Glossary of Aviation Terminology |

**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;