Relative wind.  Direction of the airflow produced by an object  moving through the air. The relative wind for an airplane in  flight flows in a direction parallel with and opposite to the  direction of flight; therefore, the actual flight path of the  airplane determines the direction of the relative wind. Remote communications outlet (RCO). An unmanned  communications facility that is remotely controlled by air  traffic personnel. Required navigation performance (RNP).  A specified level  of accuracy defined by a lateral area of confined airspace in  which an RNP-certified aircraft operates. Restricted area. Airspace designated under 14 CFR part  73 within which the flight of aircraft, while not wholly  prohibited, is subject to restriction. Reverse sensing.  The VOR needle appearing to indicate the  reverse of normal operation. RF.  Radio frequency. Rhodopsin.  The photosensitive pigments that initiate the  visual response in the rods of the eye. Rigidity.  The characteristic of a gyroscope that prevents its  axis of rotation tilting as the Earth rotates. Rime ice. Rough, milky, opaque ice formed by the  instantaneous freezing of small supercooled water droplets. Risk.  The future impact of a hazard that is not eliminated  or controlled. RMI.  See radio magnetic indicator. RNAV.  See area navigation. RNP.  See required navigation performance. Runway end identifier lights (REIL).  A pair of synchronized  flashing lights, located laterally on each side of the runway  threshold, providing rapid and positive identification of the  approach end of a runway. Runway visibility value (RVV).  The visibility determined  for a particular runway by a transmissometer. Runway visual range (RVR).  The instrumentally derived  horizontal distance a pilot should be able to see down the  runway from the approach end, based on either the sighting  of high-intensity runway lights, or the visual contrast of  other objects. RVR.  See runway visual range. RVV.  See runway visibility value. St. Elmo’s Fire.  A corona discharge which lights up the aircraft  surface areas where maximum static discharge occurs. Satellite ephemeris data. Data broadcast by the GPS  satellite containing very accurate orbital data for that  satellite, atmospheric propagation data, and satellite clock  error data. Scan.  The first fundamental skill of instrument flight, also  known as “cross-check;” the continuous and logical observation  of instruments for attitude and performance information. SDF.  See simplified directional facility. Selective availability.  A satellite technology permitting the  Department of Defense (DOD) to create, in the interest of  national security, a significant clock and ephemeris error in  the satellites, resulting in a navigation error. Semicircular canal.  An inner ear organ that detects angular  acceleration of the body. Sensitive altimeter. A form of multipointer pneumatic  altimeter with an adjustable barometric scale that allows the  reference pressure to be set to any desired level. SIDS.  See standard instrument departure procedures. SIGMET. The acronym for Significant Meteorological  information. A weather advisory issued concerning weather  significant to the safety of all aircraft. Signal-to-noise ratio. An indication of signal strength  received compared to background noise, which is a measure  of how adequate the received signal is.  G-16