Avionics and Aircraft Instrumentation

Summary

Flight instrumentation refers to the indicators and instruments that inform a pilot of the position of the aircraft and give navigational information. Avionics comprises all the devices that allow a pilot to give and receive communications, such as air traffic control directions and navigational radio and satellite signals. Early in the history of flight, instrumentation and avionics were separate systems, but these systems have been vastly improved and integrated. These systems allow commercial airliners to fly efficiently and safely all around the world. Additionally, the integrated systems are being used in practically all types of vehicles—ships, trains, spacecraft, guided missiles, and unmanned aircraft—both civilian and military.

Definition and Basic Principles

Flight instrumentation refers to the instruments that provide information to a pilot about the position of the aircraft in relation to the Earth's horizon. The term avionics is a contraction of “aviation” and “electronics” and has come to refer to the combination of communication and navigational devices in an aircraft. This term was coined in the 1970s after the systems were becoming one integral system.

The components of basic flight instrumentation are the magnetic compass, the instruments that rely on air-pressure differentials, and those that are driven by gyroscopes and instruments. Air pressure decreases with an increase in altitude. The altimeter and vertical-speed indicator use this change in pressure to provide information about the height of the aircraft above sea level and the rate that the aircraft is climbing or descending. The airspeed indicator uses ram air pressure to give the speed that the aircraft is traveling through the air.

Other instruments use gyroscopes to detect changes in the position of the aircraft relative to the Earth's surface and horizon. An airplane can move around the three axes of flight. The first is pitch, or the upward and downward position of the nose of the airplane. The second is roll, the position of the wings. They can be level to the horizon or be in a bank position, where one wing is above horizon and the other below the horizon as the aircraft turns. Yaw is the third. When an airplane yaws, the nose of the airplane moves to the right or left while the airplane is in level flight. The instruments that use gyroscopes to show movement along the axes of flight are the turn and bank indicator, which shows the angle of the airplane's wings in a turn and the rate of turn in degrees per second; the artificial horizon, which indicates the airplane's pitch and bank; and the directional gyro, which is a compass card connected to a gyroscope. Output from the flight instruments can be used to operate autopilots. Modern inertial navigation systems (INS) use gyroscopes, sometimes in conjunction with a global positioning system (GPS), as integrated flight instrumentation and avionics systems.

The radios that comprise the avionics of an aircraft include communications radios that pilots use to talk to air traffic control (ATC) and other aircraft and navigation radios. Early navigation radios relied on ground-based radio signals, but many aircraft have come to use GPS receivers that receive their information from satellites. Other components of an aircraft's avionics include a transponder, which sends a discrete code to ATC to identify the aircraft and is used in the military to discern friendly and enemy aircraft, and radar, which is used to locate rain and thunderstorms and to determine the aircraft's height above the ground.

Background and History

Flight instruments originally were separated from the avionics of an aircraft. The compass, perhaps the most basic of the flight instruments, was developed by the Chinese in the second century B.C.E. Chinese navy commander Zheng He's voyages from 1405 to 1433 included the first recorded use of a magnetic compass for navigation.

The gyroscope, a major component of many flight instruments, was named by French physicist Jean-Bernard-Léon Foucault in the nineteenth century. In 1909, American businessman Elmer Sperry invented the gyroscopic compass that was first used on U.S. naval ships in 1911. In 1916, the first artificial horizon using a gyroscope was invented. Gyroscopic flight instruments along with radio navigation signals guided American pilot Jimmy Doolittle to the first successful all-instrument flight and landing of an airplane in 1929. Robert Goddard, the father of rocketry, experimented with using gyroscopes in guidance systems for rockets. During World War II, German rocket scientist Wernher von Braun further developed Goddard's work to build a basic guidance system for Germany's V-2 rockets. After the war, von Braun and 118 of his engineers immigrated to the United States, where they worked for the U.S. Army on gyroscopic inertial navigation systems (INS) for rockets. Massachusetts Institute of Technology engineers continued the development of the INS to use in Atlas rockets and eventually the space shuttle. Boeing was the first aircraft manufacturer to install INS into its 747 jumbo jets. Later, the Air Force introduced the system to their C-141 aircraft.

Radios form the basis of modern avionics. Although there is some dispute over who actually invented the radio, Italian physicist Guglielmo Marconi first applied the technology to communication. During World War I, in 1916, the Naval Research Laboratory developed the first aircraft radio. In 1920, the first ground-based system for communication with aircraft was developed by General Electric. The earliest navigational system was a series of lights on the ground, and the pilot would fly from beacon to beacon. In the 1930s, the nondirectional radio beacon (NDB) became the major radio navigation system. This was replaced by the very high frequency omnidirectional range (VOR) system in the 1960s. In 1994, the GPS became operational and was quickly adapted to aircraft navigation. The great accuracy that GPS can supply for both location and time was adapted for use in INS.

How It Works

Flight Instruments

Flight instruments operate using either gyroscopes or air pressure. The instruments that use air pressure are the altimeter, the vertical speed indicator, and the airspeed indicator. Airplanes are fitted with two pressure sensors: the pitot tube, which is mounted under a wing or the front fuselage, its opening facing the oncoming air; and the static port, which is usually mounted on the side of the airplane out of the slipstream of air flowing past the plane. The pitot tube measures ram air; the faster the aircraft is moving through the air, the more air molecules enter the pitot tube. The static port measures the ambient air pressure, which decreases with increasing altitude. The airspeed indicator is driven by the force of the ram air calibrated to the ambient air pressure to give the speed that the airplane is moving through the sky. The static port's ambient pressure is translated into altitude above sea level by the altimeter. As air pressure can vary from location to location, the altimeter must be set to the local barometric setting in order to receive a correct altimeter reading. The vertical speed indicator also uses the ambient pressure from the static port. This instrument can sense changes in altitude and indicates feet per minute that the airplane is climbing or descending.

Other flight instruments operate with gyroscopes. These instruments are the gyroscopic compass, the turn and bank indicator, and the artificial horizon. The gyroscopic compass is a vertical compass card connected to a gyroscope. It is either set by the pilot or slaved to the heading indicated on the magnetic compass. The magnetic compass floats in a liquid that allows it to rotate freely but also causes it to jiggle in turbulence; the directional gyro is stabilized by its gyroscope. The magnetic compass will also show errors while turning or accelerating, which are eliminated by the gyroscope. The turn and bank indicator is connected to a gyroscope that remains stable when the plane is banking. The indicator shows the angle of bank of the airplane. The artificial horizon has a card attached to it that shows a horizon, sky, and ground and a small indicator in the center that is connected to the gyroscope. When the airplane pitches up or down or rolls, the card moves with the airplane, but the indicator is stable and shows the position of the aircraft relative to the horizon. Pilots use the artificial horizon to fly when they cannot see the natural horizon. The artificial horizon and directional gyro can be combined into one instrument, the horizontal situation indicator (HSI). These instruments can be used to supply information to an autopilot, which can be mechanically connected to the flight surfaces of the aircraft to fly it automatically.

Ground-Based Avionics

Ground-based avionics provide communications, navigational information, and collision avoidance. Communication radios operate on frequencies between 118 and 136.975 megahertz (MHz). Communication uses line of sight. Navigation uses VOR systems. The VOR gives a signal to the aircraft receiver that indicates the direction to or from the VOR station. A more sensitive type of VOR, a localizer, is combined with a glide slope indicator to provide runway direction and a glide path for the aircraft to follow when it is landing in poor weather conditions and the pilot does not have visual contact with the runway. Collision avoidance is provided by ATC using signals from each aircraft's transponders and radar. ATC can identify the aircrafts' positions and advise pilots of traffic in their vicinity.

Satellite-Based Systems

The limitation of line- of-sight for ground-based avionic transmitters is a major problem for navigation over large oceans or in areas of the world that have large mountain ranges or few transmitters. The U.S. military was very concerned about these limitations and the Department of Defense spearheaded the research and implementation of a system that addresses these problems. GPS is the United States' satellite system that provides navigational information. GPS can give location, movement, and time information. The system uses a minimum of twenty-four satellites orbiting the Earth that send signals to monitoring receivers on Earth. The receiver must be able to get signals from a minimum of four satellites in order to calculate an aircraft's position correctly. Although originally designed solely for military use, GPS is widely used by civilians.

Early unmanned aerial vehicles (UAVs) were also constrained by line-of-sight limitations. Because of the curvature of the earth, the UAV, which resembled an airplane, would have it to increase its altitude the further away it ventured from its ground control system. This more easily subjected it to harsher flying condiditons, notably ice, in cold weather conditions. Also, the optics the UAV carried, such as a photographic or video camera, was pulled further away from its target the higher the vehicle had to climb. This challenge was overcome as unmanned systems began to use satellites to relay its control signal. This enable the vehicle to be flown much lower to the ground at further distances.

Inertial Navigation Systems (INS)

The INS is a self-contained system that does not rely on outside radio or satellite signals. INS is driven by accelerometers and gyroscopes. The accelerometer houses a small pendulum that will swing in relation to the aircraft's acceleration or deceleration and so can measure the aircraft's speed. The gyroscope provides information about the aircraft's movement about the three axes of flight. Instead of the gimbaled gyros, more precise strap-down laser gyroscopes have come to be used. The strap-down system is attached to the frame of the aircraft. Instead of the rotating wheel in the gimbaled gyroscopes, this system uses light beams that travel in opposite directions around a small, triangular path. When the aircraft rotates, the path traveled by the beam of light moving in the direction of rotation appears shorter than the path of the other beam of light moving in the opposite direction. The length of the path causes a frequency shift that is detected and interpreted as aircraft rotation. To work correctly, an INS must be initialized. The system has to be able to detect its initial position or it must be programmed with its initial position before it is used or it will not have a reference point from which to work.

Applications and Products

Military INS and GPS Uses

Flight instrumentation and avionics are used by military aircraft as well as civilian aircraft. Military avionics, nonetheless, have many other applications. INS is used in guided missiles and submarines. It can also be used as a stand-alone navigational system in vehicles that do not want to communicate with outside sources for security purposes. INS and GPS are used in bombs, rockets, and, with great success, UAVs have employed GPS with great success. These are employed for reconnaissance as well as delivering ordnance without placing a pilot in harm's way. GPS is used in almost all military vehicles such as tanks, armored vehicles, cars, ships, and submarines (though not at depth, as the satellite signals will not penetrate deep water). GPS is also used by the United States Nuclear Detonation Detection System as the satellites carry nuclear detonation detectors.

Navigation

Besides the use of flight instrumentation and avionics for aircraft navigation, the systems can also be used for almost all forms of navigation. The aerospace industry has used INS for guidance of spacecraft that cannot use earthbound navigation systems, including satellites that orbit the planet. INS systems can be initialized by manually inputting the craft's position using GPS or using celestial fixes to direct rockets, space shuttles, and long-distance satellites and space probes through the reaches of the solar system and beyond. These systems can be synchronized with computers and sensors to control the vehicles by moving flight controls or firing rockets. GPS can be used on Earth by cars, trucks, trains, ships, and handheld units for commercial, personal, and recreational uses. One limitation of GPS is that it cannot work where the signals could be blocked, such as under water or in caves.

Cellular Phones

GPS technology is critical for operating cell phones. GPS provide accurate time that is used in synchronizing signals with base stations. If the phone has GPS capability built into it, as smartphones do, it can be used to locate a mobile cell phone making an emergency call. The GPS system in cell phones can be used in cars for navigation as well for recreation such as guidance while hiking, biking, boating, or geocaching.

Tracking Systems

In the same manner that GPS can be used to locate a cell phone, GPS can be used to find downed aircraft or pilots. GPS can be used by biologists to track wildlife by placing collars on the animals, a major improvement over radio tracking that was line-of-sight and worked only over short ranges. Animals that migrate over great distances can be tracked by using only GPS. Lost pets can be tracked through GPS devices in their collars. Military and law enforcement use GPS to track vehicles.

Other Civilian Applications

Surveyors and mapmakers use GPS to mark boundaries and identify locations. GPS units installed at specific locations can detect movements of the Earth to study earthquakes, volcanoes, and plate tectonics. GPS-enabled UAVs can be helpful in aerial photography, disaster management, search and rescue operations, border control surveillance, and crop monitoring.

Next Generation Air Transportation System (NextGen)

While ground-based navigational systems such as the VOR are still used by pilots and radar is used by ATC to locate airplanes, the Federal Aviation Administration (FAA) is researching and designing NextGen, a new system for navigation and tracking aircraft that will be based on GPS in the National Airspace System (NAS). Using NextGen GPS navigation, which is expected to be in place by 2025, aircraft will be able to fly shorter and more direct routes to their destinations, saving time and fuel. In 2020, as part of NextGen, the FAA started implementing a system called automatic dependent surveillance-broadcast (ADS-B) to track and monitor most aircraft. ADS-B transmits more data than radar and can track aircraft over the oceans, the Earth's poles, and inaccessible terrains. This is part of a switch to ATC using a satellite-based system of managing air traffic. The European Space Agency (ESA) collaborated with the satellite communications company Inmarsat in 2021 to provide digital information to radio channels through satellites to manage air traffic. It would also help in the future as the number of flights was estimated to double by 2035.

Careers and Course Work

The possible careers associated with flight instruments and avionics include both civilian and military positions ranging from mechanics and technicians to designers and researchers. The education required for these occupations usually requires at least two years of college or technical training, but research and design may require a doctorate.

Maintenance and avionics technicians install and repair flight instruments and avionics. They may work on general aviation airplanes, commercial airliners, or military aircraft. With more and more modes of transportation using INS and GPS, mechanics and technicians may also be employed to install and repair these systems on other types of vehicles—ships, trains, guided missiles, tanks, or UAVs. NASA and private companies employ technicians to work with spacecraft. Most of these positions require an associate's degree with specialization as an aircraft or avionics technician, or the training may be acquired in the military.

As computers are becoming more and more important in these fields, the demand for computer technicians, designers, and programmers will increase. Jobs in these fields range from positions in government agencies such as the FAA or National Aeronautics and Space Administration (NASA) or the military to private-sector research and development. The education required for these occupations varies from high school or vocational computer training to doctorates in computer science or related fields. Universities such as Oklahoma State University and Embry-Riddle Aeronautical University in Florida offer related courses.

Flight instrument and avionics systems are being designed and researched by persons who have been educated in mechanical, electrical, and aeronautical engineering, computer science, and related fields. Some of these occupations require the minimum of a bachelor's degree, but most require a master's or doctorate. Aspirants can work in organizations such as Virgin Galactic, Raytheon Technologies, and Boeing.

Social Context and Future Prospects

Aviation, made possible by flight instrumentation and avionics, has revolutionized how people travel and how freight is moved throughout the world. It has also dramatically changed how wars are fought and how countries defend themselves. In the future, flight instrumentation and avionics will continue to affect society not only through aviation but also through applications of the technology in daily life.

Military use of UAVs controlled by advances in flight instrumentation and avionics will continue to change how wars are fought. However, in the not-too-distant future this technology may be used in civilian aviation. UAVs could be used to inspect pipelines and perform surveys in unpopulated areas or rough terrain, but it is unclear whether they will be used for passenger flights. Many people will certainly be fearful of traveling in airplanes with no human operators. The FAA would have to develop systems that would incorporate unmanned aircraft into the airspace. To this end, in 2021, NASA worked with the FAA to integrate unmanned aircraft systems (UAS) into air traffic at low altitudes by using UAS Traffic Management (UTM) to avoid interference with helicopters, airplanes, or airports. Furthermore, the use of unmanned vehicles may be an important part of future space exploration.

Perhaps the avionics system that has had the most impact on society is GPS. As GPS devices are being made more compact and more inexpensively, they are being used more and more in daily life. GPS can permit underdeveloped countries to improve their own air-navigation systems more rapidly without the expense of buying and installing ground-based navigational equipment or radar systems used by air traffic control facilities.

By the mid-2020s, GPS, as a military application, was shown to have limitations. This was evidenced following the Russian invasion of Ukraine in 2022. The Russians employed types of GPS jammers that could interfere with the ability of such systems to direct precision munitions to a target on a battlefield. This type of hindrance had also begun to appear in civilian airline operations. The number of incidents of civilian aircraft subjected to GPS interference began to show steady increases. These types of attacks caused GPS systems to go offline or to report false locations. Thus, the future of both civilian and military avionics may move beyond a dependence on GPS.  In 2024, the U.S. Air Force began experimenting with artificial intelligence (AI) as an alternative to GPS navigation. Theoretically, an aircraft or missile navigational system empowered with AI could determine its global position without receiving inputs from a satellite-based GPS system. It would instead rely on terrestrial features, or even celestial objects such as stars, to auto-direct itself from its current position to its target. This is essentially how humans performed navigation before the advent of GPS.

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