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A localizer is a radio navigation aid used for lateral guidance by aircraft approaching an airport to land.

A localizer (or localiser, if you're British) is a ground-based aid which provides lateral guidance to approaching aircraft.

A localizer consists of an antenna array which transmits in two narrow beams - one slightly to the left of the approach course and one slightly to the right - which use the same radio frequency, but pulse at different rates. An aircraft's localizer receiver listens to the signal from the two beams; if the overall signal pulses at the rate of the left-of-course beam, the aircraft is too far to the left (and the pilot's tell them to fly further right), and vice versa.

A localizer usually forms one part of an ; however, it can usually also be used on its own, without the glideslope, to provide guidance for a (this is known as a localizer approach or localizer-only approach, abbreviated LOC). There are two main types of localizer approach:

  • A front-course localizer approach is basically an ILS approach minus the glideslope; it uses the signal emitted from (surprise surprise) the front side of the localizer array (the side that it's designed to provide the best guidance on). Newer localizer antennae emit essentially their entire signal from the front of the array, making a front-course approach the only type possible with these localizers.
  • Older localizers, however, can sometimes be used for a back-course (or, in Europe, back beam) localizer approach. As you might have guessed from the name, this takes advantage of the fact that older designs of localizer arrays emitted significant amounts of signal in non-front directions (known as sidelobes), with a major portion of this being emitted directly backwards. Although less reliable than a front-course approach (due to the weaker signal), a back-course approach - if properly flown - is still quite safe (assuming, of course, that the localizer in question is old enough to support this kind of approach).

The vast majority of localizers are located some distance past the far ends of the runways they serve, and mark out an approach path that leads straight to, and down, the runway. However, for some airports, terrain considerations make a straight-in approach impossible or inadvisable, and the localizer beam is, instead, at a considerable angle to the runway (and may be located a considerable distance therefrom); at a certain point in such an approach, aircraft wishing to land rather than crash must deviate from the localizer path and maneuver visually to the runway. Such a non-aligned localizer is called a localizer-type directional aid (LDA); in a few rare cases, an LDA is paired with a to form an instrument guidance system (IGS), an unusual subtype of ILS.

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