glide-path transmitter

An element in the ILS (instrument landing system) that defines a glide-path beam and assists an aircraft to make an approach at a correct glide angle. A glide-path transmitter is located 750 to 1250 ft from the runway threshold and 250 to 600 ft to the side of the runway. Its radio beam extends upward approximately 21/2 to 31/4° from the approach end of an instrument runway. The glide slope guides an aircraft along the desired glide path when it is making an ILS approach along the localizer path. The glide-slope transmitter transmits a directional pattern on one of forty channels in the VHF (very high frequency) band of 329.15 to 335.4 MHz, modulated by two signals (transmitted on 90 and 150 Hz, respectively). When received with equal intensity, they are displayed by compatible airborne equipment as an “on path” indication. In this case, the horizontal needle of the crosspointer indicator shows the pilot whether he or she is above, below, or on the glide slope. The glide slope is also referred to as the glide path. False glide slope signals may exist in the area of the localizer back course approach, which can cause the glide-slope flag alarm to disappear and present unreliable glide-slope information. The pilot should disregard all such glide-slope signal indications when making a localizer back course approach unless a glide slope is specified on the approach and the landing chart. See also false glide slope.