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Global Positioning System (GPS)

GPS, Global Positioning Syste, Space segment, Receiver, satellites, Earth orbit, latitude, longitude, ground station, Time of Arrival, US Defense, Atomic cloc, almanec, ephimerous, selective ability, base station,

Global Positioning System (GPS)

GPS is the most modern approach towards solving one of the great questions of human history “where are we?” Global Positioning System is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations.The Global Positioning System (GPS) is developed by U.S. Department of defense. Additional 5 more satellites are added into the constellation to make this network of 29 satellites to obtain more precise and accurate information. Originally GPS was built to use for the military applications, however, the US government realising its importance for civilian use made it available for public in 1980. The GPS can be used free of cost for the civilian purposes.

GPS can be used for determining a basic position of object or the people on the earth, knowing the movement from one place to other place, monitoring the movement of people and things, creating maps of the world, and bringing precise timings to the world. The current GPS consists of three major segments: space segment (SS), a control segment (CS), and a user segment (US). The space segment is composed of the orbiting GPS satellites, or Space Vehicles (SV). The GPS is designed such a way that 24 SVs are always to be distributed equally among six circular orbital planes of the earth. The orbital planes are centered on the earth and the six planes have approximately 55° inclination and are separated by 60° right ascension of the ascending node.

These satellites are orbiting at an altitude of approximately 20,200 kilometers (12,600 miles or 10,900 nautical miles; orbital radius of 26,600 km (16,500 mi or 14,400 NM)). Each SV makes two complete orbits each day, so it passes over the same location on Earth twice each day. The orbits are arranged so that at least six satellites are always within line of sight from almost anywhere on Earth.

The orbiting paths of the satellites are monitord by US Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, along with monitor stations operated by the National Geospatial-Intelligence Agency (NGA) These mmonitoring stations are the eyes and ears of GPS and monitor satellites as they pass overhead by measuring distances to them every 1.5 seconds. This data is then smoothed using ionospheric and meteorological information and sent to Master Control Station at Colorado Springs. The ionospheric and meteorological data is needed to get more accurate delay measurements, which in turn improve location estimation. Master control station estimates parameters describing satellites' orbit and clock performance. It also assesses health status of the satellites and determines if any re-positioning may be required. This information is then returned to three uplink stations (stationed at the Ascension Island, Diego Garcia and Kwajalein monitor stations) which transmits the information to satellites.

GPS receiver is the user segment of the GPS system. GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock (often a crystal oscillator). This may be built to display location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years such that, as of 2006, receivers typically have between twelve and twenty channels.

GPS receivers can display position data to a PC or other device using the NMEA 0183 protocol. NMEA 2000 is a new and less widely adopted protocol. Both are proprietary and controlled by the US-based National Marine Electronics Association. References to the NMEA protocols have been compiled from public records, allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Other proprietary protocols exist as well, such as the SiRF protocol. Receivers can interface with other devices using methods including a serial connection, USB or Bluetooth.

Satellite transmits Ephemeris and Almanac Data to GPS receivers. Ephemeris data contains important information about status of satellite (healthy or unhealthy), current date and time. This part of signal is essential for determining a position. Almanac data tells GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing orbital information for that satellite and for every other satellite in the system.

GPS uses concept of time of arrival (TOA) of signals to determine user’s position. This involves measuring the time taken for a signal at a known location to reach a user receiver. The signals are transmitted by an emitter (satellite). Time interval is basically signal propagation time. Time interval (signal propagation time) is multiplied by speed of signal (speed of light) to obtain distance from satellite to receiver. By measuring propagation time of signals broadcast from multiple satellites at known locations, receiver can determine its position.

Assuming we have precise clocks, how do we measure signal travel time? In order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond. Accurate time measurements are required. (If we are off by a thousandth of a second, at the speed of light, that translates into almost 200 miles of error)
To make a satellite positioning system using only synchronized clocks, it was required to have atomic clocks not only on all the satellites, but also in the receiver itself. Atomic clocks are highly accurate and precise clocks which utilize the natural vibrations found within some atoms. But atomic clocks cost somewhere between $50,000 and $100,000, which makes them too expensive for everyday consumer use.

The Global Positioning System has a clever solution to this problem. Every satellite contains an expensive atomic clock, but the receiver uses an ordinary quartz clock which it constantly resets. In a nutshell, the receiver looks at incoming signals from four or more satellites and gauges its own inaccuracy.

When we measure the distance to four located satellites, we can draw four spheres that all intersect at one point. Three spheres will intersect even if the numbers are way off, but four spheres will not intersect at one point if we measured incorrectly. Since the receiver makes all its distance measurements using its own built-in clock, the distances will all be proportionally incorrect. The receiver can easily calculate the necessary adjustment that will cause the four spheres to intersect at one point. Based on this, it resets its clock to be in sync with the satellite's atomic clock.

The receiver does this constantly whenever its on, which means it is nearly as accurate as the expensive atomic clocks in the satellites. The receiver can easily calculate the necessary adjustment that will cause the four spheres to intersect at one point. Based on this, it resets its clock to be in sync with the satellite's atomic clock.
The GPS receiver simply stores an almanac that tells it where every satellite should be at any given time. Things like the pull of the moon and the sun do change the satellites' orbits very slightly. However, the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites' signals.

There are two kinds of error experienced in GPS and these can be categorized as intentional and unintentional. Intentional errors: government can and does degrade the accuracy of GPS measurements. This is done to prevent hostile forces from using GPS to full accuracy. Policy of inserting inaccuracies in GPS signals is called Selective Availability (SA). SA was single biggest source of inaccuracy in GPS and was deactivated in 2000.

To ward off inaccuracies due to SA or other errors, the technique called differential correction can yield accuracies within 1-5 meters, or even better, with advanced equipment. Differential correction requires a second GPS receiver, a base station, collecting data at a stationary position on a precisely known point.

Because physical location of base station is known, a location with GPS location is determined by using satellites. A GPS receiver essentially determines the receiver's position on Earth. Once the receiver makes this calculation, it can tell us the latitude, longitude and altitude of its current position. To make the navigation more user-friendly, most receivers plug this raw data into map files stored in memory. We can use maps stored in the receiver's memory, connect the receiver to a computer that can hold more detailed maps in its memory, or simply buy a detailed map of our area and find way using the receiver's latitude and longitude readouts.

Some receivers let us download detailed maps into memory or supply detailed maps with plug-in map cartridges. A standard GPS receiver will not only place us on a map at any particular location, but will also trace our path across a map as we move. If we leave our receiver on, it can stay in constant communication with GPS satellites to see how our location is changing.

This is what happens in cars equipped with GPS. With this information and its built-in clock, the receiver can give us several pieces of valuable information: how far we've traveled, how long we've been traveling, our current speed, our average speed, a trail showing us exactly where we have traveled on the map, and the estimated time of arrival at our destination if we maintain our current speed.
Isn’t GPS amazing?


Published: 2007-01-25
Author: Rama Kant Mishra

About the author or the publisher
I have done Masters in Fisheries Management and have written and published articles on Fisheries, Agriculture, Medical, Pharmaceutical, political, self Improvement and Career. Professionally working for an IT company as a Technical Writer. I love to write and publish and have worked as freelance writer, ghost writer and as special correspondent for print media. You may find me contributing on www.merinews.com and www.groundreport.com You may reach me at mishraramakant@gmail.

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