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The correct name of the system is NAVSTAR (Navigation System for Timing and Ranging), but commonly it is referred to as GPS (Global Positioning System).
The GPS System was produced by the American Department of Defence and was initially based on running with 24 satellites (21 satellites being mandatory and 3 satellites as substitute). Today, about 30 active satellites rotate around the earth in a distance of 20200 km. Global positioning system satellites broadcast signals which allow the exact location of a GPS receiver, if it is located on the surface of the earth or in the earth atmosphere. The GPS signal can be used by anyone who owns a GPS receiver.
In 1973 the decision to make a satellite navigation system was made. From 1974 to 1979 they began to test the system. 1977 a receiver test was performed on earth before the satellites where sent into space, this help to ensure that no defaults were to occur in space. 11 satellites where launched over a 7 year period, years 1978 to 1985. When a civilian airplane of the Korean Airline was shot down in 1983 after it had gone lost over Sovjet territory, it was decided to allow the civilian use of the GPS system. At first only 18 satellites should be operated. 1988 the number of satellites is again raised to 24, as the functionality is not satisfying with only 18 satellites. On 08th of December it was decided that the use of GPS was to be use free of charge worldwide. In the year 2000 accuracy for civilian users from about 100 m to 20 m was enhanced.
Wherever you are on the planet, four GPS satellites are visible at any time. Each one transmits information about its location and the present time at standard intervals. These signals, travelling at the speed of light, are received by your GPS receiver, the GPS then calculates the distance between you and the satellite based on how long it took for the messages to arrive.
Once it has gained information on the distance from the GPS to at least three satellites your GPS receiver can locate your location using a procedure called trilateration.
Imagine yourself standing anywhere on Earth with three satellites in the sky above you. If you know how far away you are from satellite A, then you know you must be situated anywhere on the blue circle. If you do the same calculating for satellites B and C, you can work out your position by seeing where the three circles meet. This is what your GPS receiver does. The more satellites there are on top of you the more precisely your GPS unit can establish where you are.D:\My Documents\daniels stuff\physics\gegrgeg.bmp
Mobile phones have a built in microphone that changes sound waves into electrical signals. These signals are digested (turned into a code of 1s and 0s) and transmitted as radio waves to a base station. The base station has a system of antennae on top of a tower; these stations each accept and transmit radio signals from three adjacent hexagonal-shaped areas called cells.D:\My Documents\daniels stuff\physics\cells.bmp
Every single base is linked to a switching centre by a cable system that carries the signal as electrical impulses. The impulses have been made by radio-wave energy interacting with the aerial. Each switching centre is joined to additional switching centres and base stations. There are three main likely paths for the signal to take.
The first path is if the telephone call occurs among a mobile and a far-away fixed telephone, the signal may be transformed into light and travel an optical fibre network to a distant switching centre nearby to the destination. It is here at the centre where the signal is moved into the copper wire network as an electrical impulse and is decoded in a receiving telephone.
The second path, if a call was to be made between a mobile and a fixed telephone that are close together, it is converted into an electrical impulse in a copper cable. It may stay as an electrical impulse in the copper cable network until it reaches a switching centre near its destination. As in the earlier situation, the signal is then sent as an electrical impulse along copper cable to a receiving telephone where the signal is changed back into sound waves.
In the third path, a call is made from a mobile to another mobile, the signal will be transferred to a switching centre near a base station servicing the cell near the receiving mobile phone is situated. The signal from the switching station is sent to the base station as an electrical signal and transmitted as radio waves to the mobile phone. Once the radio waves have reached the mobile the radio signal is changed back into electrical impulse signal, the speaker in the mobile then converts the waves into sound.
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Radio waves were initially predicted in 1865 by James Clerk Maxwell, who discovered the equations for electromagnetism, afterward known as Maxwell's equations. The existence of these waves was verified by Heinrich Hertz only 12 years later, in 1887, when he generated radio waves in his lab.
Radio waves are one type of electromagnetic radiation; they have the longest wavelengths in the electromagnetic spectrum. These waves can be up to 100,000km long or just a few millimetres long. We use radio waves extensively for communication, radio waves move back and forth at frequencies between a few kilohertz (kHz or 1000 hertz) and a few terahertzes (THz or 1012 hertz).
Radio waves are notoriously used to send audio, images, and text in the structure of signals. Radio waves have long wavelengths allowing it to get around obstacles and travel lengthy distances, unlike visible light and other spectra of higher frequencies. The first wireless telegraphy system was patented in 1896 by Marconi.
Radio waves with a wavelength of less than about 10 meters are absorbed by the atmosphere, otherwise they rebound back and forth among the ionosphere and the ground, and this makes radio waves ideal for transmitting over the horizon. The smallest frequencies are used for communication with submarines, because of their low energy and high penetrating power. Lower frequencies penetrate further, especially through thick substances such as water.
Radio is a very simple technology. With just a few electronic parts that are not very expensive you can build simple radio transmitters and receivers, this is a great advantage to society because this means sharing media from is not difficult financially.
These are just a small number of the everyday technologies that rely on radio waves:
AM and FM radio broadcasts
Garage door openers
JACARANDA HSC SCIENCE, 2009, John Wiley and sons Australia, ltd, physics preliminary course one third editions.