air navigation


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air navigation,

science and technology of determining the position of an aircraft with respect to the surface of the earth and accurately maintaining a desired course (see navigationnavigation,
science and technology of finding the position and directing the course of vessels and aircraft. Early Navigational Techniques

In ancient times, mariners navigated by the guidance of the sun and stars and landmarks along the coast.
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).

Visual and Instrument Flight

The simplest and least sophisticated way to keep track of position, course, and speed is to use pilotage, a method in which landmarks are noted and compared with an aeronautical chart. Whether these landmarks are observed visually or on radar, this technique of air navigation is usually called flying under visual flight regulations (VFR). These establish the minimum weather conditions under which pilotage is permissible.

Pilotage is not satisfactory for long trips, especially over water or terrain lacking distinctive features. In these cases, or when weather conditions do not permit navigation by visual reference, planes must fly according to instrument flight regulations (IFR), which require that the aircraft be equipped with the necessary position-finding instruments and that the pilot be trained in operating those instruments. Also required under IFR is the filing of a flight plan with air traffic controlair traffic control,
the system by which airplanes are safely routed into and out of major airports. Air traffic control in the United States is centered in a number of regional control centers that route airplanes along established airways to airport traffic control centers.
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 authorities at the departure point. The aircraft is then cleared for a given course and a given altitude. Air traffic controllers monitor the craft until it reaches its destination.

Aircraft Instruments

Light aircraft, flown by pilotage, typically have a simple set of navigational instruments, including an airspeed indicator (see pitot static systempitot static system
, device for measuring the rate at which a fluid flows. Among the principal applications of the device are an airspeed indicator for aircraft and a distance and speed indicator for ships.
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), an aneroid altimeter, and a magnetic compass. For supersonic and hypersonic aircraft the airspeed indicator is altered to show the airspeed as a Mach numberMach number
[for E. Mach], ratio between the speed of an object and the speed of sound in the medium in which the object is traveling. An airplane that has the velocity of Mach 3.0 is traveling at three times the speed of sound as measured in the prevailing atmospheric conditions.
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, which is the ratio of the speed of an aircraft to the speed of sound. Advanced aircraft also use electronic systems to give the pilot highly accurate positional information for use during landing. The Instrument Landing System enables an airplane to navigate through clouds or darkness to an airport's runway; the Microwave Landing System, installed in U.S. airports beginning in 1988, is capable of landing the plane automatically, although the pilot always has the option of overriding manually.

Other navigational aids include the radio altimeteraltimeter
, device for measuring altitude. The most common type is an aneroid barometer calibrated to show the drop in atmospheric pressure in terms of linear elevation as an airplane, balloon, or mountain climber rises.
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, a radar device that indicates the distance of the plane from the ground; the ground-speed indicator, which operates by measuring the Doppler shift in a radio wave reflected from the ground; and, in commercial airliners, the flight management computer, which can display altitude, speed, course, wind conditions, and route information, as well as monitor the airplane's progress through the airway. Other similar systems use inertial devices such as free-swinging pendulums and gyroscopes as references in determining position. These automated and semiautomated procedures free the pilot from many of the activities previously necessary for navigation and thus allow the pilot to concentrate on actually flying the aircraft. Another device which is useful in this way is the automatic pilot, which interprets data on direction, speed, attitude, and altitude to maintain an aircraft in straight, level flight on a given course at a given speed.

Airways and Radio Ranges

Basic to air traffic control are special air routes called airways. Airways are defined on charts and are provided with radio rangesradio range,
geographically fixed radio transmitter that radiates coded signals in all directions to enable aircraft and ships to determine their bearings. An aircraft or ship can determine its line of position and drift if it knows its bearing relative to the radio transmitter
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, devices that allow the pilot whose craft has a suitable receiver to determine the plane's bearing and distance from a fixed location. The most common beacon is a very high frequency omnidirectional radio beacon, which emits a signal that varies according to the direction in which it is transmitted. Using a special receiver, an air navigator can obtain an accurate bearing on the transmitter and, using distance-measuring equipment (DME), distance from it as well.

The system of radio ranges around the United States is often called the VORTAC system. For long distances other electronic navigation systems have been developed: Omega, accurate to about two miles (3 km); Loran-C, accurate to within .25 mi (.4 km) but available only in the United States; and the Global Positioning System (GPS), a network of 24 satellites that is accurate to within a few yards and is making radio ranging obsolete.

Bibliography

See J. Elliott and G. Guerny, Pilot's Handbook of Navigation (1977).

Navigation, Air

 

the science of the methods and equipment for guiding aircraft (airplanes, helicopters, missiles, and so on); the aggregate of operations used by ground control or on-board facilities to determine navigational elements, and also their use to guide the aircraft. The principles of air navigation originated in sea navigation, which was developed in ancient times. In particular, the method of the magnetic compass and nautical astronomy were borrowed from it.

Air navigation ensures that an aircraft is guided along a trajectory determined by the flight route and profile, according to a prescribed plan that regulates the flight conditions from takeoff to landing at a prescribed time. In addition, air navigation solves particular navigation problems, such as the maintenance of prescribed distances and time intervals between aircraft on routes with dense air traffic or upon leaving a route for a landing approach, the prevention of collisions of airplanes with terrain obstacles (such as mountains), and the convergence of two aircraft in flight (for example, a rendezvous with a tanker for refueling). When a flight is carried out according to a predetermined route and plan, the task of air navigation, in contrast to pilotage, reduces primarily to the acquisition of continuous or periodic information on the current navigational elements of the translational motion of the center of mass of the aircraft with respect to a coordinate system correlated to the surface of the earth.

Various types of equipment are used to determine the navigational elements (including course, drift angle, course angle, airspeed and ground speed, altitude, and the coordinates of the aircraft’s location). The equipment is divided into four main groups, according to the primary source of navigational information as follows.

(1) Geotechnical devices, which make possible determination of the relative altitude of flight, the magnetic course, and the location of the aircraft by measuring various parameters of the earth’s geophysical fields (such as the magnetic and gravitational fields). Such equipment includes altimeters, devices for measuring airspeed and ground speed, magnetic and gyromagnetic compasses, directional gyroscopes, optical sights, and inertial navigation systems.

(2) Radio aids, which make possible determination of the true altitude, ground speed, and location of an aircraft by measuring various parameters of the electromagnetic field on the basis of radio signals from special transmitters. Such aids include radio altimeters, radio beacons, radio compasses, and radio navigation systems.

(3) Astronomical equipment, which makes possible determination of the course and location of an aircraft. This includes astrocompasses, sextants, and stellar trackers.

(4) Illumination devices, which are intended to facilitate the landing of aircraft under complex weather conditions and at night, and also to facilitate orientation (beacons).

Since advantages and shortcomings are inherent in each group of technical navigational aids, aids that use various principles are combined as sensors to form unified integrated systems to ensure the accurate flight of an aircraft along a prescribed route under any weather conditions. In such systems the main navigation problems are solved by means of analog or digital computers, and a flight plan is drawn up (including such factors as the coordinates of the points on the route, the altitudes and speeds of flyover of points, and the coordinates of radio navigation systems). Integrated navigation systems that are linked to an autopilot can accomplish automatic flight over the entire route, and also a landing approach, even when the surface of the earth cannot be seen. The integrated navigation system usually determines the position of the aircraft on the basis of three coordinates: two are the projections of the aircraft’s center of mass onto a horizontal plane (longitude and latitude), and one is the altitude. To orient an aircraft it is sufficient to know the two coordinates in the horizontal plane. The flight route is monitored on the basis of the route line, which is determined by a projection of the ground speed vector. This vector is found by adding the measured vectors of airspeed (the speed of the aircraft with respect to the air) and the speed of movement of the air with respect to the surface of the earth. The altitude is measured with an altimeter.

Various methods are used to determine the instantaneous coordinates of the aircraft’s position in flight. The three main methods are as follows: (1) dead reckoning, which is based on determination of the lines (surfaces) of the aircraft’s position by discrete or continuous summation of its measured velocity or acceleration over time; (2) the positional method (position lines), which is used for direct determination of the position lines (surfaces) of an aircraft without taking into consideration the distance it has traveled, by finding the coordinates of the aircraft’s position with respect to known ground reference points or heavenly bodies; (3) the comparative scanning method (orientation), which is used to determine an aircraft’s location either by comparing the actually observed picture of the terrain according to identified ground reference points (including visual, radar, and magnetic references) with a geographic map or standard model of the terrain or by comparing a section of the sky with a star chart. The specific characteristics of the guidance of various types of aircraft, and also the class and function of the aircraft, the regions where the craft are used, and the nature of the route, determine the composition of integrated air navigation systems. The equipment and methods to be used in air navigation are selected in accordance with a plan drawn up in advance by the navigator.

The necessity of ensuring the greatest possible safety of air traffic in spite of its increasing density, the growth in the number and length of air routes, and the further increase in the flying speeds of aircraft has led to the development and introduction of automatic integrated systems for air navigation and air traffic control.

REFERENCES

Spravochnik aviatsionnogo shturmana. Edited by V. I. Sokolov. Moscow, 1957.
Kirst, M. A. Navigatsionnaia kibernetika poleta. Moscow, 1971.

M. M. RAICHEV

air navigation

[¦er ‚nav·ə′gā·shən]
(navigation)
The process of directing and monitoring the progress of an aircraft between selected geographic points or with respect to some predetermined plan. Also known as avigation.
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