Brunei DCA CPL(A) Radio Navigation key terms

An electromagnetic wave that carries energy through space, produced whenever electric charge accelerates. Its frequency and wavelength determine how it travels, bends and reflects, which decides how useful it is for navigation.

The physical distance between two successive crests of a radio wave, equal to the speed of light divided by frequency. Short wavelengths behave almost like light, while long wavelengths bend around terrain and follow the Earth's curve more readily.

A named range of radio frequencies, such as VHF, UHF or SHF, grouped because waves in that range share similar propagation behaviour and are allocated to particular navigation services under international rules.

Propagation in which a signal travels in an essentially straight path and is blocked by obstacles or by the Earth's curvature. VHF and UHF navigation aids are normally usable only within this horizon, which extends with aircraft altitude.

The effective distance at which a transmitter becomes hidden by the Earth's curvature, made slightly greater than the geometric horizon because the atmosphere refracts radio waves downward. It increases with the square root of height.

The bending of a radio wave as it passes between layers of different density, notably in the atmosphere. Normal downward refraction extends range, while abnormal temperature or moisture layers can bend signals sharply and cause navigation errors.

Strong downward bending of a radio wave in a layer where temperature rises with height or moisture falls sharply, trapping the signal near the ground. It extends range well beyond normal but can produce false or distorted readings.

Trapping of a radio signal between atmospheric layers so it travels unusually far with little loss. Ducted signals may reach receivers far beyond the normal service range and cause unexpected interference or misleading navigation indications.

A wave that travels upward and is returned to Earth by reflection from the ionosphere. It allows long-distance reception at lower frequencies but is unreliable for short-range navigation and can cause night-time interference.

The portion of a transmitted signal that follows the Earth's surface, useful at low and medium frequencies. It provides stable reception near the transmitter but weakens quickly as frequency and distance increase.

The deliberate variation of a carrier wave's amplitude, frequency or phase in order to carry information such as Morse identification, audio tones or navigation data. The modulation type determines what a receiver can extract from the signal.

The steady radio-frequency oscillation that is modulated to carry information. Its frequency defines the channel tuned by the receiver, while the modulation placed on it conveys the identification or guidance data.

The band of frequencies above or below the carrier created by modulation, containing the transmitted information. Suppressing the carrier and one sideband saves bandwidth and power, a technique used in some navigation and communication transmissions.

A very high frequency omnidirectional range station that transmits a rotating phase reference so an aircraft receiver can determine the magnetic bearing of the aircraft from the station. It provides course guidance along airways and for approaches within line of sight.

The magnetic bearing, measured from the VOR station, along which an aircraft lies. It is expressed as the direction from the station to the aircraft, so it differs from the aircraft's heading and from the bearing to the station.

The course deviation indicator, a needle on the navigation display that shows how far the aircraft is left or right of the selected course. Its sensitivity increases with distance from the station for VOR and is fixed for ILS.

The omni-bearing selector, a control that rotates the selected course or radial on the navigation display. It sets the reference against which the course deviation indicator and the to-from flag are interpreted.

An indicator that tells whether the selected VOR course, if flown in the direction set, would take the aircraft toward or away from the station. It removes the ambiguity that otherwise exists when the needle is centred.

The region directly overhead a VOR or NDB transmitter where the signal is weak or absent because the aircraft is above the radiating elements. Guidance is unreliable there and should be disregarded until the aircraft leaves the region.

Distance measuring equipment that measures slant range from the aircraft to a ground transponder by timing the round trip of interrogating pulses. The indication is line-of-sight distance, not track distance along the ground.

A military ultra high frequency navigation system providing bearing and distance from a ground beacon. Its distance element is compatible with civil DME, and a VOR combined with TACAN distance is presented to civil users as VORTAC.

The difference between the distance actually measured to a station and the horizontal distance over the ground, greatest when the aircraft is close to and high above the station. It makes DME read higher than the true track distance.

The moment an aircraft passes directly over or abeam a navigation station. DME distance reaches a minimum roughly equal to altitude, bearing changes rapidly, and the course deviation indicator may swing before settling.

A non-directional beacon transmitting a continuous signal at low or medium frequency. It provides no bearing information itself, so the aircraft's automatic direction finder must determine the direction from which the signal arrives.

Automatic direction finder, airborne equipment that senses the direction of arrival of an NDB signal and displays it as a relative bearing. Interpreting that bearing against heading and magnetic variation yields the magnetic bearing to the beacon.

The angle measured clockwise from the aircraft's nose to the direction of the station, as shown on the ADF dial. Adding it to the aircraft heading gives the magnetic bearing to the station.

The magnetic direction from the aircraft to the beacon, obtained by adding relative bearing to heading. It is the value used to plot a position line and to intercept or track a selected bearing.

Bending of a low or medium frequency wave as it crosses a coastline at an oblique angle, because the wave travels faster over water than over land. It causes ADF bearing errors near the shore.

Instrument landing system, a precision approach aid combining a localizer for lateral guidance and a glide path for vertical guidance, with marker beacons or distance information to fix approach progress. It supports approaches down to low minima.

The lateral guidance component of ILS, transmitting on VHF so that the receiver shows displacement from the extended runway centreline. Its sensitivity is greatest at the runway end and reduces with distance from the threshold.

The vertical guidance component of ILS, normally defining a descent of about three degrees to the runway aiming point. It is transmitted on UHF and displayed by a vertical needle or flight director command.

An unwanted glide path indication that can appear above the true path, typically at a steeper angle, because of reflections or the shape of the transmitted lobe. Following it leads to an incorrect and unstabilised approach.

A low-power transmitter that illuminates a cockpit light and sounds a tone as the aircraft passes overhead, indicating a fixed distance from the runway threshold. It helps the crew judge approach progress and height.

The height or altitude on an approach at which a go-around must be initiated if the required visual reference has not been established. It depends on the approach category and the available ground and airborne equipment.

A radar system that transmits pulses and detects the energy reflected back from the target itself, requiring no equipment in the aircraft. It gives range and azimuth but no identity or altitude information.

A radar system that interrogates a transponder in the aircraft, which replies with coded pulses. It provides identification, pressure altitude and, with modern modes, more data, but depends on the aircraft equipment working.

Airborne receiver and transmitter that replies to ground radar interrogations with a coded signal and, when mode C or S is selected, pressure altitude. Its correct operation is essential for identification and separation.

The four-digit octal code selected in the transponder, which identifies the aircraft to secondary surveillance radar. A specific code may be assigned by air traffic control or reserved for emergencies and radio failure.

The transponder mode that transmits pressure altitude derived from the aircraft's static source, referenced to standard pressure. Controllers use it to verify level and separation, so an incorrect setting causes altitude errors.

Radar carried in the aircraft that detects precipitation by measuring reflected energy, displaying intensity and range ahead. It shows precipitation, not wind shear or clear air turbulence directly, so interpretation requires care.

The portion of a transmitted radar pulse returned to the receiver by a target such as precipitation or terrain. The strength and timing of the echo give information about the target's reflectivity and distance.

Loss of radar or radio signal strength as it travels through a medium, especially heavy precipitation. Strong attenuation can hide storms beyond the nearest one, so the displayed picture may understate the hazard ahead.

A method that allows an aircraft to follow any desired path within the coverage of navigation aids, rather than flying directly between ground stations. It enables parallel routes, direct routing and flexible terminal procedures.

A defined geographical position, stored in the navigation database, that the aircraft flies to or over as part of a route or procedure. Waypoints may be defined by coordinates, by radials and distances, or by named fixes.

Area navigation that uses onboard computing to determine position and guidance along a defined path, without requiring continuous reception of a specific ground aid. Its accuracy depends on the sensors and the navigation specification applied.

Required navigation performance, a specification that adds onboard performance monitoring and alerting to area navigation. The aircraft must warn the crew if the navigation system's accuracy fails to meet the stated navigation specification.

A framework that specifies navigation performance in terms of accuracy, integrity, continuity and availability rather than by the ground equipment used. It covers RNAV and RNP specifications applied to airspace and procedures.

A waypoint at which the aircraft begins turning before reaching the point, so as to roll out on the next leg. It is used for smooth route transitions, unlike a fly-over waypoint which must be passed directly.

Global navigation satellite system, a constellation of satellites broadcasting timing signals from which a receiver computes position, velocity and time. Examples include GPS and other international systems, all sharing similar principles.

The apparent distance from a satellite to the receiver, calculated from signal travel time using a receiver clock that is not perfectly synchronised. Errors from the clock are removed by solving for several satellites simultaneously.

Receiver autonomous integrity monitoring, a function in which the GNSS receiver checks the consistency of its satellite measurements and warns the crew if a faulty signal could cause misleading guidance. It requires sufficient satellites in view.

A GNSS integrity function that not only detects an inconsistent satellite signal but also removes it from the position solution, allowing navigation to continue. It requires more satellites in view than detection alone.

Satellite-based augmentation system, which uses geostationary satellites to broadcast corrections and integrity information for GNSS. It improves accuracy and provides a warning of unsafe signals, supporting approaches in some regions.

Ground-based augmentation system, which uses a ground station to monitor GNSS signals and transmit corrections and integrity data to nearby aircraft. It supports precise approach guidance within a limited area around the station.

Unwanted energy that degrades or masks a wanted radio signal, whether from other transmitters, atmospheric noise or deliberate jamming. It can cause loss of guidance, erroneous indications or complete failure of a navigation receiver.

Reception of a signal by more than one path because of reflection from terrain, buildings or aircraft structure. The combined signal is distorted, producing bearing or distance errors and, in the worst case, loss of lock.

The displacement of a navigation needle that corresponds to the edge of the display scale, representing a defined angular or linear error. Knowing that value allows the pilot to judge how far off course the aircraft is.