The local area augmentation system (LAAS) ground facility  receives the data from the GPS ground reference receivers.  GPS reference receivers, which are serving the local area (such as an airport) receive the GPS signal from the GPS  constellation (one or more satellites).  The corrected signal is then sent and transmitted as a VHF signal, called a VHF data broadcast.  The broadcast signal is received by appropriately equipped aircraft which provide the pilot with highly refined GPS guidance.  GPS Reference  Receiver  GPS Reference  Receiver  LAAS Ground  Facility  VHF Data  Broadcast  LAAS Ground  Facility  VHF Data  Broadcast  4  3  2  1  GPS Reference  Receiver  Figure 9-31. LAAS representation. Figure 7-27b. WAAS satellite representation  Figure 9-30. WAAS provides performance enhancement for GPS  approach procedures through real-time monitoring.  50 ft.  10 ft.  You are here  GPS Accuracy  WAAS Accuracy  Inertial Navigation System (INS) is a system that navigates  precisely without any input from outside of the aircraft. It is  fully self-contained. The INS is initialized by the pilot, who  enters into the system the exact location of the aircraft on the  ground before the flight. The INS is also programmed with  WPs along the desired route of flight. INS is considered a stand-alone navigation system, especially  when more than one independent unit is onboard. The  airborne equipment consists of an accelerometer to measure  acceleration—which, when integrated with time, gives  velocity—and gyros to measure direction. Later versions of the INS, called inertial reference systems  (IRS), utilize laser gyros and more powerful computers;  therefore, the accelerometer mountings no longer need to  be kept level and aligned with true north. The computer  system can handle the added workload of dealing with the  computations necessary to correct for gravitational and  directional errors. Consequently, these newer systems are  sometimes called strap down systems, as the accelerometers  and gyros are strapped down to the airframe rather than being  mounted on a structure that stays fixed with respect to the  horizon and true north. The principal error associated with INS is degradation of  position with time. INS computes position by starting with  accurate position input which is changed continuously as  accelerometers and gyros provide speed and direction inputs.  Both accelerometers and gyros are subject to very small  errors; as time passes, those errors probably accumulate. Inertial Navigation System (INS) INS Components INS Errors 9-34