Global Positioning System

Global Positioning System

The Global Positioning System (GPS) is a Global Navigation Satellite System (GNSS) developed by the United States Department of Defense. It is the only fully functional GNSS in the world. It uses a constellation of between 24 and 32 Medium Earth Orbit satellites that transmit precise microwave signals, which enable GPS receivers to determine their current location, the time, and their velocity. Its official name is NAVSTAR GPS. Although NAVSTAR is not an acronym, a few backronyms have been created for it. The GPS satellite constellation is managed by the United States Air Force 50th Space Wing. GPS is often used by civilians as a navigation system.

The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. Using a constellation of five satellites, it could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timation satellite which proved the ability to place accurate clocks in space, a technology that GPS relies upon. In the 1970s, the ground-based Omega Navigation System, based on signal phase comparison, became the first worldwide radio navigation system. The design of GPS is based partly on similar ground-based radio navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s, and used during World War II. Additional inspiration for the GPS came when the Soviet Union launched the first Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion.

Basic concept of GPS
A GPS receiver calculates its position by carefully timing the signals sent by the GPS satellites high above the Earth. Each satellite continually transmits messages containing the time the message was sent, precise orbital information (the ephemeris), and the general system health and rough orbits of all GPS satellites (the almanac). The receiver measures the transit time of each message and computes the distance to each satellite. Geometric trilateration is used to combine these distances with the location of the satellites to determine the receiver's location. The position is displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units also show derived information such as direction and speed, calculated from position changes.
It might seem three satellites are enough to solve for position, since space has three dimensions. However a very small clock error multiplied by the very large speed of light the speed at which satellite signals propagate—results in a large positional error. The receiver uses a fourth satellite to solve for x, y, z, and t which is used to correct the receiver's clock. While most GPS applications use the computed location only and effectively hide the very accurately computed time, it is used in a few specialized GPS applications such as time transfer and traffic signal timing.
Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known (for example, a ship or plane may have known elevation), a receiver can determine its position using only three satellites. Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a degraded position when fewer than four satellites are visible.

GARMIN GPSmap 276C GPS Receiver

Product Description
This WAAS-enabled, 12 parallel channel GPS receiver provides color chart plotting for both land and sea, making it an excellent addition to any boat or automobile dashboard. Its display-- 3.8-inch, 480 x 320 pixel, 256-color backlit TFT--provides enhanced sunlight-readability, and its adjustable quad helix receiving antenna with remote antenna capability delivers superior data reception.
Used with your car, the 276c's auto routing capability lets you select a destination and get turn-by-turn directions. There's even voice prompt guidance; just hook up a 12-volt speaker cable (sold separately) and the GPSMAP 276C will audibly tell you when you need to turn and how much farther you have to go.
Data provided with the unit includes 3000 user waypoints with an expanded list of waypoint symbols; 50 reversible routes with 1,200 waypoints per route; 10,000 point automatic track log with 15 saved tracks and 700 saved track points each; position formats including Lat/Lon, UTM, Loran TDs, Maidenhead, MGRS, User, and more; a large numbers option for easy viewing, and much more.
In addition to its built-in basemap detail (cities, interstates, coastline detail and US tide stations), the GPSMAP 276C accepts data from MapSource CD-ROMs, including BlueChart for offshore detail, Recreational Lakes with Fishing Hot Spots for detail of selected inland lakes, and City Select for street-level maps. The package includes the GPSMAP 276c, USB interface cable, power/data cable, marine mount, Trip & Waypoint Manager CD, rechargeable lithium-ion battery, AC adapter, owner’s manual, and a quick reference guide. Manufacturer rating of the battery lists it at up to 15 hours of use depending on backlight setting.

The GPSMAP 276C is a versatile color chartplotter and automobile navigator, perfect for land or water. Along with its counterpart, the GPSMAP 276C, this portable navigator features a sunlight-readable 256-color TFT display and a built-in autorouting basemap that can route you on major interstates and highways. On water, the GPSMAP 276C is a dependable color chartplotter with built-in basemap enhanced by Garmin's marine cartography. For extra-precise navigation, the 276C has a Course Deviation Indicator (CDI) - a feature normally found on aircraft, but equally practical for the marine environment. The CDI quickly indicates whether your boat has veered off-course and helps you get back on track quickly. The 276C also supports external NMEA sources for water depth, water temperature, and water speed through two serial interfaces. On land, the GPSMAP 276C's autoroute basemap can route you on interstates and major highways. The 276C is compatible with BlueChart maps, which add extra detail when you're out on the seas. It also accepts optional MapSource data cards, so you can add other types of map data easily. For convenience and easy map transfer, you can connect the 276C to your computer with the included USB cable. NMEA 0183 input/output Maps & Memory - Basemap with the ability to add maps through Garmin data cards 3000 Waypoints/favorites/locations 50 Routes 10,000 points Track log Unit Dimensions - Width 5.7 x Height 3.2 x Depth 1.9 (14.5x8.1x4.8cm) Unit Weight - 13.6 ounces (385 grams) .