GPS systems

Precisely locating positions on Earth is not a new phenomenon. Navigators, sailors, explorers and surveyors have done this for centuries as they traveled about the world. Most maps and globes display longitude and latitude or some other coordinate projection information. Points on Earth are given unique addresses on maps using specific coordinate systems. Agriculturists commonly use either a geographic system of latitude and longitude measured in degrees or a Universal Transverse Mercator coordinate system that locates positions in meters measured from a specific point.

The GPS system uses measured distances to the precisely located GPS satellites to locate positions on Earth. Radio receivers in GPS units monitor radio signals broadcast from the GPS satellites.

A GPS position is determined by simultaneously measuring the distance to at least three satellites. The distance to a satellite is measured by the time it takes a radio signal to travel from the satellite to the GPS receiver. Computers in GPS units use information from the radio signals, including broadcast time and unique satellite information, to calculate positions. Information from at least four satellites is needed to calculate elevation. Signal reception from more satellites increases position accuracy.

The global positioning system includes a constellation of 24 systematically arranged satellites orbiting the earth in 6 orbital planes with 4 satellites in each plane.

The satellite orbits are approximately 12,500 miles above the earth. The constellation is arranged to guarantee radio reception from at least 4 satellites from any location anytime, anywhere on Earth (Figure 3). GPS receivers normally receive signals from 8 to 9 satellites in location without obstructions like buildings or trees.

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List of Cell Phones with GPS:
Amoi (2) BlackBerry (4) HTC (7) Nokia (5)
Asus (4) Eten (8) i-mate (3) O2 (3)
Benefon (4) Fujitsu Siemens Mitac (4) Samsung (3)
BenQ (1) Gigabyte (3) Motorola (12) Siemens (1)
BenQ-Siemens HP (6) NEC (3) Thuraya (1)
      Toshiba (2)