Star Chart and Compass

Thousands of years ago, ancient people already know how to use stars to find direction and where they were. When seamen sailed out into the open sea, they could use the position of the stars to chart their courses. Later, a significant development was the compass was invented and used for navigation. With star charts and a compass, Christopher Columbus sailed across the Atlantic ocean and discovered the Americas.

On October 4, 1957, Soviet Union launched the first artificial satellite Sputnik 1. Since then, scientists found that the "artificial stars" could be used for navigation. They found that the satellite's radio signal increased as it approached and decreased as it left. So the fact that a satellite's position could be tracked from the ground. And in reverse, the receiver of the radio signals also could be determined from the satellite.

The U.S. Navy developed a satellite navigation system - Transit System in 1960's. It was designed for submarines carrying Polaris nuclear missiles. This system has six satellites that circled the earth in polar orbits. By decoding the radio signals from the satellite, the submarines could locate its position.

NAVSTAR GPS

The GPS System was created and realized by the American Department of Defense (DOD) at a cost of $12 billion. The full name of the system is NAVSTAR (Navigation System for Timing and Ranging), but commonly it is referred to as GPS (Global Positioning System). It was originally based on and run with 24 satellites. The first GPS satellite was launched in 1978. The original use of GPS was as a military positioning, navigation, and weapons aiming system. It has higher accuracy and stable atomic clocks on board to achieve precise time transfer. Nowadays, about 30 active satellites orbit the earth in a distance of 20200 km. GPS satellites transmit signals which enable the exact location of a GPS receiver.

GPS for Civilian Use

The first products for civilian consumers appeared in the mid 1980's. 1983 When a civilian airplane of the Korean Airline (Flight 007) was shot down after it had gone lost over Sovjet territory, the US decided to allow the civilian use of the GPS system. It was in 1984 that President Reagan announced that a portion of the capabilities of GPS would be made availabe to the civil community. On August 12, 1993, The Initial Operational Capability (IOC) is announced. In the same year it is also definitely decided to authorize the world wide civilian use free of charge. In March 1996, the White House announced that a higher level of GPS accuracy will be made available to everyone, and the practice of degrading civil GPS signals will be phased out within a decade. The White House reaffirmed that the federal government will providing GPS services for world-wide civilians for peaceful, commercial, and scientific use free of charge.

Still controlled by the US Department of Defense, GPS satellites now broadcast two signals:

• For civilian use: a signal that is accurate to within 100 feet;
• For military use: a signal that is accurate to within 60 feet.

The Pentagon has also reserved the ability to reduce the accuracy of the civilian signal to about 300 feet at any time.