پروژه زبان تخصصی نوروز

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Global Positioning System: The GPS consists of 24 satellites that circle the earth in six,  12- hour orbits. These satellites, known as NAVSTAR (NAVigation by Satellite Timing

And Ranging), have been launched and maintained by the department of defense (DOD)

to provide all weather ranging 24 hours a day anywhere on the surface of the earth. Each

orbit is at an altitude of 20,200 km (12,500 miles) and contains four satellites. These

satellites are equipped with four atomic clocks each to provide accurate timing. They

transmit two radio frequencies on two separate L-bands (L1 = 1575.42 MHz and

L2=1227.60 MHz). The L1 signal consists of a Course/Acquisition (C/A) and a

Precision (P) code. The L2 signal contains only the P-code. Only the C/A code of the L1

signal, known as the standard positioning service (SPS), is available for civilian use. The

L1 and L2 signals are available for military and other authorized users and provide a precise positioning service (PPS). The part of the GPS that consists of satellites is known as the space segment. The GPS satellites are continuously monitored by four ground-based stations located at

Ascension island, Diego Garcia, Hawaii, and Kwajalein. The master control station

located at Colorado Springs, CO receives the information on the health of the satellites

and their exact locations from monitoring stations and uploads necessary corrections to

the satellites. These ground-based stations form the control segment of the GPS.

The GPS receivers that receive information from the GPS satellites form the user

segment of the GPS. The receiver computes the time required for the radio signal to

reach the receiver from each of the satellites that is “visible.” From these timing signals,

the receiver calculates its distance from each of the “visible” satellite by multiplying the

time by the speed of radio signal (300,000 km/s or 186,300 miles/s). This process is

known as ranging. If the distances of the receiver from at least three satellites are

available, the location of the receiver on the surface of the earth can be calculated.

However, the timing measurements are never completely accurate because the receiver

clock is not as accurate as the atomic clocks in the satellites. The GPS receivers try to

minimize timing errors using information from an additional (fourth) satellite. The

distance of the receiver from each of the “visible” satellite is called “pseudo range” since

this range is not accurate due to clock error. Note that pseudo ranges from at least four

satellites are necessary to obtain accurate estimation of receiver location (figure 2).