Reduced vertical separation minimum (RVSM).  Reduces  the vertical separation between flight levels (FL) 290 and 410  from 2,000 feet to 1,000 feet, and makes six additional FLs  available for operation. Also see DRVSM.  Reference circle (also, distance circle).  The circle depicted  in the plan view of an IAP chart that typically has a 10 NM  radius, within which chart the elements are drawn to scale.  Radio magnetic indicator (RMI).  An electronic navigation  instrument that combines a magnetic compass with an ADF or  VOR. The card of the RMI acts as a gyro-stabilized magnetic  compass, and shows the magnetic heading the aircraft is flying.  Radiosonde.  A weather instrument that observes and reports  meteorological conditions from the upper atmosphere. This  instrument is typically carried into the atmosphere by some  form of weather balloon.  Radio wave.  An electromagnetic (EM) wave with frequency  characteristics useful for radio transmission.  Regions of command.  The “regions of normal and reversed  command” refers to the relationship between speed and the  power required to maintain or change that speed in flight.  Region of reverse command.  Flight regime in which flight  at a higher airspeed requires a lower power setting and a  lower airspeed requires a higher power setting in order to  maintain altitude.  RAIM.  See receiver autonomous integrity monitoring.  RAM recovery.  The increase in thrust as a result of ram air  pressures and density on the front of the engine caused by  air velocity.  Relative bearing (RB).  The angular difference between the  aircraft heading and the direction to the station, measured  clockwise from the nose of the aircraft.  Random RNAV routes.  Direct routes, based on area  navigation capability, between waypoints defined in terms  of latitude/longitude coordinates, degree-distance fixes, or  offsets from established routes/airways at a specified distance  and direction.  Ranging signals.  Transmitted from the GPS satellite, signals  allowing the aircraft’s receiver to determine range (distance)  from each satellite.  Relative bearing indicator (RBI).  Also known as the fixed- card ADF, zero is always indicated at the top of the instrument  and the needle indicates the relative bearing to the station.  Relative humidity.  The ratio of the existing amount of  water vapor in the air at a given temperature to the maximum  amount that could exist at that temperature; usually expressed  in percent.  Rapid decompression.  The almost instantaneous loss of  cabin pressure in aircraft with a pressurized cockpit or cabin.  RB.  See relative bearing.  RBI.  See relative bearing indicator.  RCO.  See remote communications outlet.  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.  Receiver autonomous integrity monitoring (RAIM).  A  system used to verify the usability of the received GPS signals  and warns the pilot of any malfunction in the navigation  system. This system is required for IFR-certified GPS units.  Recommended altitude.  An altitude depicted on an instrument  approach chart with the altitude value neither underscored nor  overscored. The depicted value is an advisory value.  Receiver-transmitter (RT).  A system that receives and  transmits a signal and an indicator.  Restricted area.  Airspace designated under 14 CFR part  73 within which the flight of aircraft, while not wholly  prohibited, is subject to restriction.  Required navigation performance (RNP).  A specified level  of accuracy defined by a lateral area of confined airspace in  which an RNP-certified aircraft operates.  Reverse sensing.  The VOR needle appearing to indicate the  reverse of normal operation.  G-25  REIL.  See runway end identifier lights. 