Glossary of Aviation Terminology |

**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