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Operation of Global Positioning System(GPS)

INTRODUCTION:
GPS - Global Positioning System - It is a satellite based navigation system. To find the location and time information anywhere on the Earth, GPS devices are used. It provides longitude as well as latitude information of the devices. The signal used in GPS should be unobstructed line of sight and can be used or telecast in all kind of weathers.



All GPS satellites synchronize operations so that the repeating signals are transmitted at the same instant. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are farther away than others. All GPS devices can answer this question "Where I am?" to anyone by calculating altitude and transmitting the details to the receiver.
OPERATION:
A GPS receiver's job is to locate four or more of these satellites, figure out the distanc­e to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration.

Global Positioning System satellites transmit signals to equipment on the ground. GPS receivers passively receive satellite signals (they do not transmit). Then, the receiver (GPS Device) calculates the positioning of the receiver to answer your location.

How does GPS Calculate information?
The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. When the receiver estimates the distance to at least four GPS satellites, it can calculate its position in three dimensions.

For elaborate process:

At a particular time, the satellite begins transmitting a long, digital pattern called a pseudo-random code. The receiver begins running the same digital pattern also exactly at midnight. When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern.


The length of the delay is equal to the signal's travel time. The receiver multiplies this time by the speed of light to determine how far the signal traveled. Assuming the signal traveled in a straight line, this is the distance from receiver to satellite. In order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond.
Advantages:
1. To lower the price of receiver, it is built with quartz clock that keeps synchronization with the satellite.

2. Real time operation is performed. The receiver resets its clock to be in sync with the satellite's atomic clock. The receiver does this constantly whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the satellites.
Disadvantages:
1. GPS receivers require an unobstructed view of the sky, so they are used only outdoors and they often do not perform well within forested areas or near tall buildings.

2. Synchronization should be there for proper functioning. In order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond. To make a satellite positioning system using only synchronized clocks, would need to have atomic clocks.
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