What is ILS
 

I hear it on United's channel 9 and see it on runways. What is it and what does it do?

ILS = Instrument Landing System. If you are interested in technical details, check out this site: http://www.allstar.fiu.edu/aero/ILS.htm

ILS stands for Instrument Landing System.

It is used when landing in weather which obscures the view of the runway.

Radio transmitters near the runway broadcast two signals: the localizer which gives the pilot information on whether they are left or right of the runway centerline, and the glideslope, which tells the pilot if they are above or below the optimum slanted approach path to the runway. In addition, there are marker beacons (outer, middle, inner) which give the pilot some indication of how far away from the runway they are.

It's much more complex than this, but you get the general idea...

Cheers,

birdstrike

ILS is the acronym for "Instrument Landing System". It is a system that transmits radio waves which are by gauges in the cockpit of a plane and helps the pilot and copilot maintain the proper glidepath for landing.

Just for the sake of clarity, the actual use of the ILS is not dictated by weather, and associated restriction of visibility. It can be, and regularly is, used in VMC (visual meterological conditions). Dozens of landings are made daily at airports with splendid weather and visibilty utilizing the ILS; Phoenix and Honolulu come to mind.