, which stays put in the sky.
- “The phone near the North Pole uses a geostationary satellite.” — Near the North Pole, a satellite above the equator is on or below the horizon. Low-orbit satellites pass over the poles.
- “The live video call uses a geostationary satellite.” — A geostationary satellite is about 36 000 km up, so the signal takes about a quarter of a second each way. Low-orbit satellites are only a few hundred kilometres up.
Scope
Communication with artificial satellites, mainly by microwaves: low-orbit satellites for some satellite phones, geostationary satellites for other satellite phones and for direct broadcast satellite television (3.3.5); Supplement: why mobile phones and wireless internet use microwaves, Bluetooth radio waves, and optical fibres visible light or infrared (3.3.7); digital and analogue signals (3.3.8); a sound sent as a digital or analogue signal (3.3.9); the benefits of digital signals, a higher rate of data and a greater range through accurate regeneration (3.3.10). The regions of the electromagnetic spectrum and their other uses are in Light and the electromagnetic spectrum; total internal reflection in optical fibres is in Refraction; orbits and their periods are in the Space physics modules; how a signal is put onto a carrier wave and how a sound is sampled are not covered.