Science & Technology in Action

10th Edition

Radar and its Uses

Commission for Communications Regulations

Air traffic control, weather forecasting and navigation are among the most important applications of radar. The lesson outlines the basic principles of radar systems. ComReg regulates the allocation of the hundreds of RF bands that are used for TV and radio, mobile phones, radar, meteorology and satellite communications.
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Contains the full lesson along with a supporting toolkit, including teachers’ notes.

Lesson excerpt

Introduction 
The word radar is an acronym for Radio Detection And Ranging. Common applications include air and marine traffic monitoring, meteorology, geology, astronomy and speed detection of motor cars. 

In a typical radar system a radio frequency signal is directed towards the object and the reflected signal is detected; using the time for the ‘echo’ to return, the distance to the object can be calculated. 

Water waves and electromagnetic waves 
When a stone is thrown into water it produces oscillations, i.e. the water moves down and then returns towards its original position. However, as it returns it gathers speed and overshoots the equilibrium level. The oscillations continue for some time and the disturbance is propagated outwards as circles of ever increasing diameter. As they travel outwards from the source the waves carry energy. The waves can be reflected by structures or boundaries; they can be refracted if they are slowed down (for example by going into shallower water); they can be diffracted around objects such as rocks or piers. 

In the early 1800s light was shown to have these same properties and so it was considered to be some kind of wave. In 1832 Michael Faraday had already come to the conclusion that light was some kind of electromagnetic wave although it took him over ten years to get the evidence he had been looking for. 

We now know that light (and all the other kinds of electromagnetic wave) can be described as a magnetic oscillation and an electric oscillation perpendicular to one another. If all the waves from a source have the electric oscillation in, for example, a vertical direction then the wave is said to be vertically polarised. (Satellite TV transmissions are generally polarised either vertically or horizontally.) 

Quiz questions

  1. Light travels faster than radio waves. false
  2. Radio waves have a lower frequency than light waves. true
  3. Radar can be used to detect aircraft over 100 km away. true
  4. Radar can be used to detect clouds. true
  5. Marconi demonstrated radar in 1897. false
  6. The wavelength of a 3 GHz RF signal is one metre. false
  7. The energy of microwave photons is greater than that of visible light photons. false
  8. The energy of a photon is proportional to its frequency. true
  9. The radio frequency spectrum includes microwaves. true
  10. The change in frequency of waves reflected off a moving target is called the Marconi Effect. false
  11. When waves travel from one medium to another they can change direction. This happens because the frequency of the waves changes. false