The Global Positioning System (GPS), 24 GPS satellites orbit the Earth, transmitting radio signals which allow GPS receivers to triangulate their speed, location, and direction. Since the first GPS satellite was launched in 1978, GPS has become indispensable for navigation around the world, and an important tool for Cartography (map-making) and land surveying. GPS is also used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.GPS receivers vary widely in accuracy based on the number of channels they can monitor simultaneously. Early consumer-grade receivers typically included six to eight channels. As of 2006, even low-cost hand held receivers typically have twelve channels. More expensive "dual-frequency receivers", which receive both L1 and L2 signals, can correct for ionospheric delays. Because receiver accuracy also depends on the amount of processing applied to the received signals, even low cost modern receivers often outperform much more expensive earlier models. Many GPS receivers can relay position data to a PC or other device using the NMEA protocol. Receivers can interface with other devices using methods including a serial connection, USB, or Bluetooth Developed by the United States Department of Defense, and officially named NAVSTAR GPS (Navigation Signal Timing and Ranging Global Positioning System). The satellite constellation is maintained by the United States Air Force Space Wing. Marine Navigation, GPS, In-Car Navigation, INCAR, PDA GPS Navigators , Marine GPS Systems, Satelite Phone, Navman, Furuno, Northstar, Garmin, Magellan, Vdo Dayton, Road Angel, Copilot Live, Destinator, Tomtom, Hawkeye, Hondex, Globalstar, High Gear, AquaPac, Oregon Scientific, Intelight sport, Sports GPS Systems, Hand held GPS, Portable Instruments, Chartplotters, Chart Plotters, Autopilot, Fishfinders, Radar, Radio, VHF, Depth Sounder, Mountaineering, Hiking, Climbing, WAAS, StreetPilot, Smart NAVI, Garmin nüvi, GPSMAP, Co Pilot Live, iCN 550, iQue 3000, iQue M5, Garmin Mobile 10, Garmin edge 205, Garmin Forerunner 305, Navman R300, Garmin GPSMap 276, Northstar 6000i, Garmin Fishfinder 320C, Navman Fish 4600, Navman Gpilot 3100, Northstar ns3300, Geonav Chartplotters, Colour LCD, Sat Nav, Satelite Navigation, global positioning system,