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The Global Positioning System

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The Global Positioning System The low quality training of the US Army showed during the Vietnam War, when it was realised that none of the troops could read a map and work out their position. The US Department of Defence (DoD) decided to try and rectify this problem, using technology and physics, rather than actually teach their troops to read maps. To begin with, the DoD experimented with 4 satellites, codenamed TRANSIT. These four satellites were in high orbit above the earth. The system was available to both the US military and any marine users. However, the system was flawed, as position fixes could only be made once every 2 hours, in favourable conditions. Obviously, in military applications, this was nowhere near good enough. In the late eighties, the NavStar system was developed, NavStar standing for Navigational Satellite Timing And Ranging. The system became operational in 1986. However, at this time, NavStar operational capabilities were limited. There were only a small number of satellites in orbit transmitting NavStar data, and so there was only about 3-4 hours coverage every day. Again, this was an obvious limitation in terms of the original specifications of the GPS system. However, the reason for the lack of satellites with NavStar was the explosion of the Space Shuttle Challenger. Challenger was used by the DoD to place NavStar in orbit, and after the disaster, NavStar satellites had no launch mechanism. However, Saddam Hussein forced the US to upgrade the NavStar system to the GPS system we know today. When the Gulf War began, the US used experimental "Block 2" satellites in the NavStar system, along with the original "Block 1". This gave a usable constellation of 21 satellites. After the war, in late 1990, the DoD made the GPS system available for civilians. In those days, America understandably didn't want her enemies buying a GPS and having 30 metres accuracy, so a limitation called Selective Availability (SA) ...read more.


The net effect of this is that the clocks of the satellite run fast, which is corrected by offsetting the clock from the real time prior to launch, and it is maintained by the operating stations. The effects of time dilation on the GPS clocks provide a rare example of both special and general relativity at work. So far, I have covered the fact that there are satellites floating around in the sky, sending out a signal, which a receiver then translates into a position. However, so far, I have not said how this signal is converted into a signal. To begin with, we will assume that there is only one satellite and nothing else, and this satellite is broadcasting a time signal. At the moment, it helps to think of the timing signal as saying "my current time is t and my current position is (x,y,z)" If the signal reaches a stationary receiver, it will not be at time t, it will be at time t + ?t. So, for example, if the satellite transmits a signal saying t = 40 seconds, and the receiver detects this signal 80 ms later, when its clock says t = 40.0800 seconds, then logically, the range of the satellite from the receiver is c?t, or the speed of light multiplied by the time taken for the signal to travel the distance. In this case, the receiver is 2.4 x 107 m from the satellite. If we then draw a circle of radius c?t, with the satellite at the centre, then the receiver must be at some point along that circle. (Figure 1) To obtain a two-dimensional position, then another satellites signal must be used in the same way. The receiver will obviously be on the point where these circles cross. To find where this is, we can use Pythagoras' Theorem, linking (x1,y1) and (x2,y2), the positions of the satellites, their ranges c?t1 and c?t2 with (X,Y), the position of the receiver. ...read more.


L1 has a 1 ms period; L2 has a 7 day period. Information about L2 signal processing is difficult to find, as one can presume that it is encrypted for a reason. It must be very similar to the signal processing that takes place in C/A and L1, but there must be subtle differences. GPS is not a perfect system. Errors can be introduced into the signal, reducing the accuracy of the receiver. Satellite clock errors caused by Master Control not correcting the clocks can cause errors of the order of several metres. Atmospheric conditions can also affect the signal. The troposphere is the lowest part of the atmosphere, and when temperature, humidity and pressure change, then microwave transmission can be affected. Again, these errors are in the order of several metres. The ionosphere can introduce errors of 10+ metres. The ionosphere composed of ionised air, which can interfere with microwave transmission in a significant way. Finally, multipath errors can occur. These occur when the signal is reflected around the ground near the receiver, and the receiver picks up both signals. This introduces only small errors, but multipath errors are the only errors that cannot be corrected by some obscure method. The user can make errors, but with the simplicity of GPS receivers on the market now, only technically retarded people could make these errors. And these types of people would not be using GPS anyway - they would brand it witchcraft, and try to burn it at the stake. In this day and age, technology is becoming easier to use, and so nobody should really be making mistakes with a simple, off the shelf GPS receiver. Those using specialist units will obviously have had some sort of training Bibliography Physics Education, Volume 34 Number 1 January 1999 The Global Positioning System, AJ Walton and RJ Black Global Positioning System Overview Peter H Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder, 1994 http://www.colorado.edu/geography/gcraft/notes/gps/gps.html GPS History No credited author http://www.safetrack.com/History.htm GPS: Theory and Practice, 3rd Edition. 1994 Hoffman-Wellenhof, BH Lichtenegger, J Collins Advancing Physics A2. ...read more.

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