Reference Receiver  GPS Satellites  LAAS Facility  VHF Transmitter  Figure 7-28. LAAS representation.  Figure 9-32. The LAAS system working with GPS satellites,  reference receivers and radio transmitters which are located on  or in the vicinity of the airport.  While the best INS/IRS display errors of 0.1 to 0.4 NM after  flights across the North Atlantic of 4 to 6 hours, smaller and  less expensive systems are being built that show errors of 1  to 2 NM per hour. This accuracy is more than sufficient for  a navigation system that can be combined with and updated  by GPS. The synergy of a navigation system consisting of an  INS/IRS unit in combination with a GPS resolves the errors  and weaknesses of both systems. GPS is accurate all the time  it is working but may be subject to short and periodic outages.  INS is made more accurate because it is continually updated  and continues to function with good accuracy if the GPS has  moments of lost signal. Most navigation systems approved for en route and terminal  operations under IFR, such as VOR, NDB, and GPS, may also  be approved to conduct IAPs. The most common systems in  use in the United States are the ILS, simplified directional  facility (SDF), localizer-type directional aid (LDA), and  microwave landing system (MLS). These systems operate  independently of other navigation systems. There are new  systems being developed, such as WAAS and LAAS. Other  systems have been developed for special use. The ILS system provides both course and altitude guidance  to a specific runway. The ILS system is used to execute a  precision instrument approach procedure or precision approach.  [Figure9-33]  The system consists of the following components: The localizer (LOC) ground antenna array is located on the  extended centerline of the instrument runway of an airport,  located at the departure end of the runway to prevent it from  being a collision hazard. This unit radiates a field pattern,  which develops a course down the centerline of the runway  toward the middle markers (MMs) and outer markers  (OMs) and a similar course along the runway centerline in  the opposite direction. These are called the front and back  The ILS uses a number of different ground facilities. These  facilities may be used as a part of the ILS system, as well as  part of another approach. For example, the compass locator  may be used with NDB approaches. ILS approaches are categorized into three different types of  approaches based on the equipment at the airport and the  experience level of the pilot. Category I approaches provide  for approach height above touchdown of not less than 200 feet.  Category II approaches provide for approach to a height above  touchdown of not less than 100 feet. Category III approaches  provide lower minimums for approaches without a decision  height minimum. While pilots need only be instrument rated  and the aircraft be equipped with the appropriate airborne  equipment to execute Category I approaches, Category II  and III approaches require special certification for the pilots,  ground equipment, and airborne equipment. The following supplementary elements, though not specific  components of the system, may be incorporated to increase  safety and utility: 2.  A glideslope (GS) providing vertical (up/down)  guidance toward the runway touchdown point, usually  at a 3° slope. 3.  Marker beacons providing range information along  the approach path. 4.  Approach lights assisting in the transition from  instrument to visual flight. 1.  Compass locators providing transition from en route  NAVAIDs to the ILS system and assisting in holding  procedures, tracking the localizer course, identifying  the marker beacon sites, and providing a FAF for  ADF approaches. 2.  DME collocated with the GS transmitter providing  positive distance-to-touchdown information or DME  associated with another nearby facility (VOR or stand- alone), if specified in the approach procedure. 1.  A localizer providing horizontal (left/right) guidance  along the extended centerline of the runway. Instrument Approach Systems Instrument Landing Systems (ILS) ILS Components Ground Components Localizer 9-35