Goniometer. As used in radio frequency (RF) antenna  systems, a direction-sensing device consisting of two fixed  loops of wire oriented 90° from each other, which separately  sense received signal strength and send those signals to two  rotors (also oriented 90°) in the sealed direction-indicating  instrument. The rotors are attached to the direction-indicating  needle of the instrument and rotated by a small motor until  minimum magnetic field is sensed near the rotors. GPS.  See global positioning system. GPS Approach Overlay Program. An authorization for  pilots to use GPS avionics under IFR for flying designated  existing nonprecision instrument approach procedures, with  the exception of LOC, LDA, and SDF procedures. Graveyard spiral.  The illusion of the cessation of a turn  while still in a prolonged, coordinated, constant rate turn,  which can lead a disoriented pilot to a loss of control of the  aircraft. Great circle route.  The shortest distance across the surface  of a sphere (the Earth) between two points on the surface. Ground proximity warning system (GPWS).  A system  designed to determine an aircraft’s clearance above the Earth  and provides limited predictability about aircraft position  relative to rising terrain. Groundspeed.  Speed over the ground, either closing speed to  the station or waypoint, or speed over the ground in whatever  direction the aircraft is going at the moment, depending upon  the navigation system used. GS.  See glide slope. GWPS.  See ground proximity warning system. HAA.  See height above airport. HAL.  See height above landing. HAT.  See height above touchdown elevation. Hazardous attitudes. Five aeronautical decision-making  attitudes that may contribute to poor pilot judgment:  antiauthority, impulsivity, invulnerability, machismo, and  resignation. Hazardous Inflight Weather Advisory Service (HIWAS).  Service providing recorded weather forecasts broadcast to  airborne pilots over selected VORs. Head-up display (HUD).  A special type of flight viewing  screen that allows the pilot to watch the flight instruments  and other data while looking through the windshield of the  aircraft for other traffic, the approach lights, or the runway. Height above airport (HAA).  The height of the MDA above  the published airport elevation. Height above landing (HAL).  The height above a designated  helicopter landing area used for helicopter instrument  approach procedures. Height above touchdown elevation (HAT).  The DA/DH or  MDA above the highest runway elevation in the touchdown  zone (first 3,000 feet of the runway). HF.  High frequency. Hg.  Abbreviation for mercury, from the Latin  hydrargyrum. HIWAS. See Hazardous Inflight Weather Advisory  Service. Holding. A predetermined maneuver that keeps aircraft  within a specified airspace while awaiting further clearance  from ATC. Holding pattern.  A racetrack pattern, involving two turns  and two legs, used to keep an aircraft within a prescribed  airspace with respect to a geographic fix. A standard pattern  uses right turns; nonstandard patterns use left turns. Homing.  Flying the aircraft on any heading required to keep  the needle pointing to the 0° relative bearing position. Horizontal situation indicator (HSI).  A flight navigation  instrument that combines the heading indicator with a CDI,  in order to provide the pilot with better situational awareness  of location with respect to the courseline. HSI.  See horizontal situation indicator. HUD.  See head-up display. Human factors.  A multidisciplinary field encompassing the  behavioral and social sciences, engineering, and physiology,  to consider the variables that influence individual and  crew performance for the purpose of optimizing human  performance and reducing errors.  G-8