Range information is provided by marker beacons located along the route. These beacons broadcast a signal straight up over a narrow area. When you receive a marker beacon signal, it is an indication you are over the beacon. An outer marker is provided near the beginning of the ILS approach and a middle marker is provided near the end of the approach. Beacons are received by a marker beacon receiver, which does not need to be tuned or identified.
When you fly over a beacon, the beacon receiver will flash a colored, labeled light and sound a tone indicating the type of beacon being overflown. To properly receive an ILS marker beacon, the receiver should be operated in low sensitivity mode. The glide slope is normally set at an angle that intercepts the middle marker at about 200 feet and the outer marker at about 1,400 feet above runway elevation.
DME may be substituted for marker beacons when installed with the ILS and specified in the approach procedure. A compass locator may also be used for a marker beacon. A compass locator is simply an NDB used for this purpose. While usually lower power than normal NDB, certain compass locators broadcast over a larger area. Finally, some ILS approaches substitute air traffic control's precision radar for marker beacons.
Visual information is provided by approach lights, touchdown zone lighting, runway centerline lights, runway lights, and precision runway markings. The intent is for the ILS to get you down to a point where you can pick up these visual cues and land.
Approach minimums for an ILS approach are normally 200 and ½, referring to a 200 foot ceiling and ½ statute mile visibility. This describes a typical Category I ILS approach. With special equipment and training, pilots can become qualified to conduct Category II or Category III ILS approaches. Category II approaches typically have minimums of 100 and ¼, while Cat III minimums go even lower. At airports with Cat II or Cat III approaches, an additional inner marker is placed on the ILS approach. Your marker beacon radio will pick up this inner marker when you are about 100 feet above the runway's elevation.
At many airports, an ILS approach is provided at each end of a runway. For example, the same airport may have an ILS approach to runway 9 and another ILS approach to runway 27. Where this situation occurs, these systems are not in service simultaneously. If a tower or approach control closes, they will leave the ILS setup for whatever they anticipate being needed during the off hours. This might result in a situation say, where you need the ILS for 27, but the ILS for 9 is being broadcast. It is situations like these that carefully tuning and indentifying your navaids is so important.
The profile view of an ILS approach chart depicts the minimum glide slope interception altitude with a small lightning bolt looking symbol, as well as the on glide slope altitude at the outer marker. The glide slope angle and threshold crossing height are also given. The TCH shown does not represent the actual on glide path indication over the runway threshold. If you are right on the glide slope and descending at the correct rate of descent to maintain centered on the glide slope, you will cross the threshold at the TCH if you maintain that same path perfectly inbound from the middle marker. Inside the middle marker, some glide slopes will take you low, while others will fly you half way down the runway. When you make the decision to continue below minimums, your primary references change from instrument to visual references.
The pilot must maintain the correct rate of descent to maintain the glide slope on an ILS approach. As groundspeed increases, the rate of descent required goes up. At slower ground speeds, a lower rate of descent is required.
The final approach fix for a precision approach is indicated by the lightning bolt symbol on the profile view. If ATC directs a glide slope intercept altitude which is lower than published, the actual point of glide slope intercept becomes the FAF.