ABSTRACT: Global Positioning System (GPS) is the only system today able to show ones own position on the earth any time in any weather, anywhere. This paper addresses this satellite based navigation system at length. The different segments of GPS viz. space segment, control segment, user segment are discussed. In addition, how this amazing system GPS works, is clearly described. The various errors that degrade the performance of GPS are also included. DIFFERENTIAL GPS, which is used to improve the accuracy of measurements, is also studied. The need, working and implementation of DGPS are discussed at length. Finally, the paper ends with advanced application of GPS.
INTRODUCTION: The Global Positioning System (GPS) is a satellite-based Navigation system developed and operated by the US Department of Defense. GPS Permits land, sea and airborne users to determine their three-dimensional position, velocity and time. This service is available to military and civilian users around the clock, in all weather, anywhere in the world.
HISTORY: Since prehistoric times, people have been trying to figure out a reliable way to tell where they are, to help guide them to where they are going, and to get they back home again. 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. 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 the perfect method of navigation were the magnetic compass and the sextant. The needle of a compass always points 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 sun above the horizon.
In the early 20th century several radio-based navigation systems were developed. 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 a larger area, but are not a good yardstick to tell you exactly where you are. A transmitter high above the Earth sending a high-frequency radio wave with a special coded signal can cover a large area and still overcome much of the "noise" encountered on the way to the ground. This is the main principle behind the GPS system.
GPS ELEMENTS: GPS has 3 parts: the space segment, the user segment, and the control segment. The space segment consists of 24 satellites, each in its own orbit 11,000 nautical miles above the Earth. The user segment consists of receivers, which you can hold in your hand or mount in your car. The control segment consists of ground stations (five of them, located around the world) that make sure the satellites are working properly.
Space segment: The complete GPS space system includes 24 satellites, 11,000 nautical miles above the Earth, which take 12 hours each to go around the Earth once (one orbit). They are positioned so that we can receive signals from six of them nearly 100 percent of the time at any point on Earth. There are six orbital planes (with nominally four Space Vehicles in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane.
Satellites are equipped with very precise clocks that keep accurate time to within three nanoseconds. This precision timing is important because the receiver must determine exactly how long it takes for signals to travel from each GPS satellite. The receiver uses this information to calculate its position.
The first GPS satellite was launched in 1978. The first 10 satellites were developmental satellites, called Block I. From 1989 to 1993, 23 production satellites, called Block II, were launched. The launch of the 24th satellite in 1994 completed the system.
Control Segment: The control segment consists of a worldwide system of tracking and monitoring stations.The 'Master Control Facility' is located at Falcon AFB in
. The monitor stations measure signals from the GPS satellites and relay the information they collect to the Master Control Station. The Master Control Station uses this data to compute precise orbital models for the entire GPS constellation. This information is then formatted into updated navigation messages for each satellite. Colorado Springs, CO
User Segment: The user segment consists of the GPS receivers, processors and antennas utilized for positioning and timing by the community and military. The GPS concept of operation is based on satellite ranging. Users figure their position on the earth by measuring their distance to a group of satellites in space. Each GPS satellite transmits an accurate position and time signal. The user's receiver measures the time delay for the signal to reach the receiver. By knowing the distance to four points in space, the GPS receiver is able to triangulate a three-dimensional position.
WORKING OF GPS:
The principle behind GPS is the measurement of distance (or "range") between the receiver and the satellites. The satellites also tell us exactly where they are in their orbits above the Earth. 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.
One trip around the Earth in space equals one orbit. The GPS satellites each take 12 hours to orbit the Earth. Each satellite is equipped with an accurate clock to let it broadcast signals coupled with a precise time message. The ground unit receives the satellite signal, which travels at the speed of light. Even at this speed, the signal takes a measurable amount of time to reach the receiver. The difference between the time the signal is sent and the time it is received, multiplied by the speed of light, enables the receiver to calculate the distance to the satellite. To measure precise latitude, longitude, and altitude, the receiver measures the time it took for the signals from four separate satellites to get to the receiver.
It works something like this: If we know our exact distance from a satellite in space, we know we are somewhere on the surface of an imaginary sphere with radius equal to the distance to the satellite radius. If we know our exact distance from two satellites, we know that we are located somewhere on the line where the two spheres intersect. And, if we take a third measurement, there are only two possible points where we could be located. By taking the measurement from the fourth satellite we can exactly point out our location.
Need for DGPS: As the GPS receivers use timing signals from at least four satellites to establish a position, each of those timing signals is going to have some error or delay, depending on what sort of perils have befallen it on its trip down to receiver. Since each of the timing signals that go into a position calculation has some error, that calculation is going to be a compounding of those errors.
The sheer scale of the GPS system solves the problem. The satellites are so far out in space that the little distances we travel here on earth are insignificant. So if two receivers are fairly close to each other, say within a few hundred kilometers, the signals that reach both of them will have traveled through virtually the same slice of atmosphere, and so will have virtually the same errors.
The underlying premise of differential GPS (DGPS) is that any two receivers that are relatively close together will experience similar atmospheric errors. Differential GPS involves the cooperation of two receivers, one that's stationary and another that's roving around making position measurements. Since the reference receiver has no way of knowing which of the many available satellites a roving receiver might be using to calculate its position, the reference receiver quickly runs through all the visible satellites and computes each of their errors. Then it encodes this information into a standard format and transmits to the roving receivers. It’s as if the reference receiver is saying: "OK everybody, right now the signal from satellite #1 is ten nanoseconds delayed, satellite #2 is three nanoseconds delayed, and satellite #3 is sixteen nanoseconds delayed..." and so on. The roving receivers get the complete list of errors and apply the corrections for the particular satellites they're using.
Implementing DGPS: The three main methods currently used for ensuring data accuracy are real-time differential correction, reprocessing real-time data, and post processing.
1. Real-Time DGPS
Real-time DGPS occurs when the base station calculates and broadcasts corrections for each satellite as it receives the data. The correction is received by the roving receiver via a radio signal, if the source is land based or via a satellite signal, if it is satellite based and applied to the position it is calculating. As a result, the position displayed and logged to the data file of the roving GPS receiver is a differential corrected procedure.
2. Reprocessing Real-Time Data
Some GPS manufacturers provide software that can correct GPS data that was collected in real time. This is important for GIS data integrity. When collecting real-time data, the line of sight to the satellites can be blocked or a satellite can be so low on the horizon that it provides only a weak signal, which causes spikes in the data. Reprocessing real-time data removes these spikes and allows real-time data that has been used in the field for navigation or viewing purposes to be made more reliable before it is added to a GIS.
3. Post processing Correction
Differentially correcting GPS data by post processing uses a base GPS receiver that logs positions at a known location and a rover GPS receiver that collects positions in the field. The files from the base and rover are transferred to the office processing software, which computes corrected positions for the rover's file. This resulting corrected file can be viewed in or exported to a GIS.
Thus, Differential GPS or "DGPS" can yield measurements good to a couple of meters in moving applications and even better in stationary situations. That improved accuracy has a profound effect on the importance of GPS as a resource. With it, GPS becomes more than just a system for navigating boats and planes around the world. It becomes a universal measurement system capable of positioning things on a very precise scale.
LIMITATIONS OF GPS:
GPS can provide worldwide, three-dimensional positions, 24 hours a day, in any type of weather. However, the system does have some limitations. There must be a relatively clear "line of sight" between the GPS antenna and four or more satellites. Objects, such as buildings, overpasses, and other obstructions, that shield the antenna from a satellite can potentially weaken a satellite's signal such that it becomes too difficult to ensure reliable positioning. These difficulties are particularly prevalent in urban areas. The GPS signal may bounce off nearby objects causing another problem called multipath interference.
APPLICATIONS OF GPS:
GPS receivers were used in several aircraft, including F-16 fighters, KC-135 aerial refuel, and B-2 bombers; Navy ships used them for rendezvous, minesweeping, and aircraft operations.
GPS has become important for nearly all military operations and weapons systems .In addition, it is used on satellites to obtain highly accurate orbit data and to control spacecraft orientation.
GPS is based on a system of coordinates called the World Geodetic System 1984 (WGS 84). The WGS 84 system provides a built-in frame of reference for all military activities, so units can synchronize their maneuvers.
GPS is also helping to save lives. Many police, fire, and emergency medical service units are using GPS receivers to determine the police car, fire truck, or ambulance nearest to an emergency, enabling the quickest possible response in life-or-death situations.
CONCLUSION: GPS, a satellite based navigation system, thus can be used to determine the position of an object on earth. As discussed above, its application field is vast and new applications will continue to be created as the technology evolves. GPS can also interface with other similar projects such EU’s GALILEO to account for unpredictable applications. Thus, the GPS constellation, like manmade stars in the sky, can be used for guiding and navigation.
REFERENCES: Satellite Communication-T.Pratt and C.W.Bostain-John Wiley and Sons.