Aircraft Maintenance, Mechanic, Industry Glossary

A

Above Ground Level (AGL)
Distance of the aircraft above the ground Synonyms: radar altitude;

Above Sea Level (ASL)
Distance of the aircraft above mean sea level

Absolute
A standard, fixed reference, as opposed to moving reference; Compare: relative;

Acceleration
Rate of change of velocity, either scalar or vector, often with subscripts such as ENU or XYZ to denote the coordinate frame; time derivative of velocity; time integral of jerk; Symbols: a, A; Typical Units: ft/s-squared, g; Dimensions: Length / Time-squared;

Acceleration East
Aircraft acceleration in true east direction; Symbols: A sub E; Typical Units: ft/s-squared; Dimensions: Length / Time-squared;

Acceleration North
Aircraft acceleration in true north direction; Symbols: A sub N; Typical Units: ft/s-squared; Dimensions: Length / Time-squared;

Accelerometer
An inertial device for measuring acceleration, usually in three orthogonal axes (lateral X, longitudinal Y, and vertical Z); accelerometers usually consist of a mass, spring, and damper; accelerometers are usually included in inertial sensors, such as AHRS and INS;

Accept
To allow to proceed, for example with a position update, usually by an operator; Compare: reject

Accuracy
Measure of exactness, possibly expressed in percent; Compare: precision

Acquire
To begin reception of useful data

Activate
To begin performing a mission objective, such as flying along a radial of a radio station; usually refers to a mode of radio navigation, such flying along that radial after capturing that radial; See Also: capture criterion; Compare: arm, capture;

ADC
Air data computer

ADDR
Air data dead reckoning

ADF
Automatic Direction Finding

Advisory
A signal to indicate safe or normal configuration, condition of performance, operation of essential equipment, or to attract attention and impart information for routine action purposes (from MIL-STD-1472D); an annunciator that is the least critical (less than a caution or a warning);

AFCS
Automatic flight control system

AGL
Above ground level

AGR
Air-ground ranging

AGR Slant Range
Straight-line distance from the aircraft to a point on the ground;

AHOV
Approach to hover

AHRS
Attitude Heading Reference System

Aiding
A process by which one or more sensors provide data to another sensor to produce results better than any single sensor; aiding occurs at the data source level or at the physical device level, depending upon specific implementation of the device and the data source (choice of implementation is transparent above the data source); aiding is automatically controlled by software without input from an operator; a basic control to a data source from navigation, radio navigation, or other devices Compare: update;

Aileron
A control surface on fixed-wing aircraft, usually mounted on the aft edge of wings, that controls roll, and is controlled by the wheel; Symbols: delta sub A; Typical Units: rad, deg;

Air Data Computer (ADC)
A primary navigation data source A navigation sensor based on atmospheric data sensors; usually measures static pressure, dynamic pressure, and outside air temperature; sometimes computes other atmospheric data, such as indicated airspeed, Mach number, calibrated airspeed As a guidance mode, ADC is least accurate of the listed modes and is used only as a last resort

Air Data Dead Reckoning (ADDR)
Dead reckoning navigation based on simple instruments as source (barometric altimeter, magnetic compass, airspeed indicator, known wind conditions); sometimes called dead reckoning;

Air-Ground Ranging (AGR)
Straight-line distance from the aircraft to a point on the ground;

Air-Mass Flight Path Angle
Angle in vertical plane of earthspeed vector and groundspeed vector; occasional definition for flight path angle; Compare: earth-referenced flight path angle; Symbols: gamma sub A; Typical Units: rad, deg;

Air Traffic Control (ATC)
Standard aviation term

Aircraft (ac)
A craft that flies in the air; either fixed or rotory wing

Airspeed (as, a/s)
See Also: state data, true airspeed, indicated airspeed, calibrated airspeed; Symbols: V sub A/S; Typical Units: kt, ft/s; Dimensions: Length / Time;

Airy
A standard model for computing earth data

Alignment
A basic control to a data source from controls and displays to align a device Also a procedure to align physical devices, usually navigation sensors, so that they provide the most accurate results possible; commonly required by INS, AHRS, barometric altimeter; See Also: boresighting, calibration, initialization;

ALTINTVAL
Altitude integral input

Altimeter
A device to measure altitude, either barometric altitude or radar altitude

Altitude
Height, usually with respect to the terrain below (radar altitude, feet above closest dirt) or fixed earth reference (barometric altitude, feet above mean sea level); Symbols: h; Typical Units: ft; Dimensions: Length;

Altitude Error
A basic output from guidance to flight director, indicating the difference between actual altitude and desired altitude; Symbols: DELTA h; Typical Units: ft; Dimensions: Length

Altitude Error Scale Factor (KZSF)
A guidance control law parameter, generated by the vertical guidance modes; varies control authority of vertical guidance

Altitude Integral Gain (KALTINT)
A guidance control law parameter, generated by the vertical guidance modes; varies control authority of the altitude integral in vertical guidance, to reduce steady-state errors in altitude error Typical Units: ft; Dimensions: Length;

Altitude Integral Input (ALTINTVAL)
A guidance control law parameter, generated by the vertical guidance modes; a reference altitude for reducing steady-state errors in altitude error Typical Units: ft; Dimensions: Length;

Altitude Integral Limit (INTMAGLIM)
A guidance control law parameter, generated by the vertical guidance modes; limits the magnitude of altitude integral value Typical Units: ft; Dimensions: Length;

Angle Of Attack
The difference between pitch and the air-referenced flight path angle; the angle between the aircraft center line and the airspeed vector in the vertical plane, positive when the nose is up; Symbols: alpha; Typical Units: rad, deg;

Angular Acceleration
Rate of change of angular velocity, either scalar or vector, often with subscripts such as XYZ to denote the coordinate frame; time derivative of angular position; time integral of angular acceleration; Symbols: alpha; Typical Units: rad/s-squared; Dimensions: 1/Time-squared;

Angular Position
Amount of rotation about an axis, either scalar or vector, often with subscripts such as XYZ to denote the coordinate frame; time integral of angular velocity; Synonyms: angle; Symbols: theta; Typical Units: rad, deg;

Angular Velocity
Rate of change of rotation about an axis, either scalar or vector, often with subscripts such as XYZ to denote the coordinate frame; time derivative of angular position; time integral of angular acceleration; See Also: tachometer; Symbols: omega; Typical Units: rad/s, rpm; Dimensions: 1/Time;

Annunciator
Any one of warning, caution, or advisory; Synonyms: alert;

Aperiodic
A process that executes based on events rather than a fixed rate, it is not synchronized to other processes of interest; Compare: periodic;

APPR
Approach

Approach (APPR)
To fly towards a point; a basic guidance mode, providing lateral guidance, longitudinal guidance, and vertical guidance to a point at an operator selected groundspeed and radar altitude; See Also: waypoint approach, ILS approach, rendezvous approach;

Approach to Hover (AHOV)
Hover approach of a rotary wing aircraft

Arm
To strive for a mission objective, such as flying toward a radial of a radio station; usually refers to a mode of radio navigation, such as striving to reach a specific radial of a radio station prior to flying along that radial; See Also: capture criterion; Compare: activate, capture;

ASL
Above sea level

Astronomical Latitude
Latitude measured with respect to vector of apparent gravity; Compare: geocentric latitude, geodetic latitude; Symbols: Phi sub A; Typical Units: rad, deg,DMS;

ATC
Air Traffic Control

Atmospheric Data
Environmental data related to the atmosphere at some point of interest

Attitude
The primary aircraft angles in the state vector; pitch, roll, and yaw;

Attitude Heading Reference System (AHRS)
Combines information from a Magnetic Heading Sensor with self-contained aircraft acceleration data to provide attitude, heading, position, body inertial velocity, and body inertial acceleration Typically a low-accuracy, self-contained navigation source using strapdwon accelerometers;

Australian National
A standard model for computing earth data

Automatic Direction Finding (ADF)
A basic guidance mode, providing lateral guidance to a radio station Equipment that determines bearing to a radio station;

Automatic Flight Control System (AFCS)
An automated system for controlling the primary flight controls, often with built-in functions for guidance and flight director, and sometimes radio navigation; many flight control systems include basic instruments similar to a AHRS; many flight control systems accept flight director inputs so that its radio navigation, guidance, and flight director can be bypassed

Autopilot
A mode of an automatic flight control system which controls primary flight controls to meet specific mission objectives, such as maintain a heading or altitude; Synonyms: automatic flight control system;

Averaging Filter
A filter for combining multiple data sources, usually of the same type, by adding with weighted averages; a simple average of the data sources; Compare: complementary filter, Kalman filter;

AVM
Avionics monitor

Axis
One direction in an orthogonal reference frame;

Azimuth
An angle in the horizontal plane, usually measured with respect to body coordinates

Aircraft Maintenance, Mechanic, Industry Glossary

B

BALT SEL
Barometric altitude select

Band-Pass Filter (BPF)
A filter that allows frequencies between two cutoff frequencies to pass while attenuating frequencies outside the cutoff frequencies; a band-pass filter can be constructed as the composition of a low-pass filter and a high-pass filter;

Bank Angle
The angle between the horizontal plnase and the right wing in the lateral plane, positive when the right wing is down; Synonyms: roll; Symbols: phi,Phi; Typical Units: rad, deg;

Barometric Altitude
Height with respect to fixed earth reference (above mean sea level); Synonyms: pressure altitude; Symbols: h sub b; Typical Units: ft; Dimensions: Length;

Barometric Altitude Select (BALT SEL)
A basic guidance mode, providing vertical guidance to an operator selected barometric altitude;

Barometric Pressure
Height with respect to fixed earth reference (barometric altitude, feet above mean sea level); Synonyms: pressure; Symbols: p sub a; Typical Units: in HG,mbar; Dimensions: Mass /Time-squared * Length;

BC
Bus controller

Beacon
A device, usually based on the ground, that aids in determining position or direction;

Bearing (BRG)
Direction on a compass; Synonyms: direction; Symbols: B; Typical Units: rad, deg;

Bessel 1841
A standard model for computing earth data

Bias
An offset applied to a measurement for error correction; Synonyms: offset

BIS
Built-in simulation

Body
The aircraft, usually referring to a coordinate system;

Body Coordinates
Coordinates referenced to the body of the aircraft; See Also: XYZ; Compare: earth coordinates, stability coordinates

Boresight Angle
The angle between the center line of a sensor and aircraft center line, either by design or by misalignment;

Boresighting
A basic control to a data source from controls and displays to boresight a device; Also, a procedure to align the center line of physical devices, usually update sensors, so that they provide the most accurate results possible; a basic control to a data source from controls and displays; commonly required by FLIR, MMR; boresight procedures commonly result in correction factors to be downloaded from the host processor to the device; during boresighting, the device is usually not available; See Also: alignment, calibration, initialization;

BPF
Band-pass filter

BRG
Bearing

Built-In Simulation (BIS)
Function in avionics software that simulates sensors, aircraft, and pilot, to exercise avionics software (including navigation, radio navigation, guidance and flight director); BIS is often used by a development team to check basic operation following installation of new software or patches; BIS is seldom used by aircraft flight crews or maintenance crews; Compare: real-time engineering simulation;

Bus Controller (BC)
Term defining role of device on a MIL-STD-1553 bus as being master; Compare: remote terminal;

Aircraft Maintenance, Mechanic, Industry Glossary

C

Calibrated Airspeed (CAS)
Indicated airspeed corrected for instrumentation errors, but not for air density; See Also: airspeed; Symbols: V sub 'CAS'; Typical Units: kt,ft/s; Dimensions: Length / Time;

Calibration
A basic control to a data source from controls and displays for calibrating a device; Also, a procedure to adjust physical devices so that they provide the most accurate results possible; calibration procedures commonly result in correction factors to be downloaded from the host processor to the device; during calibration, the device is usually not available; See Also: alignment, boresighting, initialization;

Cant Angle
Angle of nacelle mounting; Typical Units: rad, deg;

Capture
To attain an objective, such as reaching a radial of a radio station; See Also: capture criterion; Compare: activate, arm;

Capture Criterion
A test case to determine if an armed objective has been captured; In avionics, an aircraft might have an objective to fly to a radial of a radio station, then to fly along it While enroute, the objective is armed, meaning that the crew and software are attempting to reach the radial The radial is captured and the objective is met when the capture criteria are met In this case, the capture criterion might be bearing to the radio station is within 5deg of 270deg An objective can have multiple criteria Capture criterion are often used with radio navigation to determine a transition from armed to active

CAS
Calibrated airspeed

Caution
A signal which alerts the operator to an impending dangerous condition requiring attention, but not necessarily immediate action (from MIL-STD-1472D); an annunciator that is more critical than an advisory but less critical than a warning;

CCLIM
Course cut limit

Center Line
Standard aviation term

CEP
Circular error probability

Channel
A number that maps to a frequency;

Circular Error Probability (CEP)
A probability that a percentage of two-dimension measurements will lie within a circle of given radius, with the circle centered at truth or mean of the measurements; Compare: radial error probability, spherical error probability; CEP specifies test cases for measurement errors of sensors of two dimensions, such as velocity east and north

Clarke 1866
A standard model for computing earth data

Clarke 1880
A standard model for computing earth data

Collective
A flight control operated by moving up or down with hand in rotary-wing aircraft, primarily to control lift (altitude); controls collective (total) pitch of the rotors on a rotary-wing aircraft;

Collective Cue
A vertical flight director cue for rotary-wing aircraft, primarily to control altitude, by changing power; Compare: yoke cue; Symbols: Gamma sub 'VERT'; Typical Units: percent,in;

Commanded
Controls given to a device, not that the device necessarily obeyed the controls

Communications
How well equipment is communicating; Values: operational, degraded, failed

Complementary Filter
A filter in which the complement of the filter is desired, giving the effect of a high-pass filter by implementing a low-pass filter; a filter for combining multiple data sources, usually of different types, by adding filtered values, where the sum of the filters in the frequency domain is unity; a Kalman filter with fixed gains; Complementary filters are often designed in the frequency domain in way that that the filters determined at build time such that the cutoff frequency of the LFP is equal to that of the HPF This provides the advantages of DNS's long-term accuracy and INS's short-term accuracy, while filtering DNS's high-frequency noise and INS's slow drift Compare: averaging filter, Kalman filter;

Computer Cycle
In a periodic, cyclical computer system, the most basic, fastest timing loop

Continuous Time
Time which can have any point expressed as a real quantity, without regard for any specific interval or processing rate; Compare: discrete time;

Continuous-Time Equation
A mathematical relationship to describe a function of time, expressed in terms of continuous time; Compare: difference equation, differential equation, discrete-time equation, Laplace transform, state-space model, Z transform; See Also: first-order filter, second-order filter, unit functions for examples;

Control Law
The mathematical definition of a system used to control or to change the dynamic response of a system;

Control Surface
An airfoil attached to an aircraft that is moved to control the attitude of the aircraft; an surface to control flight of an aircraft indirectly, such as a swashplate to control pitch of rotor blades; See Also: aileron, rudder, spoiler, elevator, flaps, trim tab, stabilizer; Symbols: delta; Typical Units: rad, deg;

Controlled Altitude (CTALT)
A guidance control law parameter, generated by the vertical guidance modes; altitude that is being controlled Typical Units: ft; Dimensions: Length;

Controlled Speed (CTS)
A guidance control law parameter, generated by the longitudinal guidance modes; speed that is being controlled Typical Units: ft/s, kt; Dimensions: Length / Time;

Coriolis Acceleration
Tangential acceleration caused by motion on a radial on a rotating surface, in aviation, it is acceleration in the earth's longitudinal direction caused by changing latitude, usually computed from system state data; Symbols: a sub c; Typical Units: ft/s-squared,g; Dimensions: Length / Time-squared;

Corrected Altitude
Measured pressure altitude corrected for instrumentation errors

Coupled
Describes operation of flight director in which automatic flight control system causes flight controls to follow commands from flight director or errors from guidance

Course
Towards a point at a specified course; Compare: direct

Course Cut Limit (CCLIM)
A guidance control law parameter, generated by the lateral guidance modes; limits the intercept angle of the flight path with a desired course, typically 45deg Typical Units: deg, rad;

Creeping Line Search
A pattern of equally spaced parallel lines followed for searching the ground from an aircraft; Compare: expanding square search, sector search;

Cross Track
Perpendicular to the course;

Crosstrack Deviation (XTKD)
A guidance control law parameter, generated by the lateral guidance modes; distance from the aircraft to a desired course measured along a prependicular to the course Typical Units: ft; Dimensions: Length;

Crosstrack Deviation Gain (KXTKD)
A guidance control law parameter, generated by the lateral guidance modes; relative weighting of crosstrack deviation in the lateral control law Typical Units: rad/ft; Dimensions: 1/L;

Crosstrack Deviation Rate (XTKR)
A guidance control law parameter, generated by the lateral guidance modes; rate of change of crosstrack deviation Typical Units: ft/s; Dimensions: Length / Time;

Crosstrack Deviation Rate Gain (KXTKR)
A guidance control law parameter, generated by the lateral guidance modes; relative weighting of the crosstrack deviation rate in the lateral control law Typical Units: s; Dimensions: Time;

CTALT
Controlled altitude

CTS
Controlled speed

Cue
A indicator to an operator for control placement, tells the operator where to place controls; Synonyms: command

Cursor
See: moding cursor, target cursor

Cutoff Frequency
The frequency at which the gain of a filter is at an edge of a band, usually taken to be when gain is 05, or -301dB; the frequency at which the output of a filter is half the power of the input; See Also: band-pass filter, high-pass filter, low-pass filter; Symbols: omega sub c; Typical Units: rad/s,Hz; Dimensions: 1/Time;

Aircraft Maintenance, Mechanic, Industry Glossary

D

Damped Frequency
The frequency of oscillation of an underdamped second-order filter; See Also: second-order filter; Symbols: omega; Typical Units: rad/s,Hz; Dimensions: 1/Time;

Damping Ratio
Control parameter for a second order filter Symbols: zeta;

Data Source Object (DSO)
Software that receives data from a physical device, translates the data into standard units, maintains equipment status, and provides a common interface for each variation of a particular device;

Data Transfer System (DTS)
A device for transferring data with avionics, similar to a diskette drive;

Dead Reckoning (DR)
A method of navigation based on basic information (barometric altitude, magnetic heading, airspeed, wind conditions) from best available source; sometimes short for air data dead reckoning;

Delta
Difference; error

Depart From Hover (dhov)
A guidance mode providing lateral guidance, longitudinal guidance and vertical guidance for a set heading or bank angle, a set speed, and a set climb rate, altitude or pitch;

Derivative
Rate of change, usually with respect to time; Symbols: x dot, x prime, x sup (1), dx/dt, Dx;

Derived
Calculated values for which no direct measurement exists; Compare: estimated, filtered, measured, raw, selected, smoothed

DESALT
Desired altitude

Desired
What must be achieved in order to match a plan; Synonyms: reference;

Desired Altitude (DESALT)
A guidance control law parameter, generated by the vertical guidance modes; altitude which controlled altitude is attempting to achieve Typical Units: ft; Dimensions: Length;

Desired Path
A trajectory in space determined by guidance to meet the current mission objectives;

Desired Speed (DESS)
A guidance control law parameter, generated by the longitudinal guidance modes; speed which controlled speed is attempting to achieve Typical Units: ft/s,kt; Dimensions: Length / Time;

DESS
Desired speed

Deviation
Difference from desired;

Device
A piece of equipment, a subsystem; Synonyms: physical device, unit

DFAD
Digitized Feature Analysis Data

DHOV
Depart from hover

DME
Distance Measuring Equipment

Difference Equation
A mathematical relationship to model a discrete function, expressed in terms of other values in the sequence; Difference equations are usually derived from differential equations Compare: continuous-time equation, differential equation, discrete-time equation, Laplace transform, state-space model, Z transform; in avionics, a difference equation usually models periodic process in terms of past values; See Also: first-order filter, integrator, second-order filter;

Differential Equation
A mathematical relationship to model a continuous function, expressed in terms of derivatives; Initial conditions are usually given or implied In avionics, differential equations are commonly used by systems engineers to model avionics systems The systems engineer usually converts differential equations to difference equations for specification and implementation in software In avionics, a differential equation usually models continuous-time phenomenon in terms of time derivatives; Compare: continuous-time equation, difference equation, discrete-time equation, Laplace transform, state-space model, Z transform; See Also: first-order filter, integrator, second-order filter

Digital Map Generator (DMG)
Digitial equipment that produces map video, and sometimes contains TRN; uses Digital Terrain Elevation Data and Digitized Feature Analysis Data; Displays reconstructed digital map data, aeronautical charts or photographs The digital map data can be annotated with natural and man-made (point and linear) features as well as threats A Terrain Referenced Navigation algorithm correlates altitude readings with digital map data to provide position data

Digital Terrain Elevation Data (DTED)
On a digitial map, data for elevation of terrain; DTED is usually provided by Defense Mapping Agency; Compare: Digitized Feature Analysis Data;

Digitized Feature Analysis Data (DFAD)
On a digitial map, data for cultural features such as buildings and roads; DFAD is usually provided by Defense Mapping Agency; Compare: Digital Terrain Elevation Data;

Dimensionless
No units, such as ratios; Synonyms: unitless;

Direct
Towards a point along the shortest distance; Compare: course

Direction
bearing

Discrete Time
Time divided into quantized intervals; in avionics, time is usually divived into equal intervals to create a periodic process; Compare: continuous time;

Discrete-Time Equation
A mathematical relationship to describe a function of time, expressed in terms of discrete time; Compare: continuous-time equation, difference equation, differential equation, Laplace transform, state-space model, Z transform; See Also: first-order filter, unit functions;

Distance
Method of measurement dependent on use; Synonyms: range;

Distance Measuring Equipment (DME)
Equipment for measuring distance, usually from an aircraft to a ground station; usually part of a Tactical Air Navigation system

DMG
Digital Map Generator

DNS
Doppler Navigation System

Doppler
A technique for measuring velocity by radiating and determining frequency shift

Doppler Navigation System (DNS, DPLR)
A navigation for measuring velocity by radiating and determining frequency shift;

DPLR
Doppler navigation system

DR
Dead reckoning

Drag
Force of air against aircraft acting in opposite direction of the airspeed vector projected into horizontal plane; Symbols: D; Typical Units: lbf,kip; Dimensions: Mass * Length / Time-squared;

Drift
Slow, monotonic change in measured data

DSO
Data source object

DTED
Digital Terrain Elevation Data

DTS
Data Transfer System

Dynamic Pressure
Total pressure

Earth coordinates
Coordinates referenced to the earth; See Also: east-north-up; Compare: body coordinates, stability coordinates

Earth data
Environmental data related to the earth at some point of interest; usually a function of latitude and longitude

Earth eccentricity
A measure of the degree to which the earth is oblate; Symbols: epsilon;

Earth model
The earth model computes data related to the earth Most data is a function of position Standard models are: International, Clarke 1866, Clarke 1880, Everest, Modified Everest, Bessel 1841, Australian National, World Geodetic Survey 1972, World Geodetic Survey 1984, Airy, Hough, South American

Earth radius
Radius of the earth, function of position, separate radii for longitudinal radius and for lateral radius; See Also: state data; Symbols: rho; Typical Units: ft; Dimensions: Length;

Earth radius best sphere
Gaussian radius of curvature

Earth radius east/west
Prime radius of curvature

Earth radius north/south
Meridian radius of curvature

Earth-referenced flight path angle
Angle in vertical plane of airspeed vector and groundspeed vector; usual definition for flight path angle; Compare: air-mass flight path angle; Symbols: gamma sub T; Typical Units: rad, deg;

Earthspeed
Total velocity measured with respect to a plane tangent to the earth's surface at the current position; a vector composed of velocity north, velocity east, and vertical velocity; See Also: state data; Symbols: V sub E; Typical Units: kt,ft/s; Dimensions: Length / Time;

East-north-up (ENU)
A standard earth coordinate frame and sign convention, where east, north, and up are positive; vertical, or V, is sometimes used in place of up; Synonyms: east-north-vertical; Compare: XYZ;

East-north-vertical (ENV)
East-north-up

ECI
Earth Centered Inertial

Elevation
An angle in the vertical plane through a longitudinal axis; height above mean sea level, usually of terrain;

Elevator
A control surface on fixed-wing aircraft, usually mounted on the aft edge of stabilizers, that controls pitch, and is controlled by the yoke; Symbols: delta sub E; Typical Units: rad, deg;

ENU
East-north-up

ENV
East-north-vertical

Environmental data
Atmospheric data and earth data

Equipment status
Operational status of a piece of equipment consisting of a status indicator and status words; Synonyms: health;

Error
Difference between desired and measured data; Synonyms: delta;

Estimated
Data that is the result of filtering two or more signals; Compare: derived, filtered, measured, raw, selected, smoothed

Euler angles
Pitch, roll, and yaw

Euler parameters
Four parameters for specifiying quaternions; Symbols: e sub < 1 , 2 , 3 , 4 >,a,b,c,s;

Everest
A standard model for computing earth data

Expanding square search
A pattern of progressively larger squares (a ``square spiral'') followed for searching the ground from an aircraft; Compare: creeping line search, sector search;

Extrapolate
Function to determine values from two or values in a table, when the given value lies outside of the range of the table; usually linear but can be higher order;

FCS
Flight control system

Filter
A device to alter a signal; software to alter a data steam; See Also: averaging filter, band-pass filter, complementary filter, first-order filter, high-pass filter, hysteresis, Kalman filter, limiter, low-pass filter, rate limiter, second-order filter, smoothing filter, wash-out filter;

Filtered
Data that is the result of filtering a signal; filtering is usually more sophisticated than smoothing Compare: derived, estimated, measured, raw, selected, smoothed;

First-order filter
A filter in which the output follows the input, only more slowly; It is usually implemented in software as a difference equation of period T The first-order filter is commonly used in avionics to smooth data, and to wash out transients at mode change It is also used as a low-pass filter When implementing a second-order filter on normalized variables, such as angles, the discontinuities require special treatment

Fix
A determination of one's position based on external data, such as a known terrain point

Fixed wing
An airplane, as opposed to a rotory wing / helicopter;

Flaps
A control surface on fixed-wing aircraft, usually mounted to the fore edge of the wings, that extends the wing to provide added lift at low speeds; Compare: slats; Symbols: delta sub F; Typical Units: rad, deg,percent;

Flight control system (FCS)
A primary flight control system or an automatic flight control system;

Flight controls
Controls in a cockpit for flying an aircraft; primary flight controls are wheel, yoke, cyclic, pedals, throttle, and collective; secondary flight controls are flight controls other than primary, such as flaps, slats, stabilizer, and landing gear;

Flight director
System, usually software, that generates stick position cues from state errors - typically three cues: pitch, roll, and throttle for fixed-wing and longitudinal cyclic, lateral cyclic, and collective for rotory wing;

Flight path angle (FPA)
Angle in vertical plane of earthspeed vector and groundspeed vector (usual definition), earth-referenced flight path angle; angle in vertical plane of airspeed vector and groundspeed vector (occasional definition), air-mass flight path angle; Symbols: gamma; Typical Units: rad, deg;

Flight plan
A predetermined route, possibly including guidance modes, communications, and mission objectives, used by guidance and mission management for moding and planning; Series of navigation reference points, waypoints, and mode commands for navigation, radio navigation, guidance, and flight director

FLIR
Forward-Looking Infrared

Flux valve
A device to measure the earth's magnetic flux; a compass

Fly over
A position update by flying directly over a known point

Forward-Looking Infrared (FLIR)
Sensor equipment used to supplement AGR, extend the aircraft visual search capability and provide position information for guidance and navigation update capability Imagery derived from the FLIR sensor is displayed in the cockpit FLIR pointing can be controlled manually using a tracking handle or automatically by the Mission Computer FLIR converts a heat image into a video image and determines azimuth, elevation, and sometimes range of a point; Synonyms: Infrared Detecting Set;

FPA
Flight path angle

GA
Go around

Gaussian radius of curvature
Radius of the earth for the best fitting sphere at a given position; Synonyms: earth radius best sphere; Symbols: rho sub G; Typical Units: ft; Dimensions: Length;

GCLP
Guidance control law parameter

Geocentric latitude
Latitude measured with respect to horizontal through mass center of the earth; Compare: astronomical latitude, geodetic latitude; Symbols: Phi sub C; Typical Units: rad, deg; Reference: Kayton69 page 16

Geodetic latitude
Latitude measured with respect to normal to reference ellipsoid; Compare: astronomical latitude, geocentric latitude; Symbols: Phi sub T; Typical Units: rad, deg; Reference: Kayton69 page 16

Gimbaled inertial sensor
Accelerometers mounted to a platform which is free to rotate, with gyroscopes to measure rotation and servomotors to maintain a fixed attitude with respect to the earth Compare: strapdown inertial sensor;

Glideslope
Angle approach a runway; Symbols: Gamma; Typical Units: rad, deg;

Global Positioning System (GPS)
A navigation sensor based on satellites; A Global Positioning System (GPS) provides highly accurate navigation data: position, velocity, and time reference GPS is often aided by the INU, AHRS, and Doppler data GPS is accurate with four or more properly oriented satellites Accuracy is degraded with improperly placed satellites or fewer than four satellites visible GPS-INS is the most accurate of modes listed, with day/night and all weather capability

Go around (GA)
A basic guidance mode, providing lateral guidance, longitudinal guidance and vertical guidance to climb then to accelerate, while maintaining a wings-level roll; in some implementations (such as SOA), individual axes can be overridden by other modes (for example, longitudinal with G/S SEL and vertical with RALT SEL or BALT SEL)

GPS
Global Positioning System

GPS-DNS
GPS using DNS

GPS-INS
GPS using INS

Gravitational acceleration
Acceleration caused by the force of gravity; Symbols: g; Typical Units: ft/s-squared,g; Dimensions: Length / Time-squared;

Gravity
Force exerted by gravity; gravity sometimes includes effects of the earth's rotation; gravity is often treated as a constant, but for greater accuracy gravity is a function of latitude, altitude, and the phase of the moon (yes, it's true); Symbols: g; Typical Units: lbf,kip; Dimensions: Mass * Length / Time-squared;

Grid north
Standard aviation term

Grivation
Symbols: nu sub g ; Typical Units: rad, deg;

Ground track angle (GTA)
Direction of ground speed vector with respect to true north; Synonyms: true track; Symbols: eta; Typical Units: rad, deg;

Groundspeed
The speed over the ground; earthspeed projected to a horizontal plane; Symbols: V sub g; Typical Units: kt,ft/s; Dimensions: Length / Time;

Groundspeed select (G/S SEL)
A basic guidance mode, providing longitudinal guidance to an operator selected groundspeed;

G/S SEL
Groundspeed select

GTA
Ground track angle

Guidance
System, usually software, that determines state errors of desired state minus current state, typically three states: heading, altitude, and speed;

Guidance control law parameter (GCLP)
One of several parameters for the guidance control laws, generated by individual guidance modes; See Also: altitude error scale factor, altitude integral gain, altitude integral input, altitude integral limit, controlled altitude, controlled speed, course cut limit, crosstrack deviation, crosstrack deviation gain, crosstrack deviation rate, crosstrack deviation rate gain, desired altitude, desired speed, path integral gain, path integral limit, path integral value, reference acceleration, reference acceleration gain, track angle error, track angle error gain, velocity error scale factor; Reference: kilmer89

Gyroscope (gyro)
An inertial device for measuring change of attitude (pitch rate, roll rate, and yaw rate); gyroscopes usually consist of a gimbled, rotating mass; gyroscopes are usually included in inertial sensors, such as AHRS and INS; See Also: ring-laser gyro;

HDG SEL
Heading select

Heading
Direction on a compass that aircraft is pointed, measured with respect to true north or magnetic north; Symbols: psi; Typical Units: rad, deg;

Heading error
A basic output from guidance to flight director, indicating the difference between actual heading and desired heading; Symbols: DELTA psi; Typical Units: rad, deg;

Heading select (HDG SEL)
A basic guidance mode, providing lateral guidance to an operator selected heading (magnetic heading or true heading, again, operator selectable);

Health
Equipment status

High-pass filter (HPF)
A filter that allows frequencies above a cutoff frequency to pass while attenuating frequencies below the cutoff frequency;

Hold
To maintain some aspect(s) of aircraft state, such as heading, airspeed, altitude, pitch

Hook
In the US Navy, a target cursor;

Hough
A standard model for computing earth data

Hover hold (HVR SYM)
A basic guidance mode, providing lateral guidance and longitudinal guidance to maintain an operator selected north velocity and east velocity; if the selected velocities are zero, then a position is held

HPF
High-pass filter

HVR SYM
Hover hold

Hysteresis
A function in which the algorithm for computing output changes at defined events or thresholds, such that output follows one path as input increases and another path as input decreases; Hysteresis can be formalized: (0) at initialization, select algorithm-0 (1)if event-1 occurs, switch to algorithm-1 (2) if event-2 occurs, switch to algorithm-2 (N) if event-n occurs, switch to algorithm-n Frequently in avionics, hysteresis prevents a test from oscillating near the transition point due to noise Implementation is usually: (0) at initialization, set y = 0 (1) if x sub c + h/2 le x, then set y = 1 (2) if x sub c - h/2 lt x lt x sub c + h/2, then let y retain its value (3) if x le x sub c - h/2, then set y = 0

IAS
Indicated airspeed

IBC
Ils Back Course

IBIT
Initiated built-in-test

IDS
Infrared Detecting Set

ILS
Instrument Landing System

ILS approach
To approach a runway using ILS

ILS Back Course (IBC)
Operation of ILS in which the runway is approached in reverse direction, giving only lateral guidance; Compare: ILS Front Course;

ILS Front Course
Operation of ILS in which the runway is approached in forward direction, giving lateral, longitudinal, and vertical guidance; sometimes called ILS Compare: ILS Back Course;

Indicated airspeed (IAS)
Airspeed as instruments would indicate, not corrected for instrumentation errors or air density; See Also: airspeed; Symbols: V sub 'IAS'; Typical Units: kt,ft/s; Dimensions: Length / Time;

Inertial
Based on inertia, such as with an INS or an AHRS

Inertial Navigation System (INS)
An Inertial Navigation System (INS) is a self-contained navigation system It consists of gyroscopes and accelerometers to provide attitude, heading, position, attitude, body/inertial velocity, and acceleration information A primary navigation data source INS loses accuracy with time due to drift of gyroscopes INS-DNS is moderately accurate over land, not good over water See Also: Inertial Navigation Unit;

Inertial Navigation Unit (INU)
A self-contained Inertial Navigation System;

Infrared Detecting Set (IDS)
Forward-Looking Infrared;

Initialization
A basic control to a data source from controls and displays for initializing a device Initiated by power-on, operator, driver, or MC; During initialization, the device is usually not available; Also, a procedure to reset physical devices to a known state; Values: initialization in progress, not initialized, unreliable, normal, redundant, degraded, failed See Also: alignment, boresighting, calibration;

Initiated built-in-test (IBIT)
Selftests running internal to a device initiated external to the device, usually an operator, causing the device to temporarily cease normal operation; Compare: periodic built-in-test;

Inner marker
Innermost marker beacon on an ILS

INS
Inertial Navigation System

Instrument Landing System (ILS)
Equipment determining glideslope, localizer (bearing), and distance (marker beacon) to a runway; ILS provides precision aiding for landing; ILS is usually part of a VOR station A basic guidance mode, providing lateral guidance, longitudinal guidance, and vertical guidance to approach a runway for landing; in ILS back course, vertical guidance is not provided

Instrumentation
Hardware to measure and to monitor a system

Integrate
To combine multiple systems; Also, to compute to integral of;

Integrator
A function that integrates; Many types of integrators exist; in fact, they constitute entire books Avionics software usually relies on rectangular, single integrators, but occassional uses trapezoidal or double integrators

International
A standard model for computing earth data

Interpolate
Function to determine intermediate values from two or values in a table; usually linear but can be higher order; endpoints are either extrapolated or limited;

Interrogation
A request of data

INTLIM
Path integral limit

INTMAGLIM
Altitude integral limit

INTVAL
Path integral value

INU
Inertial Navigation Unit

Invalid
An indication that data from a device is bad and cannot be trusted

Jerk
Rate of change of acceleration, either scalar or vector, often with subscripts such as ENU or XYZ to denote the coordinate frame; time derivative of acceleration; Symbols: j,J; Typical Units: ft/s-cubed; Dimensions: Length / Time-cubed;

Kalman filter
A filter for combining multiple data sources, usually of different types, to produce an estimate better than any single source; Compare: averaging filter, complementary filter;

KALTINT
Altitude integral gain

KINT
Path integral gain

KLA
Reference acceleration gain

KTAE
Track angle error gain

KVSF
Velocity error scale factor

KXTKD
Crosstrack deviation gain

KXTKR
Crosstrack deviation rate gain

KZSF
Altitude error scale factor

Laplace transform
A mathematical relationship to model a continuous function in the complex frequency domain (S-plane); Laplace transforms are commonly used by systems engineers to describe avionics systems; Compare: continuous-time equation, difference equation, differential equation, discrete-time equation, state-space model, Z transform; See Also: first-order filter, integrator, second-order filter, unit functions;

Laser obstacle avoidance / terrain avoidance sensor (OA/TA)
A sensor that provides warnings for long, thin objects (like wires) It has at least a 20deg x 30deg FOV, which is both velocity tracked and pitch stabilized It provides warnings (every 25 seconds) for long, thin obstacles (like wires) at 400 meters detection range and contour flight at 80-120 knots

Lateral
Related to latitude; across an aircraft left to right

Lateral cue
A cue to control heading; lateral cyclic cue See Also: wheel cue,

Lateral cyclic
A flight control operated by moving left or right with hand in rotary-wing aircraft, primarily to control roll (heading); controls differential pitch of the rotors as they rotate from one side to the other on a rotary-wing aircraft;

Lateral cyclic cue
A lateral flight director cue for rotary-wing aircraft, primarily to control heading, by changing roll; Compare: wheel cue; Symbols: Gamma sub 'LAT'; Typical Units: percent,in;

Lateral guidance
Calculations for the lateral axis of the appropriate guidance modes The control law lateral axis input data are: Cross Track Deviation, Cross Track Deviation Rate, Cross Track Deviation Rate Gain, Track Angle Error, Track Angle Error Gain, Course Cut Limit, Path Integral Limit, Path Integral Gain The major output from Longitudinal Guidance is the speed error for the selected longitudinal guidance mode

Latitude
Position on earth, north or south of the equator; See Also: astronomical latitude, geocentric latitude, geodetic latitude; Symbols: Phi; Typical Units: rad, deg;

Leg
A segment of a flight plan; flight path between two waypoints

LF ADF
Low Frequency Automatic Direction Finding

LOC
Localizer

Lift
Force, created primarily by wings (fixed wing) or by rotors (rotary wing), acting in opposite direction of gravity vector; Symbols: Length; Typical Units: lbf,kip; Dimensions: Mass * Length / Time-squared;

Limiter
A filter that passes the input to the output, except that the output is limited to a minimum value and a maximum value; Compare: rate limiter;

Localizer (LOC)
Part of ILS that provides lateral deviations from a preset course;

Longitude
Position on earth, east or west of the prime meridian; Symbols: lambda; Typical Units: rad, deg;

Longitudinal
Related to longitude; lengthwise along the center line of an aircraft forward

Longitudinal cue
A cue to control pitch; longitudinal cyclic cue See Also: throttle cue,

Longitudinal cyclic
A flight control operated by moving fore or aft with hand in rotary-wing aircraft, primarily to control pitch (speed); controls differential pitch of the rotors as they rotate from nose to tail on a rotary-wing aircraft;

Longitudinal cyclic cue
A longitudinal flight director cue for rotary-wing aircraft, primarily to control speed, by changing pitch; Compare: throttle cue; Symbols: Gamma sub 'LONG' ; Typical Units: percent,in;

Longitudinal guidance
Calculations for the longitudinal axis of the guidance modes The control law longitudinal axis input data are: Reference Acceleration, Reference Acceleration Gain, Desired Velocity, Velocity Error Scale Factor The major output from Longitudinal Guidance is the speed error for the selected longitudinal guidance mode

Low Frequency Automatic Direction Finding (LF ADF)
Equipment that determines bearing to a radio station on a low frequency band, usually the standard AM band;

Low-pass filter (LPF)
A filter that allows frequencies below a cutoff frequency to pass while attenuating frequencies above the cutoff frequency; See Also: first-order filter;

LPF
Low-pass filter

Mach number
Ratio of airspeed to the local speed of sound (Mach 1 is the speed of sound under current atmospheric conditions); Symbols: M;

Magnetic heading
Heading of the aircraft relative to magnetic north; A Magnetic Heading Sensor provides this heading data Symbols: psi sub M; Typical Units: rad, deg;

Magnetic variation (MVAR, MAGVAR)
Difference between true north and magnetic north, varying with position; magnetic variation drifts with time; Symbols: nu; Typical Units: rad, deg;

MAGVAR
Magnetic variation

Maintenance
Indicates device is in a maintenance mode; Values: non-maintanance, calibration, alignment, boresight

Map
Equipment that produces a map image; See Also: Digital Map Generator

Marker beacon (MB)
Part of Instrument Landing System that signals crew members of distance to runway, consisting of three markers: inner, middle, and outer;

Massaged
Filtered, estimated, or derived, or some combination of the three

Master caution
A signal which indicates that one or more caution lights has been activated (from MIL-STD-1472D);

Master warning
A signal which indicates that one or more warning lights has been activated (from MIL-STD-1472D);

MB
Marker Beacon

MC
Mission computer

Mean sea level (MSL)
Standard aviation term See Also: above sea level;

Measured
Raw data converted to standard units; Compare: derived, estimated, filtered, raw, selected, smoothed

Meridian radius of curvature
Radius of the earth in the east/west direction at a given position; Synonyms: earth radius north/south; Symbols: rho sub M; Typical Units: ft; Dimensions: Length;

Middle marker
Marker beacon located where the center of the glideslope is 200ft above the runway

Mission computer (MC)
Mission processor;

Mission objectives
Goals to be accomplished during a specific mission, including flight plan, NRPs, legs, and a plan on how to accomplish these objectives; plan includes, usually on a leg-by-leg basis, navigation modes, radio navigation modes, guidance modes, flight director modes, data source control information such as frequencies; See Also: capture criterion

Mission processor (MP)
A general purpose computer to host avionics software Synonyms: mission computer;

MLS
Microwave Landing System

Mode
A selection of one of several alternatives, such as guidance mode (VOR, TACAN, or Waypoint), or navigation mode (INS, Doppler, or dead reckoning)

Modified Everest
A standard model for computing earth data

Moding cursor
A symbol on a display, moved by an operator much like arrow keys for menu selection, to select one of several options

MP
Mission processor

MSL
Mean sea level

Multi-mode radar (MMR)
A Multi-Mode Radar is used for Terrain Following (TF) and Terrain Avoidance (TA), Ground Mapping (GM) and Air-to Ground Ranging (AGR) The TF mode supplies commands which are processed and displayed as climb/dive commands on the Flight Director display and E-squared video used by the operators in anticipating near term TF commands When in TA or GM modes, the operator is provided with a Plan Position Indicator (PPI) display The AGR mode allows the operators to determine the range to a designated target, which can be used for position updates

MVAR
Magnetic variation

Nacelle
An enclosure on an aircraft

Nap-of-the-earth flight
Flight with a goal to remain close to the earth, usually below the height of surrounding trees and less than 100 ft above the terrain; Compare: terrain following

Natural frequency
Standard engineering term See Also: second-order filter; Symbols: omega sub n; Typical Units: rad/s,Hz; Dimensions: 1/Time;

Navigation
A system, usually software, in which the primary purpose is to generate position relative to a coordinate frame, usually fixed earth frame, such as latitude and longitude or UTM;

Navigation aid
A device or process to help with navigation, such as a VOR station or a position update;

Navigation reference point (NRP)
A point, usually fixed in earth coordinates but possibly moving; Also, a basic guidance mode, providing lateral guidance to an NRP, either by course or by direct (operator selectable); Synonyms: point;

Noise
Part of received data that is undesired, consisting of random sinusoidal terms added to a signal; Compare: offset, signal;

Normalizer
Function to restrict input to a specific range, such as restricting an angle alpha in radians so that -pi le alpha le +pi; angles usually require normalizing following any computation; normalized variables often present problems for filters and other functions at their discontinuities;

NRP
Navigation reference point

OA
Obstacle avoidance

OAT
Outside Air Temperature

Obstacle avoidance (OA)
Flight cues designed to avoid obstacles, such as terrain, buildings, and power lines; Compare: terrain avoidance, threat avoidance;

Off
Device is powered off (power switch is off; no response to communications) - no data and function is available

Offset
Part of received data that is undesired, consisting of a random, time-invariant term added to a signal; Synonyms: bias; Compare: noise, signal

OMEGA
An OMEGA receiver provides position information that can be used to update the aircraft navigation position OMEGA is the least accurate method of obtaining position information

Operations
How well is equipment operating; Values: operational (all function and data is available), degraded (equipment has partially failed with some function or data unavailable and some available), failed (equipment has failed with no function or data available);

Orientation
Direction in reference to a coordinate frame

Outer marker
Marker beacon located 5-7mi from the end of the runway; See Also: marker beacon

Outside Air Temperature (OAT)
The temperature just outside the aircraft; Symbols: T; Typical Units: deg; Dimensions: Temperature

Override
To alter selection made automatically by software

Past status words
Status words that are logically combined over time (such as ``and''ing or ``or''ing) to provide history of what has been set in the past

Path integral gain (KINT)
A guidance control law parameter, generated by the lateral guidance modes;

Path integral limit (INTLIM)
A guidance control law parameter, generated by the lateral guidance modes; Typical Units: rad;

Path integral value (INTVAL)
A guidance control law parameter, generated by the lateral guidance modes; Typical Units: ft; Dimensions: Length

PBIT
Periodic built-in-test

Pedal
A flight control operated by pushing with feet, primarily to control yaw via the rudder in fixed-wing aircraft or thrust to tail rotor in rotary-wing aircraft; pedals are automatically controlled in modern aircraft;

Period
Time of a periodic process; 1/f where f is the sampling frequency; Symbols: T; Typical Units: s; Dimensions: Time

Periodic
A process that executes at a fixed rate; Compare: aperiodic;

Periodic built-in-test (PBIT)
Selftests running internal to a device as part of normal operation; Compare: initiated built-in-test;

Personnel Locating System (PLS)
A basic guidance mode, providing lateral guidance to a PLS transmitter from range and bearing inputs Equipment that determines range and bearing to a personnel with a PLS transmitter; Provides range and bearing to locate ground personnel Coded continuous or periodic interrogations of the portable ground radios are used to provide the information

PFCS
Primary flight control system

Physical device
A piece of equipment, a subsystem; Synonyms: device

Piloted simulation
Real-time engineering simulation

Pitot pressure
Total pressure

Pitch
The angle of a rotor measured in the plane of rotation; Symbols: theta,Theta; Typical Units: rad, deg;

Pitch cue
Flight director cue to control pitch; in fixed-wing aircraft, a yoke cue; in rotary-wing aircraft, a longitudinal cyclic cue

Pitch rate
Rate of change of pitch; time derivative of pitch; Symbols: p; Symbols: theta dot; Typical Units: rad/s,deg/s; Dimensions: 1/Time;

PLS
Personnel Locating System

Position
Location, usually in fixed earth coordinates such latitude and longitude; location, either scalar or vector, often with subscripts such as ENU or XYZ to denote source or coordinate frame; time integral of velocity; Symbols: p,P,x,y,z; Typical Units: ft,nmi; Dimensions: Length;

Position update
To cause navigation sensors, devices, or algorithms to reset position to value known to be more accurate due to inaccuracies and drift in the devices and algorithms

Precision
Measure of exactness, possibly expressed in number of digits, for example, computed to the nearest millimeter; Compare: accuracy

Present status words
Most recently reported status words

Pressure
Barometric pressure

Pressure altitude
Barometric altitude

Primary flight control system (PFCS)
The most basic part of the flight controls operated by a pilot, including wheel (fixed wing), yoke (fixed wing), cyclic (rotary wing), pedals (fixed wing and rotary wing), throttle (fixed wing), and collective (rotary wing);

Primary units
A standard set of four units to which all units can be resolved; primary units are Mass(M), Length(L), Time(theta), and Temperature(T); for example, standard units for velocity might be kt (nmi/hr), ft/s, m/s, mph, but primary are always Length / Theta; sometimes written with negative subscripts

Prime radius of curvature
Radius of the earth in the east/west direction at a given position; Synonyms: earth radius east/west; Symbols: rho sub P; Typical Units: ft; Dimensions: Length;

Quaternion
A system of representing attitude by measuring angle of aircraft center line with respect to three orthoginal axes plus rotation about centerline; quaternions are used over Euler angles (pitch, roll, yaw) when pitch can approach 90deg because of a singularity on Euler angles at 90deg; discrete-time computations using quaternions can run more slowly than those with Euler angles while producing results of the same accuracy See Also: Euler parameters;

RADALT
Radar Altimeter

Radar altimeter (RADALT)
Measures height above terrain The altitude is monitored to provide a low altitude warning during TF operations and landing operations It can also be used as input to the Terrain Reference Navigation algorithm for position updates

Radar altitude
Height with respect to the terrain below (distance above closest dirt); Synonyms: above ground level; Symbols: h sub r; Typical Units: ft; Dimensions: Length;

Radar altitude select (RALT SEL)
A basic guidance mode, providing vertical guidance to an operator selected radar altitude;

Radial error probability (REP)
A probability that a percentage of one-dimension measurements will lie on a radial (line) of given length, with the origin centered at truth or mean of the measurements; used to specify test cases for measurement errors of sensors of one dimension, such as vertical velocity; Compare: circular error probability, spherical error probability;

Radio navigation
Navigation relative to radio station, providing, for example, of relative bearing, range, lateral deviation, and glideslope; Examples include VOR, TACAN, and PLS Radio navigation differs from other navigation in that the transmitter signals often dropout for a long period of time, like minutes This can occur because of natural obstructions, or because the transmitter was shut down intentionally In hostile territory, a PLS can locate a downed pilot, who would be foolhardy to be continuously transmitting, but would transmit infrequently with small bursts of data The Radio- Navigation system accommodates this phenomenon by simulating range and bearing to the fixed site when it is not transmitting After reacquiring a mobile transmitter, the mobile station's position is re-determined The navigation component supports wash-out filters on output data

RALT SEL
Radar altitude select

Range (rng)
Standard aviation term Synonyms: distance; Symbols: r; Typical Units: ft,nmi - method of measurement dependent on use; Dimensions: Length;

Ranging
Act of determining a range

Rate limiter
A filter that passes the input as the output, except that rate of change of the output is limited to a maximum absolute value; Compare: limiter;

Raw
Data taken directly from the sensor; Compare: derived, estimated, filtered, measured, selected, smoothed

Real time
Time in a computational process which runs at the same rate as a physical process; for example, algorithms designed to run a fixed period t (filter time constants at set for t) and actually execute with frequency 1/t execute in real time; Avionics systems must run in real time;

Real-time engineering simulation (rtes)
A simulator designed to test avionics algorithms with a pilot in the loop, consisting of a simulated cockpit, an aircraft model, sensor models, and algorithms to be tested; rtes is often used during development to check algorithms, such as the navigation, radio navigation, guidance and flight director, prior to full-scale software development; rtes gives systems engineers early insight in human factors problems, pilot complaints, algorithm bugs, unstability in the algorithms, expected performance (accuracy), and pilot-machine interaction; Synonyms: piloted simulation; Compare: built-in simulation;

Reasonableness
A test to determine if data is reasonable, for example, radar altitude must be positive, and two devices should return similar data within known limits of each other

Receive
To absorb rf energy

REFACC
Reference acceleration

Reference
What must be achieved in order to match a plan; Synonyms: desired;

Reference acceleration (REFACC)
A guidance control law parameter, generated by the longitudinal guidance modes; Typical Units: ft/s-squared, g; Dimensions: Length / Time-squared

Reference acceleration gain (KLA)
A guidance control law parameter, generated by the longitudinal guidance modes; Typical Units: s; Dimensions: Time

Reject
To disallow a position update, usually by an operator; Compare: accept

Relative
Applies to measurements, in a non-standard, moving reference, as opposed to fixed reference; Compare: absolute;

Relative bearing
Angle from aircraft center line to bearing of the destination; Symbols: B sub R; Typical Units: rad, deg;

Remote terminal (RT)
Term defining role of a device on a MIL-STD-1553 bus as being a slave; Compare: bus controller;

Remote Terminal Unit
Signal Data Converter;

Rendezvous
To meet with another aircraft in the air, for refueling or other mission objectives

Rendezvous approach
To approach a planned rendezvous point

REP
Radial error probability

Reported
Data from a device

Ring-laser gyro (RLG)
A gyroscope based on a laser beam instead of a rotating mass, providing to same data as a gyroscope;

RLG
Ring-laser gyro

RMS
Root mean square;

RNG
Range

Roll
Bank angle; Symbols: phi,Phi; Typical Units: rad, deg;

Roll cue
Flight director cue to control roll; in fixed-wing aircraft, a wheel cue; in rotary-wing aircraft, a lateral cyclic cue

Roll rate
Rate of change of roll; time derivative of roll; Symbols: q; Symbols: phi dot; Typical Units: rad/s,deg/s; Dimensions: 1/Time;

Root mean square (RMS)
A statistical measure of data; the root of the mean of the square; for variables with mean of zero, the standard deviation is equal to the rms; Compare: root sum square;

Root sum square (RSS)
A statistical measure of data; the root of the sum of the square; for a vector, its length is equal to the rss of its scalar elements; Compare: root mean square;

Rotary wing
A helicopter;

RSS
Root sum square;

RT
Remote terminal

RTES
Real-time engineering simulation

RTU
Remote Terminal Unit

Rudder
A control surface on fixed-wing aircraft, usually mounted at aft end of the fuselage sticking up (like a dorsal fin), that controls yaw (heading), and is controlled by the pedals; Symbols: delta sub R; Typical Units: rad, deg;

S-plane
Continuous complex frequency plane; S-plane is used in control systems engineering in the design of control laws See Also: Laplace transform;

Sampling frequency
Rate of a periodic process; 1/T where T is the period; Symbols: f sub s; Typical Units: Hz; Dimensions: 1/Time

SDC
Signal Data Converter

Search pattern
Basic guidance mode; provides lateral steering guidance to fly an expanding square search pattern, creeping line search pattern, or sector search pattern

Second-order filter
A smoothing filter in which the output follows the input, only more slowly; It is usually implemented in software as a difference equation of period T When the second-order filter is used in avionics, it is commonly to smooth data, and to wash out transients at mode change Usually, a first-order filter suffices, and it being less expensive, is chosen over a second-order filter Typical values for omega sub n, a, and b are 01-2 rad/s, and zeta 01-09 It should also be noted that two first-order filters can be chained together to form a second-order filter that is critically damped or overdamped When implementing a second-order filter on normalized variables, such as angles, the discontinuities require special treatment

Sector search
A pattern of concentric arcs followed for searching the ground from an aircraft; Compare: creeping line search, expanding square search;

Selected
Measured data picked from one of many sensors; Selecting is the process of choosing the "best" parameter from multiple copies of that parameter, from multiple, identical devices or similar devices Compare: derived, estimated, filtered, measured, raw, smoothed

Selftest
A test internal to a device

Semi-circle
A measure of angle, 1 semi-circle = pi rad = 180 deg; angles from physical devices are often reported in semi-circles in order to compress data

Sensor
A device that measures, receives, or generates data, for example, an INS, a FLIR, a map See Also: Data Source Object

Sensor Fusion/Correlation (SFC)
Measure of convergence of sensor data

SEP
Spherical error probability

SFC
Sensor Fusion/Correlation

Shutdown
To cease normal operations

Sideslip angle
Symbols: beta; Typical Units: rad, deg;

Signal
Part of received data that is desired; Compare: noise, offset

Signal Data Converter (SDC)
A device that converts unique signals to a standard protocol, usually MIL-STD-1553B; Synonyms: Remote Terminal Unit;

Signal-to-noise ratio (SNR,S/N)
A ratio of magnitude of a desired signal to the magnitude of the noise received with it; Typical Units: dB;

Situational awareness
Situational awareness provides the pilots with information relative to the current surroundings, such as other aircraft or threats in the immediate area

Slant range
Direct line distance, not along the ground;

Slats
A control surface on fixed-wing aircraft, usually mounted to the aft edge of the wings, that extends the wing to provide added lift at low speeds; Compare: flaps; Symbols: delta sub S; Typical Units: rad, deg;

Smoothed
Data that is the result of conditioning a signal with a simple filter; Compare: derived, estimated, filtered, measured, raw, selected; smoothing is usually less sophisticated than filtering

Smoothing filter
A filter to reduce quick changes of a signal by attenuating high frequencies; See Also: first-order filter, second-order filter, wash-out filter;

S/N
Signal-to-noise ratio

SNR
Signal-to-noise ratio

South American
A standard model for computing earth data

Speed
Scalar velocity;

Speed cue
Flight director cue to control speed; in fixed-wing aircraft, a throttle cue; in rotary-wing aircraft, a longitudinal cyclic cue

Speed error
A basic output from guidance to flight director, indicating the difference between actual speed and desired speed; Symbols: DELTA V; Typical Units: ft/s,kt; Dimensions: Length / Time

Spherical error probability (SEP)
A probability that a percentage of three-dimension measurements will lie within a sphere of given radius, with the sphere centered at truth or mean of the measurements; SEP specifies test cases for measurement errors of sensors of three dimensions, such as velocity east, north, and vertical Compare: circular error probability, radial error probability;

Spoiler
A control surface on fixed-wing aircraft, usually mounted to the wings, that provides roll control and lift; Symbols: delta sub S; Typical Units: rad, deg;

Stability coordinates
Coordinates referenced to the air mass; Compare: body coordinates, earth coordinates;

Stabilizer
A control surface, usually mounted at aft end of the fuselage parallel to the wings, that provides pitch stability, some aircraft have an adjustable stabilizer; Symbols: delta sub H; Typical Units: rad, deg;

Stagnation pressure
Total pressure

Standard units
Units commonly encountered for a particular quantity;

State data
Data that defines aircraft parameters, such as position, velocity, attitude; Some standard terms for state data include: Groundspeed vector, wind speed vector, true airspeed vector, true bearing, true track, ground track angle, relative bearing, sideslip angle, drift angle, true heading, magnetic variation, grivation earthspeed vector, vertical velocity, air mass flight path angle, earth-referenced flight path angle, angle of attack, pitch, radar altitude, barometric altitude, earth radius, glideslope, gravity vector, lift vector, lateral acceleration vector, bank angle

State-space model
A mathematical relationship of a system in time using state variables, inputs, outputs, and constants; The state-space model is composed of n state variables (x sub 1 , x sub 2 , , x sub n), m input variables (u sub 1 , u sub 2 , , u sub m), k output variables (y sub 1 , y sub 2 , , y sub k), and four constants a, b, c, and d Alternatively, a state-space model can be expressed with matrices Compare: continuous-time equation, difference equation, differential equation, discrete-time equation, Laplace transform, Z transform;

Static pressure
A measure of barometric pressure as if the sensor were not moving with respect to the air; Compare: total pressure; Symbols: p sub s; Typical Units: psi,lbf/in-squared; Dimensions: Mass /Time-squared * Length

Station
A structure on the ground, perhaps containing VOR or TACAN

Status
An indicator of how well a system or subsystem is working

Status indicator
An binary indicator of a particular aspect of a device; status indicators are independent of each other; status indicators listed in this dictionary are derived from existing programs See Also: off, warning, operations, communications, useability, initialization, test, maintenance, unknown;

Status words
Data words reported by devices to indicate status; Each bit is defined on a device-by-device basis The number of words vary from device to device Status words are used by maintenance personnel and maintenance software Present, past, and test status words are reported

Strapdown inertial sensor
Accelerometers mounted to a platform fixed to the aircaft; Compare: gimbaled inertial sensor

Surface
Related to surface of water;

Surface Wind Wave Motion (SWWM)

Survivability
A survivability subsytem detects and counters hostile actions

Swashplate
A device to control the pitch of rotors; on rotary-wing aircraft, the swashplate is controlled by a collective and a cyclic;

SWWM
Surface Wind Wave Motion

System
Applied to measurements, means the best value that the system can determine

TA
Terrain avoidance

TACAN
Tactical Air Navigation;

TACAN Point-to-Point (TCNP)
A basic guidance mode, providing lateral guidance to a point relative to a TACAN station by specified range and bearing;

Tachometer
A device for measuring angular velocity;

Tactical Air Navigation (TACAN)
A basic guidance mode, providing lateral guidance to a TACAN station; Equipment that determines range and bearing to a radio station with a TACAN transmitter;

TAE
Track angle error

Target
Object or point pointed by FLIR or radar

Target cursor
A symbol on a display, moved by a track handle or similar device, to select objects on the display

TAS
True airspeed

Taut line
Basic guidance mode; provides vertical guidance to maintain a taut line for a dipping sonar, or other such device

TCN
Tacan

TCNP
Tacan Point-to-Point

Terrain
The contour of the earth;

Terrain avoidance (TA)
Flight such that the aircraft maintains a constant barometric altitude but flies around obstacles; Compare: obstacle avoidance, threat avoidance;

Terrain following (TF)
A basic guidance mode, providing vertical guidance to maintain an operator selected radar altitude above the terrain Flight such that the aircraft tries to maintain a constant height above the terrain, usually in the range of 100-1,000 ft; Uses a g-command from the Multi-Mode Radar to generate a flight director cue This controls the aircraft flight path so that the set clearance altitude is achieved over major high points in the terrain with zero flight path angle Compare: nap-of-the-earth flight;

Terrain Referenced Navigation (TRN)
A navigation mode based on comparison of barometric altitude and radar altitude with a map; Kalman filters correlate the terrain data and the altitudes A primary navigation data source TRN combines INS with map references It is most accurate over rough terrain Does not give accurate data while over flat areas or water See Also: Digital Map Generator;

Test status words
Status words reported at the conclusion of a test Values: test in progress, not tested, normal, redundant, degraded, failed;

TF
Terrain following;

ThA
Threat avoidance

Threat avoidance (ThA)
Flight cues designed to avoid enemy threats, such as anti-aircraft artillery and aircraft; Compare: obstacle avoidance, terrain avoidance;

Throttle
A flight control operated by moving fore or aft with hands, primarily to control thrust (speed) in fixed-wing aircraft;

Throttle cue
A longitudinal flight director cue for fixed-wing aircraft, primarily to control speed, by changing power; Compare: longitudinal cyclic cue; Symbols: Gamma sub LONG; Typical Units: percent,in;

Thrust
Force, created by engines and rotors, acting in the direction of the engine; Symbols: T; Typical Units: lbf,kip; Dimensions: Mass * Length / Time-squared;

Tilt rotor
An aircraft with tilting rotors for fixed-wing flight or rotary-wing flight;

Time constant
Constant for a first-order filter determining time at which the output of the filter reaches nearly 06321 percent of a step input; Symbols: tau; Typical Units: s; Dimensions: Time;

Tolerances
Allowed error in measurements

Total pressure
A measure of barometric pressure in the moving air; Synonyms: dynamic pressure, Pitot pressure, stagnation pressure; Compare: static pressure; Symbols: p sub t; Typical Units: psi,lbf/in-squared; Dimensions: Mass /Time-squared * Length

Track (TRK)
A basic guidance mode, providing lateral guidance to an operator selected ground track;

Track angle error (TAE)
A guidance control law parameter, generated by the lateral guidance modes; Typical Units: rad;

Track angle error gain (KTAE)
A guidance control law parameter, generated by the lateral guidance modes;

Track handle
A device to move a cursor in two axes on a display, much like a mouse

Tracking
Tracking is performed by the TRN

Transceivers
A device that receives and transmits

Transient-free switch
A switch with a wash-out filter so that the output contains no transients (steps) at switch time

Transmit
To radiate RF energy

Transmitter
A device that transmits

Trim tab
A secondary control surface, usually mounted to primary control surface such as aileron, elevator, rudder, or stabilizer, that controls the position of the primary control surface, and is controlled by the an operator or an autopilot; Symbols: delta sub T; Typical Units: rad, deg;

TRK
Track

TRN
Terrain Referenced Navigation;

TRN-INS
TRN with INS

True
Best available estimate, such as true airspeed; referenced to true north, such as true heading;

True airspeed (TAS)
Airspeed corrected for instrumentation errors and air density; See Also: airspeed; Symbols: V sub 'TAS'; Typical Units: kt,ft/s; Dimensions: Length / Time;

True bearing
Symbols: B sub T; Typical Units: rad, deg;

True heading
Heading of the aircraft relative to true north; Symbols: psi sub T; Typical Units: rad, deg;

True track
Ground track angle; Symbols: T sub T; Typical Units: rad, deg;

Tune
To set the operating frequency or channel for a device

UHF/VHF Automatic Direction Finding (U/V ADF)
An Automatic Direction Finder that determines relative bearing to a transmitter to which it is tuned, in either the UHF band or VHF band;

Unit functions
A collection of functions used as standard test cases in control systems engineering; The primary unit functions of interest in avionics are the unit impulse, the unit step, and the unit ramp Symbols: u sub k ( t );

Unit impulse
A function used as a standard test case in control systems engineering; a spike of ``area'' one at time t = 0; Synonyms: impulse; See Also: unit functions; Symbols: u sub <0> ( t ), delta ( t );

Unit ramp
A function used as a standard test case in control systems engineering; a line of slope 1 starting at zero at time t = 0; Synonyms: ramp; See Also: unit functions; Symbols: u sub < -2> ( t );

Unit step
A function used as a standard test case in control systems engineering; a step from zero to one at time t = 0; Synonyms: step; See Also: unit functions; Symbols: u sub< -1>( t ), u( t );

Unitless
No units, such as ratios; a quantity with standard units of 1; a quantity with primary units of 1; Synonyms: dimensionless;

Units
A standard quantity, such as ft or mi; Synonyms: dimension;

Universal Transverse Mercator (UTM)
A system of fixed earth coordinates, sometimes used instead of latitude and longitude, accurate relative to others close by, such as ground troops;

Update
A process by which position is reset with a known better position; updates occur at the data source level or at the physical device level, depending upon specific implementation of the device and the data source (choice of implementation is transparent above the data source); all updates are operator initiated, but the new position may be derived from other sources such as FLIR, MMR, or radio navigation; a basic control to a data source from controls and displays for updating a device Compare: aiding;

UTM
Universal Transverse Mercator

U/V ADF
UHF/VHF Automatic Direction Finding

Velocity
Rate of change of location, either scalar or vector, often with subscripts such as ENU or XYZ to denote the coordinate frame; time derivative of position; time integral of acceleration; Symbols: v,V; Typical Units: kt,ft/s; Dimensions: Length / Time;

Velocity east
Aircraft velocity in true east direction; Symbols: V sub E; Typical Units: kt, ft/s; Dimensions: Length / Time;

Velocity error scale factor (KVSF)
A guidance control law parameter, generated by the longitudinal guidance modes;

Velocity north
Aircraft velocity in true north direction; Symbols: V sub N; Typical Units: kt,ft/s; Dimensions: Length / Time;

VERT SPD
Vertical speed hold

Vertical
Reference to earth radial, for example, vertical velocity is velocity along earth radial; See Also: East-North-Vertical;

Vertical acceleration
Aircraft acceleration in earth vertical direction; Symbols: A sub V; Typical Units: ft/s-squared,g; Dimensions: Length / Time-squared;

Vertical cue
A cue to control altitude; See Also: yoke cue, collective cue;

Vertical guidance
Calculations for the vertical axis, rather than the longitudinal axis The control law vertical axis input data are: Desired Altitude, Altitude Integral, Altitude Integral Gain, Altitude Integral Limit, Altitude Error Scale Factor, Magnitude limit for delta altitudes The major output from Vertical Guidance is the altitude error for the selected vertical guidance mode

Vertical speed hold (VERT SPD)
A basic guidance mode, providing vertical guidance to maintain an operator selected vertical speed;

Vertical velocity
Aircraft velocity in earth vertical direction; Symbols: V sub V; Typical Units: ft/s; Dimensions: Length / Time;

VHF Omnirange (VOR)
Equipment that determines bearing to a radio station with a VOR transmitter; VOR transmitters usually contain ILS transmitters in addition to VOR; A basic guidance mode, providing lateral guidance to a VOR station A VOR/ILS/MB radio receiver provides a VHF Omni-Direction Range (VOR) function and Instrument Landing System with Marker Beacon (MB) function VOR provides bearings to a fixed point (Localizer function); ILS/MB provides course (localizer function) and glideslope deviations (Glide Slope (G/S) function) during approach to a runway, along with marker beacon indications If VOR is collocated with a TACAN (VOR-TAC, or VORTAC), bearing and range information is provided

VOR
VHF Omnirange;

VORTAC
Collocation of VOR and TACAN providing distance and bearing to station; a basic guidance mode, providing lateral guidance to a set of a VOR station and a TACAN station that are collocated

Warning
A signal which alerts the operator to a dangerous condition requiring immediate action (from MIL-STD-1472D); an annunciator that is the most critical (more than an advisory or a caution); Also, an indicator of potential failure soon; Values: none, hot, low-power, high-power, other;

Warning, Caution, Advisory (WCA)
See: warning, caution, advisory, annunciator, alert;

Wash-out filter
A filter to smooth a transition due to change of input source, such as when changing modes; See Also: transient-free switch;

Waypoint (WYPT)
A point on the ground, predefined as a point of interest for the flight; a basic guidance mode, providing lateral guidance to a waypoint, either by course or by direct (operator selectable);

Waypoint approach
To approach a waypoint

WCA
Warning, Caution, Advisory

Weight on wheels (WOW)
Indication of whether the aircraft has weight on its wheels, meaning airborne or on the ground; weight on wheels can be detected by a sensor on the wheels, computed from other state data, or a combination;

WGS72
World Geodetic Survey 1972

WGS84
World Geodetic Survey 1984

Wheel
A flight control operated by turning with hands in fixed-wing aircraft, primarily to control roll (heading) via the ailerons; wheel is connected to yoke;

Wheel cue
A lateral flight director cue for fixed-wing aircraft, primarily to control heading, by changing roll; Compare: lateral cyclic cue; Symbols: Gamma sub 'LAT' ; Typical Units: percent,in;

Wind bearing
Symbols: B sub W; Typical Units: rad, deg;

Wind speed
Symbols: V sub W; Typical Units: kt,ft/s; Dimensions: Length / Time;

World Geodetic Survey 1972 (WGS72)
A standard model for computing earth data;

World Geodetic Survey 1984 (WGS84)
A standard model for computing earth data;

WOW
Weight on wheels

Wrap test
A test to send data to a device having it sent back unaltered

WYPT
Waypoint

XTKD
Crosstrack deviation

XTKR
Crosstrack deviation rate

XYZ
A standard aircraft coordinate frame and sign convention, where nose, right wing, and down are positive X, Y, and Z, respectively; often used as subscripts; Compare: east-north-up;

Yaw
Angle of heading; Symbols: psi,Psi; Typical Units: rad, deg;

Yaw rate
Rate of change of yaw; time derivative of yaw; Symbols: r; Symbols: psi dot; Typical Units: rad/s, deg/s; Dimensions: 1/Time;

Yoke
A flight control operated by pushing and pulling with hands in fixed-wing aircraft, primarily to control pitch (altitude) via the elevators; yoke is mounted on a column between the operator's legs, positioned much like a steering wheel in a car; yoke control is achieved by pushing and pulling the wheel to move the column (yoke) fore and aft;

Yoke cue
A vertical flight director cue for fixed-wing aircraft, primarily to control altitude, by changing pitch; Compare: collective cue; Symbols: Gamma sub VERT; Typical Units: percent,in;

Z-plane
Discrete complex frequency plane; Z-plane is used in control systems engineering in the design of control laws See Also: Z transform;

Z transform
A mathematical relationship to model a discrete function in the complex frequency domain (Z-plane); Z transforms are commonly used by systems engineers to describe avionics systems; Compare: continuous-time equation, difference equation, differential equation, discrete-time equation, Laplace transform, state-space model; See Also: first-order filter, second-order filter, unit functions;

ZOC
Zone of confusion

Zone of confusion (ZOC)
A circular area centered at a TACAN station in which bearing is extremely noisy;