on a gyro-stabilized platform. This averts the introduction  of errors that may result from acceleration due to gravity.  An INS uses complex calculation made by an INS computer to  convert applied forces into location information. An interface  control head is used to enter starting location position data  while the aircraft is stationary on the ground. This is called  initializing.  [Figure 11-155]  From then on, all motion of  the aircraft is sensed by the built-in accelerometers and run  through the computer. Feedback and correction loops are used  to correct for accumulated error as flight time progresses. The  amount an INS is off in one hour of flight time is a reference  point for determining performance. Accumulated error of less  than one mile after one hour of flight is possible. Continuous  accurate adjustment to the gyro-stabilized platform to keep it  parallel to the Earth’s surface is a key requirement to reduce  accumulated error. A latitude/longitude coordinate system is  used when giving the location output. INS is integrated into an airliner’s flight management  system and automatic flight control system. Waypoints can  be entered for a predetermined flightpath and the INS will  guide the aircraft to each waypoint in succession. Integration  with other NAV aids is also possible to ensure continuous  correction and improved accuracy but is not required. location information is analyzed, and correction instructions  are sent to communication satellites in geostationary orbit  over the NAS. The satellites broadcast GPS-like signals  that WAAS enabled GPS receivers use to correct position  information received from GPS satellites. A WAAS enable GPS receiver is required to use the wide  area augmentation system. If equipped, an aircraft qualifies  to perform precision approaches into thousands of airports  without any ground-based approach equipment. Separation  minimums are also able to be reduced between aircraft that  are WAAS equipped. The WAAS system is known to reduce  position errors to 1–3 meters laterally and vertically.  An inertial navigation system (INS) is used on some  large aircraft for long range navigation. This may also be  identified as an inertial reference system (IRS), although  the IRS designation is generally reserved for more modern  systems. An INS/IRS is a self-contained system that does  not require input radio signals from a ground navigation  facility or transmitter. The system derives attitude, velocity,  and direction information from measurement of the aircraft’s  accelerations given a known starting point. The location of the  aircraft is continuously updated through calculations based on  the forces experienced by INS accelerometers. A minimum  of two accelerometers is used, one referenced to north, and  the other referenced to east. In older units, they are mounted  11-71 Inertial Navigation System (INS)/Inertial  Reference System (IRS) Figure 11-155.  An interface panel for three air data and inertial  reference systems on an Airbus. The keyboard is used to initialize  the system. Latitude and longitude position is displayed at the top.  Figure 11-154.  The wide area augmentation system (WAAS) is used  to refine GPS positions to a greater degree of accuracy. A WAAS  enabled GPS receiver is required for its use as corrective information  is sent from geostationary satellites directly to an aircraft’s GPS  receiver for use.  GPS satellites Wide area reference station Ground Earth station Communication satellites L1 L1 Wide area master station SYS OFF TEST TK/GS PPOS WIND HDG STS 1 3 2 1 N 2 3 W 4 H 5 E 6 7 S 8 9 ENT 0 CLR OFF IR1 IR3 DATA DISPLAY IR2 ADR1 ADR3 ADR2 NAV ATT OFF NAV ATT OFF NAV ATT