In the civilian market, Global Positioning System navigation is used most often for turn-by-turn navigation devices, which, as they’ve matured, have plummeted in price and increased in features. Now, the technology even works as a smartphone application. Other uses of GPS include marine and aviation navigation systems, Internet network timekeeping, and monitoring the time and position of delivery trucks, pet collars and even children.
How GPS Works
The GPS system consists of 24 satellites in an orbiting constellation around the earth, with four in-orbit spares, as of October 2012. The U.S. Air Force launches replacement satellites on a regular basis every 2 to 3 years, replacing the oldest ones currently in service to keep features updates. Each satellite broadcasts a constant signal that includes a time stamp derived from an on-board clock, and the satellite’s orbital information.
A ground-based receiver tries to “spot” four or more satellites at once to get a positional fix, treating each signal as the radius of a sphere – four overlapping spheres will have a unique intersection point, which is converted into a latitude, longitude and altitude.
Accuracy of GPS Positions
In the United States, GPS receivers are good for positional fixes down to around ten yards on the horizontal plane, and anywhere from fifteen to twenty yards in altitude. Military GPS receivers also use a ground based triangulation point that also broadcasts a signal, and, within the map coordinates selected, are accurate down to a foot or less.
Prior to 2000, civilian GPS systems had integrated timing errors limiting precision to around 150 meters on the ground. Current regulations by the US Department of Commerce prevent GPS receivers from reporting altitudes in excess of 60,000 feet — airliners travel at 35,000 feet — and positions and speeds greater than 999 miles per hour.
GPS Devices and Mapping Software
What most people think of when they hear the term GPS is actually an integrated GPS device and map display. Map displays show cartographic data with terrain features, street names and distances between intersections. In concert, a combined GPS and Geographical Information System can show where your vehicle is on a street map.
Because street maps and addresses change regularly, maps need to be updated frequently; most mapping software is switching to a subscription model where updated maps are rolled out at a regular basis as long as the annual fee is paid.
Other GPS Navigation Tools
GPS coordinate fixes and charting software is used for civilian and military marine navigation. Similar to land maps, these have maps of the depths of various bodies of water, and integrate tidal patterns. Higher end models overlay weather information on top of the marine chart.
GPS mapping software is also available as a smartphone mapping application; while the screen of a typical smartphone isn’t ideal for using in a car — at least not without a phone dashboard mount — they can be quite handy for pedestrians, or people hiking in the woods. There are also dedicated hand-held GPS systems that don’t have integrated map displays – they just provide GPS coordinates, and are used for land orienteering and hiking.
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