series BY THOMAS INMAN Glideslope T his month's Theory & Practice column examines the final component of an instrument landing system. So far in previous months, we have addressed localizer that gives pilots lateral guidance along with the marker beacon, which marks specific locations along the path toward the end of the runway. Some systems include distance measuring equipment in place of the marker beacon, and a few use an automatic direction finder transmitter known as an outer compass locater Inner marker Middle Marker FIGURE 1: Aircraft above the glidepath and to the right of the runway Outer Marker 3000 to 6000 ft Radio shack 200 ft Aircraft above the glide path and to the left of the runway 90 Hz (yellow) 150 Hz (blue) Glideslope modulation frequency 4-7 miles Aircraft below the glide path and to the left of the runway Minimum vectoring altitude Aircraft centered on both the glidepath and the runway This illustration of an instrument landing system shows marker beacons and localizer, but features glideslope. placed at the outer marker. This month, we cover the glideslope system. The glideslope system provides vertical guidance to the runway. The glideslope operational theory is much like that of the localizer. A directional antenna array broadcasts UHF radio signals from the approach end of a runway. One signal is modulated with 90 hertz, and the other is modulated with 150 Hz. These signals overlap, creating a glidepath, at an angle of 2.5 to 3 degrees rising from the touchdown zone of 48 avionics news * july 2023