You look out your passenger window, it’s raining out there. The Captain has come over the PA system and gave his usual “prepare for landing” speech. You glance out, not being able to make out more than the wingtip. Then it suddenly dawns on you. How does the pilot know where the runway is? How does he land?
The answer, is an aid called an Instrument Landing System or ILS. The ILS is actually two systems coupled in one- the Glide Slope and the Localizer.
Localizer: Provides the lateral (side to side guidance) to the runway. At the far end of the runway there are a set of antennae. These transmit a signal that falls between the frequency range of 108.10 MHz to 111.95 MHz. These are transmitted at different wavelengths from each other. Special Equipment on the aircraft can decode the distance between each wave and figure out the aircraft’s position in relation to the runway. (Either on course to it’s center line or to it’s left or right) This is displayed to a pilot usually by means of a vertical needle. If the needle sways to the left, than the course is altered to the left until it is intercepted again.
Glideslope: The glideslope functions in a way very similar to the localizer, but it provides vertical guidance down to the runway. It is represented by a horizontal needle. If it drifts down, you intercept it by going down, if up, you hold your altitude until it is re-intercepted.
The aim of the game is to keep both needles crossed against each other. Marker Beacons, positioned on the approach path allow pilots to know how far they are from touchdown. To intercept the ILS, pilots either follow published procedures printed on charts, or are vectored by Air Traffic Controllers.
Traditionally the ILS would only take you to 200ft above the runway. At this point, if you saw the runway you could carry on visually, if not, the pilot would throttle up and go around. Nowadays, full auto land capability exists, and the aircraft cannot only land on it’s own, it can keep itself aligned with the runway during the roll out. This type of ILS is called CAT III and additional training and measures have to be undertaken for an airline to earn it’s CAT III certification.
There have been other systems besides ILS. The Microwave Landing System (MLS) was intended as a replacement for the ILS, but it never really caught on. New developments are allowing for GPS approaches. Since this system is cheaper to operate, and less prone to interference, it is currently being developed and introduced.