An instrument landing system localizer, or simply localizer (LOC, or LLZ prior to 2007), is a system of horizontal guidance in the instrument landing system, which is used to guide aircraft along the axis of the runway. Principle of operation In aviation, a localizer is the lateral component of the instrument landing system (ILS) for the runway centerline when combined with the vertical glide slope, not to be confused with a locator, although both are parts of aviation navigation systems. A localizer (like a glideslope) requires both a transmitting airport runway system and receiving cockpit instruments. An older aircraft without an ILS receiver cannot take advantage of any ILS facilities at any runway, and much more importantly, the most modern aircraft have no use of their ILS instruments at runways which lack ILS facilities. In parts of Africa and Asia large airports may lack any kind of transmitting ILS system. Some runways have ILS only in one direction, this can however still be used for the opposite direction (with a lower precision) known as back beam or "Back Course" which is not associated with a glide slope. Two signals are transmitted on one of 40 ILS channels. One is amplitude modulated at 90 Hz, the other at 150 Hz. These are transmitted from co-located phased array antenna elements. Each antenna transmits a narrow beam. In addition, a clearing signal is transmitted at one tenth of the power with a wider beam to prevent receivers from picking up the side lobes of the main beam. The signals' phases at the antenna elements are arranged such that the 150 Hz signal is more prominent (has a greater depth of modulation) at a receiver located to the right of centerline, and the 90 Hz signal is more prominent to the left. The cockpit instrument uses the difference between the modulation strengths of the two received signals to indicate left or right deviation from centerline. Carrier frequency pairings Localizer (LOC) and glide slope (G/S) carrier frequencies are paired so that the navigation radio automatically tunes the G/S frequency which corresponds to the selected LOC frequency. The LOC signal is in the 110 MHz range while the G/S signal is in the 330 MHz range. LOC carrier frequencies range between 108.10 MHz and 111.95 MHz (with the 100 kHz first decimal digit always odd, so 108.10, 108.15, 108.30, etc., are LOC frequencies and are not