The ups and downs of Australian air traffic control
Glossary
automatic dependent surveillance. A computer on board the plane uses the Global Positioning System to determine its position. It then sends this information regularly via satellite or a VHF (very high frequency) radio link to the Australian Advanced Air Traffic System, which then plots the aircraft's position on the controller's screen. This surveillance is used when an aircraft is out of radar range. Not all aircraft currently have this facility, but it seems likely that it will be installed on most international passenger aircraft.
electromagnetic radiation. Electromagnetic radiation is simply energy which travels through space at about 300,000 kilometres per second – the speed of light. We 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. Other than wavelength, frequency and energy there is no difference between a radio wave, an X-ray and the colour green. They all possess the same physical nature. For more information see Back to Basics: Electromagnetic radiation (Australian Academy of Science) and Electromagnetic Spectrum (NASA Goddard Space Flight Center, USA).
flight data processing. This plots an aircraft's expected position as calculated by computer from the aircraft flight plan (stored electronically by the Australian Advanced Air Traffic System).
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).
Global Positioning System. The Global Positioning System (GPS) is a collection of 24 earth-orbiting satellites which allows any person who owns a GPS receiver to determine their location on the planet. More information on the Global Positioning System can be found How a GPS receiver works (How Stuff Works, USA) and The Global Positioning System: The role of atomic clocks (Beyond Discovery, National Academy of Sciences, USA).
radar. The use of reflected radio waves to determine the location of an object and its speed if it is moving. It is an acronym derived from radio detecting and ranging. For more information see How radar works (How Stuff Works, USA).
radio frequency. This is lowest of the electromagnetic radiation frequencies. Radio frequencies, or radio waves, have wavelengths ranging from less than a centimetre to as long as 100 kilometres. (See also electromagnetic radiation).
We divide the radio wave part of the electromagnetic spectrum into bands that are allocated to different uses. These include AM (amplitude modulation), FM (frequency modulation) and CB (citizens' band) radio, television, aircraft communications, satellites, mobile phones and pagers. Within each band, no two transmissions can use the same part of the spectrum – or frequency - at the same time. For this reason, each band within the radio wave spectrum, itself a part of the broader electromagnetic spectrum, must be managed carefully to ensure the best use of this limited resource. For more information see How the radio spectrum works (How Stuff Works, USA).
VHF (very high frequency) radio. Radios that use frequencies in the range 30 to 300 megahertz (millions of oscillations per second). The wave length of these VHF radio waves range from 1 metre to 10 metres.
Page updated May 2001.