How GPS works

A detailed Summary of How GPS works

The GPS consists of three main segments: space, control, and user. Satellites, which are launched into specific orbits and monitored by ground station, transmit information in form of radio signals. Ground stations update the information from satellites and send update signals back to satellites. Users with GPS receivers can decode satellite signals to find helpful information such as a location in a desert.

The space segment includes 24 satellites that are launched into six specific orbits to ensure coverage of the whole globe. The six orbits are 20,200 km above the earth. They are 60 degrees apart with 55 degrees inclination with respect to the equatorial plane.

The satellites are equipped with atomic clocks that provide accurate time to within three billionths of a second. The satellites transmit signals on two L-band frequencies (L1 = 1575.42 MHz and L2 = 1227.6 MHz) to equipment on the ground. The satellite signals consist of a Pseudo Random Code (PRC), ephemeris, and almanac data. The Pseudo Random Code is a complicated sequence of pulse signals. This code ensures not only that the same frequency can be used for all satellites, but also that GPS receivers

In fact, three satellites are needed to locate a 2D position. That means the information from three satellites can only reveal a precise latitude/longitude position. In case of an airplane position, a receiver needs information from at least four satellites to determine its latitude, longitude, and altitude (X, Y and Z locations).

Since the satellites transmit signals at the speed of light, the distance from a satellite to receiver is calculated by multiplying the speed of light by the travel time from satellite to receiver. For a difference of a thousandth of a second, an error will be 340 km. That means high precision atomic clocks need to be set up on satellites. With an imperfect clock in a receiver, a crosscheck of a fourth measurement is included. When the fourth measurement is not matched with the other three measurements of satellites, the receiver computer uses a correction factor to calculate the position.

can receive the GPS signals without the need of big satellite dishes. Moreover, Pseudo Random Code also identifies which satellite is sending. When a receiver receives a Pseudo Random Code, the number of the satellite shows up on a screen. The ephemeris data reflects status of satellites, current date and time. The position of a satellite is encoded in almanac data. A

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Approximate Word count = 878
Approximate Pages = 4 (250 words per page double spaced)

Category: Miscellaneous

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How GPS works

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