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<br />. non directional radio beacon <br />(NDB), and <br /> <br />. a global positioning system <br />(GPS). <br /> <br />The instrument landing system <br />(ILS) is an approach and landing aid <br />designed to identify the exact alignment <br />of the runway approach path. The ILS <br />systems are installed to allow <br />approaches during periods of poor <br />visibility. San Marcos Municipal <br />Airport has a published ILS approach to <br />Runway 12. <br /> <br />ILS systems provide three functions: 1) <br />guidance, provided vertically by a glide <br />slope beacon, and horizontally by a <br />localizer beacon; 2) range, furnished by <br />marker beacons; and 3) visual <br />alignment, supplied by the approach <br />lighting system and runway edge lights. <br /> <br />The localizer (LOC) antennas for each <br />approach is located beyond the far end <br />of a runway. The localizer antenna for <br />Runway 12 is situated on an extended <br />centerline, 1,350 feet southeast of the <br />Runway 30 threshold. The antenna <br />emits very high frequency (VHF) <br />signals that provide the pilot with <br />course deviation left or right of the <br />runway centerline and the degree of <br />deviation. The ultra high frequency <br />(UHF) glide slope (GS) transmitter for <br />Runway 12 is located approximately <br />1,150 feet northeast of the Runway12 <br />landing threshold. The transmitter <br />provides a signal indicating whether the <br />aircraft is above or below the desired <br />glide path. <br /> <br />To further assist the ILS approach, an <br />initial approach fix, usually a trans- <br /> <br />1-14 <br /> <br />mitted signal intersection, has been <br />established with the Gary NDB. Also, <br />outer and middle markers have been <br />established to aid pilots in determining <br />their progress along the approach. The <br />Runway 12 ILS approach has a 3.0 <br />degree glide slope that intercepts the <br />initial approach fix at a distance of 5.5 <br />nautical miles and 2,445 feet above the <br />threshold. The approach intercepts the <br />GARYS locator/outer marker at 5.1 <br />nautical miles from the middle marker <br />at 2,445 feet above the threshold. The <br />middle marker is 0.5 nautical miles <br />from the runway threshold, and serves <br />as the missed approach point. <br /> <br />The VOR provides azimuth readings to <br />pilots of properly equipped aircraft by <br />transmitting a radio signal at every <br />degree to provide 360 individual <br />navigational courses. Frequently, <br />distance measuring equipment (DME) <br />is combined with a VOR facility to <br />provide distance as well as direction <br />information to the pilot. In addition, <br />military tactical air navigation aids <br />(TACANs) and civil VORs are <br />commonly combined to form a VORTAC. <br />A VORTAC provides distance and <br />direction information to civil and <br />military pilots. Although there is no <br />instrument approach associated with a <br />VOR at San Marcos, navigation <br />between and utilizing VORs is <br />extremely important. The VOR airway <br />system is described below. <br /> <br />There are several VOR facilities located <br />in the region which can be utilized by <br />pilots flying to or from San Marcos <br />Airport. Area VOR facilities that are <br />available for navigational purposes in <br />the San Marcos Airport vicinity are: <br />