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A GPS receiver's job is to locate four or more of these satellites, figure out the distance to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration. The accuracy of a position determined with GPS depends on the type of receiver. Most hand-held GPS units have about 10-20 meter accuracy. Other types of receivers use a method called Differential GPS (DGPS) to obtain much higher accuracy. DGPS requires an additional receiver fixed at a known location nearby.
Sources:
The primary two companies that appeared to be at the forefront of the industry are Garmin and Magellan. Garmin has a much larger and more diverse product line. the Magellan MapSend Direct Route software the first street routing software designed specifically for recreational GPS handhelds. I read that the color display is easier to read in daylight. CSx designation on Garmin - C: color; S: built in compass (drain's batteries); x: microSD memory card.
Sources: Areas where you may have difficulty garnering reception are very dense forests, slot canyons, caves or downtown Manhattan.
Maps:
MapSend (Magellan)
MapSource (Garmin)
MapTech
Garmin MetroGuide USA
TOPO! (National Geographic) 2007 forum at hWildSnow.com/ Great review! Hey, have you seen the new Magellan Triton line of GPS units? Looks like the first units that allow direct downloading from NG TOPO maps all the way down to 1:24,000 level. Plus allows you to include all 5 levels of maps provided on the TOPO software. Takes an SD card so you can throw 2GB in there and load it up. I’ve got the model 500 on order for Christmas, they say mid-Dec shipping. EZE, I’ve not seen the new Triton but am testing the Delorme version of a topo map GPS — their Earthmate. Delorme’s uses vector maps (computer drawn contour lines) so it’s quite different than having raster maps such as those public domain USGS government sourced topos that National Geographic uses. I like the raster maps better, but the vector maps load faster and take up less space. All super interesting. WildSnow will be reviewing the Delorme unit in a few weeks (if we like it). Glossary: 2D RMS or 2drms - Twice-distance Root Mean Square - A horizontal measure of accuracy representing the radius of a circle within which the true value lies at least 95 percent of the time. Anywhere fix - The ability of a receiver to start position calculations without being given an approximate location and approximate time. A-GPS - Assisted GPS - Provides supplementary information enabling reduced 'time to first fix' so that users get position very quickly at power on. DGPS - Differential GPS - A form of corrected GPS giving enhanced accuracy in the few decimeter range. EGNOS is a dGPS service. (better than stand alone GPS, not as good as RTK). ARNS - Aeronautical radio-navigation service - Uses a special GPS frequency. C/A Code (Coarse/Acquisition Code) - The standard positioning signal for the civilian user. DMRs - Delay-mapping receiver - Special GPS which measures signal reflected from the earth. DOP - Dilution of Precision - A factor based on signal strength and location of satellites that indicates accuracy. DOP of 2 is really good, 20 is really bad. EGNOS - European Geostationary Navigation Overlay Service - Provides GNSS augmentation. Consisting of three geostationary satellites and a network of ground stations. ETA - Estimated time of arrival ETE - Estimated-Time-Enroute Galileo - A European operated GNSS scheduled to start in 2008. Galileo and GPS combined will lead to improvements in position quality, even in very harsh environments. GDOP - Geometric dilution of recision - See DOP. GLASNOSS - Similar Russian System GLONASS - Global Navigation Satellite System GNSS - Global Navigation Satellite System - Generic term for systems such as GPS and Galileo. GPS - Global Positioning System - System of 24 U.S. Satellites. 10-20 m accuracy w/o WAAS. GPS III - A new initiative by the U. S. government to upgrade the current GPS system, which includes replacing all current satellites. GPX - GPS eXchange format - An XML schema designed for transferring GPS data between software applications. It can be used to describe waypoints, tracks, and routes. Marlin A-GPS Chip Set - Acquires satellites with signals down to −160dBm. Enables position availability in harsh signal environments (deep indoor and urban canyon). For use in mobil phones. Multi-channel receiver - A GPS receiver that can simultaneously track more than one satellite signal. Even though you only need 4 satellites a 12-channel system will keep track of others so when one disappears over the horizon it can quickly switch. NEMA - National Marine Electronics Association - Interface Standard (NMEA 0183) for data exchange between marine electronic devices. PPS - Precise Positioning Service - The highest level of military dynamic positioning accuracy provided by GPS, using the dual-frequency encrypted P-code. Was military only. SA is turned off. Accuracy of at least 22m (2d rms, 95%) in the horizontal plane and 27.7m (95%) in the vertical plane. Users in mid latitudes (especially in North America) can expect significantly better horizontal accuracy (less than 10 meters) Quadrifilar Helix Antenna - Also known as Quadhelix Antenna. A type of antenna in which four spiraling elements form the receiving surface of the antenna. Unlike Patch Antennas which are flat, Quadrifilar Helix Antennas are encased either in a plastic cylinder or within the GPS unit itself, resulting in a GPS unit that gets the best reception possible. P-Code - The precise code of the GPS signal typically used only by the U.S. military. RNSS - Radionavigation satellite service RTK - Real Time Kinematic - A form of corrected GPS giving accuracy down around 20 mm. SA - Selective Availability - A policy adopted by the Department of Defense to introduce some intentional clock noise into the GPS satellite signals thereby degrading their accuracy for civilian users. This policy was discontinued as of May 1, 2000, and now SA is turned off. SPS - Standard Positioning Service - Uses single frequency C/A code. Accuracy is 100 m 2DRMS (95% of the time.) SBAS - Space Based Augmentation System - Any system that uses a network of geostationary satellites and ground stations to enhance the performance of a GNSS. SiRF - Fast acquisition GPS chipset from SiRF techlology Static positioning - Location determination when the receiver's antenna is presumed to be stationary on the earth. This allows the use of various averaging techniques that improve accuracy by factors of over 1,000. UTM - Universal Transverse Mercator - A nearly worldwide coordinate projection system using north and east distance measurements from reference point(s). UTM is the primary coordinate system used on U.S. Geological Survey topographic maps. WAAS - Wide Area Augmentation System. Developed by FAA for aircraft navigation. Provides accuracy of better than 3 m. 95% of the time. WGS-84 - World Geodetic System, 1984 - The primary map datum used by GPS.A Glossary of GPS Terms at U. New South Wales, AU Map GPS Glossary at maps-gps-info.com GPS Glossary at REI
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When people talk about "a GPS," they usually mean a GPS receiver. The Global Positioning System (GPS) is actually a constellation of 27 Earth-orbiting satellites (24 in operation and three extras in case one fails). They circle the globe at about 12,000 miles (19,300 km), making two complete rotations every day.