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(3) NAVAIDs <br />The presence of adequate NAVAIDs and lighting is critical to an airport's <br />ability to fulfill its role in the airport system. NAVAIDs include both <br />precision and nonprecision (visual) aids. Precision NAVAIDs provide <br />approaching aircraft with data on distance to the runway, the approach slope, <br />and alignment. Precision aids allow aircraft to land during poor weather <br />conditions. Table I-5 depicts existing NAVAIDs at study airports. Typical <br />NAVAIDs include: <br />Nondirectional Radio Beacon (NDB) - Nonprecision nondirectional <br />beacons transmit signals (low or medium frequency) that are used by <br />the pilot of an aircraft equipped with a loop antenna to determine <br />his bearing and "home" on the NDB location. These facilities can be <br />used for enroute navigation and as an airport approach aid. <br />Instrument Landing_ System (IL Sl - A precision instrument approach <br />system consisting of a localizer (azimuth guidance), glide slope <br />(altitude guidance), marker beacons (distance guidance), and an <br />approach light system. <br />Approach Light System (ALS) - A system of approach lights that are <br />configured along the extended runway centerline for a distance of <br />up to 3,000 feet; mcludes ALSF-1, MALS or MALSR, and SSALR. <br />Runway/Taxiway Lighting - Lights having a prescribed angle of <br />emission used to define the lateral limits of a runway. <br />Runway End Identifier Lights (REM) - Two synchronized flashing <br />light groups, one on each side of the runway threshold, which <br />provide rapid and positive identification of the approach end of a <br />particular runway. <br />Visual Approach Slone Indicator (VASI) - An airport lighting facility <br />providing vertical visual approach slope guidance to landing aircraft. <br />Precision Approach Path Indicator (PAPI) - This system gives more <br />precise visual indications of the approach path of the aircraft to the <br />pilots and utilizes only one set of lights, as opposed to the minimum <br />of two required by the VASI system. <br />Airport Rotating Beacon - A visual NAVAID with alternating white <br />and green flashes that indicate the airport's location. <br />Automatic Terminal Information Service (ATIS) - The continuous <br />broadcast of recorded noncontrol information in selected terminal <br />areas. Its purpose is to improve controller effectiveness and to <br />I-21 <br />