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Technology - Satellite navigation system GPS

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3. Satellite Navigation System GPS

Soon after the first earth satellites were launched, people began to search how to use them for navigation purposes. It was decided to develop a passive distance-measuring system in which the ground-based part - GPS receiver - has only the receiving function and the appropriate navigation information is transmitted by the satellite. On one hand, the radio passivity of the user brings a number of complications to the system; on the other hand it has a great advantage for the user. The ground-based equipment itself is only a special receiver, which features a low power requirement so that it can be constructed even as a battery supplied unit with small dimensions.

Motion of satellites on the orbit is determined by the Kepler`s laws and beside this they have to meet the conditions of the Newton’s gravitation law. This means that if we know the so-called ephemeredes of the satellite (which in fact is a list of data about the satellite’s position as a function of time which as we shall see later is continuously transmitted by each satellite), we have a possibility to exactly determine its position in each particular moment.

Navigation System

The navigation system consists of three parts called segments. The space segment incorporates 21 basic and 3 stand-by satellites. All 24 satellites are working. The satellites are located in 6 orbit levels mutually shifted by 60° with an inclination (which is the angle of the satellite’s orbit with respect to the equator) of 55°. Orbits of the satellites are roughly circular with the elevation of about 20200 km. The satellite’s orbit time (period) is 11 hours and 58 minutes. Each satellite has an engine, which enables modification of its orbit, if necessary. At present, the space segment consists of fourth-generation satellites that feature enhanced resistance against radiation and capability to operate autonomously for 180 days. Coverage of the territory of our country is guaranteed at least by 7 satellites for 24 hours a day. With the minimum elevation angle of 5°, up to 10 space segment satellites can be “seen” in one time.

The control segment monitors the activity of all satellites and transmits to them the information about their orbits, clock operation and other data. The control segment consists of the main control station, group of five monitoring stations and three control stations. The main control station located at Falcon base in Colorado Springs gathers data from measurements of the monitoring stations, calculates ephemeredes for each satellite and parameters of their clock. The monitoring stations are places at suitable locations on the earth ground. They form network, which checks the transmitted ephemeredes of each satellite and the accuracy of their clock. Values calculated on the main control station are transmitted to the control stations from where they are transmitted to the corresponding satellites. These values are part of the satellite signal and are available for real time navigation. The exact ephemeredes and time corrections are transmitted to the satellites several times within 24 hours.

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