Technology - Satellite navigation system GPS

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 . δ

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 δ. 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 interference. It is capable of holding the connection even in case when level of the received signal drops below the noise level. This is not possible with any other transmission system.