Communications (CIE IGCSE Physics)

• Geostationary and polar orbiting (low orbit) satellites are both used for communicating information

Geostationary and polar orbits around the Earth

Geostationary Satellites

• Geostationary satellites orbit above the Earth’s equator
  • The orbit of the satellite is 24 hours
  • At a height of 36 000 km above the Earth’s surface, much higher than polar satellites
  • Used for radio and telecommunication broadcasting around the world due to its high orbit

Some satellite phones and direct broadcast satellite television use geostationary satellites

Polar Satellites

• Polar, or low orbit, satellites orbit around the Earth’s north and south poles
• These orbit much lower than geostationary satellites, at around 200 km above sea level
  • Used for monitoring the weather, military applications, and taking images of the Earth’s surface
  • There is a much shorter time delay for signals compared to geostationary orbit signals
  • The signals and images are much clearer due to the lower orbit
  • However, there is limited use in any one orbit because more than one satellite is required for continuous operation

Some satellite phones use low-orbit artificial satellites if a more detailed signal is required

EXTENDED

• Many important systems of communications rely on long wave electromagnetic radiation, including:
  • Mobile phones, wireless internet & satellite television (using microwaves)
  • Bluetooth, terrestrial television signals & local radio stations (using radio waves)
  • Optical fibres (using visible or infrared waves)

Radio Waves

• Radio waves can be used to transmit signals over short distances 
  • Terrestrial (local) television signals, radio station transmissions and Bluetooth all work using radio waves

• Radio station signals are transmitted at a longer wavelength than terrestrial television signals
• In hilly areas, it may be possible to receive radio signals but not receive terrestrial television signals
  • This is because radio signals are more prone to diffraction around the hills

• Radio signals tend to have wavelengths of around a kilometer, so the radio signals are more likely to have wavelengths similar to the size of the hill
  • This leads to diffraction, so radio signals can reach locations not in the line of sight of the transmitter, whereas TV signals are not diffracted

• Bluetooth uses radio waves instead of wires or cables to transmit information between electronic devices, over short distances, such as phones and speakers
  • Bluetooth signals tend to have shorter wavelengths than radio or television signals
  • This enables high rates of data transmission, but can only be used over a short distance (for example, within a household)
  • This means they can pass through walls but the signal is significantly weakened on doing so

Radio signals diffract around hills because they are a similar wavelength to the hill

  

Microwaves

• Microwaves can be used to transmit signals over large distances 
  • Microwaves are used to send signals to and from satellites
  • Mobile phones, wireless internet, satellite (global) television and monitoring Earth systems (for example, weather forecasting) all utilise microwave communication
• As with radio waves, microwave signals will be clearer if there are no obstacles in the way which may cause diffraction of the beam
• On the ground, mobile phone signals use a network of microwave transmitter masts to relay the signals from the nearest mast to the receiving phone 
  • They cannot be spaced so far apart that, for example, hills or the curvature of the Earth diffract the beam

• When microwaves are transmitted from a dish, the wavelength must be small compared to the dish diameter to reduce diffraction
  • Also, the dish must be made of metal because metal reflects microwaves well

• Mobile phones and wireless internet use microwaves because microwaves are not refracted, reflected or absorbed by the atmosphere or ionosphere
  • This means satellites can relay signals around the Earth enabling 24-hour-a-day communication all around the world
  • Also, they can penetrate most walls and only require a short aerial for transmission and reception

Optical Fibres

• Optical fibres (visible light or infrared) are used for cable television and high-speed broadband
  • This is because glass is transparent to visible light and some infrared
  • Also, visible light and short-wavelength infrared can carry high rates of data due to their high frequency

Optical fibres use visible light or infrared for transmitting cable television and high-speed broadband signals