The Instrument Landing System (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 and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument meteorological conditions, such as low ceilings or reduced visibility due to fog, rain, or blowing snow.
A localizer is one of the instrument landing system. A localizer (LOC, or LLZ in europe) antenna array is normally located beyond the departure end of the runway and generally consists of several pairs of directional antennas. Two signals are transmitted on one out of 40 ILS channels between the carrier frequency range 108.10 MHz and 111.95 MHz (but only the odd kHz, so 108.10 108.15 108.30 and so on are LaC frequencies but 108.20 108.25 108.40 and so on are not). One is modulated at 90 Hz, the other at 150 Hz and these are transmitted from separate but co-located antennas. Each antenna transmits a narrow beam, one slightly to the left of the runway centerline, the other to the right.
The localizer receiver on the aircraft measures the difference in the depth of modulation (DDM) of the 90 Hz and 150 Hz signals. For the localizer, the depth of modulation for each of the modulating frequencies is 20 percent. The difference between the two signals varies depending on the position of the approaching aircraft from the centerline.
A localizer antenna element has a relationship with obstacles. As a result, when we analyzed the ground check system data what using each of localizer 20 element log periodic antenna and localizer 14 Element Log Periodic Antenna, we confirmed that 20 Element Log Periodic Antenna was little influence by Terrain obstacles compared with 14 Element Log Periodic Antenna.