Instrument Landing System ILS is a ground-based instrument approach system that provides precision lateral and vertical guidance to an aircraft approaching and landing on a runway, using a combination of radio signals from radio beacons with a meter range. Ground equipment consists of two radio beacons – localizer and glide slope, which are placed on an aerodrome.
The localizer provides lateral guidance. It formed two directional pattern in horizontal plane (fig. 79) . The localizer normally located along the runway axis with the aim to set the lane which will pass through the runway axis. Localizer carrier frequencies are selected from range between 108.10 MHz and 111.95 MHz. Two signals are transmitted on one of 40 ILS channels. One is modulated at 90 Hz, the other at 150 Hz. These are transmitted from co-located antennas. Each antenna transmits a narrow beam, one slightly to the left of the runway centreline, the other slightly to the right. The localiser receiver on the aircraft measures the difference in the depth of modulation (DDM). If there is a predominance of either 90 Hz or 150 Hz modulation, the aircraft is off the centreline. In the cockpit, the needle on the instrument part of the ILS shows that the aircraft needs to fly left or right to correct the error to fly toward the centre of the runway. If the DDM is zero, the aircraft is on the localiser centreline coinciding with the physical runway centreline. The pilot controls the aircraft so that the indicator remains centered on the display. The localizer’s coverage area is 50 km.
Fig. 79. Principle of operation of Instrumental Landing System
The glide slope provides vertical guidance. The pilot controls the aircraft so that the glide slope indicator remains centered on the display to ensure the aircraft is following the glide path to remain above obstructions and reach the runway at the proper touchdown point. The glide slope signal is transmitted on a carrier frequency using a technique similar to that for the localiser. Lower lobe is modulated on the 150 Hz, higher – 90 Hz. If an aircraft is descending according to glide slope, then DDM is zero. If there is some deviation from the ideal direction, the DDM will increase proportionally to that deviation. And according to sign it is possible to determine in which side an aircraft is deviated (up or down). The glide slope’s coverage area is 18 km. Glide slope uses frequencies from 328.6 to 335.4 MHZ, and it depends from localizer’s frequency. The principle of displaying information about the deviation from the desired aircraft trajectory decline shown in Fig. 80.
Fig. 80. Deviation indication
ILS landing system also provides listening the crew call-ground radio beacons.
ILS on-board equipment consists of several receivers localiser and glide slope antennas. Each antenna is connected to the receivers separate entrances. Information about the deviation from the line glide path shown on a special display (Fig. 81) or by using electronic display.
Fig. 81. Instrument landing system indicator
The disadvantage of ILS in the meter wavelength range is a strong influence of the reflected signals and as a result - curvature of the trajectory when placing of the aircraft on the runway. For decreasing of this factor are used microwave landing system operating in the centimeter wavelength range. Compared to the ILS it has the following advantages:
- Less dependent on terrain and obstacles,
- Wider angular size of zone of activity,
- Higher accuracy of the position.
Microwave landing system (Microwave Landing System -MLS)  performs the same function as the ILS: takes two signals from beacons MLS which are located at the aerodrome, first of which specifies the path approaching the runway elevation, and the second - in azimuth . Directivity pattern azimuth beacon has form as shown below (Fig. 82) and the measurement makes two moves from left to right within the sector of minus 62 ° to +62 °.
Fig. 82. The principle of the MLS
Through the antenna attenuating vibrations are radiated. Due to the rapid movement on board aircraft taken two pulse signals. Duration between them proportional to the azimuth of aircraft. Similar moves up and down on the desired direction provides directivity pattern of the glide slope. Zone of MLS azimuth of ± 40 °, elevation - 0,9 ° -20 °, the range of - 37 km. The range of frequencies allocated for Azimuth measurement is 5031,0-5090,7 MHz for measurements rangefinder - 1000 MHz.
In modern aircrafts are set multifunction receivers which are capable to receive signals from several types of beacons, for example, ILS, MLS and VOR.