Differential Global Positioning System (DGPS).  A system  that improves the accuracy of Global Navigation Satellite  Systems (GNSS) by measuring changes in variables to  provide satellite positioning corrections. Direct indication.  The true and instantaneous reflection of  aircraft pitch-and-bank attitude by the miniature aircraft,  relative to the horizon bar of the attitude indicator. Direct User Access Terminal System (DUATS).  A system  that provides current FAA weather and flight plan filing  services to certified civil pilots, via personal computer,  modem, or telephone access to the system. Pilots can request  specific types of weather briefings and other pertinent data  for planned flights. Distance circle.  See reference circle. Distance measuring equipment (DME). A pulse-type  electronic navigation system that shows the pilot, by an  instrument-panel indication, the number of nautical miles  between the aircraft and a ground station or waypoint. DME.  See distance measuring equipment. DME arc.  A flight track that is a constant distance from the  station or waypoint. DOD.  Department of Defense. Doghouse.  A turn-and-slip indicator dial mark in the shape  of a doghouse. Domestic Reduced Vertical Separation Minimum  (DRVSM).  Additional flight levels between FL 290 and FL  410 to provide operational, traffic, and airspace efficiency. Double gimbal.  A type of mount used for the gyro in an  attitude instrument. The axes of the two gimbals are at right  angles to the spin axis of the gyro, allowing free motion in  two planes around the gyro. DP.  See departure procedure. Drag.  The net aerodynamic force parallel to the relative  wind, usually the sum of two components: induced drag  and parasite drag. Drag curve.  The curve created when plotting induced drag  and parasite drag. DUATS.  See direct user access terminal system. Duplex.  Transmitting on one frequency and receiving on a  separate frequency. Eddy currents.  Current induced in a metal cup or disc when  it is crossed by lines of flux from a moving magnet. EFAS.  See En Route Flight Advisory Service. EFC.  See expect-further-clearance. Electronic flight display (EFD).  For the purpose of  standardization, any flight instrument display that uses  LCD or other image-producing system (Cathode Ray Tube  [CRT], etc.) Elevator illusion. The sensation of being in a climb or  descent, caused by the kind of abrupt vertical accelerations  that result from up- or downdrafts. Emergency.  A distress or urgent condition. Emphasis error.  The result of giving too much attention  to a particular instrument during the cross-check, instead of  relying on a combination of instruments necessary for attitude  and performance information. EM wave.  Electromagnetic wave. Encoding altimeter.  A special type of pressure altimeter  used to send a signal to the air traffic controller on the ground,  showing the pressure altitude the aircraft is flying. En route facilities ring.  Depicted in the plan view of IAP  charts, a circle which designates NAVAIDs, fixes, and  intersections that are part of the en route low altitude airway  structure. En Route Flight Advisory Service (EFAS).  An en route  weather-only AFSS service. En route high-altitude charts.  Aeronautical charts for en  route instrument navigation at or above 18,000 feet MSL. En route low-altitude charts.  Aeronautical charts for en  route IFR navigation below 18,000 feet MSL. Equivalent airspeed.  Airspeed equivalent to CAS in  standard atmosphere at sea level. As the airspeed and pressure  altitude increase, the CAS becomes higher than it should be,  and a correction for compression must be subtracted from  the CAS.  G-6