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
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;
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;
C
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 0.5, or -3.01dB; 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;
Cyclic.
See: longitudinal cyclic, lateral cyclic;
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
E
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;
F
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
G
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;
H
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
I
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
J
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;
K
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
L
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 2.5 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
M
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
N
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
O
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
P
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: |
