Global Posstsoning Syste

Global Positioning System

GLOBAL POSITIONING SYSTEM Introduction to GPS ;Since ancient times, human have looked for new ways to find there way out of the geospatial puzzle. The dependence started with stars as direction finder. The mariners use compass and modem navigation system. But star shift position, compasses gets affected by magnetic field or magnets. So if you wonder what could be near perfect solution, bank on "Global Positioning System" (GPS). It not only established your exact location but also ensures that you are going in the right direction. In 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 them back to home again. The earliest mariners followed the coast closely to them to keep them from getting lost when navigators first sailed in to the open ocean, they discovered they could chart their course by following stars. The ancient phoencians used the North stars during journey. But unfortunately for all mariners the stars are only visible at night and only on clear nights. In the early 20th century several radio based navigation systems were developed, which were used widely during world war II. One draw back of using radio waves generated on the ground is that you must choose between the system that is very accurate, high frequency radio waves like UHF (Ultra High Frequency) can provide accurate location but only be picked up in a small localized area. Low frequency radio waves like AM wave i.e. amplitude modulated wave can cover large area but are not good to tell your exact location. Scientist therefore decided that the only way to provide coverage for the entire world was to place 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 one of the main principles behind GPS system. The GPS system can tell you your location anywhere on or behind above the earth to within about 300 feet. Even grater accuracy, usually within less than three

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Global Positioning System

feet, can be obtained with correction calculated by a GPS receiver at a known fixed location. What is GPS? GPS is the word which stands for “Global Positioning System” is the only system today able to show your exact position on the earth anytime, any where. In the GPS system there are 24 satellite in all orbit at 11,000 nautical miles above the earth. They are continuously monitored by ground stations located world wide. The satellite transmit signals that can be detected by anyone with a GPS receiver, you can determine your exact position, location with a great precision with GPS. GPS is one of history most exciting and revolutionary development.

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Global Positioning System

GPS is founded by and controlled by U.S. department of defense. The system was originally designed for and is operated by U.S. military. GPS provides specially coded satellite signal that can be processed in GPS receiver, enabling the receiver to computer position, velocity and time. GPS was meant for military application. But today civilian application have out numbered those of defense by one almost 10:1 because of shrinking sizes and cost of GPS receiver. GPS offers two level of services. 1. The standard positioning system (SPS):It is the positioning and timing service which will be available to all public GPS users on continuous world wide basis.(In the range of 10 meters) 2. The precise positioning system (PPS):It is highly accurate military positioning, velocity and timing service use by the department of defense.( in the range of 1meters) Elements of GPS GPS consist of three of key element. •

The space segment

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The control segment

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The user segment

Space segment: Space segment is placed above the earth. It consists of 24 satellite in all, which are placed in 6 orbit planes and each orbit is then equipped with 4- satellites which is about 20.000 km above the earth at an inclination of 55 0. Each satellite take 12 hour to go around the earth once. Which means one orbit. The whole set of satellite is called as “Contellation”. They are positioned so well that we can receive signals from six of them nearly 100% of the time at any point on the earth. These satellites are equipped with very precise clock that keeps accurate time to within

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Global Positioning System

three nanoseconds. This precise time is important because the receiver must determine exactly how long it takes for signals to travel. First GPS satellite was launched in 1978. the first 10 satellite were developments satellites. From 1989 to 1993, 23 production satellites were launched further. The launched of 24th satellite in 1994 completed the GPS system. The control system:It consists of master control station at Falcon Air Force based in Colorado Springs, Colorado with five monitor stations in specific ocean, in India ocean, Ascension is land in Atlantic ocean and Colorado springs in Colorado and there ground antennas located through out the world. The monitor stations tract all GPS satellite in view and collect ranging information from the satellite board casts. The monitor stations send the information they collect from each of the satellite back to the master control stations, which computers extremely precise messages for each satellite. Te updated information is transmitted to each satellite via the ground antennas which also transmit and receive the satellite control and monitoring signals. User segment:A GPS receiver can calculate altitude, latitude, co-ordinates by measuring the traveling time of the signals transmitted by these satellite multiplied by the speed of light. A receiver locks in to and measures the signals from at least three different satellite. This is known as “ TRANGULATION”. Typically there are two types of receiver. •

Multiplexing receivers.

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Parallel channel receivers.

GPS receivers can be hand held or installed on aircraft, ships, tanks, submariners, cars trucks. These receiver detects decode and process the signals transmitted from GPS satellite 100 different types of receiver models are in use.

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Global Positioning System

How GPS works? It`s necessary to know some of the basic terms that are used in connection with any earth orbiting satellite. Co-ordinate system:This is the most commonly used co-ordinate system today. The prime meridian and the equator are the reference planes used to define latitude and longitude. Lines of latitude are the equator and are perpendicular to the equator and so many degrees east of west of the prime meridian running through Greenwich. UK at 0 longitude. Earth centered, Earth fixed x, y and z:Earth centered, earth fixed Cartesian co-ordinates are also used to define three dimensional positions. These Cartesian co-ordinates (XYZ) defines three dimensional positions with respect to the center of mass of the earth. The X-axis defined the intersection of the plane defined by the prime meridian and the equatorial plane. The X-axis is directed by a plane of 900 east of the X-axis. The Z-axis points towards North-pole.

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Global Positioning System

Indication:It is the angle between the orbital plane and earths equatorial plane. It is measured at the point where the orbit crosses the equatorial plane while going from south to north. This point is known as the ascending node. The opposite to this is the descending node which is the point where the orbit crosses the equatorial plane while going from north to south. Working of GPS:As mentioned earlier, the GPS system consists of 24 satellite orbiting the earth at the height of 20,000 km twice a day. There are total six orbits with four satellites in each of them. The ascending nodes of the orbits of the GPS satellites are separated by 60 degree.

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Global Positioning System

The GPS system uses one way transmission i.e. from satellites to users. So that the user does not required transmitter, only receiver i.e. GPS receivers are required. But the GPS receiver must be able to measure the time, from which the propagation delay and hence the range of each satellite in view can be determined. The GPS system uses earth centered, earth fixed co-ordinate system. As mentioned above. If the positions of three points relative to the co-ordinate system are known then the distance from an observer relative to co-ordinate system can be calculated. Here is a simple geometrical explanation of how the position of the 3 satellites and the distance from them are used to calculate the position. If we know our distance from one satellite in space, we know we are somewhere on the surface of an imaginary sphere with radius equal to the distance between the satellite and the receiver taking the satellite as the center. If we determine another distance from another satellite, we narrow dawn our possible location to somewhere on the surface where these two possible points there we are located. One of these is usually impossible (for examples inside the planet) and the GPS receivers has the mathematical methods of eliminating the impossible location. Each satellite broadcast the signal which a table of the orbital parameter. The receiver reads the information regarding of the position of the satellite which is used in determination of its position. But as explain above the receiver must also know its distance from these satellites for this receiver calculates this distance by multiplying the speed of the signal with the time required by the signal to reach the receiver from the satellite. Time markers are needed to show when transmission leaves the satellite so that by measuring propagation times and working speed of propagation, the ranges can be calculated. There lies a problem as the user station have no direct way of telling when a transmission from a satellite is commenced. The problem is overcome by having a satellite transmit continues a wave carrier which is modulated by a clocking signals. Both the carrier signals and clocking signals being derived from the atomic clock abroad the satellite. At a users station the receiver generates a replica of the modulated signals from its own atomic clock. The satellite signal and its replica are compared in a comparator as the receiver clock kept

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Global Positioning System

exactly at the same time as the satellite clock. Time shift is also measured and would give the propagation delay. The monitor station track all GPS satellite in view and collect ranging information in the satellite. The monitor station send the information they collect from each of the satellite back to the master control station. Which computes extremely précised satellite orbit. The information is then formatted into updated navigation message for each satellite. The updated information is then transmitted to each satellite via ground antenna. The time broadcast from the satellite is also monitored by the controlled and transmit back if any error is detected in timing. Application of GPS:GPS systems is used in the various application as follows 1. Aviation GPS receivers are used for navigation and altitude determination. These are wide ranges receivers. 2. Mapping:GPS is used for mapping items for latter inclusions in database, maps, and drawings 3. Marine:GPS is exclusively used for navigation as the direction finder 4. Surveying:GPS is used for survey purpose i.e. to drive measurement rather than position for finding accurate position. 5. Timing:GPS is used for determining the time the precise time for completing any transmission and reception. 6. Tracing:GPS is also used in monitoring the movement of peoples.

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Global Positioning System

Future of GPS:In general, GPS devices are used for 5 key objective location. Navigation, tracking, mapping, surveying, and timing. Although the GPS satellite constellation satellite was completed only recently, it has already proved to be most valuable aid to US military. GPS has becomes important for nearly all the military application and weapon systems. In addition, it used in satellite to obtained highly accurate orbit data and to control space craft orientation GPS is best on a system of coordinates called “World Geodetic System”(1984). Similar to the longitude and latitude lines you see on wall map in the school, the WGS 84 systems provides a built in frame of references. So the futures of GPS is unlimited as your imagination. New application will continual to be created as the technology evolves. The GPS satellite like handmade in sky will be guiding us in the 21st century.

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Global Positioning System

CONCLUSION: Using GPS, the mapping facilities and resources in a fraction of time taken. If synchronized video cameras or coupled with kinematics GPS. It also possible to map the position of the road furniture simply by driving along the road. The full impact of this technology is still to be felt. The GPS offers navigational precise positioning in all weather conditions 24 hours a day. This facility offers people from a wide variety of non- technical backgrounds.

REFERENCES: www.gps.world.com www.aero.org/communication Electronic communication systems- Wyane Tomsi

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