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Author : Mahesh Patil

This paper reviews the Global Positioning System (GPS) technology in the sense that makes it possible to obtain precise geographic coordinates for virtually any place on earth in an efficient and convenient way. GPS Technology allows precise determination of location, velocity, direction, and time. GPS is a satellite-based radionavigation system developed and operated initially by the U.S. Department of Defense. GPS permits land, sea, and airborne users to determine their three-dimensional position, velocity, and time 24 hours a day, in all weather, anywhere in the world with a precision and accuracy far better than any other radionavigation systems available today or in the foreseeable future. GPS is used to support land, sea, and airborne navigation, surveying, Geophysical exploration, mapping and geodesy, vehicle location systems, and a wide variety of additional applications.

INTRODUCTION

The concept of satellite navigation was conceived following the launch of the first satellite, Sputnik, in 1957. Scientists at the Applied Physics Laboratory of Johns Hopkins University (APL) developed a method of tracking the satellite's orbit by observing the Doppler Shift of the radio signal broadcast by Sputnik I. By reversing this process, it was proposed that a navigator's position could be determined by tracking the Doppler frequency of a radio signal broadcast by a satellite that had a precisely known orbit.

The U.S. Navy were quick to take advantage of this new technology to provide accurate position updates to inertial navigation equipment on ships and submarines around the globe. The TRANSIT satellite navigation system became operational on U.S. Polaris submarines in 1964. TRANSIT was also made available for commercial use, starting in 1967, and commercial users of the navigation system rapidly exceeded military users.

The TRANSIT navigation system was based on measuring the Doppler shift of the satellite radio signal broadcast as it transited the sky. This meant that a position fix was only available roughly every 40 minutes and users had to be nearly stationary to get an accurate navigation fix.

With the development of precision atomic clocks in the 1960s, it became possible to design a satellite constellation that carried a network of clocks precisely synchronized to a common time reference. By broadcasting time-coded signals, a receiver could measure the distance (or range) to a satellite by observing the transit time of the radio signal from the satellite. The first satellite to implement this passive ranging technique was the TIMATION I satellite launched by the U.S. Navy in 1967.

The USAF initiated a program in 1964 to develop and test a coded transmission technique that would provide precise ranging and timing data using a signal modulated with a pseudo-random noise (PRN) code. This has the effect of spreading the signal spectrum over the bandwidth of the modulation code. When the satellite signal is regenerated by correlating with the modulation code at the receiver, any other interfering signals are spread and weakened relative to the desired satellite signal. This feature also allows all of the satellites in the constellation to broadcast on the same frequency without interfering with each other. This feature is known as code division multiple access.

In 1973, the USAF and US Navy programs were combined into the Navigation Technology Program which later evolved into the NAVSTAR GPS program.

The Phase I of the GPS program was a concept validation phase, during which the Block I satellite prototypes (Navistar's 1 through 11) were developed. The Phase II full scale development and test program started in 1978 with the launch of the first four NAVSTAR satellites. By 1985 there were seven operational satellites which provided about 5 hours of navigation coverage daily for testing.

GPS was developed as a military navigation system. As such, it was designed to offer two classes of navigation services: a precision positioning service (PPS) restricted to government users; and a standard positioning service (SPS) that is open for commercial use.

The global positioning system (GPS) provides a high accurate real time location. Originally, this system has been developed by the U.S. Department of Defense and is partially used for commercial purpose, that is, available only for limited operation hours. To dissolve this inconvenience, complete GPS satellite constellation consists of lots of satellites in orbits and at least four satellites are in view at the same time anywhere on the earth. Each satellite, rotating around the earth at an altitude of 20,000 km about twice a day, continuously transmit the information such as, standard time, orbital information and other parameters. This system is now widely used even on the ocean and expected to expand its usage. It also has infinite potential even for fishing industry in the future.

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Last Updated : September 03, 1999 09:46:33 AM -0400

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