Published by Australian Academy of Science
Glossary | GPS and never having to say: 'Where am I?'

atomic clock.  An extremely accurate clock that is based on the vibrations of an atom or molecule. For more information see How atomic clocks work (How Stuff Works, USA), The most accurate clocks in the world (Miami University, USA) and How atomic clocks work (Science Museum, UK).

Döppler shift. The change in the perceived frequency of waves (electromagnetic or sound) when the source of the waves moves in relation to the receiver.

The phenomenon is named for Christian Döppler, who realised that the pitch of a sound emanating from a moving source varies for a stationary observer, depending on the speed of the source and the direction in which it is moving. Imagine you are on a train in a station and you can hear the signals ringing at a rail crossing just down the track. Since both you and the signals are stationary, the signals sound normal. They continue to ring at the same rate as the train starts to move, but now because you are travelling towards them they seem to get faster.

In effect, the time between arrival of pulses of sound is being compressed (or shortened) and the apparent frequency is increasing. The result is that the signals sound higher-pitched. This change in frequency is called a 'Döppler shift'.

For more information see Introduction to the Döppler effect for sound (Fear of Physics, USA) and How radar works (How Stuff Works, USA).

electromagnetic radiation.  Electromagnetic radiation is simply energy which travels through space at about 300,000 kilometres per second – the speed of light. We can imagine radiation moving like a wave. The distance between two adjacent wave crests is called a wavelength. The shorter the wavelength, the more energetic the radiation is said to be. Also, the shorter the wavelength, the greater the frequency of the radiation. The highest frequencies in the spectrum of electromagnetic radiation are gamma-rays; the lowest frequencies are radio waves.

For more information see Electromagnetic Spectrum (NASA Goddard Space Flight Center, USA) and Measuring the electromagnetic spectrum (High-Energy Astrophysics Learning Center, Goddard Space Flight Center, USA).

frequency. A measure of how frequently an electromagnetic wave goes up and down (oscillates) or the number of waves passing by in a second. A hertz is a unit of frequency – 1 oscillation per second; a kilohertz (kHz) is 1000 hertz – 1000 oscillations per second; a megahertz is 1 million hertz – 1 million oscillations per second. For more information see Sound properties and their perception – pitch and frequency (The Physics Classroom, USA).

Galileo. A satellite radio navigation system initiated by the European Union and developed for non-military applications. The final system will be based on a collection of 30 satellites. For more information see What is Galileo? (European Space Agency) and Galileo: European satellite navigation system (Europa, European Union).

GLONASS. Global Navigation Satellite System, operated by the Russian Federation Ministry of Defence. When completed, it will have a constellation of 24 satellites, and is intended to service maritime and aviation users throughout the world. For more information see GLONASS – summary (Andrews Space and Technology, USA).