Introduction to Instrument Landing Systems (ILS)

Introduction

The ILS (Instrument Landing System) is a navigation aid used internationally to facilitate approach and landing. It is comprised of a localizer (LLZ or LOC), a glide path (GP or GS) and a series of marker beacons that includes an outer and middle marker and, in special cases, an inner marker. Each group generates radio signals independently and simultaneously. The localizer supplies left−right navigation information, the glide path supplies up−down navigation information, and the marker beacons supplies distance−to−threshold information. 

The system includes equipment in the control tower that can be used to remotely control, monitor, and maintain the localizer, glide slope and markers. The localizer and the glide path principle of operation is based on measurements of the difference in depth of modulation (DDM) between two signals with frequencies 90 Hz and 150 Hz. These are the navigation frequencies used to detect the correct approach course (DDM = 0) and the specified glide path angle (DDM = 0).

The localizer operates in the frequency range of 108 to 112 MHz and generates a vertical guidance plane, which permits the aircraft pilot to select a left/right approach course from a distance of up to about 30 km. The antenna radiation pattern reveals exactly the same amplitude for the two modulation frequencies of 90 and 150 Hz in the guidance plane. If the pilot deviates to the left of the guidance plane, the 90 Hz modulation signal will predominate causing the cockpit indicator to show a fly right indication. If the pilot deviates to the right, the 150 Hz modulation signal will predominate causing the cockpit indicator to show a fly left indication.

The glide path operates in the frequency range between 328 and 336 MHz and generates the glide path plane, which is elevated above the runway by the glide angle. The antenna radiation pattern results from an interaction with the earth’s surface, and contains predominantly 150 Hz modulation below the glide path plane and predominantly 90 Hz modulation above the glide path plane. In the glide path plane itself the amplitudes of the two modulation signals are equal.

The beam which shows the aircraft the correct landing approach path is formed by the intersection of the vertical course guidance plane and the horizontal glide path plane. In addition, the marker beacons provide marks that indicate the distance from the runway thresholds. The marker beacon transmitters radiate vertically upwards at the same carrier frequency, and are characterized by various continuously keyed Morse code signals and different modulation frequencies.