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J.C.D.M. COLLEGE OF ENGINEERING P.O.Box no.81,Barnala Road Sirsa-125055 (Haryana)

Seminar Report On Global Positioning System

Submitted to: Ms.Gagandeep (Lect. in C.S.E. deptt.)

Submitted by: Yashika Goel CSE-VII Sem 7362

Global Positioning System

The Global Positioning System (GPS) is a space-based global navigation satellite system (GNSS) that provides reliable location and time information in all weather and at all times and anywhere on or near the Earth when and where there is an unobstructed line of sight to four or more GPS satellites. It is maintained by the United States government and is freely accessible by anyone with a GPS receiver. GPS was created and realized by the U.S. Department of Defense (USDOD) and was originally run with 24 satellites. It was established in 1973 to overcome the limitations of previous navigation systems

History: The design of GPS is based partly on similar ground-based radio navigation systems, developed in the early 1940s, and used during World War II. The first satellite navigation system, Transit used by the United States Navy, was first successfully tested in 1960. Over the Labor Day, a meeting of about 12 military officers at the Pentagon discussed the creation of a Defense Navigation Satellite System (DNSS). It was at this meeting that "the real synthesis that became GPS was created." Later that year, the DNSS program was named Navstar. With the individual satellites being associated with the name Navstar, a more fully encompassing name was used to identify the constellation of Navstar satellites, Navstar-GPS, which was later shortened simply to GPS. Initially, the highest quality signal was reserved for military use, and the signal available for civilian use was intentionally degraded ("Selective Availability", SA). This changed with U.S. President Bill Clinton ordering Selective Availability turned off at midnight May 1, 2000, improving the precision of civilian GPS from 100 meters (about 300 feet) to 20 meters (about 65 feet). The U.S. military by then had the ability to deny GPS service to potential adversaries on a regional basis.

Structure of GPS: The current GPS system consists of three major segments: y y y Space Segment Control Segment User Segment

Space Segment: The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three circular orbital planes but this was modified to six planes with four satellites each. The orbital planes are centered on the Earth, not rotating with respect to the distant stars. The six planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection).The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface. The result of this objective is that the four satellites are not evenly spaced (90 degrees) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30, 105, 120, and 105 degrees apart which, of course, sum to 360 degrees. Orbiting at an altitude of approximately 20,200 kilometers (about 12,550 miles or 10,900 nautical miles; orbital radius of approximately 26,600 km (about 16,500 mi or 14,400 NM)), each SV makes two complete orbits each sidereal day, repeating the same ground track each day. This was very helpful during development because even with only four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones. As of March 2008, there are 31 actively broadcasting satellites in the GPS constellation, and two older, retired from active service satellites kept in the constellation as orbital spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail. About eight satellites are visible from any point on the ground at any one time.

Control segment The control segment is composed of 1. 2. 3. 4. a master control station (MCS), an alternate master control station, four dedicated ground antennas and six dedicated monitor stations

The MCS can also access U.S. Air Force Satellite Control Network (AFSCN) ground antennas (for additional command and control capability) and NGA (National Geospatial-Intelligence

Agency) monitor stations. The flight paths of the satellites are tracked by dedicated U.S. Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs, Colorado and Cape Canaveral, along with shared NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC.The tracking information is sent to the Air Force Space Command's MCS at Schriever Air Force Base 25 km (16 miles) ESE of Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the United States Air Force (USAF). Then 2 SOPS contacts each GPS satellite regularly with a navigational update using dedicated or shared (AFSCN) ground antennas (GPS dedicated ground antennas are located at Kwajalein, Ascension Island, Diego Garcia, and Cape Canaveral). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter that uses inputs from the ground monitoring stations, space weather information, and various other inputs. Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must be marked unhealthy, so receivers will not use it in their calculation. Then the maneuver can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again.

User Segment The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service. In general, 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). They may also include a display for providing 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 so that, as of 2007, receivers typically have between 12 and 20 channels

Basic Concept of GPS:

The actual theory behind GPS is very easy to appreciate, since it is exactly the same as traditional triangulation. If one imagines an orienteer needing to locate themselves on a map, they first need to be able to find at least three points that they recognize in the real world, and pinpoint their locations on the map.

They can then measure, using a compass, the heading that would be needed to take them from the point on the map to their current position. A line is then drawn from each of the three points, and where the three lines meet is where they are on the map. Translating this into the GPS world, we can replace the known points with satellites, and the direction with time taken for a signal to travel from each of the known points to the GPS receiver. This enables the system to work out roughly where it is located ± it is where the circles representing the distance from the satellite, calculated on the basis of the travel time of the signal, intersect. Of course, this requires that the GPS locator has the same coordinated time as the satellites, which have atomic clocks on board. To do this, it cross checks the intersection of the three circles with a fourth which it acquires from another satellite. If the four circles no longer intersect at the same point, then the GPS system knows that there is an error in it¶s clock, and can adjust it by finding one common value (one second, half a second and so on) that can be applied to the three initial signals which would bring the circles to intersect in the same place. Behind the scenes, there are also many complex calculations taking place which enable the system to compensate for atmospheric distortion of the signals, and so forth, but the principle remains the same. Working of GPS:

Signals In order for GPS to work, a network of satellites was placed into orbit around planet Earth, each broadcasting a specific signal, much like a normal radio signal. This signal is powerful enough that it can be received by a reasonably low cost, low technology aerial. Rather than carrying an actual radio or television program, the signals that are broadcast by the satellites carry data that is passed from the aerial to the GPS software. The information is specific enough that the GPS software can identify the satellite, it¶s location in space, and the time that the signal took to travel from the satellite to the GPS receiver.

Using many different signals, the GPS software is able to triangulate the position of the receiver. The principle is very similar to that which is used in orienteering ± if you can identify three places on your map, take a bearing to where they are, and draw three lines on the map, then you will find out where you are on the map. The lines will intersect, and, depending on the accuracy of the bearings, the triangle that they form where they intersect will approximate to your position, within a margin of error. GPS software performs a similar kind of exercise, using the known positions of the satellites in space, and measuring the time that the signal has taken to travel from the satellite to Earth. The result of the triangulation of at least three satellites, assuming that the clocks are all synchronized enables the software to calculate, within a margin of error, where the device is located in terms of its latitude (East-West) and longitude (North-South). Timing & Correction In a perfect world, the accuracy should be absolute, but there are many different obstacles which prevent this. Principally, it is impossible to be sure that the clocks are all synchronized. Since the satellites each contain atomic clocks which are extremely accurate, and certainly accurate to each other, we can assume that the problem lies with the GPS unit itself. Keeping the cost of the technology down to a minimum is a key part of the success of any consumer device, and it is simply not possible to fit each GPS unit with an atomic clock costing tens of thousands of dollars. Luckily, in creating the system, the designers also defined how GPS works out whether it¶s clock is accurate or not. There are a few solutions. The first is to fit a separate receiver which can receive a terrestrial signal from a nearby atomic clock. This technology exists for clocks which cost a fraction of the tens of thousands that would be required for a true atomic clock, but would still add tens, if not hundreds of dollars to the price of the GPS receiver. The solution that was chosen uses a fourth satellite to provide a cross check in the triangulation process. Since triangulation from three signals should pinpoint the location exactly, adding a fourth will move that location; that is, it will not intersect with the calculated location. This indicates to the GPS software that there is a discrepancy, and so it performs an additional calculation to find a value that it can use to adjust all the signals so that the four lines used in the triangulation intersect.

Usually, this is as simple as subtracting a second (for example) from each of the calculated travel times of the signals. Thus, the GPS software can also update its¶ own internal clock; and means that not only do we have an accurate positioning device, but also an atomic clock in the palm of our hands. Mapping Knowing where the device is in space is one thing, but it is fairly useless information without something to compare it with. Thus, the mapping part of any GPS software is very important; it is how GPS works our possible routes, and allows the user to plan in advance. In fact, it is often the mapping data which elevates the price of the GPS solution; it must be accurate and updated reasonably frequently. There are, however, several kinds of map, and each is aimed at different users, with different needs. Road users, for example, require that their mapping data contains accurate information about the road network in the geographical location that they will be traveling in, but will not require detailed information about the lie of the land ± they do not really worry about the height of hills and so forth. On the other hand, hiking GPS users might wish to have a detailed map of the geographical surroundings, rivers, hills and so forth, and perhaps tracks and trails, but not roads. They might also like to adorn their map with specific icons of things that they find along the way and that they wish to keep a record of ± not to mention waypoints; locations to make for on their general route. Finally, marine users need very specific information relating to the sea bed, navigable channels, and other pieces of maritime data that enables them to navigate. Of course, the sea itself is reasonably featureless, but underneath quite some detail is needed to be sure that the boat is safe.

Applications: GPS has become a widely used and useful tool for commerce, scientific uses, tracking and surveillance. GPS's accurate timing facilitates everyday activities such as banking, mobile phone operations, and even the control of power grids. Farmers, surveyors, geologists and countless others perform their work more efficiently, safely, economically, and accurately.

Civilian:

y y y y y y

Cellular Telephony Disaster Relief Vehicle /person/pet Tracking System GPS Aircraft Tracking Map Making TECTONICS

Military: y y y Navigation Target Tracking Search and Rescue

GPS Augmentations: To meet the specific user requirements for positioning, navigation, and timing (PNT), a number of augmentations to the Global Positioning System (GPS) are available. An augmentation is any system that aids GPS by providing accuracy, integrity, reliability, availability, or any other improvement to positioning, navigation, and timing that is not inherently part of GPS itself. Such augmentations include, but are not limited to:
y

Nationwide Differential GPS System (NDGPS): The NDGPS is a ground-based augmentation system operated and maintained by the Federal Railroad Administration, U.S. Coast Guard, and Federal Highway Administration, that provides increased accuracy and integrity of the GPS to users on land and water. Modernization efforts include the High Accuracy NDGPS (HA-NDGPS) system, currently under development, to enhance the performance and provide 10 to 15 centimeter accuracy throughout the coverage area. NDGPS is built to international standards, and over 50 countries around the world have implemented similar systems.

y

Wide Area Augmentation System (WAAS): The WAAS, a satellite-based augmentation system operated by the U.S. Federal Aviation Administration (FAA), provides aircraft navigation for all phases of flight. Today, these capabilities are broadly

used in other applications because their GPS-like signals can be processed by simple receivers without additional equipment. Using International Civil Aviation Organization (ICAO) standards, the FAA continues to work with other States to provide seamless services to all users in any region. Other ICAO standard space-based augmentation systems include: Europe's European Geostationary Navigation Overlay System (EGNOS), India's GPS and Geo-Augmented Navigation System (GAGAN), and Japan's Multifunction Transport Satellite (MTSAT) Satellite Augmentation System (MSAS). All of these international implementations are based on GPS. The FAA will improve the WAAS to take advantage of the future GPS safety-of-life signal and provide better performance and promote global adoption of these new capabilities.

y

Global Differential GPS (GDGPS): GDGPS is a high accuracy GPS augmentation system, developed by the Jet Propulsion Laboratory (JPL) to support the real-time positioning, timing, and orbit determination requirements of the U.S. National Aeronautics and Space Administration (NASA) science missions. Future NASA plans include using the Tracking and Data Relay Satellite System (TDRSS) to disseminate via satellite a real-time differential correction message. This system is referred to as the TDRSS Augmentation Service Satellites (TASS).

Advantages: y y y Ease of Navigation Search Nearby Area Water Navigation

Disadvantages: y y Possible Failures Inaccuracy

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