![]() It is the center section of the vertical green course indicator. The localizer CDI can be seen at the bottom center of the display. The diamond is shown centered indicating the aircraft is on course vertically. The green diamond moves up and down to reflect the aircraft being above or below the glidepath. The narrow vertical scale on the right of the attitude indicator with the “G” at the top is the deviation scale for the glideslope. On the right, a Garmin G-1000 PFD illustrates an aircraft during an ILS approach. The horizontal white line is the deviation indicator for the glideslope. A traditional course deviation indicator is shown on the left. The VOR/ILS localizer CDI and the glideslope are displayed together on whichever kind of instrumentation is in the aircraft. The glideslope indicator operates identically to the localizer CDI only 90° to it. It drives a vertical course deviation indicator known as the glideslope indicator. The aircraft’s glideslope receiver deciphers the signals similar to the method of the localizer receiver. Like the localizer, the glideslope transmits two signals, one modulated at 90 Hz and the other modulated at 150 Hz. When the VOR/ILS receiver is tuned for the approach, the glideslope receiver is automatically tuned. The frequency is paired to the localizer frequency of the ILS. ![]() The transmitting frequency range is UHF between 329.3 MHz and 335.0 MHz. The glideslope transmitter antenna is horizontally polarized. A glideslope antenna broadcasts radio signals to guide an aircraft vertically to the runway. It transmits in a wedge-like pattern with the field narrowing as it approaches the runway. The transmitting glideslope antenna is located off to the side of the approach runway approximately 1,000 feet from the threshold. Radio signals funnel the aircraft down to the touchdown point on the runway at an angle of approximately 3°. The vertical guidance required for an aircraft to descend for a landing is provided by the glideslope of the ILS. The pilot must turn toward the right to center the CDI and the aircraft with the runway center line. If the 90 Hz signal is received by the VOR receiver, the CDI deflects to the right. This centers course deviation indicator on the display and centers the aircraft with the centerline of the runway. The pilot must correct course with a turn to the left. ![]() This indicates that the runway is to the left. If the aircraft receives a 150 Hz signal, the CDI of the VOR/ILS display deflects to the left. The signals received are passed through filters and rectified into DC to drive the course deviation indicator. The circuitry specific to standard VOR reception is inactive while the receiver uses localizer circuitry and components common to both. The aircraft’s VOR receiver is tuned to the localizer VHF frequency that can be found on published approach plates and aeronautical charts. The right side of the approach contains a 150 MHz modulated signal. The left side of the approach area is filled with a VHF carrier wave modulated with a 90 Hz signal. The field tapers to runway width near the landing threshold. They create an expanding field that is 21⁄2° wide (about 1,500 feet) 5 miles from the runway. Two modulated signals are produced from a horizontally polarized antenna complex beyond the far end of the approach runway. The localizer broadcast is a VHF broadcast in the lower range of the VOR frequencies (108 MHz–111.95 MHz) on odd frequencies only. Components of an instrument landing system (ILS). Together, all of these radio signals make an ILS a very accurate and reliable means for landing aircraft. Marker beacons provide distancefrom- the-runway information. Compass locator transmissions for outer and middle approach marker beacons aid the pilot in intercepting the approach navigational aid system. A separate glideslope broadcast provides vertical guidance of the aircraft down the proper slope to the touch down point. It is used to provide horizontal guidance to the center line of the runway. A localizer is one of the radio transmissions. Multiple radio transmissions are used that enable an exact approach to landing with an ILS. This radio navigation system guides the aircraft down a slope to the touch down area on the runway. An ILS is used to land an aircraft when visibility is poor.
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