Takeoffs are optional, landings are mandatory

The middle marker works on the same principle, but the 75 MHz signal is now saddled with a 1,300 Hz modulation. When the aircraft passes through the cone of the middle marker, the second frequency detector within the Nav receiver is activated and illuminates an amber "M" accompanied by an audible frequency spelling "M" in Morse code.

Airports incorporating an inner marker use the same principle as the others. The 75 MHz carrier is delivered skyward, but this time a 3,000 Hz signal. Now the third frequency detector within the receiver is active and switches on a white light labeled "A" for azimuth as well as giving audible accompaniment with a Morse code "A."

The lateral mode of ILS is called the "localizer." This is also a radio transmitter with a series of loop antennas located 1,000 feet off the nonapproach end of the runway. An operating frequency of 108 to 111.95 MHz will permit the use of a standard navigation receiver as well as the aircraft very high omni directional range (VOR) antenna. Two radio frequency (RF) transmitters will supply a five degree wide beam with 150 Hz modulation to the right of the runway centerline and a 90 Hz modulation to the left.

As long as the aircraft is on the centerline of the runway, both signals are of an equal strength. Should the aircraft start to drift to the right, the 150 Hz signal predominates and is detected by the navigation receiver which queues the pilot. VHF transmission is considered line of sight and may be impeded by such things as taxiing aircraft and snow banks. The range of localizers is limited to a maximum of about 30 miles and is affected by terrain but still has an accuracy within 25 feet at the runway threshold.

Vertical guidance is provided by the glideslope and in principle is similar to the localizer. A glideslope transmitter is located about 1,000 feet down the runway from the landing end and displaced about 600 feet from the centerline. This device includes a UHF transmitter with an operating frequency range of 328.6 to 335.4 MHz, and like the glideslope, two modulated signals are transmitted, one at 90 Hz above the glidepath and another at 150 Hz below the glidepath. As long as the aircraft descends on the 2 1/2 to 3 degree vertically inclined beam, neither the 150 Hz or the 90 Hz predominates.

The range of the glideslope is usually less than the localizer (with 15 miles being the maximum), but still maintains an accuracy of 7 feet at 100 feet. These UHF frequencies are captured by a dedicated glideslope antenna frequently mounted in the nose of the aircraft. Although UHF frequencies are used, many newer navigation receivers can sort out and utilize the glideslope frequencies.

Pilots are also not required to tune a glideslope and a localizer. The FAA has assigned specific glideslope frequencies to specific localizer frequencies. These frequency relationships are published in their entirety in the "Airman's Information Manual" and the selection of a specific localizer commands the auto tuning of the corresponding glideslope channel.

Microwave landing systems (MLS) were developed to overcome some of the weaknesses in the localizer — glideslope systems such as obstruction of the signals by terrain or manmade obstacles. As a joint venture between the Department of Transportation, Department of Defense, and NASA, the MLS was designed to provide improved performance over the present aging ILS and was to have been fully implemented by 1998. One of the key features of MLS is the ability to deviate from the standard, straight-in approach thereby providing a precision approach to airports that are surrounded by mountains or have other obstacles in the flight path.

The MLS presently in use is a time reference scanning beam (TRSB) MLS and operates on a frequency range of 3 to 30 Ghz. By using UHF frequencies, a wider choice of ground locations are available with less concern for signal reflection from buildings or hills. Like ILS, the MLS uses two transmitters; one supplying a vertical beam and the other a lateral or azimuth beam. The precise timing of the beams provides a very accurate position to the onboard MLS receiver which in turn processes the information and determines the aircraft relative position to the airport on an internal grid.