Glossary of Aviation Terminology |

**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 0.1-2 rad/s, and zeta 0.1-0.9. 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;

**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.