30-07-2014, 12:36 PM
GLOBAL POSITIONING SYSTEMS
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Abstract
GPS, which stands for Global Positioning System, is the only system today able to show you your exact position on the Earth anytime, in any weather, anywhere. GPS uses satellite technology to enable a terrestrial terminal to determine its position on the Earth in latitude and longitude. GPS satellites, 24 in all, orbit 11,000 miles above the Earth. Ground stations located worldwide continuously monitor them. The satellites transmit signals that can be detected by anyone with a GPS receiver. Using the receiver, you can determine your location with great precision.
Introduction
Since prehistoric times, people have been trying to figure out a reliable way to tell where they are and to help guide them to where they are going. 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 they could chart their course by following the stars. The ancient Phoenicians used the North Star to journey from Egypt and Crete. The next major developments in the quest for the perfect method of navigation were the magnetic compass and the sextant.
About 50 years ago, when the space technology was born, it was actually giving birth to an entirely new method of tracking and guiding, employing man made stars, now known as Satellite Navigation Systems and Global Positioning Systems is the latest of its out come. The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. GPS uses navigation satellites as reference points to calculate positions accurate to a matter of meters. In fact, with advanced forms of GPS you can make measurements to better than a meter! In a sense it's like giving every square meter on the planet a unique address. GPS receivers have been miniaturized to just a few integrated circuits and so are becoming very economical. And that makes the technology accessible to virtually everyone.
GPS
The nominal GPS Operational Constellation consists of 27(24 in operation and three extras in case one fails). The U.S. military developed and implemented this satellite network as a military navigation system, but soon opened it up to everybody else. These satellites orbit the earth once in 12 hours. There are often more than 24 operational satellites as new ones are launched to replace older satellites. The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier each day). There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about 55 degrees with respect to the equatorial plane. This constellation provides the user with between five and eight SVs visible from any point on the earth.
This Calculation of distance is based on a simple mathematical principle called trilateration. That is, finding the location, if distance from a few number of fixed points are known. Below figure will help us to know the various terminologies used in satellite navigation such as Orbital planes, inclination etc
The Space Segment
The space segment consists of a constellation of 24 active satellites orbiting the earth every 12 hours. There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. Four satellites are located in each of six orbits. The orbits are distributed evenly around the earth. The satellites orbit at an altitude of about 20,200 km at a velocity of 26.61 km per second. Satellites are positioned so that we can receive signals from six of them nearly 100 percent of the time at any point on Earth. You need that many signals to get the best position information. This constellation provides the user with between five and eight SVs visible from any point on the earth. These satellites are equipped with very precise clocks that keep accurate time to within three nanoseconds —that’s 0.000000003, or three billionths, of a second. This precision timing is important because the receiver must know exactly how long it takes for its signal to get to each satellite and return. By knowing the exact amount of time the signal has taken to get back from each satellite, it can calculate its position. Each SV contains four atomic clocks (two cesium and two rubidium). SV clocks are monitored by ground control stations.
The Control Segment
The GPS control, or ground, segment consists of unmanned monitor stations located around the world (Hawaii and Kwajalein in the Pacific Ocean; Diego Garcia in the Indian Ocean; Ascension Island in the Atlantic Ocean; and Colorado Springs, Colorado); These stations track and monitor the GPS satellites. The Master Control facility is at Falcon Air Force Base in Colorado Springs, Colorado; and four large ground antenna stations broadcast signals to the satellites. These monitor stations measure signals from the SVs which are incorporated into orbital models for each satellites. The models compute precise orbital data and SV clock corrections for each satellite. The Master Control station uploads orbital data and clock data to the SVs. The SVs then send subsets of the orbital data to GPS receivers over radio signals
Navigation Message
The GPS Navigation Message contains parameters that describe the location of the GPS satellites, their clock offsets, and various other system parameters. The Navigation Message consists of 25 data frames, each divided into five sub-frames. The sub-frames are 300 bit sequences. Bits are transmitted at 50 bits per second. Each sub-frame takes 6 seconds, each frame 30 seconds, and the entire set of 25 frames takes 750 seconds (12.5 minutes) to complete.
Sub-frames one, two, and three contain messages pertaining to the satellite that is transmitting them (complete orbit and clock descriptions) , while sub-frames 4 and 5 contain system data relating to all the satellites (abbreviated orbit and clock data for all the satellites and common system).
Within a GPS receiver the received data bits are aligned, checked for errors (with a parity algorithm), separated into sets of bits representing each individual parameter, scaled, converted into various numeric formats, and then converted where required into specific units. These units include meters, meters squared, semi-circles, radians, seconds, seconds per second, seconds per second per second, and weeks
. Normally, a receiver gathers new Orbital data each hour, but can use old data for up to four hours without much error. The accurate orbital parameters are used with an algorithm that computes the SV position for any time within the period of the orbit described by the orbital parameter set. The approximate orbital data is used to preset the receiver with the approximate position and carrier Doppler frequency (the frequency shift caused by the rate of change in range to the moving SV) of each SV in the constellation. Each complete SV data set includes an ionosphere model that is used in the receiver to approximate the phase delay through the ionosphere at any location and time.
The GPS receiver produces replicas of the C/A and/or P (Y)-Code. Each PRN code is a noise-like, but pre-determined, unique series of bits. The receiver produces the C/A code sequence for a specific SV with some form of a C/A code generator. Modern receivers usually store a complete set of precompiled C/A code chips in memory.
Conclusion
The future of GPS is as unlimited. New applications will continue to be created as technology evolves. The GPS satellites are like handmade stars in the sky.
But no technology last forever. In the future GPS will also get replaced with more advanced technologies. But it had really played a great role in its journey for more perfect and more convenient Navigational Systems.