The modulation type of all ILS transmitters is the good old amplitude modulation (AM). The carrier oscillation in the localizer frequency range is 108.00 MHz to 111.975 MHz modulated with a 90Hz and a 150Hz tone signal.

The ILS works by sending 2 beams up from the landing runway, one telling the pilots if they or high or low and the other telling them if they are left or right of the runway centreline. The ILS receiver on the aircraft measures the difference in depth of modulation (DDPM) between the signals For most ILS’s the pilots should be lined up with the runway centreline and on a 3 degree glide path, but on some ILS’s, like London City have a steeper approach of 5.5 degrees.

The vertical beams of modulated signals are produced from a horizontally polarized antenna complex beyond the far end of the approach runway. They create an expanding field that is 21⁄2° wide (about 1,500 feet) 5 miles from the runway. The field tapers to runway width near the landing threshold. The left side of the approach area is filled with a VHF carrier wave modulated with a 90 Hz signal. The right side of the approach contains a 150 MHz modulated signal. The aircraft’s VOR receiver is tuned to the localizer VHF frequency ( is the same for both beams, only the modulation of te carrier wave changes !) that can be found on published approach plates and aeronautical charts. The circuitry specific to standard VOR reception is inactive while the receiver uses localizer circuitry and components common to both. The signals received are passed through filters and rectified into DC to drive the course deviation indicator. If the aircraft receives a 150 Hz signal, the CDI of the VOR/ILS display deflects to the left. This indicates that the runway is to the left. The pilot must correct course with a turn to the left. This centers course deviation indicator on the display and centers the aircraft with the centerline of the runway. If the 90 Hz signal is received by the VOR receiver, the CDI deflects to the right. The pilot must turn toward the right to center the CDI and the aircraft with the runway center line. When the aircraft is on the exact centerline during the final approach stage the difference in deph modulation is 0 (annex1)

Like the localizer, the glideslope transmits two signals, one modulated at 90 Hz and the other modulated at 150 Hz. The aircraft’s glideslope receiver deciphers the signals similar to the method of the localizer receiver. It drives a vertical course deviation indicator known as the glideslope indicator. The glideslope indicator operates identically to the localizer CDI only 90° to it. The VOR/ILS localizer CDI and the glideslope are displayed together on whichever kind of instrumentation is in the aircraft (annex2)