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The positions of the satellites are determined in an x, y, z-system with origo in the centre of the earth. The calculated range creates a sphere of possible positions
1. The coordinates of the satellite expressed in a threedimensional coordinate system with the centre of the Earth as origo, are known to the receiver because of information transmitted in the signals.
1. Control Segment (GPS)
In order that the system will work it is necessary for the positions of the satellites to be know from second to second - in spite of the fact that they are hurtling through space at 4 km/sec.
Clocks are necessary to time the radio signals and only the extreme accuracy of atomic clocks makes this level of precision possible. But even their extraordinary accuracy has to be monitored and corrected by the control system.
It is the function of the control station in Colorado Springs to ensure that the satellites are in their correct positions and to correct each of the four atomic clocks on board every satellite. This is done using data from the monitoring stations. The specification for the error of each atomic clock is to keep this to less than 7 nanoseconds, which is the equivalent of a 2 meter error in the position of the vessel.
The locations of monitoring stations on the surface of the globe and the control station in USA.
This diagram is from www.gps.gov where up-to-date information about the GPS is available
The system keeps track of the positions of the satellites with the help of the monitoring stations. The control station sends orbital information to the satellites, which in turn relay this to the receivers. This informs the receivers of where the satellites are located in space at any given instant in time.2. GPS satellites
The GPS system needs more than 24 satellites that circle the globe in six orbital planes with an orientation of 55° to the equatorial plane. Each plane contains four satellites. This ensures that at any moment there are at least six satellites whose height over the horizon is at least 9.5° everywhere on the surface of the earth. This is considered to be the lowest angle at which an acceptable signal can be received by a simple antenna. The oldest satellites have stopped working but new generations have been launched. Their useful life is expected to be from 7 to 12 years.
A measurement of distance to a satellite gives a position circle on the surface of the earth
Satellites transmit signals continuously on two frequencies, LI on 1575.42 MHz for civilian use (also called C/A, Coarse Acquisition) and L2 on 1227.60 MHz (P, Precise code). This is for military applications and provides greater protection from interference.
Owing to energy restrictions in space the signals are weak, about 50 W. The energy source is solar panels and, on the dark side of the earth, batteries. The carrier wave is coded with the unique code that identifies a satellite and the navigational information. This is divided into 25 blocks, each containing 1500 bits, and is sent continually. The transmission rate is 50 bits/second so it takes 12.5 minutes for complete transmission. The information contains:
4. Satellite receiver
Receivers for the signals from navigational satellites come in a variety of shapes and sizes, for different applications.
A receiver from Gecko Systems which can handle GPS/G!onass and other systems
A little GPS receiver without a screen with a Bluetooth connection to a computer. Also available with a UBS connecto
КWhen GPS is combined with a processor, memory, screen and electronic chart one gets a chart plotter
Receiver that displays position, COG and SOG. Can be connected to a navigational system
4.1. Boot-up (Start)
The time required to boot a receiver is quite variable, depending, for example, on whether this is the initial boot-up or if the system has been moved when switched off, the age of the unit, its quality and the manufacturer. Newer units boot faster.
4.2. Hot Start
The fastest boot-up is obtained when the satellite receiver has access to the current position, time, almanac and orbital data. This situation occurs when the unit is booted in more or less the same location and within a few hours of the latest position plot.
4.3. Warm Start
The time and almanac are valid but the orbital data are not absolutely up-to-date. Each satellite broadcasts its ephemeris data every 30 seconds.
4.4. Cold or Factory start
If the receiver has been switched off for some weeks, stored without batteries or has been repositioned on board since the latest plot then boot-up will be even slower. Then the receiver must search for all possible satellites in order to obtain information of all satellites, called the almanac. This information is transmitted repeatedly over 12,5 minutes.
These functions are under continual development. Some of the newer receivers have a very short boot-up time.