8. Resultados 86
8.3. Aplicación de Path Análisis 104
1.3.1 History
GPS are an established technology in modern society with most people carrying one in their pocket or bag, some without knowing it. Most smart phones will carry the capabilities to utilise GPS to find their position on the ground and direct them to their required place. However this technology is relatively young and has only been available for commercial use since the early 2000s.
The earliest attempts to accurately define a bodies position using radio signals were during the First World War when the USA LORAN radio navigation system was used (Kumar, and Moore, 2002). Modern GPS has arisen from this type of military technology and has been advanced by USA and specifically the USA-Soviet Union Cold War (Figure 1.8).
Although USA was the main driving force behind the development of GPS receivers on Earth it was Soviet Union who first showed the possibility of being able to send and receive radio signals from space when they launched Sputnik into orbit in 1957 (Figure 1.8). This was a trigger for US military forces to create a national council to study and develop a navigation system based in space. This council commissioned studies into the type, number and orientation of satellites required to accurately define a body’s position on Earth down to less than 100 m. From this council, Navstar GPS (Navigation System Using Timing and Ranging) was developed and the first satellites were launched into orbit in 1978 (Figure 1.8) (Kumar and Moore, 2002).
66 | P a g e The principle of GPS is based on trilateration and the speed, distance, time equation (Speed = Distance/Time) to work out the position of a receiver on Earth. Figure 1.6 shows trilateration in a 2D scenario. GPS works by signals coming from 3 or more satellites to a receiver point on Earth. The central dot spot in this Figure is the receiver on Earth and A, B and C are the satellites in space. Each satellite produces a circular signal and the point at which these crossover can be used to calculate the position. The actual signals are sent in a spherical shape (not a 2D circle) as can be seen by Figure 1.7. The satellites produce 3D spheres which intersect at two points. However as one of these points is in space it is disregarded and the point on Earth is used to define the bodies position (Bajaj, Ranaweera and Agrawal, 2002).
Figure 1.7. A basic 2-Dimensional model of GPS satellites and the use of trilateration to calculate a position on Earth. Reproduced from Dzurisin (2006)
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Figure 1.8. The interaction GPS satellites and the use of trilateration to calculate a position on Earth. Reproduced from Bajaj, Ranaweera and Agrawal (2002)
To calculate the distance, and subsequently the position of the object on Earth, accurate speeds and times are required from each of the satellites. This is acquired via the use of radio waves and atomic clocks. Both of these provide accurate data on the speed of signal (speed of light) and the time which it is sent and received. Calculation provides a distance for each satellite and this is used to define the position on Earth. Although it is possible to do this with three satellites it is more accurate when more satellites are used. A minimum of 24 satellites are required for the network to function, however currently there are 32 functioning satellites in orbit with 27 of these operational (Dzurisin, 2006). At most points on Earth there are between 8 and 11 satellites visible at any one time allowing for greater accuracy.
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Figure 1.9. NAVSTAR satellite constellation. Reproduced from Dzurisin (2006)
Initially GPS was only for military use. However in 1983 an attack on a Korean airliner by Soviet Union air defences resulted in the US president of the time, Ronald Reagan, to open up the network to commercial airlines (Figure 1.8). The first prominent use of the newly developed US Global Navigation Satellite System (NAVSTAR) GPS network was the Persian Gulf War in 1990 (Figure 1.8). It allowed military forces to be able to accurately direct personnel and ammunition towards their opposition. Post-war the US authorities degraded the signal for commercial users meaning only the US military were able to accurately use the system. This brought about the development of deferential GPS devices which were ground based technologies to increase the signal accuracy. Eventually the US
69 | P a g e government removed this degradation and commercial use increased exponentially. In 2004 the first mobile telephones were tested with GPS devices leading to a wide variety of publicly available GPS-enabled devices (Figure 1.8) (Kumar and Moore, 2002).
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Figure 1.10. An overview of the history of Global Positioning Systems (GPS).
• 1940s • LORAN (Long
Range Navigation system) device used in WWI • Uses radio signals
to calculate positions from ground based receivers • 1950s • Sputnik launched into space in 1957 • This showed the
US that orbiting satellites could transmit radio signals from space and be visible all over the world
• 1960 – 70s • Post Cold War
the US Air Force and Navy developed the NAVSTAR GPS system • First satellite launched in 1978 • 1980s • After a Korean Air civilian aircraft was shot down President Ronald Regan made GPS available to commercial airliners • 1990s • NAVSTAR widely used in the Gulf War • Post war the US
switches on a system to degrade the GPS signals for commercial receivers
• 2000s
• The US switches off the degradation of GPS signal allowing for world wide commercial use • First mobile phone
devices are tested in 2004 • 2010s • New range of satellites developed and a series of deployments scheduled to upgrade the system