Global Positioning System

Global Positioning System (GPS) Yongli Shang Group A1 Nowadays, GPS (Global Positioning System) has been gaining its popularity in all over the world. It has been widely used in different fields, especially in electronic industry. It is a useful tool for map-making, rescuing, and military use. GPS plays a dominant role in transportation systems worldwide, which includes aviation, ground, and maritime operations. 1. So, what is GPS? It is a satellite navigation system. To make it simple, we can see it like that, a user’s device send a message to the master ground station, then the station send it to the satellite which moves around the surface of the earth, at last, the satellite send the message back to the station, and the computer calculate the position due to the time transmitter and the speed of signal carrier. The GPS receive the data from the station and show it on the screen. The first satellite was successfully tested in 1960, which consisted of five satellites and could provide a navigational fix approximately once per hour. 1It only can measure the fixed position, and not synchronous to the moving object. In order to get a continuous data, the GPS always use a high frequency carrier. Satellite frequency carriers are usually between 1000 MH to 1600 MH. Since all of the satellite signals are modulated onto a certain carrier frequency, there is also need to separate the signals after demodulation.2 However, a real GPS is a very complex technique, which needs amount of data collection and measurement. It consists of three main parts, which are space equipment, ground support systems and the equipment of users own. In space, 24 satellites move around the medium earth orbit, usually eight each in three circular orbital planes and 6 planes are built by every 4 satellites to cover nighteight percent of the earth. People can obverse 4 satellites at anytime and anywhere on the ground, so that can collect the attitude and longitude at that position. On the ground, there should be one master control station which use to manage and consort the whole control system on the ground. The ground aerial controlled by master control station sends message to satellites. The monitoring stations which collect and analysis information automatically. Finally, there also 1 Astronautica Acta II, 25 (1956) 2 GPS World. November 5, 2007

need the communication system to transmit the data. The user’s equipment consists of two main parts: GPS receiver and satellite aerial. Now, we have a very basic understanding of the structure of GPS. How does it work? We can find the position of satellite precisely, because these twenty-four satellites move around the earth at a height about 20,200kilometers, with a certain period of 12 hours. 3During the observation of GPS, we can get the distance between receiver and satellite by the product of time and velocity, as the formula of distance in three-dimension place; we use three satellites to make three functions to get the position(X, Y, and Z) of the observation point.4 However, what I have told above are based an ideal situation, which have no errors. In the real situation, there must be some errors during the sending and receiving message. For insistence, signal will be affected by troposphere and ionosphere. One of the most important significant error sources is GPS receiver’s clock. Because the signal which is sent by user should go to the ground station first, and then it will be sent to the satellite, so there must be a time difference between satellite’s clock and receiver’s clock. As we know, distance is equal to the time multiply by velocity, and the light velocity is a very large value, so a very small time difference may lead a big mistake. So, how to solve this problem? We can find that if we make the time difference as the fourth unknown data which is labelled as t, we need another satellite to make the fourth function to get the position(X, Y, Z, and T) of the observation point.5 Actually, a receiver can lock at least four satellites each group; it can choose the minimum error one from several groups to enhance its precise. If we want to determine a moving object’s position, we need to use a high frequency signal to display that moving object on an electronic map or show the change of attitude and longitude. Initially, GBS is mainly used in aviation, ground, and maritime operations. It can provide a whole-weather and global navigation services. In military uses, it can be used in intelligence collection, emergency communication, and monitoring of some nuclear test or any radiated test. It is important to collect information before action. One important advance is to combine it with cruise missile, which makes the long-distance and precise missile possible. It also can be used in police’s system to monitor some dangerous people in order to protect civilians from dangerous. Disaster relief and emergency services also depend upon GPS for

location and timing capabilities in their life-saving missions. It also can help rescuer’s originations to find the most safety and shortest way to find and save people, which enhance the percentage of survival. In scientific uses, people can use GPS to make the world’s map more precisely, to explore some place where people cannot or hard to reach. In recent years, GPS is widespread use in human’s daily life and work. Many civilian use one or more of three basic components of the GPS: absolute location, 3 Agnew, D.C. and Larson, K.M. (2007) 4 (http://en.wikipedia.org/wiki/Global_Positioning_System) 5 (http://en.wikipedia.org/wiki/Global_Positioning_System)

relative movement, and time transfer. It accelerate the development of commerce of the country, GPS makes people’s work more efficiently. For example, the car’s GPS equipments let people to choose the fastest way to their destinations; it let people to know where to go in an unfamiliar environment. For airplane, it provides the automatic navigation system which let the plane fly on a selected path accurately. GPS has started to move into mobile phones in the past ten years, the first handsets

with integrated GPS were used already in the late 1990’s, and were available for broader consumer availability on networks. 6For example, it is possible for people’s handset positioning in emergency calls. It is a huge advance that makes it smaller and more convenient. During a long time research, GPS’ mobile phone has already been used by most of people. However, GPS is a so powerful tool that it may violate other’s secrecy. So the governments stipulate that the precise of measurement of civilian’s GPS is above a height above the ground. For example, the U.S government controls the civilian receivers capable of

functioning above 18 km (60,000 ft) altitude and 515 m/s (1,000 knots).7 GPS is a powerful technique, it is used as an advanced method of measurement and new productivity, and it has become an indispensable part of development of commerce and national defence. It used as eyes of a country. However, each coin has its opposite sides, so I think it should be used carefully, at least restrict its using in a certain field. People can benefit from GPS if it is used in a right way. Oppositely, it will bring a huge disaster.

6 http://en.wikipedia.org/wiki/Global_Positioning_System#Civilian 7 http://en.wikipedia.org/wiki/Global_Positioning_system#civilian