Aviation Maintenance Terms beginning with S
Satellite-Based Augmentation System (SBAS)
A wide coverage augmentation system in which the user receives augmentation information from a satellite-based transmitter. In the United States, this is referred to as Wide Area Augmentation System (WAAS).
Secondary Surveillance Radar (SSR)
A radar sensor that listens to replies sent by transponders carried on board airborne targets. SSR sensors, in contrast to primary surveillance radar (PSR) sensors, require the aircraft under surveillance to carry a transponder.
Selected Traffic
Traffic for which additional information is requested by the flight crew.
Selective Availability (SA)
A protection technique employed by the Department of Defense that degraded GPS accuracy. Selective availability was discontinued on May 1, 2000.
Sensor
A measurement device. An air data sensor measures atmospheric pressure and temperature, to estimate pressure altitude, and pressure altitude rate, airspeed, etc. A primary surveillance radar sensor measures its antenna direction and the times of returns of echoes of pulses that it transmits to determine the ranges and bearings of airborne targets. A secondary surveillance radar sensor measures its antenna direction and the times of returns of replies from airborne transponders to estimate the ranges and bearings of airborne targets carrying those transponders.
Separation
The minimum distance between aircraft/vehicles allowed by regulations. Separation requirements vary by factors such as radar coverage (none, single, composite), domain (terminal, en route, oceanic), and flight rules (instrument or visual).
Simple Action
A flight crew action that may be performed within a short period of time and without requiring significant concentration that would distract from the main aviation tasks (e.g., a button press). Please refer to AC 25-11( ) for more details.
Source Integrity Level (SIL)
The probability of the reported horizontal position exceeding the radius of containment defined by the NIC without alerting, assuming the avionics has no faults. A SIL of 3 is required by § 91.227.
Spacing
A distance maintained from another aircraft for specific operations.
State Vector
An aircraft’s current horizontal position, vertical position, horizontal velocity, vertical velocity, and navigational accuracy and integrity.
System Design Assurance (SDA)
The failure condition that the position transmission chain is designed to support. An SDA of 2 or greater is required by § 91.227. Refer to A.2.29 for more information.
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