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Aim

There have been many developments in the field of aviation ever since the introduction of aircrafts. Different techniques and aids are constantly being improved for the execution of safe and smooth landings so as to complete this trickiest phase of a flight with less difficulty and more perfection. The aim of this paper is to look into the characteristic features of various prominent precision approaches such as, Instrument Landing System (ILS), Microwave Landing System (MLS), Wide Area Augmentation System (WAAS) and Local Area Augmentation System (LAAS); their components, the principle of operation; their limitations and probability of improvements in future. It also focuses on the comprehensive study of the future precision approach system called Smart Path, a ground based augmentation system, developed by Honeywell, and selected by FAA as an upcoming replacement for ILS in its Next Gen program. This paper also evaluates the importance of these systems in providing safety and reducing operational expenditures of airlines. Hence, the aim of this paper is to analyze the adequacy and accuracy of the landing aids and their implementation with respect to their requirements and practicability in aviation at present and in the years to come.

Executive Summary

Landing is the most crucial and demanding phase of a flight. For a smooth and safe landing of an aircraft it is necessary that the pilot is assisted with some landing aids. There are several technological navigation aids that help in the execution of a precision approach and landing.

The ILS is a combination of both approach lighting and radio beacons including their best qualities within one system. Though it is an important landing aid providing the lateral and vertical guidance necessary for a precision approach but in view of the future aviation requirements, it has certain limitations which prohibit or restrict its use in many circumstances.

MLS offers many advantages over ILS besides overcoming its confines. Accurate precision navigation guidance is provided by this system relative to the alignment and landing of aircraft. It is appropriate for the autoland approaches where the aircraft is controlled by the equipment as it allows safer landings in extreme weather, with zero visibility.

Further, two augmentations systems are developed, known as LAAS and WAAS, to assist the GPS in the civil aviation with space based navigation. Global, standardized navigation services for all types of aircraft operations and all phases of flight can be made available with the use of differentially corrected GPS service, with the support of WAAS and the LAAS.

The requirement of GPS to get some assistance from the ground for rapid descending of an aircraft quite near to the ground paved the way for a ground based augmentation system (GBAS), which is also called Smart Path and is an effective system to smarten up air-traffic control. The replacement of ILS with GBAS technology has been recognized crucial for the enhancement of the air traffic capacity in the FAAs Next Gen program and Euro controls Single European Sky ATM Research Program. GBAS avionics have become an integral part of most current-production Airbus and Boeing aircraft. GBAS, described as SmartPath by Honeywell, is the initial stage of an infinite plan to refurbish air-traffic control.

The limitations of older technologies in ensuring safe landings where only short-length airstrips are available due to deserts or vegetated areas and in rough seas, led to the development of autoland system. Different climate and weather conditions affect the approach and landing phase of an aircraft considerably. Autoland systems are used to ensure a safe landing in extreme weather conditions and intense levels of visibility.

Advancements in avionics such as the SmartPath, introduced by Honeywell and the autoland system are not only cost effective but also suggest an increasing role for flight inspections chances, in the coming years, with respect to the navigation and landing aids.

Development in Aircraft Landing Technology

Introduction

Landings are the most crucial phase of a flight and carry significant safety messages with their execution. Missing of any essential correction and misjudgments at any point may result into an immense calamity. Most of the accidents are caused by inaccurate procedures in the landing phase (Horne 2011).

The complexities involved in the process generate chances of accidents taking place. So, in order to return the aircraft back to the earth safely and firmly, a skillful execution of the landing phase is highly required. It is essential to have an ideal knowledge of the various aspects such as: what should be the speed, knowledge regarding the rate of descent and glide path along with an absolute judgment of glide path (Nirjharini 2011).

To achieve a smooth and secure landing a number of technological navigation aids can be used by the pilot. Initially, there were only people-powered landing aids where people used to stand on the field with flags to provide direction to the aircraft. They used to wave red, green, or white cloths symbolizing different signals (Mola n.d.).

Gradually, the radio controllers were developed to provide some landing aid to the pilots. Moreover, red or green lights were also utilized to present the view of the runway threshold and sides in the airport. This helped the pilot to judge the remaining distance and angle to be taken for landing (Mola n.d.).

History

Development of aircrafts: Before the existence of airports and introduction of any landing aids, the pilots used to be guided by the direction of the wind, in landing the aircrafts. With the constant growth of air traffic, the airports were developed and landing was restricted to definite directions. The complexities faced during the landing of an aircraft were resolved to some extent with the development of the landing aids that enabled pilots to stick to a correct landing course, thus making it safer (Mola n.d.).

Use of lights: Airports began the use of lights in the late 1920s.Landing fields were marked with rotating lights that offered guidance to the pilots in the dark. The use of approach lighting in the airports in the early 1930s helped the pilots by guiding them to take proper descent and ensured that they were on the right direction. The approach path was called the glide path or glideslope (Mola n.d.). In course of time, the International Civil Aviation Organization (ICAO) provided a standardized form of the colors of the lights and their rates of flash to be recognized in all over the world (Molan.d.).

Introduction to slop-line approach: The slope-line approach was the first system implemented as landing aids that was a cone shaped formation of rows of lights (funnel) providing guidance to the pilot to reach the finishing point of the runway. In case of some deviation to the right or left, or too high or low other patterns of lights were exhibited. Although this system, developed in 1940s was economical to build and operate but some constraints restricted it from being used in certain airports (Molan.d.).

Introduction of radio navigation aids: Next, radio navigation aids were developed for getting assistance in landing. In 1929, the establishment of a four-course radio-range with the power of Morse code signals guided the pilot in taking right decisions in maneuver. Another types included a low-frequency radio beam, which flickered outward from the landing point forming a v shape. Although, the wide space offered for flying towards the end of the runway offered convenience to the pilot but the narrow space at the landing point often misled the pilot in striking the center point for landing. The method of tuning into a certain frequency at a distant checkpoint, and using a stop watch to come down at an exact rate to the touchdown area also proved to be complicated (Molan.d.).

Development of Instrument Landing System: The instrument Landing System was the next step in the development of landing aids. Tests of this system began in 1929, and it got approval for installation in 1941 at six locations by the Civil Aeronautics Administration (CAA).In the Instrument Landing System the approach lights and radio beacons both were included to build a system combining the best features of both. The pilot could view the glide path personally with the help of the electronic picture of glideslope in the cockpit instruments. The ILS system was tested for the first time on January 26, 1938, when a scheduled U.S. passenger airliner Pennsylvania-Central Airlines Boeing 247-D, from Washington, D.C., to Pittsburgh, using ILS landed in a snowstorm (Molan.d.).

Adoption of the standardized ILS: Several types of ILS system were tried before standardizing it. At last, a system comprising a localizer for the lateral guidance, a glide path or landing beam to provide vertical guidance, and two marker beacons denoting the progress of approach to the landing field, was adopted as a standardized form of ILS. The pilot was enabled to receive the information with the help of the equipment in the airplane so that he could see the runway and bring the aircraft on the right flight path (Mola n.d.).

In 1949, the International Civil Aviation Organization (ICAO) adopted the standard set by the U.S. Army, by introducing an ILS with a higher frequency transmitter to reduce static and create straighter courses, for all member countries. It was called the Army Air Forces Instrument Approach System Signal Set 51. The first ILS equipment, without any visual aids, achieved a safe landing in 1960s (Molan.d.).

During World War II, the development of radar showed the way to the introduction of a new precision-beam landing aid which is known as the ground control approach (GCA). GCA performed well in association with the ILS in making the landing easier for the pilots at more active airports. By 1948, distance measuring equipment (DME) for measuring the distance of the airplane from the ground was set up. The data acquisition relating to the planes distance from the ground was made available with the help of distance measuring equipment (DME). Along with the air-route supervision type of radar other radar were also installed at a number of airports in the mid-1950s helping air traffic controllers in dealing with air-traffic efficiently (Molan.d.).

Instrument Landing System:

ILS description: The ILS is an important landing aid providing the lateral and vertical guidance necessary for a precision approach. A precision approach suggests an approved procedure of descending an aircraft with the help of a navigation facility associated with a runway. An integrated ILS system along with the approved approach procedure leads to the proper execution of a precision approach (Carnegie 2008).

ICAO Categorization with respect to RVR: The current ILS was developed and standardized in the United States during the World War II. Three categories of visibility are defined by ICAO relative to the runway visual range (RVR) (Powell 1981).

This categorization is based on the principle that there should be adequate short visual reference for the range of permitted decision heights. These categories are defined graphically in the following illustration (Pallet & Coyle 1993).

Category 1: Category 1 illustrates that there should be minimum 200ft decision height and runway visual range of 800m for the possibility of successful approach.

Category2: A high possibility of success rate is described below 200ft decision height and runway visual range from 800m, and as low as 100ft decision height and 400m runway visual range (Pallet & Coyle 1993).

Category3: Category 3 illustrates the visual range of minima 200m, with external visual reference during the final phase of the landing down to the runway.

Category3B: The visibility to and along the runways and taxiways is to be sufficient for visual taxiing equivalent to runway visual range of minima 50m (Pallet & Coyle 1993).

Category3C: No external visual reference is required to and along the surface of the runway and taxiways (Pallet & Coyle 1993).

Main Components of the ILS: The total system is divided into three parts consisting of a transmitter on the ground and receiver and signals in the aircraft. A localizer is provided for the lateral steering for front-course and back-course. Vertical navigation, for the front course, is provided by the glide slope. For getting distance checks, marker beacons are used (Powell 1981).

The above illustration is depicting the vertical needle of the localizer and the horizontal needle of the glide slope. The dots are representing center of the ILS or distance from center of ILS. The round button is used for the airport selection. The signs provided by marker beacons are shown below. Important runway information is put on view in floating text beside instrument (040_ILS Instrument Landing System n.d, Para 4).

Instrument landing system (ILS) provides accurate and reliable navigation facility to an aircraft in IFR conditions. In this system the position of the aircraft is ensured with reference to the instruments. ILS comprises four main instruments.

The above picture illustrates the sequence of 18 important tasks performed by the pilot in changing over from cruise flight to touchdown with the use of ILS (Niquette 1996).

Localizer

Ground Equipment: The localizer is the main component of the ILS. It makes the lateral guidance available. It is a VHF radio transmitter and antenna system with similar general range as VOR transmitters (between 108.10 MHz and 111.95 MHz).The frequencies are only on odd-tenths, with 50 kHz spacing between each frequency (Carnegie 2008, Para B.1).The transmitter and antenna are located on the centerline on the other end of the runway from the approach entrance. Some ILS systems also include the localizer back course which is normally available with a75MHz back marker facility or NDB positioned 3to 5 NM from touchdown. To ensure its positioning with specified tolerances the course is checked time to time (Carnegie 2008).

Signal Transmission: The localizer spreads out the signal in the air. These signals generate two vertical overlapping patterns identical to the shape of a fan. These patterns are aligned absolutely with the centerline of the runway. The right side of this pattern is symbolized by blue and is called the blue area. It is modulated at 150 Hz. The left side of the pattern which is symbolized by yellow is called the yellow area. It is modulated at 90 Hz. The overlap existing in the middle of the two areas is helpful in providing the signal relating to the runway. The navigational beam is regulated so as to enable about 700 ft wide track signal acquisition at the runway threshold. The localizer is recognized by a two-letter audio signal (preceded by the letter I for example I-OW) overlaying the navigational signal (Carnegie 2008).

Since the primary strength of the signal is aligned with the runway centerline, the reception range of the localizer is at least 18 NM within 10º degrees and at least 10NM between l0º to 35º of the on-track signal (Carnegie 2008).

Localizer Receiver: There is a localizer receiver in the aircraft which functions in amalgamation with the VOR receiver in a single unit to receive signals in the aircraft. The localizer and the VOR use common electronic circuits. There is a common frequency selector, a volume control, and ON-OFF control for the two receivers (Carnegie 2008, Para 5).

From left to right, the aircraft is 1° Right of course, two dots (turn left to return); On course; and 1° Left of course (turn right to return)(Wood 2008, Para 10).

On receiving a localizer signal, the vertical needle or the track bar(TB) is set in motion. Suppose, a final approach track is aligned north and south, an aircraft taking position in the east of the extensive centerline of the runway (position 1) is in the area modulated at 150 Hz, will avert the TB to the left. On the other hand, if the aircraft is taking position to the west of the runway centerline, the 90 Hz signal the TB will deviate to the right (position 2).Force is applied to the needle by both the angles in the overlap area. This force redirects the needle in the direction of the strongest signal which helps in placing the aircraft in accurate position. The point showing equivalent intensity of both the 90 Hz and 150 Hz signals, TB being centered, indicates the precision approach (Carnegie 2008, Para 10).

However, it is advisable that the pilot should set the track selector most of the times for the approach track. This is because the track selector setting in the localizer function is not dependable in most cases. The obstructions in between the transmitting antenna and the aircraft often cause difficulty in receiving the signals clearly. The weak signals make the vertical needle to deflect a little or indicate an OFF flag in front of it.

Glide Slope Equipment

Transmitter: The vertical guidance for the landing approach is provided by the glide slope which is formed with the help of a ground-based UHF radio transmitter and antenna system (Carnegie 2008). These channels in the u.h.f. band function at a range of328.6-335.4 MHz at 150kHz spacing. The frequencies, allocated to the glide slope, are paired with the localizer frequency as both serve the same runway (Powell 1981, p.70-71). The positioning of the glide slope transmitter is about 750 to 1,250 feet down the runway from the entrance, offset 400 to 600 ft from the centerline of the runway (Carnegie 2008).

The glide slope signal also consists of two beams where one beam lie on top of the other. These beams are adjusted at 90 Hz and 150 Hz. The signals spread out by the glide slope are aligned above each other and emitted mainly down the approach track. The thickness of the overlap area is 1.4º or.7º above and.7º below the optimum glide slope.(Carnegie 2008).

The adjustment of this glide slope signal varies between 2º and 4.5º above a horizontal plane. In view of obstructions along the approach path and the runway slope, it is adjusted between 2.5º to 3º.Several glide path angles along the glide slope may produce false signals. The false signal approximately 6º degrees above horizontal will be a mutual signal for the fly up and fly down commands and these commands will be reversed. The false signal at 9º will be sloping just as the true glide slope. However, there are no false signals generated below the genuine slope (Carnegie 2008, Para 11).

Signal Receiver: A UHF receiver is made available in the aircraft to receive the glide slope signals in combination with the VOR controls to receive the proper glide slope frequency by tuning it automatically when the localizer frequency is selected (Carnegie 2008).

A glide slope needle, positioned in conjunction with the TB, is set in motion by the glide slope signal. There is a special OFF flag for the glide slope needle which is visible in the navigation indicator in case of a weak signal. There is a constant deviation in the glide slope needle and it stops deflecting only after reaching that point where the signals overlie on one another. The needle deviates towards the stronger signal and slowly comes to the centered position horizontally when both the signals are equal indicating that the aircraft is proceeding in the right direction on the glide path (Carnegie 2008).

An accurate location relative to the approach path can be accessed by referring to a single instrument. The Glide Slope Signal Pattern figure, above, position1, illustrates that the aircraft is located in the center of the approach path as both the needles are in the centered position. In Position 2 it is evident that the aircraft needs to fly down and towards the left to rectify the approach path whereas position3 is shows the requirements to fly up and towards right for catching the proper path(Carnegie 2008, Para 12).

With the aircraft coming closer, the instrument picks up its operation accordingly. It is important that at this point of time; the pilot must monitor the deflection of the needle to know whether the aircraft is high, low or in a centered position (Carnegie 2008).

ILS Marker Beacon: The marker beacon is used for the modulation of signals. It radiates upwardly using the frequency of 75MHz (Powell 1981).These are low-power transmitters; that operate with 3 W or less rated power output. There are some preset points along the approach track in Instrument Landing System which are recognized by the marker beacons to provide information regarding distance from the runway. They radiate an oval beam upward from the ground at an altitude of 1,000 ft. which is 2,400 ft long and 4,200 ft wide and considerably increases with higher altitudes (Carnegie 2008).

Descriptions of the ILS components

Here is a three-dimensional depiction of the Instrument Landing system. We can see the Localizer antennas close to the end of runway (Wood 2008, Para 4).

Outer Marker (OM): Within 250 ft of the extended runway centerline, the outer marker is located 3 1/2 to 6 NM from the entry. At around 1,400 ft above runway elevation a point is marked by OM crossing the glide slope vertically. It also marks the estimated point from where an aircraft starts the final approach plan. The signal is adjusted at 400 Hz, which is a low tone but easy to hear with continuous Morse code dashes. For receiving these signals, a 75 MHz marker beacon receiver is incorporated in the aircraft. The pilot can hear a tone over the speaker or headset in harmonization with flashing of a blue light. Geographical conditions may affect the positioning of the outer marker. So, a DME unit may be used to enable the pilot in making a positive position fix on the localizer. An NDB is used in place of the OM in the majority of ILS installations (Carnegie 2008, Para 14).

Middle Marker (MM): The middle marker is located about.5 to.8 NM starting from the threshold and going along the complete runway centerline. It intersects the glide slope at about 200 to 250 ft above the runway elevation and is close to the missed approach point for the ILS Category l approach (Carnegie 2008).

Back Marker (BM): About four to six miles from the runway threshold, usually, the back course marker (BM) is located. These are located on the localizer back course. The BM low pitched tone (400 Hz) is heard as a series of dots(Carnegie 2008,para4).

The aircrafts white marker beacon light is enlightened by these dots. The BM can be replaced by an NDB or DME fix quite often (Carnegie 2008).

Lighting Systems

There are many approved runway environment lighting systems in the ILS system. These lighting systems ensure the safe landing of the aircraft by providing proper visual guidance to the pilot. These are: approach light system (ALS), sequenced flashing light (SFL), touchdown zone lights (TDZ) and centerline lights (CLL-required for Category II [Cat II] operations.)(Carnegie 2008,para 4).Out of these any one or all the lighting systems may be provided at a specified facility (Carnegie 2008).

Runway Visibility Measurement: Before coming downwards to the decision height (DH) or the missed approach point (MAP) it is essential that the pilot has a clear view of the approach path with the help of the appropriate visual aids. The transmissometer is located adjoining the runway to provide visibility information. The photo electric cell receiver separates the light source by 500 to 700ft.The relative transparency or mistiness of the air is measured by the receiver, connected to the instrument readout in the airport tower. The readout is standardized in feet in respect of the visibility and is known as the runway visual range (RVR) (Carnegie 2008).

Runway Visual Range (RVR)

At the runway the maximum distance utilized for taking-off or landing is known as RVR. It is specified with light or markers so that it can be seen from a height matching to the pilots average eye-level at touchdown. It is normally expressed in hundreds of feet e.g. RVR24signifies that the visual range along the runway is 2,400 ft. Visibility differs in different runways so the RVR value differs accordingly. Even different points along the same runway may have varying visibility. To deal with this condition, there are many types of equipment fixed at the departure end and the central point of the runway. Runway Visual Range reports denote the capability of a pilot to have the visibility of the runway in the touchdown zone. However it is to be noted that the positioning of the transmissometer establishes the actual visibility at other points down the runway. RVR enables the pilots to decide their executions in the ultimate stages of an instrument approach. The information regarding the Runway visual range is sent to the ATC arrival control, and the control tower or FSS. The pilots are offered this information as per their requirement. The light settings can be adjusted on demand by the pilot. Two different light settings can be provided for the landing of consecutive flights. Since there are complex equipment involved, RVR is usually made available at busier airports only(Carnegie 2008).

NDBs

Supplementary aids like NDB also exist for assisting the pilot in reaching the final approach fix. It is a non-directional beacon which can be sited with the outer marker (OM) or back marker (BM) and also used to replace the OM or BM. Its transmitting power is about 25 watts (W) and frequency range is 200 kilohertz (kHz) to 415 kHz. The radio beacon can receive the signal spread out in the area of at least 15 nautical miles (NM)(Carnegie 2008).

ILS limitations

Even after providing navigation precision guidance well in the past so many years and adopting a lot of measures for improving its performance and consistency, the Instrument Landing System(ILS) has come up with certain limitations. In view of the future aviation requirements, the ILS has some basic limitations such as: site sensitivity and high installation costs; it adopts single approach path; multi path interference; and has only 40 channels (Carnegie 2008).

The limitations prevailing in the ILS generated a necessity of a more fool proof landing aid that could prove to be accurate for safe landings. The following limitations in the ILS cause its degradation (Crabtree 2009).

There are certain restrictions relative to ILS techniques and operations which affect the flight path flexibility and airport throughput (Crabtree 2009,para 6).
ILS gets easily affected by signal interference due to weather and other impediments resulting in major chaos in the airport traffic, causing delays (Crabtree 2009).
There are many ground related constraints with ILS. It requires a relatively flat terrain as ground irregularities and poor weather affect its performance (M.L.S. 1995).
The channel range is limited to 40 channels only (M.L.S. 1995).
ILS uses a single approach path causing much confusion and chaos in the air traffic control (M.L.S. 1995).
It has a high user cost and is dependent on expensive ground equipment (Parkinson 1996).

Basic ILS is suitable only for the FAAs non precision and Category I landing requirements and only the improved ILS system can execute a landing with Category III required accuracy (Parkinson 1996).
Moreover, the noisy ILS signals need to be processed with a smoothing filter (Parkinson 1996).

Disasters with ILS

Although, ILS has been perceived as an efficient landing aid for past so many years, inaccuracy and errors in the guidance information through this system have led to several disasters in the aviation history (Phelan 2001).

A false instrument landing system (ILS) is assumed to have caused the disastrous Korean Air Boeing 747-300 accident at Guam in August 1997.The glideslope indication may have been a chief contributory factor to this mishap. Recent studies by Air New Zealand (ANZ), have declared that the problem may have led the crew to believe that they were on the correct descent when the aircraft crashed into Nimitz Hill 5km (3nm) from touchdown. ANZ believes that the new findings indicate that 40 other controlled flight into terrain (CFIT) investigations call for review now (Phelan 2001).

On July 29, 2000, during a standard instrument landing system (ILS) approach to Faleolo at night, an Air New Zealand Boeing 767 with 165 passengers and 11 crew on board commenced a go-around after descending to an altitude of about 400 ft, some 6 mi. short of the runway (ILS System Failure A Free Lesson 2010, Para 2). It was revealed after thorough investigations that flawed guidance information was being transmitted by the ILS glide path (GP) transmitter. The cockpit GP and localizer indications were normal. The Civil Aviation Authority (CAA) of New Zealand has publicized the matter widely so as to warn the ILS operators about the risks involved with this approach. Hence, a valuable insight into the recognized weakness in ILS systems is provided by this serious incident. It has annunciated the necessity to adhere to ICAO standards and guidance material during ILS operations and maintenance to avoid extreme hazards (ILS System Failure A Free Lesson 2010, Para 2).

Another instance indicating the failures of the ILS system came into light when a First Air Boeing 737-200, regi

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