Long before the space age, people used the heavens for navigation. Besides relying on the Sun, Moon, and stars, the early travellers invented the magnetic compass, the sextant, and the seagoing chronometer. Eventually, radio navigation in which a position could be determined by receiving radio signals broadcast from multiple transmitters, came into existence. Improved high frequency signals gave greater accuracy of position, but they were blocked by mountains and could not bend over the horizon. This limitation was overcome by moving the transmitters into space on Earth-orbiting satellites, where high frequency signals could accurately cover wide areas.
The principle of satellite navigation is relatively simple. When a transmitter moves toward an observer, radio waves have a higher frequency, just like a train's horn sounds higher as it approaches a listener. A transmitter's signal will have a lower frequency when it moves away from an observer. If measurements of the amount of shift in frequency of a satellite radiating a fixed frequency signal with an accurately known orbit are carefully made, the observer can determine a correct position on the Earth.
The United States Navy developed such a system, called Transit, in the late 1960s and early 1970s. Transit helped submarines update their on-board inertial navigation systems. After nearly ten years of perfecting the system, the Navy released it for civilian use. It is now used in surveying, fishing, private and commercial maritime activities, offshore oil exploration, and drifting buoys. Transit did have some drawbacks--it was not accurate enough, a user had to wait until the satellite passed overhead, position fixes required some time to determine, and an accurate fix was difficult to obtain on a moving platform.
As a result of these shortcomings, the United States military developed another system: Navstar (Navigation Satellite for Time and Ranging) Global Positioning System (GPS). The new system can measure to within 33 feet, (10.05 m), whereas Transit was accurate only to 0.1 mile (582 feet [176.5 m]). The Navstar system was fully deployed as of April 1995, with 24 satellites orbiting Earth with 12-hour periods.
Both Transit and Navstar use instantaneous satellite position data to help users travelling from one place to another. But another satellite system uses positioning data to report where users have been. This system, called Argos, is more complicated: an object on the ground sends a signal to a satellite, which then retransmits the signal to the ground. Argos can locate the object to within 0.5 mi (.80 km). It is used primarily for environmental studies. Ships, and buoys can collect and send data on weather, currents, winds, and waves. Land-based stations can send weather information, as well as information about hydrologic, volcanic, and seismic activity. Argo can be used with balloons to study weather and the physical and chemical properties of the atmosphere. In addition, the system is being perfected to track animals.
In the future, navigational satellites will continue to improve. The equipment will shrink in size and cost, while it increases in reliability. The number of people able to use the systems will also increase. Railroads are exploring ways that satellites may improve traffic management. In addition, new satellite navigation systems will be developed. In the United States, work is proceeding on Geostar, in which a user transmits a signal to three geosynchronous satellites which in turn relay the signals to a ground station. The ground station calculates the exact location and sends the information to the user or to another location. Work is also underway on an improved search and rescue operation through the use of satellites that can pinpoint emergency locator beacons. The benefits of the new navigational satellite systems also rapidly became accessible to the public, as handheld GPS displays became available and rapidly gained acceptance among hikers, travelers, and the simply curious.
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