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Global Positioning System

From Wikipedia, the free encyclopedia

  (Redirected from Gps)
Artist's conception of GPS satellite in orbit
Artist's conception of GPS satellite in orbit
Civilian GPS receiver in a marine application.
Civilian GPS receiver in a marine application.

The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Utilizing a constellation of at least 24 Medium Earth Orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its location, speed, direction, and time. Other similar systems are the Russian GLONASS (incomplete as of 2008), the upcoming European Galileo positioning system, the proposed COMPASS navigation system of China, and IRNSS of India.

Developed by the United States Department of Defense, GPS is officially named NAVSTAR GPS (Contrary to popular belief, NAVSTAR is not an acronym, but simply a name given by John Walsh, a key decision maker when it came to the budget for the GPS program).[1] The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining the system is approximately US$750 million per year,[2] including the replacement of aging satellites, and research and development.

Following the shooting down of Korean Air Lines Flight 007 in 1983, President Ronald Reagan issued a directive making the system available for free for civilian use as a common good.[3] Since then, GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, and scientific uses. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.

[edit] Simplified method of operation

A typical GPS receiver calculates its position using the signals from four or more GPS satellites. Four satellites are needed since the process needs a very accurate local time, more accurate than any normal clock can provide, so the receiver internally solves for time as well as position. In other words, the receiver uses four measurements to solve for 4 variables - x, y, z, and t. These values are then turned into more user-friendly forms, such as latitude/longitude or location on a map, then displayed to the user.

Each GPS satellite has an atomic clock, and continually transmits messages containing the current time at the start of the message, parameters to calculate the location of the satellite (the ephemeris), and the general system health (the almanac). The signals travel at the speed of light through outer space, and slightly slower through the atmosphere. The receiver uses the arrival time to compute the distance to each satellite, from which it determines the position of the receiver using geometry and trigonometry (see trilateration[4])

Although four satellites are required for normal operation, fewer may be needed in some special cases. If one variable is already known (for example, a sea-going ship knows its altitude is 0), a receiver can determine its position using only three satellites. Also, in practice, receivers use additional clues (doppler shift of satellite signals, last known position, dead reckoning, inertial navigation, and so on) to give degraded answers when fewer than four satellites are visible.

[edit] Technical description

Unlaunched GPS satellite on display at the San Diego Aerospace museum
Unlaunched GPS satellite on display at the San Diego Aerospace museum

[edit] System segmentation

The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).[5]

[edit] Space segment

See also: GPS satellite and List of GPS satellite launches
A visual example of the GPS constellation in motion with the Earth rotating.  Notice how the number of satellites in view from a given point on the Earth's surface, in this example at 45°N, changes with time.
A visual example of the GPS constellation in motion with the Earth rotating. Notice how the number of satellites in view from a given point on the Earth's surface, in this example at 45°N, changes with time.

 

All text is available under the terms of the GNU Free Documentation License. (See Copyrights for details.)
WikipediaŽ is a registered trademark of the Wikimedia Foundation, Inc., a U.S. registered 501(c)(3) tax-deductible nonprofit charity.




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  • GPS not a license to turn off your brain

    By ROBERT L. JAMIESON Jr.
    P-I COLUMNIST

    There's an episode of NBC's hit show "The Office" where Michael, played by Steve Carell, follows his Global Positioning System device -- and drives right into a lake.

    Art imitated life in a serious way Wednesday, when a bus carrying a Seattle high school softball team plowed into a brick-and-concrete footbridge.

    The impact sheared the roof off the bus. The driver reported that he was simply following the route provided by his GPS.

    I suppose that in a country that already has given us the "Twinkie Defense," we shouldn't be surprised by what's coming down the road -- The GPS Defense.

    Even though car navigation systems are godsends for the terminally lost and eternally map-challenged, they are popping up on the radar of blame.

    Last May, the BBC reported that a British university student got in a wreck after she followed her satellite navigation system onto a railway track.

    A train smashed into her car.

    "I put my complete trust in the sat nav," Paula Ceely said, using British shorthand for satellite navigation. "It led me right into the path of a speeding train."

    It would be nice if such antics were isolated. They're not. Vince Yearley, spokesman for the Institute for Advanced Motorists, told a blogger for Computerworld.com: "We've heard some very hilarious stories where people just blindly follow (GPS) instructions. Like if the (GPS) says, 'Drive into this muddy field,' they think, 'That's weird,' but they do it anyway."

    Wednesday's bus accident in the Washington Park Arboretum had a happy ending. Members of the girls softball team from Garfield High were shaken up, but no one was seriously hurt. One thing died at the scene, however: common sense.

    The footbridge clearance is 9 feet.

    The bus was nearly 12 feet tall.

    Do the math.

    The president of the bus company blamed the GPS for routing the bus into danger's way. "We just thought it would be a safe route," Steve Abegg, president of Lynnwood-based Journey Lines, told reporters.

    Abegg's way of thinking hints at the underlying problem linked to GPS dependency -- people being so reliant on all kinds of technological doodads that they become stuck on stupid.

    They stop thinking. GPS devices are driving aids -- not excuses to turn off the brain.

    A mild confession: I was a GPS virgin until recently.

    When a night news assignment beckoned in a hard-to-find neighborhood near Sea-Tac Airport, Seattle P-I reporter Casey McNerthney lent me his hand-held navigator.

    Heading south on state Route 99, an authoritative female GPS voice guided me.

    Right turn coming up, the voice cooed.

    I was going a bit too fast, and flew into the turn only to see potential trouble a hundred yards away -- concrete abutments that marked a detour. Hitting the brakes, I stopped the car in time.

    That's when the GPS piped up: Make a safe U-turn as soon as possible, it said, after "reconfiguring."

    Had I been distracted, the GPS warning would have come too late. That experience gave me a bit of perspective on Wednesday's bus wreck.

    Yes, the bus driver may have been unfamiliar with the road. And, yes, he may have failed to see the hard-to-miss flashing lights or the sign -- in black letters against blazing yellow -- that mentions the bridge height.

    But a professional driver is supposed to be an expert, whether that means negotiating a passenger bus in rain or snow, or realizing if a road is washed out or a footbridge clearance is too low.

    So, don't blame the GPS. Do take personal responsibility. A GPS device is helpful; it got me to my destination the other night. But it isn't a magic box.

    People should not exercise less awareness or caution driving with a GPS than they would without one. Traffic experts say drivers need to realize that rules of the road and signs have priority over GPS commands.

    After all, the siren call of the GPS voice may be reassuring, even alluring. But you never quite know where it might lead you.

    P-I columnist Robert L. Jamieson Jr. can be reached at 206-448-8125 or robertjamieson@seattlepi.com.
     
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