20-11-2012, 02:16 PM
Seminar On GLOBAL POSITIONING SYTEM Report
1GLOBAL.doc (Size: 392.5 KB / Downloads: 29)
ABSTRACT
The GPS (Global Positioning System) is a “constellation” of 24 well-spaced satellites that orbit the Earth and make it possible for people with ground receivers to pinpoint their geographic location. The location accuracy is anywhere from 100 to 10 meters for most equipment. Accuracy can be pinpointed to within one (1) meter with special military-approved equipment. GPS equipment is widely used in science and has now become sufficiently low-cost so that almost anyone can own a GPS receiver.
GPS receivers are becoming consumer products. In addition to their outdoor use (hiking, cross-country skiing, ballooning, flying, and sailing), receivers can be used in cars to relate the driver’s location with traffic and weather information. Here are some Web locations that describe GPS receiver productsThe GPS is owned and operated by the U.S. Department of Defense but is available for general use around the world. Briefly, here’s how it works:
21 GPS satellites and three spare satellites are in orbit at 10,600 miles above the Earth. The satellites are spaced so that from any point on Earth, four satellites will be above the horizon.
Each satellite contains a computer, an atomic clock, and a radio. With an understanding of its own orbit and the clock, it continually broadcasts its changing position and time. (Once a day, each satellite checks its own sense of time and position with a ground station and makes any minor correction.) On the ground, any GPS receiver contains a computer that “triangulates” its own position by getting bearings from three of the four satellites. The result is provided in the form of a geographic position - longitude and latitude - to, for most receivers, within 100 meters. If the receiver is also equipped with a display screen that shows a map, the position can be shown on the map. If a fourth satellite can be received, the receiver/computer can figure out the altitude as well as the geographic position.
History of Man & Navigation
Ever since man first left his home in search of food, wealth, or whatever, he has needed a way to find his way back home. In the earliest days, this was done by simply following landmarks, either along the road, or landmarks that consisted of prominent features on the coastline. This method of navigation was obviously very limited when it comes to traveling across oceans, or even large bodies of water. Navigation by reference to the sun, moon, and the stars was the next logical progression. Techniques related to celestial navigation have improved over the last couple of thousand years to the point where now it is possible to locate a position to an accuracy of better than one hundred feet. However, the instruments required to measure latitude and longitude to an accuracy of one hundred feet or so are definitely not suited to the rapid, dynamic measurements that are necessary for navigation.
Navigation remained little changed from the late 1700’s to the early 1900’s. The primary instrument was the sextant and a highly accurate chronometer, coupled with a compass and something to measure the distance traveled since the last “fix”. The problem with this technique, obviously, is that you can only fix your position with any real degree of accuracy when the sun or stars are visible. During cloudy weather, no reliable means of locating one’s position existed. Furthermore, although a compass can tell you in which direction you are headed, it can not tell you in what direction you are actually traveling. These two directions could be somewhat different, depending on conditions of wind, waves, and currents. Thus, dead reckoning is often not a very satisfactory method of navigating for airplanes and ships.
The GPS Classification
The GPS (Global Positioning System) is a “constellation” of 24 well-spaced satellites that orbit the Earth and make it possible for people with ground receivers to pinpoint their geographic location. The location accuracy is anywhere from 100 to 10 meters for most equipment. Accuracy can be pinpointed to within one (1) meter with special military-approved equipment. GPS equipment is widely used in science and has now become sufficiently low-cost so that almost anyone can own a GPS receiver.
GPS receivers are becoming consumer products. In addition to their outdoor use (hiking, cross-country skiing, ballooning, flying, and sailing), receivers can be used in cars to relate the driver’s location with traffic and weather information. Here are some Web locations that describe GPS receiver productsThe GPS is owned and operated by the U.S. Department of Defense but is available for general use around the world. Briefly, here’s how it works:
21 GPS satellites and three spare satellites are in orbit at 10,600 miles above the Earth. The satellites are spaced so that from any point on Earth, four satellites will be above the horizon.
Each satellite contains a computer, an atomic clock, and a radio. With an understanding of its own orbit and the clock, it continually broadcasts its changing position and time. (Once a day, each satellite checks its own sense of time and position with a ground station and makes any minor correction.) On the ground, any GPS receiver contains a computer that “triangulates” its own position by getting bearings from three of the four satellites. The result is provided in the form of a geographic position - longitude and latitude - to, for most receivers, within 100 meters. If the receiver is also equipped with a display screen that shows a map, the position can be shown on the map. If a fourth satellite can be received, the receiver/computer can figure out the altitude as well as the geographic position.
GPS as a Satellite Navigation
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, the system was designed for and is operated by the U. S. military.
GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time.
Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock.
Space Segment
The Space Segment of the system consists of the GPS satellites. These space vehicles (SVs) send radio signals from space.
The nominal GPS Operational Constellation consists of 24 satellites that orbit the earth 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 orbitalplanes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane.
Control segment
The Master Control facility is located at Schriever Air Force Base (formerly Falcon AFB) in Colorado. These monitor stations measure signals from the SVs which are incorporated into orbital models for each satellites. The models compute precise orbital data (ephemeris) and SV clock corrections for each satellite. The Master Control station uploads ephemeris and clock data to the SVs. The SVs then send subsets of the orbital ephemeris data to GPS receivers over radio signals.
2.4 User Segment
The GPS User Segment consists of the GPS receivers and the user community. GPS receivers convert SV signals into position, velocity, and time estimates. Four satellites are required to compute the four dimensions of X, Y, Z (position) and Time. GPS receivers are used for navigation, positioning, time dissemination, and other research.Navigation in three dimensions is the primary function of GPS. Navigation receivers are made for aircraft, ships, ground vehicles, and for hand carrying by individuals.
The Big Idea Geometrically
Suppose we measure our distance from a satellite and find it to be 11,000 miles.Knowing that we're 11,000 miles from a particular satellite narrows down all the possible locations we could be in the whole universe to the surface of a sphere that is centered on this satellite and has a radius of 11,000 miles.Next, say we measure our distance to a second satellite and find out that it's 12,000 miles away.That tells us that we're not only on the first sphere but we're also on a sphere that's 12,000 miles from the second satellite. Or in other words, we're somewhere on the circle where these two spheres intersectIf we then make a measurement from a third satellite and find that we're 13,000 miles from that one, that narrows our position down even further, to the two points where the 13,000 mile sphere cuts through the circle that's the intersection of the first two spheres.So by ranging from three satellites we can narrow our position to just two points in space.To decide which one is our true location we could make a fourth measurement. But usually one of the two points is a ridiculous answer (either too far from Earth or moving at an impossible velocity) and can be rejected without a measurement.A fourth measurement does come in very handy for another reason however, but we'll tell you about that later.