History of Navigation
GPS is primarily a navigational system, so a background on navigation will give insight as to how extraordinary the Global Positioning System is.
People first navigated only by means of landmarks - mountains, trees, or leaving trails of stones. This would only work within a local area and the environment was subject to change due to environmental factors such as natural disasters.
For traveling across the ocean a process called dead reckoning, which used a magnetic compass and required the calculation of how fast the ship was going, was applied. The measurement tools were crude and inaccurate. It was also a very complicated process.
When traveling over the ocean, people began using the stars as guidelines. The stars appear different from different locations on Earth so analyzing the stars gave sailers the basic direction to follow. Celestial navigation was our primary means of navigation for hundreds of years. It was a time-consuming and complicated task of measuring the angles between stars - a process of triangulation. The degree of precision was limited. The sextant was developed during this time but since it only measured latitude, a timepiece was also invented so that the longitude could also be calculated. This type of navigation only worked at night and in clear weather which was a great disadvantage.
It was not until the 20th century that ground-based radio navigation systems were introduced. Some are still in use today. GPS is a satellite radio navigation system, but the first systems were ground-based. They work in the same way as does GPS: users (receivers) calculate how far away they are from a transmitting tower whose location is known. When several towers are used, the location can be pinpointed. This method of navigation was a great improvement, yet it had its own difficulties. An example of such a system is LORAN. Each tower had a range of about 500 miles and had accuracy good to about 250 meters. LORAN was not a global system and could not be used over the ocean. Because ground based systems send signals over the surface of the earth, only two-dimenstional location can be determined. The altitude cannot be calculated so this system could not be applied to aviation. The accuracy of such systems could be affected by geography as well. The frequency of the signal affected accuracy; a higher frequency would allow for greater accuracy, but the user would need to remain within the line of sight. The first global navigation system was called OMEGA. It was a ground-based system but has been terminated as of 1997.
Satellite navigation systems can provide high frequency signals allowing for high accuracy, as well as global access because the satellites are so high up that remaining within the line of sight of the satellites is easy.
History of GPS
Prior to the development of the GPS system, the first satellite system was called Transit and was operational begining in 1964. Transit had no timing devices aboard the satellites and the time it took a receiver to calculate its position was about 15 minutes. Yet, much was learned from this system. GPS is a great improvement over the Transit system. The original use of GPS was as a military positioning, navigation, and weapons aiming system to replace not only Transit, but other navigation sytems as well. It has higher accuracy and stable atomic clocks on board to achieve precise time transfer. The first GPS satellite was launched in 1978 and the first products for civilian consumers appeared in the mid 1980's. It was in 1984 that President Reagan announced that a portion of the capabilities of GPS would be made availabe to the civil community. The system is still being improved and new, better satellites are still being launched to replace older ones.