Global Positioning System (GPS)

    The Global Positioning System is a "gift" from the US Government to the world.  We spent $12 billion on a satellite network, ground communications stations and receivers so that our military could pinpoint accurately any spot on the planet.

    On March 29, 1996, President Clinton signed a Presidential Decision Directive making our GPS system into an international information utility.  We paid to build it and now the technology has been given away to the world for free.

    The "space" component of GPS is a network of 24 geosynchronous satellites that provide signal coverage for the entire planet.  Satellites have a limited useful life and are replaced from time to time by the Department of Defense to keep at least 24 aloft at all times.  The satellites are arranged in one of six orbital planes that ensure that between five and eight satellites would be visible from any one point on earth, all the time.

    A system of ground stations was constructed to upload synchronization data to the satellites.  It's this information that the satellites use to send to GPS receivers that we use.

    GPS receivers take many forms; the ones most useful for GeoCaching are hand-held units made chiefly by Garmin and Magellan for the consumer market.

    How it works

    A GPS receiver is a unit that uses advanced triangulation techniques to compute your location.  The receiver gets transmitted information from satellites and uses transmissions from four or more satellites to locate your position.  It can also compute speed and direction data.  The unit offers a display of the information in a form that a user finds helpful (usually a handheld device).  The format that the data is displayed in is one of the biggest items that differentiates different GPS receivers.  All data from GPS receivers are calculated from satellite-transmitted data, not from a compass, speedometer, or altimeter.  The altitude data that a GPS receiver outputs can often be off by 30 meters or so and can fluctuate.  Some GPS systems have a built in compass, but other, less expensive ones do not.  When you are in the wilderness, a compass and a altimeter are good compliments for a GPS receiver.

    Here's a table of recent list prices for the most common GeoCaching GPS receivers:

    Manufacturer Model Price
    Garmin eTrex $119.00
    GPS12 $150.00
    eTrex Camo $158.00
    eTrex Venture $170.00
    eTrex Summit $250.00
    eTrex Legend $250.00
    GPS III Plus $300.00
    eTrex Vista $350.00
    GPSMap 76 $350.00
    GPSMap 76S $450.00
    GPS V $500.00
         
    Magellan GPS Companion $99.99
    GPS 310 $119.94
    GPS 315 $149.99
    GPS Companion Handspring Visor $149.99
    Meridian GPS $220.00
    Map 330 $249.99
    Meridian Gold GPS $300.00
    Meridian SportTrak Pro GPS $300.00
    Meridian Platinum GPS $350.00

    If you have an REI store nearby, you can likely compare them all.

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