ILS NAVIGATION

INTRODUCTION:



The Instrument Landing System (ILS) is an internationally normalized system for navigation of aircrafts upon the final approach for landing. It was accepted as a standard system by the ICAO, (International Civil Aviation Organization) in 1947.
Since the technical specifications of this system are worldwide prevalent, an aircraft equipped with a board system like the ILS, will reliably cooperate with an ILS ground system on every airport where such system is installed.

The ILS system is nowadays the primary system for instrumental approach for category I.-III-Aconditions of operation minimums and it provides the horizontal as well as the vertical guidance necessary for an accurate landing approach in IFR (Instrument Flight Rules) conditions, thus in conditions of limited or reduced visibility.The accurate landing approach is a procedure of permitted descent with the use of navigational equipment coaxial with the trajectory and given information about the angle of descent.

The equipment that provides a pilot instant information about the distance to the point of reach is not a part of the ILS system and therefore is for the discontinuous indication used a set of two or three marker beacons directly integrated into the system. The system of marker beacons can however be complemented for a continuous measurement of distances with the DME system (Distance measuring equipment), while the ground part of this UKV distance meter is located co-operatively with the descent beacon that forms the glide slope. It can also be supplemented with a VOR system by which means the integrated navigational-landing complex ILS/VOR/DME is formed.
Analysis

CATEGORIES OF OPERATION MINIMUM:

CATEGORY I

• A minimal height of resolution at 200 ft (60,96 m), whereas the decision height represents an altitude at which the pilot decides upon the visual contact with the runway if he’ll either finish the landing maneuver, or he’ll abort and repeat it.
• The visibility of the runway is at the minimum 1800 ft (548,64 m)
• The plane has to be equipped apart from the devices for flying in IFR (Instrument Flight Rules) conditions also with the ILS system and a marker beacon receiver.

CATEGORY II

• A minimal decision height at 100 ft (30,48 m)
• The visibility of the runway is at the minimum 1200 ft (365,76 m)
• The plane has to be equipped with a radio altimeter or an inner marker receiver, an autopilot link, a raindrops remover and also a system for the automatic draught control of the engine can be required. The crew consists of two pilots.

CATEGORY III A

• A minimal decision height lower than 100 ft (30,48 m)
• The visibility of the runway is at the minimum 700 ft (213,36 m)
• The aircraft has to be equipped with an autopilot with a passive malfunction monitor or a HUD (Head-up display).

CATEGORY III B

• A minimal decision height lower than 50 ft (15,24 m)
• The visibility of the runway is at the minimum 150 ft (45,72 m)
• A device for alteration of a rolling speed to travel speed.

CATEGORY III C

• Zero visibility

Basic elements of the ILS system and THEIR brief description:

• The ILS system consists of four subsystems:
• VHF localizer transmitter
• UHF glide slope transmitter
• marker beacons
• approach lighting system

Marker beacons

For the purpose of discontinuous addition of navigation data with the value of a momentary distance from the aircraft to the runway’s threshold, the following marker beacons are used:

Outer Marker (OM)

• The outer marker is located 3,5÷6 NM (5.556÷11.112 km) from the runway’s threshold. Its beam intersects the glide slope’s ray at an altitude of approximately 1400 ft (426.72 m) above the runway. It also roughly marks the point at which an aircraft enters the glide slope under normal circumstances, and represents the beginning of the final part of the landing approach.
• The signal is modulated at a frequency of 400 Hz, made up by a Morse code – a group of two dots per second. On the aircraft, the signal is received by a 75 MHz marker receiver. The pilot hears a tone from the loudspeaker or headphones and a blue indicative bulb lights up. Anywhere an outer marker cannot be placed due to the terrain, a DME unit can be used as a part of the ILS to secure the right fixation on the localizer.
• In some ILS installations the outer marker is substituted by a Non Directional Beacon (NDB).

Figure 15 – The outer position marker (blue).

Middle Marker (MM)

• The middle marker is used to mark the point of transition from an approach by instruments to a visual one. It’s located about 0,5÷0,8 NM (926÷1482 m) from the runway’s threshold. When flying over it, the aircraft is at an altitude of 200÷250 ft (60,96÷76,2) above it. The audio signal is made up of two dashes or six dots per second. The frequency of the identification tone is 1300 Hz. Passing over the middle marker is visually indicated by a bulb of an amber (yellow) colour . It was removed in some countries, e.g. in Canada.

Figure 16 – The middle marker (yellow).

Inner Marker (IM)

• The inner marker emits an AM wave with a modulated frequency of 3000 Hz. The identification signal has a pattern of series of dots, in frequency of six dots per second. The beacon is located 60m in front of the runway’s threshold. The inner marker has to be used for systems of the II. and III. category.

Figure 17 – The inner marker (white).