Inertial Guidance System
An inertial guidance system (also known as an inertial navigation system) uses gyroscopes, accelerometers, and computers to calculate the position, velocity, and attitude of a vehicle in motion. An inertial guidance system electronically samples the current placement and acceleration of a vehicle on a continuous basis to determine its future course. Gyroscopes, often three, furnish information on fixed reference directions. Accelerometers monitors velocity changes. Computers process the data from the gyroscopes and accelerometers, ensuring the vehicle is on course. The system is self-contained and supplies navigational information without communicating with a satellite or base station. Inertial guidance systems have been used on airplanes, submarines, and missiles since the 1970s in the United States.
There are several kinds of inertial guidance systems, the gimbaled inertial system and the strap-down inertial system, with the former the more common. The gimbaled inertial system has up to four gimbals (at least one a stable platform) with three gyroscopes and accelerometers attached. When vehicle with the attached inertial guidance system is in motion, the gyroscopes drive the gimbals. The strap-down inertial system has three gyroscopes and three accelerometers mounted directly to the vehicle, and their outputs are read every one hundredth of a second. Because of the amount of data produced, computations were an encumbrance in early models.
As computers have increased in speed and capabilities, strap-down systems have improved. New gyroscopes have been developed that are more precise and less prone to error, including the laser gyroscope and the electrostatic gyroscope. In the future, strap-down systems might replace gimbaled.
Gyroscopes and accelerometers much be precise, even small errors can put a vehicle completely off course. Gyroscopes have several potential problems. Their bearings are subject to friction and drift putting the vehicle off course. Errors can also come from incorrect initial route information as it is entered in the computer. There are several solutions. In airplanes flying over land, pilots can receive signals from VOR/DME transmitting stations, helping them keep on course, though the signal has strength limitations and can be subject to interference. A newer solution is using information from the global positioning system (GPS), a series of 24 satellites that orbit Earth. Fully functional in the 1990s, GPS allows precision information on location when a vehicle is equipped with a special receiver. Indeed, as early as 1993, combination of an inertial guidance system and a GPS receiver became available on the market. Inertial guidance systems have one advantage over GPS: because they are self-contained, they are not vulnerable to any terrorist activity or other interferences. The combination of the two technologies would seem to be the ideal guidance system for airplanes and similar vehicles.
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