Glide Slope Subsystem
The glide slope provides the pilot with vertical guidance. This signal gives the pilot information on the horizontal needle of the CDI to allow the aircraft to descend at the proper angle to the runway touchdown point. The glide slope radiates on a carrier frequency between 329 and 335 MHz and is also modulated with 90 Hz and 150 Hz tones. The glide slope frequencies are paired with the localizer, meaning the pilot has to tune only one receiver control.
The radiation patterns of a typical glide slope system are shown in Figure 4. The null in the sideband-only (SBO) signal produces essentially a straight glide path angle for the aircraft. The patterns are arranged so that 90 Hz modulation predominates above the glide path and the 150 Hz modulation predominates below.
The glide path angle is normally referenced at 3 degrees. If the aircraft is on this three-degree glide path, equal amounts of the 90 Hz and 150 Hz are received and the CDI will be centered. If the aircraft is above the glide path, the 90 Hz modulation exceeds that of the 150 Hz and produces a deflection on the CDI downwards. If the aircraft is below the established glide path, the 150 Hz modulation predominates and produces a similar but opposite deflection. This deflection corresponds to the direction the pilot must fly to intercept the glide path and is proportional to the angular displacement from the glide path angle. As with the localizer, the full scale deflection is 150 microamperes. Typically, the glide slope sensitivity is set so that the full-scale indications occur at approximately 2.3 and 3.7 degrees elevation.
The FAA presently maintains five types of glide slope systems. They are the null-reference, sideband-reference, capture-effect, endfire, and waveguide. The null-reference, sideband-reference, and capture-effect glide slope systems use the terrain in front of the antenna mast to double, effectively, the vertical aperture of the radiating system and produce the path in space. These three systems are typically referred to as image glide slope systems. Where the ground plane in front of the glide slope mast is irregular or absent, endfire or waveguide types are used. These systems are called non-image and do not rely on the terrain to form the path in space.