Global Positioning System (GPS)

2.3.1 Global Positioning System (GPS)

GPS (Global Positioning System) was originally established for the star war plan by the US Department of Defense, the aim of which was to pilot the aircrafts and vessels for the US forces. In the Gulf War, GPS was widely used.

GPS is a tremendous project, which is considered as the third space project after Apollo project and space shuttle. During the gulf war, GPS was not finished yet, but the preliminary use demonstrated great triumph. As the GPS satellite network was established completely in 1993, the scope of application has been extended continuously. In 1994, the US government announced that it would

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provide the world with access to GPS free of charge in the next 10 years. On Feb

29 1996, the US government formally announced that GPS was opened only for military and civil applications; however, the positioning precision was reduced to restrict civil applications. On May 1st, the US president announced that SA was set to zero, which greatly promoted the development of GPS civil applications.

There is a popular saying in the GPS industry: “GPS is only restricted by people’s imagination”. Now the application areas of GPS include land transportation, marine transportation, civil aviation, communication, mapping, mining, agriculture, power system, medical application, scientific research and entertainment. The positioning precision has evolved from 10 meters to 1mm.

GPS is a system composed of 24 satellites for global positioning and piloting, the GPS receiver at any place will receive the space signal from at least 4 satellites. Therefore, GPS has the powerful function of positioning. The receiver interprets the positioning information from each satellite to determine the position, thus providing highly precise 3-dimensional positioning and piloting.

The space part of GPS is composed of 24 satellites evenly distributed in six orbit planes: the ground supervision part, in charge of supervising the satellite and calculating satellite calendar, includes a principal controlling station, 3 upload stations and 5 monitoring stations. The client device of GPS is primarily composed of hardware and processing software. The user receives GPS satellite signal via client device, and obtains the information about location and speed and so on so as to finally implement GPS piloting and positioning. Figure 2.10 illustrates GPS system.

Figure 2.10 GPS illustration Generally speaking, GPS is composed of three parts: space part, controlling

part and client part. The space part is composed of satellite constellation, the controlling part is managed by the ground satellite controlling center; the client part is developed and maintained by military or civil manufacturers. The space part and controlling part are now controlled by the US Department of Defense.

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According to statistics from the US Department of Commerce, in 2000 the turnover of GPS client machines is 8 to 9 billion dollars. In 2003, GPS civil products will exceed 16 billion dollars. It is no exaggeration to say that after 2010, the GPS products will become an indispensable part of people’s life and a platform for R&D of various products.

No matter whether it is analyzed from military use or civil use, GPS has aroused the concern of countries all over the world. It has been realized that the owner of the global GPS satellite network and GPS controlling network has much stronger national power and huge commercial markets. The former USSR tried to establish its own GPS (GLONASS) as early as 1982, and Russia continued to carry on this project. Although the economy of Russia is not good enough, it still launched 9 satellites belong to the GLONASS. Europe also speedup its progress to construct its Galileo system. At the present China does not have the capability to construct its own system, it chooses to take part in the Galileo project.

The implementation of GPS system is divided into three phases: The first phase witnessed solution argumentations and initial design. From

1973 to 1979, there were 4 experimental satellites launched, and ground receiver was developed and tracking network was constructed.

The second phase is the time when comprehensive research and development and experiments were made. From 1979 to 1984, there are 7 satellites launched and receivers for different purposes developed. The experiment result suggested that the accuracy of GPS far exceeded the design standard.

The third phase is applicable network construction. On Feb 4th 1989, the first GPS satellite was successfully launched, indicating that GPS system has formally entered the stage of construction. By the end of 1993, the GPS constellation

to planning. GPS is composed of three independent parts: (1) Space part: 21 satellites, 3 backups. (2) Ground support system: One master controlling station, 3 upload stations,

5 monitoring stations.

(3) Client devices: Receiving GPS satellite signal to obtain sufficient positioning information about navigation and positioning. The hardware usually includes receiver, antenna and power supply.

The principle of GPS is: The satellites continuously transmit the parameters, and the user receives the information and works out the 3-d position, 3-d direction, velocity and time of the receiver through computation. Currently the accuracy of GPS is within 10 meters, and for a higher accuracy, we usually adopt different GPS technology: plant a GPS receiver at the reference station to obtain measurement data with GPS; and then calculate the modification value against the precise coordinates of the reference station and transmit the value synchronically. Once

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the client receives satellite signal, it also receives the correction transmitted by the station. There are two kinds of difference GPS: pseudo-range difference and carrier phase difference.

1. Pseudo-range difference

This is a most widely used difference. At the reference station, all satellites are observed and their true distance to the station is worked out according to the known coordinates. And then the true distance is compared with the measured value of GPS to obtain the modification of the pseudo-distance values. The modification is sent to the clients to improve accuracy. This method can achieve meter accuracy.

2. Carrier phase difference

This technology is also called RTK (Real Time Kinematic), in which carrier phase of two monitoring stations is measured. This method can achieve centimeter accuracy.