GPS stands for Global Positioning System by which anyone can always obtain the position information anywhere in the world.
GPS consists of the following three segments.
Firstly, the signal of time is sent from a GPS satellite at a given point. Subsequently, the time difference between GPS time and the point of time clock which GPS receiver receives the time signal will be calculated to generate the distance from the receiver to the satellite. The same process will be done with three other available satellites. It is possible to calculate the position of the GPS receiver from distance from the GPS receiver to three satellites. However, the position generated by means of this method is not accurate, for there is an error in calculated distance between satellites and a GPS receiver, which arises from a time error on the clock incorporated into a GPS receiver. For a satellite, an atomic clock is incorporated to generate on-the-spot time information, but the time generated by clocks incorporated into GPS receivers is not as precise as the time generated by atomic clocks on satellites. Here, the fourth satellite comes to play its role: the distance from the fourth satellite to the receiver can be used to compute the position in relations to the position data generated by distance between three satellites and the receiver, hence reducing the margin of error in position accuracy.
The Fig 1-3 below illustrates an example of positioning by two dimensions (position acquisition by using two given points). We can compute where we are at by calculating distance from two given points, and the GPS is the system that can be illustrated by multiplying given points and replacing them with GPS satellites on this figure.
GPS satellites broadcast beams in two carrier frequencies; L1 (1,575.42 MHz) and L2 (1,227.60 MHz). Beams that can be accessible to the general public are encoded in C/A (Coarse/Acquisition) code, and the beams that can be used only by the US military force are encoded in P (Precise) code. C/A code consists of identification codes of each satellite and is broadcast together with navigation messages. The data of the orbit of each satellite is called the ephemeris*, and the data of orbit of all satellite is called the almanac**. The navigation messages are broadcast at a rate of 50 bits per second. Utilizing this collection of data, GPS receiver calculates distance between satellites and the receiver in order to generate position data. In the Fig 1-4, the details of C/A code is described, and in the Fig 1-5, navigation messages are described.
*The ephemeris provides the precise orbit for the satellite itself, which can be used to generate precise location of the satellite, necessary information for calculating position information. It is the indigenous data that is used only by each of the GPS satellites with specific identification number.
**The almanac can be regarded as simplified ephemeris data and contains coarse orbit and status information for all satellites in the network. It is used to locate available satellites in order a GPS receiver to generate current position and time. It takes 12.5 minutes to receive all the almanac data.
DOP is a value that shows the degree of degradation of the GPS positioning accuracy. The smaller the value is, the higher the positioning accuracy is. This value depends upon the positions of the GPS satellites tracked for positioning. If the tracked satellites spread evenly over the earth, the positioning accuracy would become higher, and if the positions of tracked satellites are disproportionate, the positioning accuracy would become lower.
State of reception of GPS depends upon the strength of GPS signals. The greater the signal strength is, the more stable the reception status is. Whereas the reception status would become unstable when the GPS signal became weaker, due to obstacles or noise sources in the vicinity of a GPS receiver.
State of reception of GPS depends upon the number of satellites tracked for positioning.
If the number of the tracked satellites is great, GPS positioning becomes greater, but if there were a fewer satellites tracked for positioning, it would be difficult to generate GPS position. The Fig. 1-11 illustrates the occasion where the GPS receiver tracks a greater number of satellites for positioning. The Fig. 1-12 illustrates the occasion where the GPS receiver tracks only a few number of satellites for positioning.
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