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GPS技术没什么新鲜的内容：一直在做原有技术的改造。卫星通信建立之后，科学家和工程师开始寻找新的途径来利用这一迷人的太空奇迹。无线电导航系统在第二次世界大战期间为飞机操作做了开发，随后演变为罗兰卫星系统。
1958年，美国海军开始针对罗兰卫星系统展开工作，开发一个叫做“Transit”的系统，用于地面接收定位。两年以后，海军用Transit-1B系统去演示利用卫星作为助航设备的可行性。一艘船上的接收器利用测量到的卫星发射信号变化，利用已知卫星轨道特性去计算船的位置。
There is really nothing new about GPS: the technology was reinvented fromthe old. After satellite communications was established, scientists and engineers started to look for different ways of utilizing this fascinating space marvel. Radio navigation systems had been developed during the World War II for aircraft operations, which subsequently evolved into Loran satellite system.
In 1958, the U.S. Navy began working on Loran satellite to develop a system“Transit” for indicating the position of a receiver on the ground. Two years later, the navy launched Transit-1B system to demonstrate thefeasibility of using satellites for navigational aids. A receiver on a ship used the measured shift of satellite’s radio signal, along with known characteristics of the satellite orbits, to calculate the ship position.
GPS has been reinvented from satellite communications
A practical system was born out of the need of the U.S. troops to pinpoint their locations during the Vietnam War. However, this system had a limitedaccuracy and was difficult to use due to its bulky terminal size. So, in the mid-1970s, the U.S. Department of Defense began a project to upgrade thenavigation devices built around this concept for classified military use.
The solution they developed required two dozen satellites, atomic clocks,microwave radio transmitters and some heavy-duty number-crunching hardware.
A more portable unit could now pinpoint an object’s exact location anywhere on the globe by receiving signals from a network of satellites in an orbitand triangulate them to determine latitude and longitude. The military called it Navstar, after the satellite constellation it used, but theindustry and users ignored this nomenclature, and technology became known to the world as Global Positioning System or GPS.
The operational system contained twenty-one satellites in three orbital planes, with three spare satellites. The GPS collection of twenty-foursatellites orbited twelve thousand miles above the Earth. These satellites constantly transmitted their precise time and position in space. With GPS, a receiver on ground or in the air could calculate its position using time signals from the satellites.
The calculation itself was based on a kind of triangulation—a math technique used to locate an object based on its distance from three points.
So signals from three satellites were necessary, although in practice a fourth satellite was used to improve the accuracy of the other threesignals. The result was that a GPS receiver could produce highly accurate coordinates of latitude, longitude, and altitude.
GPS was originally developed for military use
The U.S. Air Force played a crucial role in nurturing the GPS technology byincorporating features like accurate digital maps and satellite photographs.
As a result, the pilots were able to spot the key target areas and hit themeffectively. Precision-guided munitions, dubbed “smart bombs,” increasingly used GPS to hone in on a fixed target such as a military installation or an airfield.
