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. The user segment consists of a group of all authorized and non-authorized users. It also comprises all national and supra-national groups and organizations established to distribute the information about the GPS system. The created system enables the users to implement a passive method to establish their position. Principle of Location All satellites of the space segment house a precise caesium clock. All satellites keep and strictly synchronous time (this is the business of the system control segment). If also the time of the user receiver is strictly synchronous with the time of satellites, three satellites would be enough to determine the position. In such case it is possible to use the following equation to establish the distance of the user from the particular n-th satellite: Sn= c. (tk- ti) Where c is the light propagation velocity, ti is the accurate time of the satellite clock at the moment of sending the signal (this time is coded into the satellite navigation message) and tk is the user clock time at the moment of reception of the satellite signal. If the user makes such measurement for three satellites at the same moment, he obtains three equations with three unknowns X, Y and Z, which are the user’s co-ordinates. However, the stability of the user’s receivers clock is much worse than that of the satellite clock and therefore each measurement will have a certain error. It means that instead of the real distance “user-satellite”, only the following “pseudo-distance” is measured: Sn= ( X-Xn)2+ (Y-Yn)2 + (Z-Zn)2 = c .tn + c . where Xn, Yn and Zn are co-ordinates of the satellite position at the moment of transmission of the message,tn is the difference of times tk- ti for the n-th satellite and it is the error of the user’s clock. The user’s clock error is another unknown, which has to be eliminated, and therefore it is necessary to measure the distance simultaneously from at least four satellites. This gives us a system of four equations for the about four unknown X, Y, Z and. Based on this, the GPS enables each user to obtain information about his position (including the altitude above the sea level) and if he is moving, it also gives him information about the direction of the motion and what the motion speed is. Moreover, software of the modern GPS receivers enables establishing the distance of the user from the possible target point and directions to this target point. Several tens of such successive target points can be pre-stored in the computer memory. Satellite-User Communication Before continuing with our clarification about the GPS system, we have to briefly explain the used method of communication between the satellite and the user. It is a distributed spectrum system (system with code multiplex). This advanced system of wireless transmission of information features a high resistance to