Global Positioning System

Transcription

1 Global Positioning System Ms. Namami Varshney, Dr. Ambuj kumar Agarwal CCSIT, TMU, BAGADPUR MORADABAD U.P Abstract Where am I? where am I going? where are you going? what is the best way to get there? when will I get there? GPS technology can answer all these questions. The Global positioning system (GPS) is a space based navigation system that shows you exact position on the earth any time, in any weather. No matter where you are! GNSS technology has made impact on navigation and positioning needs with the use of satellites and ground stations the ability to track aircrafts, cars, cell- phones, boats and even the individuals has become a reality. It uses the constellation of between 24 and 32 earth orbit satellites that transmit precise radio signals, which allow GPS receivers to determine their current location, the time and the velocity. These satellites are high orbit, circulating at 14,000Km/hrs and 20,000Km above the earth s surface. The signal being sent to the earth at the speed of light is what is picked up by any GPS receiver that are now commonplace worldwide. Keywords: Navigation, Constellation, Speed of light, Tracking, GNSS. I. INTRODUCTION (GPS) technology is a great boon to anyone who has the need to navigate either great or small distances. The Global Positioning System (GPS) is a burgeoning technology, which provides unequalled accuracy and flexibility of positioning for navigation, surveying and GIS data capture. This wonderful navigation technology was actually first available for government use back in the late 1970s. The Global Positioning System (GPS) is a radio based navigation system that gives three dimensional coverage of the Earth, 24 hours a day in any weather conditions throughout the world. The technology seems to be beneficiary to the GPS user community in terms of obtaining accurate data up to about 100 meters for navigation. The GPS technology has tremendous amount of applications in Geographical Information System (GIS) data collection, surveying, and mapping. The first GPS satellite was launched by the U.S. Air Force in early There are now at least 24 satellites orbiting the earth at an altitude of about 11,000 nautical miles. The high altitude insures that the satellite orbits are stable, precise and predictable, and that the satellites' motion through space is not affected by atmospheric drag. These 24 satellites make up a full GPS constellation. The satellites orbit the Earth every 12 hours at approximately 12,000 miles above the Earth. There are four satellites in each of 6 orbital planes. Each plane is inclined 55 degrees relative to the equator, which means that satellites cross the equator tilted at a 55 degree angle. The system is designed to maintain full operational capability even if two of the 24 satellites fail. The GPS system consists of three segments: 1) The space segment: the GPS satellites themselves, 2) The control system, operated by the U.S. military, and 3) The user segment, which includes both military and civilian users and their GPS equipment. The Russian government has developed a system, similar to GPS, called GLONASS. The first GLONASS satellite launch was in October The full constellation consists of 24 satellites in 3 orbit planes, which have a 64.8 degree inclination to the earth's equator. The GLONASS system now consists of 12 healthy satellites. GLONASS uses the same code for each satellite and many frequencies, whereas GPS which uses two frequencies and a different code for each satellite. Galileo is Europe's contribution to the next generation Global Navigation Satellite System (GNSS). Unlike GPS, which is funded by the public sector and operated by the U.S. Air Force, Galileo 560

2 will be a civil-controlled system that draws on both public and private sectors for funding. The GPS system is passive, meaning that the satellites continuously transmit information towards the Earth. If someone has a GPS receiver they can receive the signal at no cost. The information is transmitted on two frequencies: L1 ( MHz), and L2 ( MHz). These frequencies are called carrier waves because they are used primarily to carry information to GPS receivers. The more information a receiver measures the more expensive the unit, and the more functions it will perform with greater accuracy. When one receiver is tracking satellites and obtaining position data, the information received has traveled over 12,000 miles and has been distorted by numerous atmospheric factors. This results in accuracy of about 25 meters. Moreover, the department of Defense (the agency running the GPS) degrades receiver accuracy by telling the satellites to transmit slightly inaccurate information. This intentional distortion of the signal is called Selective Availability (SA). With SA turned on and one receiver is used, the greatest accuracy a user can expect is 100 meters. To improve the accuracy of GPS, differential, or Relative Positioning can be employed. If two or more receivers are used to track the same satellites, and one is in a known position, many of the errors of SA can be reduced, and in some cases eliminated. Differential data can be accomplished using common code or carrier data (L1 or L2).The most accurate systems use differential data from a GPS base station that continually tracks twelve satellites and transmits the differential data to remote units using a radio link. II. Generations of Satellites EVOLUTION Block I Prototype (test) satellites. 10 launched between 1978 and All retired. Block II Initial operational satellites. 9 launched between 1989 and still functioning. Block IIA Slightly modified Block IIs. 19 launched between 1990 and still functioning. Block IIR Replenishment satellites.6 orbited to date. First in C/A code on L2 plus higher power on last 12 satellites launched from 2003 onwards. Block IIF Follow-on satellites. New civil signal at MHz. First launch expected in Block III Conceptual. GPS History Consolidation of several U.S. DoD developmental programs into the Navstar Global Positioning System First prototype satellites launched Korean Airlines Flight 007 shot down. President Reagan reafirms U.S. policy on civil use of GPS First operational satellites launched Initial Operational Capability (24 satellites) Full Operational Capability Selective Availability turned off Block Launch Period I II IIA IIR IIR-M IIF IIIA From 2010 From 2017 Succ ess Satellite launches Fail ure In prepar ation Planned IIIB IIIC Total Currently in orbit and healthy 561

3 III. COMPONENTS OF GPS SYSTEM The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US). The U.S. Air Force develops, maintains, and operates the space and control segments. GPS satellites broadcast signals from space, and each GPS receiver uses these signals to calculate its three-dimensional location (latitude, longitude, and altitude) and the current time. Space Segment The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS arlance. The GPS design originally called for 24 SVs, eight each in three approximately circular orbits, but this was modified to six orbital planes with four satellites each. The six orbit planes have approximately 55 inclination (tilt relative to the 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 orbital period is one-half a sidereal day, i.e., 11 hours and 58 minutes so that the satellites pass over the same locations or almost the same locations every day. The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on the 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 sum to 360 degrees. Control segment Fig1 The control segment is composed of: 1. a master control station (MCS), 2. an alternate master control station, 3. four dedicated ground antennas, and 4. six dedicated monitor stations. Fig2 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- 562

4 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. Fig3 IV. FEATURES: 12 parallel satellite tracking channels. Supports NMEA-0183 data protocol & Binary data protocol. Direct, differential RTCM SC 104 data capability. Static navigation improvements to minimize wander due to SA. Active or Passive antenna to lower cost. Max accuracy achievable by SPS. Enhanced TTFF when in Keep Alive power condition. Auto altitude hold mode from 3D to 2D navigation. Maximum operational flexibility and configurable via user commands. Standard 2x10 I/O connector. User selectable satellites. 5.Need of GPS Technology: Trying to figure out where you are is probable man s oldest pastime. Finally US Dept of Defense decided to form a worldwide positioning system. Also known as NAVSTAR ( Navigation Satellite Timing and Ranging Global positioning system) provides instantaneous position, velocity and time information. V. THE FUTURE OF GPS TECHNOLOGY Further miniaturization of the technology (smaller and smaller) Integration of GPS receivers into PDAs, cameras, sports equipment, etc. Pet, child, and disabled tracking systems and services Bluetooth (short range RF) connectivity between GPS receivers and other Bluetooth-equipped devices (GPS + Bluetooth = positioning inside buildings?) New GPS signals; higher power signals GPS + GLONASS + Galileo VI. APPLICATION AREAS OF GPS TECHNOLOGY MILITARY USE Navigation: Soldiers use GPS to find objectives, even in the dark or in unfamiliar territory, and to coordinate troop and supply movement. In the United States armed forces, commanders use the Commanders Digital Assistant and lower ranks use the Soldier Digital Assistant. Target tracking: Various military weapons systems use GPS to track potential ground and air targets before flagging them as hostile. These weapon systems pass target coordinates top recision-guided munitions to allow them to engage targets accurately. Military aircraft, particularly in air-toground roles, use GPS to find targets. for use in 155-millimeter. Search and rescue. 563

5 Fig4 Communication: The navigational signals transmitted by GPS satellites encode a variety of information including satellite positions, the state of the internal clocks, and the health of the network. These signals are transmitted on two separate carrier frequencies that are common to all satellites in the network. Two different encodings are used: a public encoding that enables lower resolution navigation, and an encrypted encoding used by the U.S. military. Forestry & GPS/GIS: As a forester, Sawchuck finds that GPS and GIS technologies enable him to more rapidly collect and geocode data and then present it in numerous formats ranging from text-based tables to detailed color maps. But the most valuable asset that the GPS/GIS combination brings to this forester s job is its analytical power. "A lot of people view GIS as a great mapmaking tool," Sawchuck notes. "It does that really well, but the real power behind GIS is the ability to do analysis of your information. Public Safety Satellite navigation is fast becoming an industry standard for location information used by emergency and other specialty fleets. Location and status information provided to public safety systems offers managers a quantum leap forward in efficient operation of their emergency response teams. The ability to effectively identify and view the location of police, fire, rescue, and individual vehicles or boats means a whole new way of doing business. Fig5 Canoeing, Kayaking & Boating: GPS provides mariners with navigational and positioning accuracy up to within 3 meters. There is no easier or safer way to navigate on the open waters. Canoeing & Kayaking Record your journey by saving points of particular interest and beauty or patches of soft shore where you can easily return on future trips. Marking hazardous areas to avoid when canoeing or kayaking at a rapid pace can be boat-saver and lifesaver. GPS allows you to easily communicate coordinates to others or to find your way. Fig6 564

6 Fig7 VII. WORKING OF GPS TECHNOLOGY GPS signals do not contain positional data. The position reported by the receiver on the ground is a calculated position based on rangefinding triangulation. GPS positioning is achieved by measuring the time taken for a signal to reach a receiver. Almost one million times a second the satellite transmits a one or a zero in a complex string of digits that appears random. In actuality this code is not random and repeats every 266 days. The receiver knows that the portion of the signal received from the satellite matches exactly with a portion it generated a set number of seconds ago. When the receiver has determined this time, the distance to the satellite can be calculated using simple trigonometry where: Distance to the satellite = speed x (tr - tto) (where speed is c, the speed of light, in a vacuum ( x 10³ ms-1). tto is the time at the origin and tr is the time at the receiver).the DoD maintains very accurate telemetry data on the satellites and their positions are known to a high level of precision. This simple operation allows the distance to a satellite to be calculated accurately. When the distance to three satellites is known then there is only one point at which the user can be standing. VIII. CONCLUSION The Global positioning system (GPS) is a space based navigation system that shows you exact position on the earth any time, in any weather. No matter where you are! GNSS technology has made impact on navigation and positioning needs with the use of satellites and ground stations the ability to track aircrafts, cars, cell- phones, boats and even the individuals has become a reality. It uses the constellation of between 24 and 32 earth orbit satellites that transmit precise radio signals, which allow GPS receivers to determine their current location, the time and the velocity. These satellites are high orbit, circulating at 14,000Km/hrs and 20,000Km above the earth s surface. The signal being sent to the earth at the speed of light is what is picked up by any GPS receiver that are now commonplace worldwide. REFERENCES [1] "What is a GPS?". [2] National Research Council (U.S.). Committee on the Future of the Global Positioning System; National Academy of Public Administration (1995). The global positioning system: a shared national asset: recommendations for technical improvements and enhancements. National Academies Press. p. 16. ISBN Retrieved August 16, 2013., Chapter 1, p. 16 [3] "Factsheets : GPS Advanced Control Segment (OCX)". Losangeles.af.mil. October 25, Retrieved November 6, [4] "Russia Launches Three More GLONASS-M Space Vehicles".Inside GNSS. Retrieved December 26, [5] Winterberg, Friedwardt (1956). "Relativistische Zeitdiiatation eines künstlichen Satelliten (Relativistic time dilation of an artificial satellite)". Astronautica Acta II (in German) (25). Retrieved19 October [6] "GPS and Relativity". Astronomy.ohio-state.edu. RetrievedNovember 6, [7] Guier, William H.; Weiffenbach, George C. (1997). "Genesis of Satellite Navigation" (PDF). Johns Hopkins APL Technical Digest19 (1): [8] Steven Johnson (2010), Where good ideas come from, the natural history of innovation, New York: Riverhead Books [9] Helen E. Worth and Mame Warren (2009). Transit to Tomorrow. Fifty Years of Space Research at The Johns Hopkins University Applied Physics Laboratory (PDF). [10] Catherine Alexandrow (April 2008). "The Story of GPS". [11] DARPA: 50 Years of Bridging the Gap. April [12] Howell, Elizabeth. "Navstar: GPS Satellite Network". SPACE.com. Retrieved February 14, [13] Jerry Proc. "Omega". Jproc.ca. Retrieved December 8, [14] "Why Did the Department of Defense Develop GPS?". Trimble Navigation Ltd. Archived from the original on October 18, Retrieved January 13, [15] "Charting a Course Toward Global Navigation". The Aerospace Corporation. Archived from the original on September 3, Retrieved October 14,