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INTRODUCTION

The GPS is a worldwide radio navigation system formed from a constellation of 24 satellites and their ground stations. GPS uses these "manmade stars" as reference points to calculate the positions accurate to a matter of meters. In fact, with advanced forms of GPS you can make measurements to better than a centimeter! In a sense it is like giving every square meter on the planet a unique address, the system was designed for and is operated by the U. S. military. GPS is funded by and controlled by the U. S. Department of Defense (DOD). While there are many thousands of civil users of GPS world wide. GPS has mainly 3 parts the space segment, the user segment and the control segment. They are collectively called as GPS Elements.

IMPORTANCE OF GPS

WHAT IS NAVIGATION?

Since prehistoric times, people have been trying to figure out a reliable way to tell where they are, to help guide them where they are going, and to get them back home again. Cavemen probably used stones and twigs to mark a trail when they set out hunting for food. The earliest mariners followed the coast closely to keep from getting lost. When navigators first sailed into the open ocean, they discovered that they could chart their course by following the stars. The ancient Phoenicians used the North Star to journey from Egypt and Crete. According to Homer, the goddess Athena told Odysseus to "keep the Great Bear on his left" during his travels from Calypso's island. Unfortunately for Odysseus and all the other mariners ,the stars are only visible at night -and only on clear nights .

The next major developments in the quest for perfect method of navigation were the magnetic compass and the sextant. The needle of a compass always points to the North, so it is always possible to know in what direction you are going. The sextant uses adjustable mirrors to measure the exact angle of the stars, moon and the sun above the horizon. However in the early days of its use, it was only possible to determine latitude (the location on the Earth located north or south from the equator) from the sextant observations. Sailors were still unable to determine their longitude (the location on the Earth located east or west).this was such a serious problem that in the 17th century British formed a special board of longitude consisting of well-known scientists. This group offered $20,000equal to a million of today's dollars, to anybody who could find a way to determine a ship's longitude within 30 nautical miles.

The generous offer paid off. In 1761,a cabinet maker named John Harrison developed a shipboard timepiece called a chronometer, which lost or gained only about one second a day -incredibly accurate for the time. For the next two centuries sextants and chronometers were used widely in combination to provide latitude and longitude information.

In the early 20th century several radio-based navigation systems were developed, which were used widely during World War II. Both allied and enemy Ships and airplanes used ground-based radio navigation systems as the technology advanced.

A few ground-based radio navigation systems are still in use today. One drawback of using radio waves generated on the ground is that you must choose between a system that is very accurate but doesn't cover a wide area, or one that covers a wide area but is not very accurate. High frequency radio waves (like UHF TV) can provide accurate position location but can only be picked up in a small, localized area. Lower frequency radio waves (like AM radio) can cover larger area, but are not a good yardstick to tell you exactly where you are.

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