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).