INSTRUMENT LANDING SYSTEM (ILS)

Instrument Landing System (ILS)

When an aircraft is about to make an approach and landing on an airport runway during bad weather conditions, there is need to radiate navigational information to carter for lost visibility. Instrument Landing System (ILS) is used for this purpose. ILS is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals, visual and aural indications to enable a safe landing during bad weather. ILS consists of two main independent sub-systems, one providing lateral guidance (localizer) and the other providing vertical guidance (glide path) to aircrafts approaching a runway. A modulation depth comparison of two radio signal beams radiated strategically from the localizer (LOC) and received by the ILS receiver in the aircraft provides lateral course-line information intended to coincide with runway center line, while a similar comparison from the glide path (GP) provides the slope information intended to coincide with inclination angle at the touch-down point of the runway. The third subsystem is the marker beacons that provide visual and aural indications.

The Principle of the Localizer

The localizer transmitter operates within the frequency band of 108 to 112MHZ with channel separation of 200KHZ. Its antenna system is strategically designed and placed symmetrically around the centerline of the runway and approximately 300 Metres behind the runway stop-end. Information about the position of an air craft is achieved by modulating the transmitted carrier with tone frequencies, 90HZ and 150HZ. The radiation pattern of the antenna system has such a form that 150HZ modulation is predominant on the right hand side of the course-line, seen in the approach direction while 90HZ modulation is predominant on the left side. 

 

The two tone frequencies are amplitude modulated to a depth of 20% with tolerance of +2% and harmonic distortion less than 10%. The course-line of a localizer is theoretically a straight line consisting of all points where equal levels of 90HZ and 150HZ are received or all points where the difference in depth of modulation (DDM) is equal to zero. The course-line is usually adjusted to coincide with the center-line of the runway. The receiving equipment of the localizer in the Air Craft (AC) is a cross-pointer instrument that reacts to the difference in depth of modulation between 90HZ and 150HZ dots. The identity (ID) of ILS facility is provided by 2 or 3 letters of the Morse code amplitude modulated at 10% and transmitted by the LOC.

Principle of the ILS Localizer

Signal Generation in ILS System

The ILS principle is based on comparison of depths of modulation for two tones. These two tones 90Hz and 150HZ are modulated on the same carrier and form a signal called CSB (Carrier and Side Bands). This is done in the electronic part of the ground equipment. Further, the same tones are used for producing a combined sideband signal designated SBO (Side Bands Only). When the two Radio Frequency (RF) signals, CSB and SBO, are mixed in the near-field and far-field of the antenna, a new modulation process called space modulation (SM) is obtained, causing the depth of modulation of CSB signal to be dependent on the amplitude and phase relationship between CSB and SBO in each point where they are mixed. By comparing depths of modulation after detection of the RF signals, the air craft receiver finds the magnitude and direction of the air craft’s displacement from the desired course-line.

Source of errors in ILS

It must be noted that the mathematical expressions for radiated CSB and SBO signals in the far field can be altered if signal attenuation in space, phase change and antenna characteristics are modified beyond tolerance. It thus means that components in CSB and SBO signals must be carefully observed and controlled to avoid deviations from nominal parameters. This research study has dwelt on finding out if roofing materials in aerodrome are such obstacles that can destabilize this equations and cause errors in ILS measurements.

Reflections from obstacles in the vicinity of the runway may interfere with direct radiating beam from the localizer or GP antenna and cause the course line to deviate from a straight line. The occurrence of interference to the ILS signal is dependent on total environment around the ILS antenna array and also the characteristics of the antenna. Any large reflecting objects including vehicles or fixed objects such as structures within the radiated signal coverage will potentially cause multipath interference to the ILS source and path structure.