An inertial navigation system (INS) is used on some large aircraft for long-range navigation. This may also be identified as an inertial reference system (IRS), although the IRS designation is generally reserved for more modern systems.
An INS/IRS is a self-contained system that does not require input radio signals from a ground navigation facility or transmitter. The system derives attitude, velocity, and direction information from measurements of aircraft acceleration from a known starting point. The location of the aircraft is continuously updated through calculations based on the forces experienced by INS accelerometers.
Older INS units commonly used accelerometers referenced to north and east. In these systems, the accelerometers were mounted on a gyro-stabilized platform to prevent errors caused by gravitational acceleration.
An INS uses complex calculations made by an INS computer to convert applied forces into location information. An interface control head is used to enter starting position data while the aircraft is stationary on the ground. This is called initializing. [Figure 1]
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| Figure 1. An interface panel for three air data and inertial reference systems on an Airbus. The keyboard is used to initialize the system. Latitude and longitude position is displayed at the top |
From then on, all motion of the aircraft is sensed by the built-in accelerometers and run through the computer. Feedback and correction loops are used to correct for accumulated error as flight time progresses. The amount of accumulated INS error after one hour of operation is a common reference for determining system performance.
Accumulated error of less than one mile after one hour of operation is possible. Continuous accurate adjustment to the gyro-stabilized platform to keep it parallel to the Earth’s surface is a key requirement to reduce accumulated error. A latitude/longitude coordinate system is used when giving the location output.
INS is integrated into an airliner’s flight management system and automatic flight control system. Waypoints can be entered for a predetermined flight path, and the INS will guide the aircraft to each waypoint in succession. Integration with other navigation aids is also possible to ensure continuous correction and improved accuracy but is not required.
Modern INS systems are known as IRS. They are completely solid-state units with no moving parts. Three ring-laser gyros replace the mechanical gyros in the older INS platform systems. This eliminates precession and other mechanical gyro shortcomings. The use of three solid-state accelerometers, one for each axis of movement, also increases accuracy. The accelerometer and gyro outputs are sent to the computer for continuous calculation of the aircraft’s position.
The most modern IRS integration is the GPS. GPS is extremely accurate on its own. When combined with IRS, it creates one of the most accurate navigation systems available. The GPS is used to initialize the IRS so the pilot no longer needs to do so. GPS also feeds data into the IRS computer to be used for error correction.
Occasional GPS service interruptions and altitude inaccuracies pose no problem for IRS/GPS. The IRS functions continuously and is completely self-contained within the IRS unit. Should GPS service become unavailable, the IRS portion of the system continues without it. Modern electronic technology has reduced the size and weight of INS/IRS avionics units significantly. Figure 2 shows a modern micro-IRS unit that measures approximately 6 inches on each side.
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| Figure 2. A modern micro-IRS with built-in GPS |
Aircraft Communications Addressing and Reporting System (ACARS)
ACARS is a two-way communication link between an airliner in flight and the airline’s ground facilities. Data is collected in the aircraft by digital sensors and is transmitted to the ground facilities. Replies from ground stations may be displayed electronically or printed so the appropriate flight crewmember(s) can have a hard copy of the response.
What makes an Inertial Navigation System (INS) self-contained?
How do modern IRS units differ from older mechanical INS units?
What is "initializing" in the context of an IRS?
What is the function of the ACARS system?
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