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magellan explorist 200 gps receiverThe GPS or Global Positioning System is a global navigation system used for military and civilian applications alike.
Each satellite repeatedly re-broadcasts the exact time according to its internal atomic clock along with a digital data packet that includes the satellite's precise position, satellite status messages, and an almanac of the approximate position of every other active GPS satellite. The almanac lets GPS receivers use data from the strongest satellite signal to locate other satellites. When it was first deployed, GPS included a feature called Selective Availability (or SA) that introduced intentional errors of up to a hundred meters into the publicly available navigation signals, making it difficult to use for guiding long range missiles to precise targets. Additional accuracy was available in the signal, but in an encrypted form that was only available to the United States military, its allies and a few others, mostly government users. GPS is coming into common use in cars these days. Most new cars allow you to purchase GPS technology that will help you plot your trip from one location to the next. One of biggest problems for GPS accuracy is that changing atmospheric conditions change the speed of the GPS signals unpredictably as they pass through the ionosphere. The effect is minimized when the satellite is directly overhead and becomes greater toward the horizon, as the satellite signals must travel through the greater "thickness" of the ionosphere as the angle increases. Once the receiver's rough location is known, an internal mathematical model can be used to estimate and correct for the error. GPS allows the military to accurately target its missile arsenal, largely composed of cruise missiles and precision-guided munitions. This was born out by the use of SOFLAM (Special Operations Forces Laser Acquisition Markers) targeting during the attacks on Tora Bora in Afghanistan. It also improves the accuracy of the US submarine launched ballistic missiles by providing precise locational information to the submarine commanders. Finally, command and control over troops is improved because commanders know precisely where their troops and the enemy are located on the battlefield. In 1998, Vice President Al Gore announced plans to upgrade GPS with two new civilian signals for enhanced user accuracy and reliability, particularly with respect to aviation safety. The author would have loved to have used GPS years ago in college. We had a class where we used maps and compasses. We were given precise coordinates and had to plot a course through woods and streams and swamps to find and sign in at specific latitude and longitude coordinates. Today, this sport, now called "geocaching", is a popular activity, particularly for outdoor hiking enthusiasts. The first experimental Block-I GPS satellite was launched in February 1978. The GPS satellites were initially manufactured by Rockwell International and now manufactured by Lockheed Martin. GPS derives in part from its predecssor, the LORAN navigation system. LORAN was designed for ship and airplane navigation in the 1940s for use during World War II. GPS receivers calculate their current position (latitude, longitude, elevation), and the precise time, using the process of trilateration after measuring the distance to at least four satellites by comparing the satellites' coded time signal transmissions.
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