INSTRUMENT LANDING SYSTEM
The instrument landing system (ILS) was developed as a result of the need for a method whereby a pilot flying an airplane could locate and fly to an airport even when visibility was very poor. The systems now being operated by the FAA are the result of development work that has been conducted since 1928. The present system, except for the glideslope, was demonstrated in a commercial form in Indianapolis in 1939 and 1940. Commercial use of the ILS was greatly delayed by the war, however, because of manpower shortages, it was not possible to obtain the necessary equipment.
The ILS provides a horizontal directional reference and a vertical reference called the glideslope. The directional reference signal is produced by the runway localizer transmitter installed approximately 1000 ft [305 m] from the end of the runway and operating at frequencies of 108 to 112 MHz. The glideslope signal is produced by the glideslope transmitter, which is located near the side of the runway on a line perpendicular to the runway centerline at the point where airplane touchdown occurs. This point is generally about 15 percent of the runway length from the approach end of the runway. The glideslope transmitter operates on a frequency of 328.6 to 335.4 MHz.
The localizer consists, essentially, of two RF transmitters and an eight-loop antenna array. The transmitters broadcast a complete system of radiation patterns which produce a null signal along the center of the runway. The radiation pattern is such that when an airplane is approaching g the runway for a landing, the signal to the right of the localizer path will be modulated with 150 Hz and the signal to the left of the localizer path will be modulated with 90 Hz. The localizer receiver on board an airplane is able to discriminate between the 90- and 150-Hz signals. The output of the receiver is fed to the vertical needle of a course deviation indicator (CDI) or to another type of instrument such as a flight-director indicator. If the airplane is to the right of the localizer centerline, the 150-Hz modulation signal will predominate, and the vertical needle of the indicator will point to the left of the centerline, indicating that the pilot should fly left in order to return to the centerline of the localizer beam.
The glideslope transmitter operates on a principle similar to that of the localizer. As previously mentioned, the glideslope transmitter is located at a distance from the approach end of the runway approximately 15 percent of the length of the runway. If An airplane is approaching the runway and is above the glide path, the 90-Hz signal will predominate; and if the airplane is below the glide path, the 150-Hz signal will predominate. The glideslope receiver will provide an output to the crosspointer indicator in such a way that the pilot will have a visual indication of the airplane position with respect to the glide path. If the horizontal pointer is above the center of the indicator, the
airplane is below the glide path.
In order to provide pilots with an indication of their distance from the runway, marker-beacon transmitters are
installed with the outer-marker transmitter at approximately 8 km from the runway and the midmarker approximately 1 km from the end of the runway.
The marker-beacon transmitter operates at a frcquency of 75 MHz and produces both aural and visual signals.
The outer-marker transmitter produces a 400-Hz intermittent signal which causes a blue indicator light on the instrument panel to glow intermittently. The midmarker transmitter produces a signal modulated at 1300 Hz which causes the amber marker-beacon light on the instrument panel to glow. Thus, when an airplane is approaching the runway and is approximately 8 km from its end, the blue light will flash. A short time later, when the airplane is within 1 km of the runway, the amber light will flash. This system provides an excellent indication to the
pilot of the plane’s distance from the runway.
THE MARKER-BEACON RECEIVER
The marker-beacon receiver for a typical large-aircraft navigation system is a fixed-frequency superheterodyne designed to operate only on a frequency of 75 MHz. The receiver is equipped with output circuits that enable it to deliver both aural and visual signals to the flight crew.
A portion of the output signal is fed through a transformer to audio filters tuned to 400, 1300, and 3000 Hz. The 75
MHz-signal from the marker-beacon transmitters is modulated with the three different audio tones, depending
upon whether the transmitter is a midmarker, an outer marker, or an airways Z, or fan, marker. Each of the
audio filters are designed to select one of the frequencies and with this signal activate a switching circuit that causes the appropriate signal light on the instrument panel to flash. The indicator lights are white, amber, and blue, thus making it possible for the pilot to know what type of marker the airplane is passing over. For example, a flashing blue light indicates that the pilot is passing over the outer marker.