Global Positioning System: Difference between revisions

Many GPS receivers can relay position data to a PC or other device using the [[NMEA 0183]] protocol. [[NMEA 2000]]<ref>[[NMEA]] [http://www.nmea.org/pub/2000/index.html NMEA 2000]</ref> is a newer and less widely adopted protocol. Both are [[proprietary]] and controlled by the US-based National Marine Electronics Association. References to the NMEA protocols have been compiled from public records, allowing open source tools like [[gpsd]] to read the protocol without violating intellectual property laws.  Other proprietary protocols exist as well, such as the [[SiRF]] protocol.  Receivers can interface with other devices using methods including a serial connection, [[Universal Serial Bus|USB]] or [[Bluetooth]].

According to the [[theory of relativity]], due to their constant movement and height relative to the Earth-centered inertial [[Special relativity#Reference frames.2C coordinates and The Lorentz transformation|reference frame]], the clocks on the satellites are affected by their speed ([[special relativity]]) as well as their gravitational potential ([[general relativity]]). For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick more rapidly, by about 45,900 [[nanoseconds]] (ns) per day, because they are in a weaker gravitational field than atomic clocks on Earth's surface. Special relativity predicts that atomic clocks moving at GPS orbital speeds will tick more slowly, by about 7,200 ns per day, than stationary ground clocks. When combined, the discrepancy is 38 microseconds per day; a difference of 4.465 parts in 10¹⁰.<ref>Rizos, Chris. [[University of New South Wales]]. [http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap3/312.htm GPS Satellite Signals]. 1999.</ref>. To account for this, the frequency standard onboard each satellite is given a rate offset prior to launch, making it run slightly more slowly than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz.<ref>[http://www.aticourses.com/global_positioning_system.htm The Global Positioning System by Robert A. Nelson Via Satellite], November 1999</ref>  

A second form of precise monitoring is called '''Carrier-Phase Enhancement''' (CPGPS). The error, which this corrects, arises because the pulse transition of the PRN is not instantaneous, and thus the [[cross-correlation|correlation]] (satellite-receiver sequence matching) operation is imperfect. The CPGPS approach utilizes the L1 carrier wave, which has a [[Periodicity|period]] 1000 times smaller than that of the C/A bit period, to act as an additional [[clock signal]] and resolve the uncertainty.  The phase difference error in the normal GPS amounts to between 2 and 3 meters (6 to 10 ft) of ambiguity. CPGPS working to within 1% of perfect transition reduces this error to 3 centimeters (1 inch) of ambiguity. By eliminating this source of error, CPGPS coupled with [[Differential GPS|DGPS]] normally realizes between 20 and 30 centimeters (8 to 12 inches) of absolute accuracy.

Having reached Fully Operational Capability on [[July 17]], [[1995]],<ref>[http://www.navcen.uscg.gov/gps/geninfo/global.htm Home | Navigation Center<!-- Bot generated title -->]</ref> the GPS completed its original design goals. However, additional advances in technology and new demands on the existing system led to the effort to "modernize" the GPS system. Announcements from the Vice Presidential and the White House in 1998 heralded the beginning of these changes and in 2000, the U.S. Congress reaffirmed the effort; referred to it as '''GPS III'''.  

*'''[[Boat]]s and [[ship]]s''' can use GPS to navigate all of the world's lakes, seas and oceans.  Maritime GPS units include functions useful on water, such as “man overboard” (MOB) functions that allow instantly marking the location where a person has fallen overboard, which simplifies rescue efforts.  GPS may be connected to the ships [[self-steering gear]] and [[Chartplotter]]s using the [[NMEA]] 0183 interface. GPS can also improve the security of shipping traffic by enabling [[Automatic Identification System|AIS]].

*'''GPS equipment for the visually impaired''' is available.  For more detailed information see the  article GPS for the visually impaired

*'''[[E911|Emergency]] and [[Location-based services]]''' — GPS functionality can be used by [[emergency services]] to locate cell phones.  The ability to locate a mobile phone is required in the United States by [[E911]] emergency services legislation. However, as of September 2006 such a system is not in place in all parts of the country.  GPS is less dependent on the telecommunications network topology than [[radiolocation]] for compatible phones.  Assisted GPS reduces the power requirements of the mobile phone and increases the accuracy of the location.  A phone's geographic location may also be used to provide location-based services including advertising, or other location-specific information.

*'''[[GPS tracking]]''' systems use GPS to determine the location of a vehicle, person, or pet and to record the position at regular intervals in order to create a log of movements.  The data can be stored inside the unit, or sent to a remote computer by radio or cellular modem.  Some systems allow the location to be viewed in [[real-time]] on the Internet with a web-browser. There are various applications for GPS tracking, namely in public safety and crime prevention.<ref>''[http://gpstrackit.com/gps-tracking-the-future/ GPS Tracking: The Future]''</ref>

The design of GPS is based partly on the similar ground-based radio navigation systems, such as [[LORAN]] and the [[Decca Navigator System|Decca Navigator]] developed in the early 1940s, and used during World War II.  Additional inspiration for the GPS system came when the Soviet Union launched the first [[Sputnik program|Sputnik]] in 1957.  A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions.  They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them.  They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion.  <!-- The converse is also true: if the satellite's position were known, they could identify their own position on Earth. (commented because I am not sure of this.  At most, they would know the rate at which the distance between themselves and the satellite was changing.  There would be at least two points (one each north and south of the equator) for which that would be true, and practically one would not get an exact position, especially with 1950s electronics, even if one knew the satellite's exact orbit, and the exact time -->

*[[Ivan Getting]], emeritus president of The Aerospace Corporation and [[engineer]] at the [[Massachusetts Institute of Technology]], established the basis for GPS, improving on the World War II land-based radio system called [[LORAN]] ('''Lo'''ng-range '''R'''adio '''A'''id to '''N'''avigation).

On [[February 10]], [[1993]], the [[National Aeronautic Association]] selected the Global Positioning System Team as winners of the 1992 [[Collier Trophy|Robert J. Collier Trophy]], the most prestigious aviation award in the United States. This team consists of researchers from the [[Naval Research Laboratory]], the U.S. Air Force, the [[Aerospace Corporation]], [[Rockwell International]] Corporation, and [[IBM]] Federal Systems Company. The citation accompanying the presentation of the trophy honors the GPS Team "for the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radio navigation 50 years ago."

* [http://www.aero.org/publications/crosslink/summer2002/index.html Crosslink] Summer 2002 issue by The Aerospace Corporation on satellite navigation.