Landing System : Flying Blind

Just another day in Delhi during the cold winter of 2008. But with very low visibility, the conditions were far from normal for most airlines and frequent fliers. Most flights stood cancelled, with nothing to do except wait for the fog to clear.

Does this mean that despite all the technological advances, aircraft have no means to land in poor visibility? Well, they can. Introducing the Instrument Landing System (ILS)—a gadget that flashes up to six different radio beacons or signals from the ground to guide an aircraft safely to terra firma.

Since it primarily uses radio beacons, the ILS falls under the category of radio navigation aids and is the most frequently used radio aid in the world of aviation. Generating information that would enable a successful landing, as though it were a normal day, with good visibility, the ILS, allows a pilot to land without looking out of the flight deck. Flying blind.

Technical Description
The ILS primarily consists of a Localizer (a VHF radio beacon that provides lateral guidance to an aircraft during the landing approach), and a Glide Slope (GS, a UHF radio beacon that provides vertical guidance). These two beacons provide the aircraft with a three-dimensional guidance that safely brings an aircraft back on the ground. In addition, marker beacons may be used, although not necessary. Lighting systems are also a part of the ILS.

Localizer
The Localizer consists of an array of log period antennae that feed off a signal from a signal generator which is enclosed in a temperature controlled room. The Localizer’s carrier frequency varies between 108.10 to 111.95 MHz. Normally, ILS Localizer frequencies are on odd tenths, with 50 KHz spacing between each frequency. The electric field vector is horizontal polarised, as shown by the horizontally placed Log periodic array. Half the array is modulated by a 90 Hz sine wave, and the other half by a 150 Hz wave. When approaching the runway, that half of the array which is seen to the left of the centerline is fed with the 90 Hz modulated carrier. An aircraft tuned to the ILS frequency on approach, on course, will experience a modulation depth of 20 per cent per frequency (90 Hz and 150 Hz). Code identification, which identifies the radio beacon, modulates a 1,020 Hz signal, which in turn 5 per cent modulates the carrier. Voice communication, if resorted to on the Localizer carrier frequency, is 50 per cent modulated.

The log periodic array, along with the double frequency modulation, provides the receiver information regarding the aircraft’s position with respect to the extended runway centerline. As the left half of the array transmits a 90 Hz modulated VHF carrier from directional log periodic antennae, the signal strength from this array decreases as you move laterally to the right, away from the beam centerline. The opposite applies to the 150 Hz modulated right half of the array. These two beams are placed such that if the aircraft drifts off course, it receives more of one beam than the other. This results in the apparent difference in the depth of modulation.

In the on-board receiver, analog circuitry compares the 150Hz component and the 90Hz component. If the aircraft is on course, and the ground based array properly aligned, the 90Hz and 150Hz components that were extracted from the carrier would be of equal voltages. However, if the aircraft drifts to the right of course, the 150Hz beam would be better received as compared to the 90 Hz beam. This mixed signal, which is seen as one by the receiver, will find the 150Hz component greater in magnitude as compared to the 90Hz component.

The difference between the two unequal magnitudes is extracted and beamed on the pilot’s display, clearly indicating by how much his aircraft has drifted off course. The pilot applies the necessary corrections, such that the difference reduces to almost zero. The correction appears on the display, thus assuring the pilot that the aircraft is now on the right course. If the Localizer’s operation was clearly understood, grasping the GS’s concepts is simple. The GS works on the same principle as the Localizer: the use of the 90Hz and 150Hz modulating signal.

However, the GS provides vertical guidance using these, as opposed to lateral guidance provided by the Localizer. The GS uses frequencies between 329.30 MHz to 335.00 MHz (UHF), with 50 kHz spacing between each channel. The transmitter is located on either side of the runway, about 1,000 ft down the runway from the runway threshold.

The Glide Slope may employ a firing angle of 2° to 4°, the angle being decided upon after assessing the obstacles around the airport. If tall buildings are found along the extended centerline, a high firing angle may be adopted. Normally, the angle is 3°. The upper beam of the GS is modulated with a 90Hz sine wave, and the lower beam with a 150Hz sine wave. An imaginary 3° (if 3° is adopted) line marks the glide path, and an aircraft flying below the GP experiences more 150Hz than 90Hz on the UHF carrier, and hence the instrument shows the aircraft below the GP.

On the other hand, an aircraft above the GP will receive more 90Hz than 150Hz, and display to the pilot the amount by which the aircraft is above the GP. As in the case of the Localizer, the ratio of the 90Hz and 150Hz signals is calculated, and the information sent to the displays in the cockpit.

Signal from the signal generator is fed to a stack of dipoles with a ground reflector. No log periodic antennae are used here. The UHF frequencies and VHF frequencies are paired. 40 ILS channels exist, and each channel has a fixed UHF and VHF frequency. When an ILS receiver is tuned to a particular VHF frequency, the corresponding UHF frequency is tuned into by the UHF module of the receiver. The system also tunes into a fixed microwave frequency associated per channel. This is for distance finding, by the use of Distance Measuring Equipment (DME).

Thus, when an ILS frequency is tuned into, three receivers are being tuned. One for the VHF Localizer, another for the UHF Glide Slope, and the third for the microwave Distance Measuring Equipment.

Marker Beacons
Marker beacons are not absolutely necessary for an ILS. For example, the HAL airport at Bangalore (VOBG) has an ILS without markers. Marker beacons operate at 75 MHz, and are of very low power of 3W or less. A vertically placed log periodic or yagi antenna fires the modulated 75 MHz wave up into the air. A complete ILS will have three marker beacons:
Outer Marker: The OM installed along the extended centerline of the runway. When an aircraft is over the beacon, it receives a 400Hz Morse code modulated on the 75 MHz carrier. The Morse code appears as two dashes per second. On reception of the OM, the pilot knows that the aircraft is about 6NM from the runway threshold.
Middle Marker: The MM, too, is installed along the extended centerline. A series of dot dash dot dash at 1,300Hz modulates the 75 MHz carrier. On reception of the OM, the pilot knows that the aircraft is about 0.8NM from the runway threshold.
Inner Marker: The IM is located along the extended centerline of the runway. A series of dots at 3,000Hz modulates the 75 MHz carrier. The aircraft receives this when on the runway threshold.

Distance Measuring Equipment
The DME has been around for quite some time. A DME unit is located near the glide slope antenna (usually collocated with it). DME operates in the 1025 MHz to 1150 MHz band. When a ground based DME unit receives a pulse train from an aircraft, it processes the signal by down frequency translating it by 63 MHz. After inserting a delay of 50us (microseconds), this processed pulse train is now transmitted.

When the aircraft receives the pulse train, it first stops a timer and then checks to see if the pulse train was what it had sent earlier. If the transmitted and received pulses match, it uses the timer’s count value (which was started when the pulse train was sent from the aircraft) to determine the distance of the aircraft from the DME equipment on ground using:

D = ½ (C X (T-50us))
T = Time taken between sending and receiving the pulse train (minus 50us due to DME internal delay)
C = Speed of light in air.
D = Distance in metres.

DMEs operate at 100W or 1kW. They are capable of servicing a maximum of 200 aircraft at a time.

A DME provides the direct or slant distance of the aircraft form the DME unit. Although the difference between the slant distance and actual ground distance is small at large distances, it grows larger as the aircraft approaches the DME unit. The DME uses a eight or 10 element collinear dipole phased array. The DME unit on board the aircraft is automatically tuned to when the VHF Localizer frequency is set, by making use if the VHF-UHF-Microwave pairing per channel for the Localizer-Glide Path-DME.

The advantage of a DME is that the pilot is continuously fed with range information: the distance of his aircraft from the runway. Even on days when the visibility is bad, the pilot can visualize the distance to runway and take necessary actions.

Compass Locator
Another radio navigation tool, a Non Directional Beacon (NDB), when used exclusively for an ILS approach, is known as a compass locator. This is nothing more than a Medium Wave (MW) or Low Frequency (LF) radio transmitter, much like a commercial AM radio station, with the exception that no music is transmitted. Compass locators differ from NDBs only in their transmission. An NDB transmits a three or two letter identification Morse code.

Compass locators may not identify themselves. On board receivers determine the direction of the transmitter relative to the aircraft, and display the beacon’s relative heading to the pilot. This enables the aircraft to be flown towards the beacon, serving as a waypoint from where the pilot may begin his ILS approach. Radio beacons are subject to disturbances that may result in erroneous bearing information. Such disturbances result from such factors as lightning, precipitation static and so on.

At night, radio beacons are vulnerable to interference from distant stations. Nearly all disturbances which affect the Automatic Direction Finder (ADF) bearing also affect the facility’s identification. Noisy identification usually occurs when the ADF needle is erratic. Voice, music or erroneous identification may be heard when a steady false bearing is being displayed. Since ADF receivers do not have a flag to warn the pilot when erroneous bearing information is being displayed, the pilot should continuously monitor the NDB’s identification.