It is essential for an air craft to know the distance with respect to a known ground point. Distance Measuring Equipment is used to provide the slant distance between aircraft and a known ground point.

INSTRUMENT LANDING SYSTEM

Introduction:

There are three main elements in the complete ILS: 

•  A localiZer radio beam to furnish directional guidance to and along the runway 
•  A glide path radio beam to furnish vertical guidance at the correct descent angle to the runway touchdown point
• Fan markers (outer marker and middle marker). In some cases DME has been authorised for use when markers are not available or cannot be installed. 

A suitable radio navigation aid is provided on most installations to assist in interception of the localiser and holding procedures. At some locations two of these aids are provided. This aid can be either a VOR or a low-powered NDB (Locator). 

Basic principle:

This is a diagram of a basic ILS installation. The localiser transmitters radiate field patterns of 90  hertz and 150-hertz modulated energy on opposite sides of the instrument runway centreline to provide a course for guidance in azimuth. The 150-hertz modulation is always on the right looking towards the runway from the outer marker and is known as the ‘blue sector’. The left side is modulated at 90-hertz and is known as the ‘yellow sector’. The course line (on-course) is a locus of points of equal 90-150-hertz modulation. It is aligned along the runway centreline extended in both directions and may be separated at the localiser antenna into a front course and a back course. In order to obtain the quality in the front course of the localiser necessary for CAT I and higher, a back course is not radiated in Australia. Glide path equipment radiates field patterns of 90-and 150-hertz modulated energy to provide a path for approach slope guidance. The field patterns are orientated so that a preponderance of the 150-hertz energy lies below the glide path and a preponderance of the 90-hertz energy is above the glide path. The line of the glide path (on-path), similar to the localiser course line, is a locus of points of equal 90-and 150-hertz modulation and is aligned at the correct approach angle for descent to the runway touchdown point.

Localizer:

One of the main components of the ILS system is the localizer which handles the guidance in the horizontal plane. The localizer is an antenna system comprised of a VHF transmitter which uses the same frequency range as a VOR transmitter (108,10 ÷ 111,95 MHz), however the frequencies of the localizer are only placed on odd decimals, with a channel separation of 50 kHz. The trasmitter, or antenna, is in the axis of the runway on it’s other end, opposite to the direction of approach. A backcourse localizer is also used on some ILS systems. The backcourse is intended for landing purposes and it’s secured with a 75 MHz marker beacon or a NDB (Non Directional Beacon) located 3÷5 nm (nautical miles), or 5,556÷9,26 km before the beginning of the runway.

The course is periodically checked to ensure that the aircraft lies in the given tolerance.

Antennas of the localizer system

Radiation pattern of the localizer’s VHF transmitter

Glide Slope

The glide slope, or angle of the descent plane provides the vertical guidance for the pilot during an approach. It’s created by a ground UHF transmitter containing an antenna system operating in the range of 329,30÷335.00 MHz, with a channel separation of 50 kHz.

The transmitter is located 750÷1250 ft (228,6÷381 m) from the beginning of the runway and 400÷600 ft (121,92÷182,88 m) from it’s axis. The observed tolerance is ±0,5°. The UHF glide slope is „paired“ with the corresponding frequency of the VHF localizer.

The UHF descent beacon draws a glide slope in the area

FLIGHT INSTRUMENTS

Flight instruments are the instruments in the cockpit of an aircraft that provide the pilot with information about the flight situation of that aircraft, such as altitude, speed and direction. The flight instruments are of particular use in conditions of poor visibility, such as in clouds, when such information is not available from visual reference outside the aircraft.

The term is sometimes used loosely as a synonym for cockpit instrument as a whole, in which context it can include engine instruments, navigational and communication equipment.

Most regulated aircraft have these flight instruments:

Attitude Indicator:

The Attitude Indicator shows rotation about both the longitudinal axis to indicate the degree of bank, and about the lateral axis to indicate pitch (nose up, level or nose down). It utilizes the rigidity characteristic of the gyro. It is gimballed to permit rotation about the lateral axis indicating pitch attitude, and about the longitudinal axis to indicate roll attitude. Once powered up, the indicator is maintain in a fixed position no matter what the aircraft attitude may be.

The principal parts of interest to the pilot are:

1. The miniature wings attached to the case remain parallel to the wings of the aircraft.
2. The horizon bar which separates the top (light) and bottom (dark) halves of the ball
3. The degree marks on the upper periphery of the dial. The first 3 on both sides of centre are 10 degrees apart, then 60 degree bank marks, and 90 degree bank arks. 

Altimeter:

The altimeter shows the aircraft's altitude above sea-level by measuring the difference between the pressure in a stack of aneroid capsules inside the altimeter and the atmospheric pressure obtained through the static system. It is adjustable for local barometric pressure which must be set correctly to obtain accurate altitude readings. As the aircraft ascends, the capsules expand and the static pressure drops, causing the altimeter to indicate a higher altitude. The opposite effect occurs when descending. With the advancement in aviation and increased altitude ceiling the altimeter dial had to be altered for use both at higher and lower altitudes . Hence when the needles were indicating lower altitudes i.e. the first 360 degree operation of the pointers was delineated by the appearance of a small window with oblique lines warning the pilot that he is nearer to the ground. This modification was introduced in the early sixties after the recurrence of air accidents caused by the confusion in the pilot's mind. At higher altitudes the window will dis-appear.

Airspeed Indicator:

The airspeed indicator is one of the basic aircraft instruments and is of importance to pilots because adherence to safe operating speeds is imperative.Each airplane has its own specified airspeed that the pilot needs to be aware of. The airspeed indicator works by comparing dynamic pressure (ram air pressure) and static pressure. This article pertains to conventional airspeed instruments as opposed to newer computerized systems found on technologically advanced aircraft. Airspeed indications on modern, computerized primary flight displays depict airspeed differently than described in this article, and are computed using modern sensor technology. Airspeed can also be obtained from a GPS unit in equipped aircraft.

Heading Indicator:

The heading indicator (also known as the directional gyro, or DG; sometimes also called the gyrocompass, though usually not in aviation applications) displays the aircraft's heading with respect to magnetic north. Principle of operation is a spinning gyroscope, and is therefore subject to drift errors (called precession) which must be periodically corrected by calibrating the instrument to the magnetic compass. In many advanced aircraft (including almost all jet aircraft), the heading indicator is replaced by a horizontal situation indicator(HSI) which provides the same heading information, but also assists with navigation.

Vertical Speed Indicator:

The vertical speed indicator (VSI) or vertical velocity indicator indicates whether the aircraft is climbing, descending, or in level flight. The rate of climb or descent is indicated in feet per minute. If properly calibrated, this indicator will register zero in level flight.

AIR TRAFFIC CONTROL

Air traffic control (ATC) is a service provided by ground-based controllers who direct aircraft on the ground and through controlled airspace, and can provide advisory services to aircraft in non-controlled airspace. The primary purpose of ATC worldwide is to prevent collisions, organize and expedite the flow of traffic, and provide information and other support for pilots.^[1] In some countries, ATC plays a security or defensive role, or is operated by the military.

To prevent collisions, ATC enforces traffic separation rules, which ensure each aircraft maintains a minimum amount of empty space around it at all times. Many aircraft also have collision avoidance systems, which provide additional safety by warning pilots when other aircraft get too close.

In many countries, ATC provides services to all private, military, and commercial aircraft operating within its airspace. Depending on the type of flight and the class of airspace, ATC may issue instructions that pilots are required to obey, oradvisories (known as flight information in some countries) that pilots may, at their discretion, disregard. Generally the pilot in command is the final authority for the safe operation of the aircraft and may, in an emergency, deviate from ATC instructions to the extent required to maintain safe operation of their aircraft.

 TRAFFIC COLLISION AVOIDANCE SYSTEM 

TCAS, short for Traffic Alert and Collision Avoidance System, is a system equipped on an aircraft that identifies the location and tracks the progress of aircraft equipped with transponders or transmitter-responder devices (Wikipedia, 2009^b 2). The goal of TCAS is to prevent mid-air collisions between aircraft operating within the same airspace by warning pilots of transponder-equipped aircraft that may present a collision threat.

TCAS operates independently of Air Traffic Control (ATC) by communicating with other transponder-equipped aircraft to build a 3-dimensional map of aircraft in the same airspace. By extrapolating the current range and altitude difference to anticipated future values, TCAS determines the potential of a collision threat. The existence of a collision threat results in subsequent communication of avoidance manoeuvres to flight crew by cockpit display or voice instructions, depending on the TCAS version installed (Wikipedia, 2009 1). Many countries have mandated the carriage of TCAS II, and ICAO has proposed a worldwide mandate of TCAS II Version 7 by 2003 (FAA. pp.40, 2000).

TCAS Versions

Passive

• Display traffic similar to TCAS but have a shorter range (generally less than 7 nautical miles)

• Rely on transponder replies triggered by ground and airborne systems that query nearby transponders for altitude information

• Monitored by third-party systems for traffic information (Wikipedia, 2009 1)

TCAS I

• Less expensive and less capable than TCAS II and TCAS III

• Designed primarily for general aviation use

• Displays the approximate bearing and relative altitude of all transponder-equipped aircraft within a range of approximately 40 miles. Figure 1 shows a TCAS display (Traffic Advisory).

• Generates collision warnings via a Traffic Advisory (TA) that warns the pilot of nearby aircraft. When the flight crew receive a TA, it is up to the Pilot in Command to decide what actions to take in response to the TA. The pilot may notify air traffic control for assistance in resolving the conflict (Wikipedia, 2009 1)

Figure 1 (image embedded from Wikipedia on 05 Aug 2009)

TCAS II

• More comprehensive than TCAS I

• Used in the majority of commercial aviation aircraft

• Operates the same as TCAS I but has greater range and bearing accuracies than TCAS I and a Resolution Advisory (RA) function

• In the event of a potential collision, TCAS II will issue a Resolution Advisory (RA) to each pilot that consists of direct, vocalised corrective or preventive vertical manoeuvring commands to avoid a collision

• TCAS II systems coordinate resolution advisories to maximise aircraft separation so that if one aircraft is instructed to climb, the other aircraft will be instructed to descend (Wikipedia, 2009 1)

TCAS III

• Whereas TCAS II only offers vertical RAs and TAs, TCAS III offers both vertical and horizontal RAs and TAs. Horizontal directives commanding left or right manoeuvres are beneficial for two conflicting aircraft close to the ground with minimal vertical manoeuvring space.

• TCAS III attempted to use its directional antenna in order to assign a bearing and assign a horizontal maneuver action (Wikipedia, 2012). However, the directional antenna was deemed to have limited accuracy (Wikipedia, 2012). Further testing and analysis determined that the concept was unworkable using available surveillance technology (Wikipedia, 2012).

• Hence, the project was suspended and there are no plans for implementation of TCAS III 

DISTANCE MEASURING EQUIPMENT

It is essential for an air craft to know the distance with respect to a known ground point. Distance Measuring Equipment is used to provide the slant distance between aircraft and a known ground point. 

In DME distance is determined by measuring the round trip time of radio pulse signals between the air craft and ground DME. The distance measured is displayed on a digital readout in the cockpit of the air craft in Nautical miles.

DME system consists of two major parts which is Interrogator (Aircraft) and Transponder (Ground). Interrogator originates the interrogation signal to receive the distance and transmits it using an Omni directional antenna. Ground Transponder receives this signal and retransmits it to interrogator as a reply signal.

Round trip time is calculated as time taken to receive the reply signal after transmitting the 

interrogate signal by the air craft.

A calibrated delay of 50μs is added to the round trip time as a standard in all DME stations. This fixed time is called ‘System delay’ or ‘Reply delay’ which allows the DME to process the interrogate signal and generate the reply.

Distance is calculated using the below formula.

There are two types of DMEs.

• En-route DME
•  Landing DME

En-route DME

En-route DME operates with Doppler VHF Omni-directional Radio Range and Landing DME operates with Instrumental Landing System. Bandaranaike International Airport currently operates with two landing DMEs at two ends of run-way and one En-route DME at 18 mile post Katunayake. Traffic handling capacity of DME is 100 aircrafts. That means 100 aircrafts can simultaneously obtain distance information. In this equipment multipath issue can be occurred. If theses multipath echoes were detected by DME it may reply to those echoes. These echoes can be Short Range echoes or Long Range echoes. So a time duration called ‘Dead Time’ has been introduced. When a valid interrogation is detected dead time will be started and during this period received interrogations will not produce reply.

Landing DME

Landing DME helps the air craft to find the slant distance to the touch down point.

RADIO DETECTION AND RANGING (RADAR)

RADAR is a technique which is used to detect the objects (mainly moving objects) in the atmosphere. In the airports this is use to detect the presence of aircraft in order to guide them safely to their destinations. RADAR equipment uses radio waves to detect these objects.

RADAR is mainly of two types as,

• Primary Surveillance Radar(PSR)
• Secondary Surveillance Radar (SSR)

Primary Surveillance Radar (PSR)

In PSR a high power radio signal is transmitted that is focused by an antenna through the atmosphere. Objects in the path of the transmitted signal will scatter the signal and some part is reflected back toward the antenna. The transmitting antenna acts also as a receiving antenna. So receiving antenna gathers back the received radiation and feed it to the receiver. Since PSR uses reflected signal to determine the position of the aircraft's it needs a very powerful signal to be transmitted as when it is reflected back it should be large enough to detect by the receiver. Normally a 2 Mw signal is transmitted and the receiving signal would be in micro watt range. If the distance to the aircraft is r received signal will reduce by a factor of 1/r⁴ compare to the transmitted signal. Frequency band used in this equipment is 1215 MHz – 1370 MHz. PSR calculates the ‘Azimuth angle’ of the radar beam with respect to magnetic north and the distance to the aircraft by measuring the round trip time of the transmitted signal.

The received signal is then processed by the RADAR processor and the data is displayed on the RADAR monitor. Stationary objects will be removed by this process by checking the position of the aircraft in few adjacent reflections. 

PSR advantages

•  Doesn’t need a cooperation of the aircraft as it detects the reflected pulse.

PSR disadvantages

• Need a high power signal to be transmitted and due to this reason it is harmful to the people who operate it.
• Can’t separately identify two aircrafts with same azimuth but in different heights.
• Doesn’t give an indication about the height information.

Secondary Surveillance Radar (SSR)

Secondary Surveillance Radar is introduced to overcome the disadvantages of PSR and it is introduced as a system to identify whether an aircraft is friend or foe at wartime. After that wartime this system developed and became SSR for worldwide usage. SSR also use radio signals to track the objects.

Typical SSR

The main disadvantage of PSR which is high power radio wave transmission is not required here because SSR doesn’t use the reflected signals to track the objects. Instead it uses Interrogation messages to detect aircrafts. So less powered radio signals are used. Due to this reason a wider coverage is detected by the SSR than PSR. In SSR ground station sends an interrogation message and aircraft replies to that message using its transponder unit. Transponder is a radio receiver and transmitter operating on the radar frequency. The target aircraft’s transponder responds to interrogation by the ground station by transmitting a coded reply signal. Since the reply signal is transmitted by the aircraft, the received signal to the ground station is stronger thus a wider coverage can be obtained due to less problems of signal attenuation. Also since the signals are electronically coded it is possible to transmit additional information between two stations. Since SSR relies on the replies from the aircraft, it is required that the target aircraft carry a transponder unit with it. Therefore SSR is a dependent system. So normally a PSR will operate in conjunction with the SSR to detect non-cooperating targets such as enemy aircrafts and light aircrafts.

SSR have a few number of modes of interrogation.Mode A, Mode C, Mode S are some of

these modes which are currently in use. Pulse streams for Mode 3/A and Mode C are as follows.

pulse streams for some SSR interrogation modes

SSR uses three signal patterns. They are Σ (Sigma), Ω (Omega), Δ (Delta).

•  Σ (Sigma)        – 1030 MHz pulse stream modulated signal
• Ω (Omega)      – Helps to detect whether the aircraft is receiving a side lobe.
• Δ (Delta)         – Used to detect the vertical plane of the main lobe.

Antenna pattern of SSR is shown below.

Antenna pattern of SSR

 In most of the circumstances data from both SSR and PSR are synchronized and shown in the radar monitor screen by the radar processor. This is happens as follows.

Synchronization of PSR & SSR data

References:

• http://www.pilotfriend.com/training/flight_training/fxd_wing/attitude.htm
• http://en.wikipedia.org/wiki/Air_traffic_control
• http://aviationknowledge.wikidot.com/aviation:tcas
• air-nav-srf.com/2012_05_01_archive.html