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overview of GPS

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Overview of GPS GPS stands for Global Positioning System, and is based on satellites and their corresponding ground stations. As the Americans were the first to introduce such a system they named it NAVSTAR with the introduction of such a system for military use the Russians had also introduced a satellite system called GLONASS, usually the American version is referred to as GPS. Satellites in orbit around the earth not just America are arranged carefully so that sufficient satellites can be seen from any point on the planets surface so as to provide an accurate position. The Transit system was the old system which formed the basis on the NAVSTAR project it was developed in the early sixties, but had major flaws in that it was slow, it gave readings every 90 minutes with an accuracy of 250m at the 95% level was very inaccurate especially for military uses. However a new form of positioning system has emerged, known as DGPS (Differential global positioning system) where a more accurate reading is given. Current new advances are being made for better accuracy positing systems everyday. Transit was switched off at the end of 1996. GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time, basically the coordinates X, Y, Z and time. To get these positions the GPS uses four satellites. Most GPS receivers display five basic sets of information to the user, they are, altitude above sea level, longitude and latitude, speed over ground and heading when moving. Navigation in three dimensions is the primary function of GPS. The main uses of navigation receivers are made for aircraft, ships, ground vehicles, and for hand carrying by individuals for example, teaching surveying techniques to students. GPS is also finding its way into everyday items such as the mobile phone and is probably going to be in most electrical equipment in the near future. ...read more.


For measurements of a shorter range the carrier phase errors can be ignored because of the high accuracy static has incorporated. It works like this, the surveyor with the GPS receiver sets up over a known position with coordinates x,y,z. then using a different receiver the user stands over a position which he/she wants to know. To understand why this process needs time is because of the 'cycle ambiguity problem' this is where it needs time to solve the integer number of cycles between the satellites. This is one of its drawbacks, in that static processing is to slow but accurate. Rapid static Very closely related to static positing but with a difference in occupation time, where static takes hours, this method only takes minutes. To achieve a shorter time has something to do with the ambiguity problem discussed above. There are two ways in which the problem can be eradicated, the problem of occupation that is. Often the problems money is thrown to solve it, one way would be is to have very expensive equipment and software, which could calculate equations very rapidly. This is the simple way of solving a problem but expensive. A less expensive approach would be to combine carrier phase with p-code measurements which would then get rid of the least squares solution for ambiguities and the combination of both the measurements would involve search routines as the GPS receiver is well capable of doing. Where static positions can take up to two hours, rapid static can reduce this time to around 20 minutes, a technique called wide laning makes this possible. Basically the GPS frequency bands L1 and L2 are put together in a linear combination, when both L1 and L2 are combined, 2 distinct signals arise. One is called narrow lane and the other wide lane. Also one is slower than the other and the wavelengths are different compared to L1 and L2. ...read more.


In the end my decision was undertaken. Using the theodolite we sighted to station2 from station1 and then extracted the data, and then we sighted to station 1 from station2, to give the distance and angles. We did this throughout the day, but because we did not know how to start off, it took sometime to get going, we called it a day at station5. Events on this day, reduced our team count to 4, Sarab Jit decided that the whole course was not for him and he left the group, Andrew Baily was then allocated group leader. Wednesday: day3 Carrying on from yesterday, we managed to finish of the final three stations, which were stations, 5, 6 and 7. We did not have any problems on this day because we knew what we were doing and I made sure that everyone was taking turns in handling the equipment so that they learn. Around the university there were other groups conducting similar exercises, but were falling behind and so we called it a day, because we were ahead of them all. Another reason was because we did not want to rush the detailing of the area, as we knew this would need concentration. Thursday: day4 Detailing of the area around the library, here we decided what should be detailed and what should not, because it was not practical to detail every item around the area, we decided to detail large objects, which ranged from pillars to flower beds. The whole day was spent detailing however we did not manage to finish the detailing. Friday: day 5 The final day, on this day detailing was completed and the group sat down and looked at the data to see if it 'looked' correct and that there were no major problems or concerns in general about the week. Members of group: Imtiaz Ali, Andrew Baily, Thomas Docker, Hoi Wing Cheung ?? ?? ?? ?? Surveying people ...read more.

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