Gilde Path or Glide Slope (GP, GS) - Signal parameters

The GP generates an RF−Signal in the frequency range of 328 to 336 MHz that is modulated in amplitude with 90 and 150 Hz. The signal to identify the "glide path plane" is achieved by a transmitter and antenna system. The transmitter can be a 2F system or 1F system, but both produce up to 5 W of power. The glide path signal is obtainable up to a distance of 10 nautical miles (approx. 18.5 km) within an azimuthal sector of ±8° relative to the localizer course line with the touch down point as reference and between the elevations 0.3θ to 1.75θ, where θ is the nominal glide path angle. Below the glide path sector the DDM increases smoothly for decreasing angle until a value of 22 % is reached. From there to 0.3θ the DDM is not less than 22 % as it is required to safeguard the glide path intercept procedure (turning to the guide beam). The characteristic values for GP within certain sectors and in relation to the runway center line are as follows:

• DDM = 0

• DDM = 17.5 % (0.175)

• θ= 2.5...3° (typical)

The plane DDM 0 radiated by the glide path antenna is hyperbolic and does not touch the ground, as the dotted line shows. According to ICAO Annex 10, section 3.1.1, the reference height of this curve has been fixed at 15 m (ILS reference datum) at the runway threshold. Taken together with the specified glide angle of
θ= 2.5 to 3° this produces an offset of the glide path antenna mast with respect to the runway threshold of the distance D. This offset is 286 to 344 m depending upon the glide path angle selected. Due to this the optimal vertical glide path is not a straight line in azimuth direction of the centre line of the extended runway, but is a hyperbola.

DDM=17.5 % is specified for glide angle deviations of ±0.24θ from the nominal glide path θ (θ = DDM 0). These values correspond to the WIDTH. The DDM characteristic is linear within this sector (±0.24θ).

Like the localizer the glide path’s angle information consists of signals amplitude−modulated with 90 Hz and 150 Hz. When the aircraft approaches the runway on the desired glide path, the airborne receiver receives both signals with equal amplitude (equivalent to DDM 0). Deviations above the nominal glide path will result in a predominant 90 Hz amplitude, and deviations below will result in a predominant 150 Hz amplitude (positive DDM).

Localizer (LLZ, LOC) Parameters

The Localizer generates an RF−signal in the frequency range of 108 to 112 MHz, which is modulated in amplitude with 90 and 150 Hz. This signal identifies the "course plane" and is produced by a transmitter and antenna system, which can be a 2F system with 25 W transmitter power or a 1F system with 30 W transmitter power. The localizer signal is obtainable up to a distance of up to 25 nautical miles (approx. 46 km) for a sector of ±10°, and it is obtainable up to a distance greater than or equal to 17 nautical miles (approx. 31 km) for a sector of ±35° relative to the course line and the Localizer−antenna. The characteristic values for LLZ within certain sectors, and in relation to the runway centre line, are as follows:

• Difference in Depth of Modulation (DDM) = 0
• DDM = 15.5 % (0.155)
• DDM greater than or equal to 18 % (0.18)

DDM 0 occurs when the approach direction corresponds exactly to the runway centre line. DDM 15.5 % occurs at the course sector selected such that the boundary at the level of the runway threshold is about 105 m to the left and right of the runway with respect to the centre line, which points are known as WIDTH points. The DDM has a linear characteristic within these points and an elevation of 0.00145 DDM per meter. This results in approx. 105 m for the half sector calculated for DDM=15.5 %. ICAO Annex10 (4th Ed., April 85, section 3.1.3.7.3).

DDM greater than or equal to18 % shows a sector of ±10°and DDM greater than or equal to15.5 % characterizes a sector of ±10° to ±35° where correct LLZ information is still obtained. In the LLZ−1F, this sector is covered by a specifically formed antenna pattern, and, in the LLZ−2F system, it is covered by an additional Clearance signal. The Course information consists of 90 and 150 Hz amplitude−modulated signals. When the aircraft is approaching the runway on the desired course, the air−borne receiver receives the two modulation signals with equal amplitudes. This state corresponds to DDM 0.

If there is a leftward deviation from the desired course, there will be a predominant 90 Hz amplitude, and if there is a rightward deviation there will be a predominant 150 Hz amplitude.

Instruments Landing System (ILS) - Principles.

Arrangement of Subsystems.

The basic subsystems belonging to the ILS system, namely

• the Localizer (LLZ)

• the Glide Path (GP)

• and the Markers, and in addition

• a Distance Measuring Equipment (DME) (optional)

• and a Far Field Monitor for the localizer (optional)

are arranged on the runway as shown in Figure below. This arrangement is valid for the single and dual frequency (1F, 2F) installations described in further detail in the next chapters.

The LLZ antenna is located 200 to 360 m beyond the end of the runway on the extended centre line. The associated LLZ transmitter is in a shelter near the antenna.

The GP antenna is located 120 to 180 m from the runway centre line. The reference height for the glide path has been fixed at 15 m above the runway threshold. The dimension "D" (286 to 344 m) between the GP antenna mast and the runway threshold is calculated from this height and the glide angle, the latter being determined on the basis of local circumstances. The associated GP transmitter is in a shelter in near the antenna.

The inner marker (IM) is 75 to 450 m ahead of the runway threshold on the extended centre line, the middle marker (MM) is 1050 m ahead, and the outer marker (OM) is 7200 m ahead. In most cases only the middle and the outer marker are used.

When a DME systems is used to supplement the marker beacons there are a number of installation
alternatives, for example:

• DME antenna on the GP mast (DME transponder in the GP shelter),

• DME antenna on the roof of the LLZ shelter (DME transponder in the LLZ shelter),

• DME transponder in a separate shelter with the DME antenna on its roof,

• DME transponder in a separate shelter and DME antenna on a separate mast.

The latter two configurations are preferable, since they permit the runway to be approached from both directions. Running time differences between touchdown and the DME installation are always included, so that touchdown is always at exactly 0 m.

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.