CNS Concepts PDF

AIS INDUCTION CNS CONCEPTS Revision 1 Jan 2024 THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Introduction Communication, Navigation and Surveillance (CNS) concept is an ICAO ideology conceived by the ICAO Future Air Navigation Systems (F...

AIS INDUCTION CNS CONCEPTS Revision 1 Jan 2024 THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Introduction Communication, Navigation and Surveillance (CNS) concept is an ICAO ideology conceived by the ICAO Future Air Navigation Systems (FANS) committee, adopted by the World-wide CNS/ATM Systems Implementation Conference (Rio de Janeiro, 1998) and recommended for approval by the ICAO Assembly. This approval was granted by the ICAO 32 session of 1998 as resolution A32-12. The concept addresses the engineering aspect of Air Navigation both in equipment to be used for effective aerial navigation operations and the personnel involved in their implementation currently referred to as Air Traffic Safety Electronics Personnel (ATSEP). The CNS systems are the main functions that form the infrastructure for Air Traffic Management (ATM) and are essential for ensuring that an ICAO State’s air traffic is safe and efficient. CNS systems can only provide maximum benefits through enhanced ATM, it is important for the State to have a robust Aeronautical Information Management (AIM) system. The AIM unit in an ANSP is responsible for the collection and dissemination of information on the serviceability and availability of airspace operation facilities. The message dissemination is usually through radio communication. It is important that AIM personnel understand the principles of operation of radio systems. The CNS concept in this subject will focus on the CNS systems used for Aerial Navigation and not CNS operation personnel. This is guided by ICAO Annex 10 (Aeronautical Telecommunication) Volumes 1 to 5. The objective of the subject is to provide the participants with Skills Knowledge and Attitudes (SKAs) to enable them appreciate communication principles in ANS applications. The areas of interest will include: 1. Principles of operation of radio communication systems in Air Navigation operations 2. The application of radio communication systems in Air Navigation operations The topics to be covered include: 1. Radio wave propagation 2. Radio communication systems 3. Radio Navigation systems THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY  4. Radio surveillance systems Note: ICAO ANNEX 10 VOL VI, which deals with Communication Systems and Procedures Relating to Remotely Piloted Aircraft Systems is not covered in this area. COMMUNICATION PRINCIPLES Communication is the back bone of Air Navigation without which the safety of an air space cannot be assured. Communication is the conveyance of intelligence from one point to another. In communication principles, the originator of the intelligence is called a “source” while the recipient is called a sink. The other definitions of communication include:  Communication is the process of exchanging messages or information between two or more parties. Organizations are heavily dependent on information to meet organizational needs. Effective communication plays a key role in fulfilling these needs and contributes significantly to organizational success. The Importance of Effective communication is crucial for the success of individuals as well as organizations. This is true within the individual organization itself as well as how that organization communicates with other organizations within its sphere of contact, influence and competition. Communication is a complex two-way process, involving the encoding, translation and decoding of messages. Effective communication requires the communicator to translate their messages in a way that is specifically designed for their intended audience. Creating and delivering an effective message requires a basic understanding of the communication process. Aviation requires clear and unambiguous communication of a message in a way that can be clearly understood by the recipient. Communication process, involves the following phases: THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   Sender  Message  Channel  Receiver The most important thing to remember is that in communication, the message that is intended to be communicated is likely to be misunderstood. The sender apart from carefully preparing and presenting their message must stay alert for any signs that their audience are mis-interpreting their message. It is up to the presenter, to continually check that their message has been received, understood, correctly interpreted and filed by the receiver. Communication can take place in many forms, these include:  Verbal communication  Non- verbal communication  Written communication  Telecommunication (usually by use of radio signals) The components of communication listed above can be grouped into three namely:  The transmitter- sender or originator of intelligence  The media – Speech, paper, radio wave, body  Receiver- recipient of the intelligence The most common form of Communication used in Air Navigation is radio communication. In radio communication, the media connecting the transmitter and the receiver is through radio waves. Radio waves are a type of electromagnetic radiation with the longest wavelength in the electromagnetic spectrum. They are typically found within the frequencies of 300 gigahertz (GHZ)) and below. Electromagnetic Waves Electromagnetic waves are also known as EM waves. Electromagnetic radiations are produced when an electric field comes in contact with a magnetic field. The electric and magnetic fields generated will be oscillating perpendicular to one another and to their direction of motion. Electric field is produced whenever there is a charged particle. A force is exerted by this electric field on other charged particles. Positive charges THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY accelerate in the direction of the field and negative charges accelerate in a direction opposite to the direction of the field. The Magnetic field is produced when there is a moving charged particle. A force is exerted by this magnetic field on other moving particles. The force on these charges is always perpendicular to the direction of their velocity and therefore only changes the direction of the velocity, not the speed. The electromagnetic field is produced by an accelerating charged particle. The electromagnetic waves are therefore electric and magnetic fields travelling through free space with the speed of light c. An accelerating charged particle occurs when the charged particle oscillates about an equilibrium position. If the frequency of oscillation of the charged particle is f, then it produces an electromagnetic wave with frequency f. The wavelength λ of this wave is given by λ = c/f. Electromagnetic waves transfer energy through space. Electromagnetic waves can be represented diagrammatically by a sinusoidal function. The Electromagnetic waves are shown by a sinusoidal graph as above. It consists of time- varying electric and magnetic fields which are perpendicular to each other and are also perpendicular to the direction of propagation of the waves. Electromagnetic waves are transverse in nature. The highest point of the wave is known as the crest while the lowest point is known as a trough. In vacuum, the waves travel at a constant velocity of 3 x 10 8 m.s-1. Electromagnetic waves are related to visible light. They are classified into different types based on their wavelength. The classification is referred to as the electromagnetic spectrum. Electromagnetic Spectrum THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The electromagnetic spectrum is a range of frequencies, wavelengths and photon energies covering frequencies from below 1 hertz to above 10 25 Hz. These frequencies correspond to wavelengths which are a few kilometres to a fraction of the size of an atomic nucleus in the spectrum of electromagnetic waves. The electromagnetic spectrum consists of a span of all electromagnetic radiation which further contains many subranges, which are commonly referred to as portions. The entire range (electromagnetic spectrum) is given by: 1. Radio waves- have longest wavelengths 2. Infrared radiation- next to visible light 3. Visible light 4. Ultra-violet radiation, 5. X-rays 6. Gamma rays and 7. Cosmic rays – shortest wavelengths, high frequencies and very high energy Type of Radiation Frequency Range (Hz) Wavelength Range Gamma-rays 1020 – 1024 < 10-12 m X-rays 1017 – 1020 1 nm – 1 pm Ultraviolet 10 – 10 15 17 400 nm – 1 nm Visible 4 x 1014 – 7.5 x 1014 750 nm – 400 nm Near-infrared 1 x 10 – 4 x10 14 14 2.5 μm – 750 nm Infrared 1013 – 1014 25 μm – 2.5 μm Microwaves 3 x 1011 – 1013 1 mm – 25 μm 11 Radio waves < 3 x 10 > 1 mm THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Radio waves are said to be the electromagnetic waves with the lowest frequencies (upto below 3Hz) but longest wavelengths (above 10,000 Km) Radio waves are also categorized into divisions referred to as radio frequency bands. Electromagnetic waves in this frequency range are widely used in modern technology, particularly in telecommunication (communication over a distance by means of cable, telegraph, telephone, or broadcasting). Radio waves can be generated artificially using electronic devices called transmitters. The process of generation and transmission of radio waves is, however, strictly regulated by national laws, coordinated by an international body called the International Telecommunication Union (ITU). This strict regulation is aimed at preventing interference between different users, The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. The body is responsible for:  Allocation of global radio spectrum and satellite orbits  Development of technical standards that ensure networks and technologies seamlessly interconnect,  Improvement of access to ICTs THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The frequency boundaries of the radio spectrum are a matter of convention in physics and are somewhat arbitrary. Since radio waves are the lowest frequency category of electromagnetic waves, there is no lower limit to the frequency of radio waves. Radio waves are defined by the ITU as: "electromagnetic waves of frequencies arbitrarily lower than 3000 GHz, propagated in space without artificial guide". At the high frequency end the radio spectrum is bounded by the infrared band. The boundary between radio waves and infrared waves is defined at different frequencies in different scientific fields. Example the terahertz band, from 300 gigahertz to 3 terahertz:  Can be considered either as microwaves or infrared.  It is the highest band categorized as radio waves by the ITU  Spectroscopic scientists consider these frequencies part of the:  Far- infrared  Mid- infrared bands. A radio band is a small frequency band (a contiguous section of the range of the radio spectrum) in which channels are usually used or set aside for the same purpose. For example, broadcasting, mobile radio, or navigation devices, will be allocated in non- overlapping ranges of frequencies. ITU has a band plan which dictates how each of these bands is to be used and shared. The plan aims at avoiding interference and to setting protocol for the compatibility of transmitters and receivers. Pursuant to convention, the ITU divides the radio spectrum into 12 bands (as shown in the table below), each beginning at a wavelength which is a power of ten (10 n) metres, with corresponding frequency of 3×108−n hertz, and each covering a decade of frequency or wavelength. Each of these bands has a traditional name (example “Tremendously Low Frequency (TLF)”- used for frequencies from 1- 3 Hz though not defined by ITU). Band Name Abbreviation ITU Frequency Wavelength Number Extremely ELF 1 3- 30 Hz 100,000- 10,000 Low Km Frequency THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY  Super Low SLF 2 30- 300 Hz 10,000- 1,000 Km Frequency Ultra Low ULF 3 300- 3,000 HZ 1,000 – 100 Km Frequency Very Low VLF 4 3- 30 KHz 100 – 10 Km Frequency Low LF 5 30 – 300 KHz 10 – 1 Km Frequency Medium MF 6 300 – 3,000 KHz 1,000 – 100m Frequency High HF 7 3- 30 MHz 100 – 10 m Frequency Very High VHF 8 30 – 300 MHz 10 – 1m Frequency Ultra High UHF 9 300 – 3,000 MHz 100 – 10 cm Frequency Super High SHF 10 3 – 30 GHz 10- 1cm Frequency Extremely EHF 11 30 – 300 GHz 10 – 1mm High Frequency Tremendously THF 12 300 – 3,000 GHz 1- 0,1mm High Frequency (Terahertz) The wavelength of a radio wave (any electromagnetic wave) is inversely proportional to its frequency because its velocity is constant. At 300 GHz, the corresponding wavelength is 1mm, which is shorter than the diameter of a grain of rice. At 30 Hz the corresponding wavelength is ~10,000 kilometers (6,200 miles), which is longer than the radius of the Earth. Like all electromagnetic waves, radio waves in a vacuum travel at the speed of light. On the Earth's atmosphere, however, they travel at a slightly slower speed due to change in media. Radio waves are generated by charged particles undergoing acceleration, such as time-varying electric currents. Naturally occurring radio waves are emitted by:  Lightening THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   Astronomic objects – example stars  Black body radiation- emitted by warm objects Radio Propagation Radio waves travelling through a media which is not a vacuum will experience changes in its propagation depending on the characteristics of the media. Radio waves will experience the following when travelling through a media that is not a vacuum:  Reflection,  Refraction  Absorption  Diffraction  Polarization. Radio Wave Reflection Reflection in waves is the phenomenon where a wave encounters a boundary or interface and is turned back into its original medium. Unlike wave refraction, which involves a change in direction and medium, reflection keeps the wave in the same medium but reverses its direction. When a wave encounters an obstacle or a boundary, part or all of the wave can be reflected, depending on various factors like the angle of incidence and the properties of the reflecting surface. The law of reflection states that the angle of incidence is equal to the angle of reflection: θ1=θ2 Both angles are measured relative to the normal line, which is the imaginary line perpendicular to the boundary at the point where the wave hits. If a wave hits a boundary head-on, it will be reflected back along the same path. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Radio Wave Refraction Refraction in waves refers to the bending or change in direction of a wave as it passes from one medium into another. This bending occurs because the speed of the wave varies in different media, causing the wave to alter its course. When a wave encounters a change in medium, its speed and wavelength can change, while its frequency remains constant. The change in speed leads to a change in the wave’s direction, causing it to bend. When working with light waves and optics, we often refer to Snell’s Law. This is the mathematical formulation for wave refraction: n1sinθ1= n2sinθ2 n1 and n2 are the indices of refraction for the first and second mediums, respectively θ1 and θ2 are the angles of incidence and refraction, also respectively THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The index of refraction is a dimensionless number that describes how fast light travels through a material. The higher the index, the slower the speed of light in that medium. For example, the index of refraction of air is approximately 1, while for water, it’s about 1.33. Light travels slower in water than in air. Radio Wave Absorption Radio wave absorption is the transfer of energy from a wave to a medium. The medium can be a solid, liquid, or gas. The absorption of electromagnetic radiation (radio waves) is how matter (typically electrons bound in atoms) takes up a photon’s energy — and so transforms electromagnetic energy into internal energy of the absorber (for example, thermal energy) A notable effect of the absorption of electromagnetic radiation is attenuation of the radiation; attenuation is the gradual reduction of the intensity of light waves as they propagate through the medium. The main effect of absorption of radio waves by materials is to heat them, similarly to the infrared waves radiated by sources of heat such as a space heater or wood fire. Radio Wave Diffraction THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Diffraction is the tendency of a wave emitted from a finite source or passing through a finite aperture to spread out as it propagates. It can also be explained as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a secondary source of the propagating wave. Diffraction results from the interference of an infinite number of waves emitted by a continuous distribution of source points. Huygens’ principle postulates that every point on a wave front of light can be considered to be a secondary source of spherical wavelets. These wavelets propagate outward with the characteristic speed of the wave. The wavelets emitted by all points on the wave front interfere with each other to produce the traveling wave. This principle is also true for electromagnetic waves. In the propagation of light, the term “any wave front” can be replaced by “a collection of sources distributed uniformly over the wave front, radiating in phase”. An example of radio wave diffraction is observed in radio communication in hilly areas Radio Wave Polarization Polarization is a property of transverse waves which specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Example of a polarized transverse wave is vibrations traveling along a taut string like a guitar string. The vibrations can be in the vertical direction, horizontal direction or at an angle perpendicular to the string depending on how the string is plucked. Longitudinal waves, like sound waves, in liquid or gas do not exhibit the same effect as the displacement of the particles in the oscillation is always in the direction of propagation- they do not exhibit polarization. Radio waves are transverse and therefore exhibit polarization. Example of Polarized Radio Wave A "vertically polarized" radio wave of wavelength λ has its:  Electric field vector E (red) oscillating in the vertical direction.  The magnetic field B (or H) is always at right angles to it (blue),  Both are perpendicular to the direction of propagation (z). One can test whether sunglasses are polarized by looking through two pairs, with one perpendicular to the other. If both are polarized, all light will be blocked. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Note: The properties listed above are very important in radio communication as they determine the type of systems to be used and their limitations. Radio waves have different propagation characteristics in the Earth's atmosphere including:  Long waves can diffract around obstacles like mountains and follow the contour of the earth while  Shorter waves can reflect off the ionosphere and return to earth beyond the horizon. Radio Wave Propagation in the Earth’s Atmosphere Electromagnetic waves (radio waves) when propagating through the Earth’s atmosphere will exhibit certain characteristics due to the condition of the environment they are passing through. The environment can have the following:  charged particles of matter,  Water particles (humid)  Solid body (mountains, hills, tall buildings) THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The conditions listed above will affect the wave differently depending on their wavelength. The study of radio propagation. Practical radio systems mainly use three different techniques of radio propagation to communicate namely: 1. Line of sight 2. Ground waves 3. Skywaves Line of Sight Wave Propagation This is a characteristic of electromagnetic radiation in which waves can only travel in a direct visual path from the source to the receiver. Electromagnetic transmission includes light emissions traveling in a straight line. The rays or waves may be:  Interfered with by the atmosphere  Obstructed by materials eg, buildings  Prevented from travelling over the horizon Radio waves of frequencies above 30 MHz (VHF and higher) any obstruction between the transmitting antenna (transmitter) and the receiving antenna (receiver) will block the signal, just like the light that the eye may sense. The ability to visually see a transmitting antenna (disregarding the limitations of the eye's resolution) roughly corresponds to the ability to receive a radio signal from it, the propagation characteristic at these frequencies is called "line-of-sight". The farthest possible point of propagation is referred to as the "radio horizon". THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Ground Wave Propagation Ground waves are radio waves propagating parallel to and adjacent to the surface of the Earth, following the curvature of the Earth beyond the visible horizon. This type of radiation is known as:  Norton surface wave  Norton ground wave, Ground waves in radio propagation are not confined to the surface. The normal line-of- sight distance to the horizon is about 80 to 100 kilometers (50 to 60 miles) at best, for an antenna on a tall tower. Signals at medium or low frequency are observed to travel several hundred miles beyond the horizon, up to several thousand kilometers in some cases. This attachment to the ground effect is called ground wave, as opposed to direct wave or sky wave. This effect does not occur for signals in the VHF or UHF frequencies, which typically propagate only about 100 km (60 miles) to the horizon. Ground propagation works because lower-frequency waves are more strongly diffracted around obstacles due to their long wavelengths. This allows them to repeatedly bend downward to follow the Earth’s curvature Ground waves propagate in vertical polarization, with their magnetic field horizontal and electric field vertical. Conductivity of the surface affects the propagation of ground waves, with more conductive surfaces such as sea water providing better propagation. Since the THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY ground is not a perfect electrical conductor, ground waves are attenuated as they follow the earth's surface. The wave fronts initially are vertical, but the ground, acting as a lossy dielectric, causes the wave to tilt forward as it travels. This directs some of the energy into the earth where it is dissipated so that the signal decreases exponentially. Sky Wave Propagation Sky wave propagation is a type of radio wave communication in which the electromagnetic wave propagates due to the reflection mechanism of the ionospheric layer of the atmosphere. It occurs as a result of radio wave propagation through the Earth’s ionosphere and this is why it is also known as inonospheric wave propagation. The radio frequencies within which sky wave propagation is dominant is between 3 MHz to 30 Mhz Electromagnetic waves in the range of 3 to 30 MHz get reflected by the ionosphere. Signals with frequency beyond 30 MHz despite undergoing reflection are able to penetrate the ionosphere making the phenomenon less noticeable within such ranges. It allows the propagation of electromagnetic waves of higher frequency from one end to another at a larger distance than ground wave propagation. And it does so by reflections of the wave from the ionosphere. This phenomenon is sometimes referred to as ionospheric wave propagation. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Sky wave propagation takes place because of the composition of the ionosphere. The ionosphere is composed of 4 different layers with each layer consisting of a different number of atoms. The outmost layer has the lowest number of atoms while the innermost layer of the ionosphere is highly dense. The reason behind this is that the atmosphere of earth is denser towards its surface and becomes rarer on proceeding upwards. The sun emits powerful cosmic rays. The cosmic rays penetrate go through the outer surface to the inner surface of the atmosphere without much interaction. This occurs because there are less number of neutral atoms in the outermost layer. The inner layer is slightly denser than the outer one so here interaction between cosmic rays and atoms takes place. The interaction between cosmic rays and the atoms increases tremendously in the E layer of the ionosphere, as this layer has a greater number of atoms. The intensity of the cosmic rays reduces as they penetrate to lower levels inside the earth’s atmosphere making it possible for only few cosmic rays to interact with the innermost layer of the ionosphere (the densest layer) The interaction between the cosmic rays and atoms present in the ionospheric layers makes electrons be emitted from the valence shell of the atom (ionization takes place). This interaction is higher in the middle layers of the atmosphere, therefore, ionization will be higher in that layer. The layer thus holds the maximum number of charged particles. The result of this phenomena is that whenever an electromagnetic signal is transmitted from the ground, it suffers reflection from the ionospheric layer and comes back to the surface of the Earth. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY. The characteristics above determine the type of radio system that would be suitable for a particular operation. An example of such a case is when the sky wave propagation phenomena is used to cover long distance.  At medium wave and short wave wavelengths, radio waves reflect off conductive layers of charged particles (ions). Radio waves directed at an angle into the sky can return to Earth beyond the horizon.  Use of multiple skips will provide multiple skips communication  Multiple skips communication will enable communication in intercontinental distances to be achieved, Skywave propagation is variable and dependent on atmospheric conditions; it is most reliable at night and in the winter. Radio waves are more widely used for communication than other electromagnetic waves mainly as a result of: their desirable propagation properties, stemming from their large wavelength. These include:  They have the ability to pass through the atmosphere in any weather, foliage,  They have the ability to pass through most building materials,  By diffraction longer wavelengths can bend around obstructions, THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   Unlike other electromagnetic waves they tend to be scattered rather than absorbed by objects larger than their wavelength. In Air Navigation, radio waves are used for several important operations, the most important operations are: 1. Radio communication 2. Radio Navigation 3. Radio Surveillance RADIO COMMUNICATION IN AIR NAVIGATION OPERATIONS A radio communication system (communication, navigation or surveillance) is composed of three (3) important components; namely: 1. The transmitter – information source 2. The media – the information path (Radio waves in radio communication) 3. The receiver – the consumer of the information or information sink THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The Components of a Radio Transmitter Radio transmitter components include:  The signal source- human being in voice communication  The signal encoder- makes the signal adapted specifically for the intended recipient  Transducer- microphone in voice communication  Modulator- Mixer of the signal produced by the transducer and the required transmission frequency  Amplifier- To provide correct signal strength  Transmission aerial- radiates the signal into space Radio Receiver Components will include:  The receiving aerial  The signal demodulator- to remove the mixed signal from the information signal  A signal filter- to remove unwanted signals (noise) introduced during signal propagation in space  Signal amplifier- to increase signal intensity for interpretation THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   Signal decoder- to convert the signal to a form which can be understood by the recipient  Signal transducer- to convert the signal to a form that can be understood by the recipient (loudspeaker converting electrical signal to sound signals)  The signal sink- the human being in the case of voice communication. In Air Navigation, voice communication is one of the most commonly used forms of communication between:  Air traffic controllers and pilots  Units The most common systems used are radio transceivers. Radio Transceivers A transceiver is an electronic device which is a combination of a radio transmitter (origin of “TRANS”) and a receiver (origin of “CEIVER”). In radio communication, such a system can both transmit and receive radio waves. These two related functions are often combined in a single device to reduce manufacturing costs. The term is also used for other devices which can both transmit and receive through a communication channel like:  Optical transceivers which transmit and receive light in optical fiber systems  Bus transceivers which transmit and receive digital data in computer data buses THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Communication transceivers are found in two (2) forms; namely: 1. Half duplex 2. Full duplex Half Duplex Transceiver The communication can only take place in one (1) direction at a time. It is said that in this type of communication, only one thing on that channel -- node -- can "talk" or transmit information at a time. Once one node has finished transmitting its data, another node can start transmitting data. In half duplex communication, if multiple nodes try to talk at the same time, a collision will occur on the network, resulting in transmission errors or data loss. Half-duplex networks require a mechanism to avoid data collisions. Example of a half- duplex transceivers is a standard walkie-talkie. The walkie-talkie can either transmit or receive communication. There is a communication procedure that speakers use to tell listeners they are done transmitting the current piece of information. For example, on a walkie-talkie, a user would say "over" when done talking or say "over and out" when getting off the line for a substantial amount of time. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Full Duplex Transceiver Full-duplex communication is a type of communication where both the transmission and reception of data can be done simultaneously. It is widely used in modern communication systems, including wireless networks, satellite communication, and radio communication. Full-Duplex communication can be achieved in two ways; namely:  By using a single physical communication channel for both directions simultaneously  By using two distinct physical channels, one for each direction (dual- simplex communication). Full-duplex communication is a powerful technology that can greatly improve the efficiency and throughput of communication systems. The benefits of full-duplex communication make it a valuable technology for a wide range of applications. Wireless technologies like Bluetooth, Wi-Fi and cellular networks use a single physical channel for communication in both directions simultaneously. In these types of full-duplex systems, communication in both directions simultaneously is achieved by using two techniques; namely: THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   Time division duplexing (TDD)  Frequency division duplexing (FDD), Time Division Duplexing (TDD) The same frequency band is used for both transmitting and receiving, but the time is divided into two parts. During the first part, one party can transmit data while the other party listens, and during the second part, the roles are reversed. This technique is commonly used in wireless networks such as Wi-Fi and Bluetooth. Frequency Division Duplexing (FDD) Two separate frequency bands are used for transmitting and receiving data. The transmitting and receiving frequencies are usually separated by a specific frequency band to prevent interference. This technique is commonly used in cellular networks such as 4 th Generation Long Term Evolution (4 G LTE) networks. Radio transceivers are commonly used in Air Navigation operations at the control tower to relay important airspace information between Air Traffic Controllers and Pilots. Air Navigation Service Providers (ANSP) use other communication systems apart from radio communication systems to enable inter-unit or administrative communication. The systems used are usually land based. These include: THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   Landline Telephones  Intercoms Land Line Telephones A telephone is a telecommunication device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into electronic signals that are transmitted via cables and other communication channels to another telephone which reproduces the sound to the receiving user. The term is derived from Greek (tēle, far) and (phōnē, voice), together meaning distant voice The essential elements of a telephone are:  Microphone (transmitter) to speak into  Earphone (receiver) which reproduces the voice at a distant location. The receiver and transmitter are usually built into a handset which is held up to the ear and mouth during conversation. The transmitter converts the sound waves to electrical signals which are sent through the telecommunication system (cable) to the receiving telephone, which converts the signals into audible sound in the receiver or sometimes a loudspeaker. Telephones permit transmission in both directions simultaneously. Most telephones also contain an alerting feature, such as a ringer or a visual indicator, to announce an incoming telephone call. Telephone calls are initiated most commonly with a keypad or dial, affixed to the telephone, to enter a telephone number THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Intercoms in Air Navigation Operations An intercom, also called an intercommunication device, inter-communicator, or interphone, is a stand-alone voice communications system for use within a building, small collection of buildings or portably within a small coverage area which functions independently of the public telephone network. Intercoms are:  Generally mounted permanently in buildings and vehicles, but can also be detachable and portable.  Can incorporate connections to:  Public address loudspeaker systems  walkie- talkies  Telephones  Other intercom systems.  Incorporate control of devices such as signal lights and door latches. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Signal Transmission Methods THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The method used for information transfer can be analog or digital. In analog communication, the modulating wave is continuous just as the carrier wave. The type of modulation can be:  Amplitude Modulation (AM)- the carrier signal amplitude is varied by the information signal  Frequency Modulation (FM)- the carrier signal frequency is varied by the information signal  Phase Modulation (PM)- the carrier signal phase is varied by the information signal. The other method of signal transmission is digital with the end communication type being referred to as Digital communication. Digital Communication The term digital communication is made from two words; namely:  Digital  Communication. The term “Digital” refers to the discrete time-varying signal and “Communication” refers to the exchange of information between two or more sources. Digital Communication can therefore be said to be the exchange of information between two or more sources by means of discrete time varying signals. The data transmission using analog methods for long-distance communication suffers from distortion, delays, interferences, and other losses. The interferences above come about because of the nature of the signals in the atmosphere. The interferences are usually positive and continuous. The effects can therefore be mitigated against if an inverted discrete time varying signal is used. Digitization and sampling of signals using different techniques help in making the transmission process more efficient, clear and accurate. Diagrammatical representation of Analog and Digital signals THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Digital Communication System The sequence of events in the signal processing is as follows:  The input signal is first applied to the input transducer  The input transducer converts the signal into electrical form for the source encoder  The source encoder converts the continuous signal to discrete time varying signal (required number of bits) for the channel encoder THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   The channel encoder removes any errors by adding the redundant bits to the input digital data for the modulator  The modulator mixes the signal with the carrier signal for transmission through the radio channel  The transmitted signal passes through the communication channel to reach the receiver  The demodulator retrieves the message signal from the carrier for the channel decoder  The channel decoder and source decoder convert the digital signal to its original format for the output transducer  The output transducer converts the digital signal into its original form of transmission. The output signal is a digital signal with no interference, noise, and error and is suitable for the user to capture. In Air Navigation, one application of digital data communication is in the Controller Pilot Data Link Communication (CPDLC) Controller Pilot Data Link Communications (CPDLC) is a means of communication between controller and pilot, using data link for ATC communications. (ICAO Doc 4444: PANS-ATM). It is a two-way data-link system by which controllers can transmit non- urgent strategic messages to an aircraft as an alternative to voice communications. The message is displayed on a flight deck visual display. It enables a number of data link services (DLS) that provide for the exchange of communication management and clearance/information/request messages which correspond to voice phraseology employed by air traffic control procedures.  The controllers are provided with the capability to issue ATC clearances  The pilots are provided with the capability to respond to messages RADIO NAVIGATION Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a body from one place to another. The field of navigation includes four general categories; namely: THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   Land navigation (movement of humans, motor vehicles)  Marine navigation (movement of vehicles (boats, ships) in water)  Aeronautic navigation (movement of aircrafts)  Space navigation (movement of space crafts) The term can also be used when referring to the art used for the specialized knowledge used by navigators (a person inside a vehicle responsible for its navigation) to perform navigation tasks. All navigational techniques involve locating the navigator's position compared to known locations or patterns. Navigators measure distance on the globe in degrees. The Navigator must be conversant with the bearings of the positions to be navigated, this involves understanding of the latitude and longitude of the initial and terminal points.  Latitude is a north-south position measured from Earth's Equator  Longitude is an east-west position measured from the prime meridian. The earliest navigation methods involved observing landmarks or watching the direction of the sun and stars. The current navigation methods are by radio. Radio navigation is similar to celestial navigation, except it replaces objects in the sky with radio waves being broadcast. The navigator can tune into a radio station and use an antenna to find the direction of the broadcasting radio antenna. Position can be determined by measuring the time it takes to receive radio signals from the stations of known locations on the ground or aboard satellites. This is the method vastly used in Air Navigation. Radio navigation is the application of radio waves to determine a position of an object on the Earth. The object can be a vessel or an obstruction. The basic principles are based on measurements from/to electric beacons. A radio beacon being a device that marks a fixed location and allows direction finding equipment to find relative bearing. In radio beacons, this is done using electromagnetic waves in the radio wave band. The beacons transmit a continuous or periodic radio signal with limited information (for example, its identification or location) on a specified radio frequency. The information is used by the receiver (vehicle) to determine the beacon location/ position from its position. The navigator (pilot in case of aircraft) can then move the vehicle towards or away from the beacon. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY In air transport, several equipment are used as radio beacons. They are collectively called Radio Navigation Aids (NAVAIDs). They are specified in ICAO Annex 10 Vol.1- Aeronautical Telecommunication- Radio Navigation Aids. The equipment used include:  Non- Directional Beacon (NDB)  Very high frequency Omni- Range transmitter (VOR)  Conventional (CVOR)  Doppler (DVOR)  Distance Measuring Equipment (DME)  Instrument Landing System (ILS)  Microwave Landing System (MLS)  Automatic Direction Finder (ADF)  Marker beacons  Global Navigation Satellite Systems with relevant augmentation namely:  Airborne Based Augmentation (ABAS)  Ground Based Augmentation (GBAS)  Space Based Augmentation (SBAS)  GNSS Receiver for Atmospheric Sounding (GRAS) Non- Directional Beacon (NDB) A non- directional beacon (NDB) sometimes called Non- Directional Radio Beacon is a radio beacon without inherent directional information. ICAO Annex 10 Vol.1 standardizes the use of NDBs in Aviation specifying that they be operated on a frequency between 190 kHz and 1750 kHz. They are to be identified by a one, two, or three-letter Morse code referred to as a callsign. In some countries like Canada, privately owned NDB identifiers consist of one letter and one number. The radio waves being emitted by NDBs, as per the ICAO regulations, are within the Low frequency (LF) and Medium Frequency (MF) radio frequency bands. Signals in this frequency band propagate as ground waves, following the curvature of the Earth. This makes it possible to receive them at much greater distances at lower altitudes. NDB signals are affected more by atmospheric conditions, mountainous terrain, coastal refraction and electrical storms, particularly at long range. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY There are four types of non-directional beacons in the aeronautical navigation service:  En route NDBs, used to mark airways  Approach NDBs  Localizer beacons  Locator beacons The last two types are used in conjunction with an instrument landing system (ILS). NDB Operation NDB navigation consists of two parts;  The ground NDB transmitter  The automatic Direction Finder (ADF) The ground transmitter sends an omnidirectional signal out. The signal is in Morse code usually providing the identity of the beacon, example Tango Hotel (TH). Diagram of NDB ground transmitter THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The information is received in the aircraft processed by the Automatic Direction Finder (ADF) inside the aircraft. The processed information is displayed to the pilot to indicate the aircraft position relative to an NDB station. The pilot can use the information to “ home” to a station or track a course from the station. The NDB transmits a continuous carrier wave that can be received by any aircraft equipped with ADF. The NDB frequency and identification information is published by states in the aeronautical chart. The Morse Codes are used to identify NDB stations while the normal commercial broadcast stations are identified by the station’s announcer. ADF equipment determines the direction or bearing to the NDB station relative to the aircraft by using a combination of directional and non-directional antennae to sense the direction in which the combined signal is strongest. This bearing may be displayed on a relative bearing indicator (RBI). The display looks like a compass card with a needle superimposed. The card is fixed with the 0-degree position corresponding to the centre line of the aircraft. In order to track toward an NDB (with no wind), the aircraft is flown so that the needle points to the 0- degree position. The aircraft will then fly directly to the NDB. Similarly, the aircraft will track directly away from the NDB if the needle is maintained on the 180- THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY degree mark. With a crosswind, the needle must be maintained to the left or right of the 0 or 180 position by an amount corresponding to the drift due to the crosswind. Aircraft heading +/- ADF needle degrees off nose or tail = Bearing to or from NDB station. Plotting fixes in this manner allow crews to determine their position. NDBs have long been used by aircraft navigators, and previously mariners, to help obtain a fix of their geographic location on the surface of the Earth. Fixes are computed by extending lines through known navigational reference points until they intersect. For visual reference points, the angles of these lines can be determined by compass, the bearings of NDB radio signals are found using radio direction finder (RDF) equipment. NDB systems have disadvantages that make them not very suitable for air navigation hence recommendation for their removal. The disadvantages affect their accuracy especially at long distances, they include effects due to:  Atmospheric conditions  Mountainous terrain  Coastal refraction  Electrical storms  Cone of confusion Note: A great advance in the RDF technique was introduced in the form of phase comparisons of a signal as measured on two or more small antennas, or a single highly directional solenoid (electromagnetic formed by a helical coil of a conducting wire like copper wire.) These receivers were smaller, more accurate, and simpler to operate. Combined with the introduction of the transistor and integrated circuits RDF systems were so reduced in size and complexity that they once again became quite common during the 1960s, and were THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY known by the new name Automatic Direction Finder (ADF). This led to their operation with simple radio beacons – the non- directional beacons. NDBs can follow the curvature of earth, hence the much greater range than the line of sight systems. They can be categorized as long range or short range depending on their power. The formula to determine the compass heading to an NDB station (in a no wind situation) is to take the relative bearing between the aircraft and the station, and add the magnetic heading of the aircraft; if the total is greater than 360 degrees, then 360 must be subtracted. This gives the magnetic bearing that must be flown: (RB + MH) mod 360 = MB. When tracking to or from an NDB, it is also usual that the aircraft track on a specific bearing. To do this it is necessary to correlate the RBI reading with the compass heading. Having determined the drift, the aircraft must be flown so that the compass heading is the required bearing adjusted for drift at the same time as the RBI reading is 0 or 180 adjusted for drift. An NDB may also be used to locate a position along the aircraft's current track (such as a radial path from a second NDB or a VOR). When the needle reaches an RBI reading corresponding to the required bearing, then the aircraft is at the position. Very high frequency Omni- Ranging (VOR) System VHF Omnidirectional Radio Ranging (VOR) is an aircraft navigation system operating in the VHF band. The system is composed of a ground transmitting station and a receiver inside an aircraft (receiving station) It is a short-range radio navigation aid that produces an infinite number of bearings that may be visualized as lines radiating from the beacon. The phase of the variable signal lags that of the reference signal by an amount equal to the azimuth angle around the beacon. The number of bearings is limited to 360, one degree apart; a radial is identified by its magnetic bearing from the VOR. VORs transmitter broadcast a VHF radio composite signal. The composite signal includes:  VOR station's Morse Code identifier (and sometimes a voice identifier) THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   Data – used by airborne receiving equipment to derive the magnetic bearing from the station to the aircraft. The station’s Morse code identifier is three letter identifier. VOR signals have a range of about 200 miles making it possible for an aircraft to receive multiple VOR signals. Pilots must identify a VOR they need before navigating to it to ensure the proper navigation aid is selected. In some cases, a voice signal is broadcast with the station name. This should contain recorded advisories. There are about 3,000 VOR stations worldwide VOR was first introduced in the 1950s as a development from Visual-Aural Range (VAR) systems. The basic principle of VOR is the same as that of a lighthouse, to provide a rotating signal such that a bearing from the station can be derived. VOR achieves this by transmitting a pair of 30Hz rotating signals, one FM and one AM and the airborne receiver will compare the phase angle of the two signals radiated from the beacon to get the bearing(‘radial’). One signal radiates Omni-directionally so that its phase is equal in all directions; this signal is referred to as the ‘Reference’ signal. The second signal, called the ‘Variable’ signal, radiates from a directional array. The phase of the variable signal received at the aircraft is dependent upon the radial on which the receiver lies with respect to Magnetic North. The aircraft receives the two 30Hz signals, demodulates them and simply compares their phase difference to derive the bearing from the station. A VOR receiver, regardless of the technology behind, receives two sine signals repeating 30 times per second: Reference and variable signals, and determines the bearing by solely measuring the difference between their phases. From a carrier standpoint, only amplitude modulation is used, but to minimize a possible influence of one signal onto the other, the VOR station uses a subcarrier, which frequency is low (9,960 Hz) but significantly distant from 30 Hz.  One phase value is conveyed by normally modulating the carrier in amplitude,  The other by modulating the subcarrier in frequency  The subcarrier then modulates the carrier in amplitude. Two antenna systems are used:  The carrier and the reference are send on one or more omnidirectional antenna(s) and all receivers see the same reference phase at the same time. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY  The variable signal is sent apart to appear different for receivers at different bearings. The type of antenna used depends on the type of VOR system. The variable signal is not created by actual modulation of the carrier, but by synthetic sidebands. This variation can be achieved mechanically or electrically. There are two (2) VOR systems, namely:  Conventional VOR (CVOR)  Doppler VOR (DVOR) Conventional VOR (CVOR) C-VOR and D-VOR differ by how the variable phase is made different for receivers at different bearings. The method relies on amplitude for a conventional VOR and on frequency for a Doppler VOR. In the CVOR, the signals being radiated are mechanically rotated. Early generations used a rotating dipole antenna to broadcast the variable signal. These were replaced by static antennas or slots on a cylinder. The mechanical signal rotation was accomplished through a physically rotating device. There are several types of conventional VORs using either rotating or static antennas. What they have in common is:  They are relatively compact.  Bearing is determined from a HF signal rotating 30 rounds per second.  They are very sensitive to multipath interference created by reflection on nearby obstacles, and must be located at isolated places. They are often used for airway beaconing. There is a ground component and an airborne component. Diagram of the ground component THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The Aircraft equipment includes:  A VOR antenna  A VOR frequency selector  A cockpit instrument to display the course information, this can be: o A Course Deviation Indicator (CDI) o A Horizontal Situation Indicator (HIS) The VOR equipment operate on radio frequency band 108.0 – 117.95 MHz. Their power output should be enough to provide coverage within their assigned operational service volume. Doppler Very High Frequency Omni- Ranging (DVOR) system The DVOR works on the principle of Doppler effect and was developed to mitigate against the atmospheric problems experienced in CVOR operations. The Doppler effect, or Doppler shift, describes the changes in frequency of any kind of sound or light wave produced by a moving source with respect to an observer. Waves emitted by an object traveling toward an observer get compressed — prompting a higher frequency — as the source approaches the observer. In contrast, waves emitted by a source traveling away from an observer get stretched out- prompting a lower frequency. This VOR consists in an array of about 50 fixed Alford loop antennas located on a 14 m circle. Two opposite antennas are activated at a time, and are electronically switched so the active antennas seem to be moving along the circle at 30 rounds per second. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY A 110 MHz source moving between -1,300 and +1,300 m/s creates a Doppler shift between -480 Hz and +480 Hz. This value, consecutive to the selection of the appropriate array diameter, is the same than the 480 Hz FM swing of the C-VOR, making the D-VOR compatible. Ground- Transmitter Operation The 30 Hz reference AM modulates the carrier which is sent in all directions by an antenna at the center of the circular array. The actual perceived Doppler shift depends on the direction of the displacement relatively to the receiver:  Maximum when "side" antennas are activated,  Null when "front" and "rear" antennas are activated. The receivers see a frequency variation repeating 30 times per second, and starting at a time varying with their own bearing. The shift can be used as the variable signal. The repeating Doppler shift being undistinguishable from frequency modulation, the variable signal must be conveyed in FM. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY  Each active antenna of the array transmits a pure frequency corresponding to a sideband of the subcarrier: f-9.96 kHz and f+9.96 kHz, f being the VOR carrier frequency. When associated with the carrier sent from the central antenna, these sidebands represent a carrier AM modulated by the 9.96 kHz signal.  When antennas radiating their pure frequency are given their angular velocity of 30 rps by the scan, a cyclic Doppler shift between -480 Hz and +480 Hz is automatically added. There are also several types of Doppler VORs, single or double sideband. The following are the general characteristics of DVORs:  They require a large 14 m diameter circular array of antennas.  Bearing is determined from the Doppler effect created by scanning the array at 1,300 m/s.  They are less efficient, inactive antennas from the array absorb a significant part of the signal emitted from the active antennas, reducing the effective range. They are less sensitive to multipath, and are easier to install at airfields Both types of VOR (and their different subtypes) are required by ICAO to produce a bearing with a ±2° tolerance. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY  The D-VOR is better in this aspect because it’s easier to produce an accurate Doppler shift than to form a precise pattern with space modulation. The VOR Receiver The signals transmitted from ground station are received by the aircraft's VOR antenna, which is usually located on the tail. The received signal is transfers it to the receiver in the cockpit for processing. The aircraft's VOR receiver compares the difference between the VOR's variable and reference phase, and determines the aircraft's bearing from the station. This bearing is the radial that the aircraft is currently on. Note: 1. Both types have the same tolerances from an ICAO requirements 2. The transmitting elements must be accurate to 2°. 3. Most stations are able to transmit a signal with an accuracy better than 0.5°. 4. A perfectly sited VOR could deliver this precision to the receivers. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Advantages of using VOR The advantages of using VOR systems for air navigation include: 1. Accuracy- VOR provides pilots with highly accurate and reliable navigation information 2. Continuous Operation- VOR operates continuously. This makes it a dependable navigation systems regardless of weather conditions 3. Long- Range Coverage- VOR provides long- range coverage, making it ideal for cross- country flights 4. Reliability- VOR systems are highly accurate and reliable, particularly when compared to NDBs (Non- Directional Beacons) DISTANCE MEASUREMENT EQUIPMENT Distance measuring equipment (DME) is a radio navigation equipment that measures the slant range (distance) between an aircraft and a ground station. It does this by timing the propagation delay of radio signals between transmission and reception. An airplane's DME interrogator uses frequencies from 1025 to 1150 MHz. DME transponders transmit on a channel in the 962 to 1213 MHz range and receive on a corresponding channel between 1025 and 1150 MHz. The band is divided into 126 channels for interrogation and 126 channels for reply. The interrogation and reply frequencies always differ by 63 MHz. The spacing of all channels is 1 MHz with a signal spectrum width of 100 kHz. Principle of operation An interrogator (airborne equipment) initiates an exchange by transmitting a pulse pair, on an assigned 'channel', to the transponder ground station. The channel assignment specifies the carrier frequency and the spacing between the pulses. The transponder (ground station) replies by:  Transmitting a pulse pair on a set frequency  The frequency is offset from the interrogation frequency by 63 MHz  The frequency has a specified separation of 150 interrogation pulse-pairs per second. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The communication has a time delay (typically 50 microseconds). The DME receiver in the aircraft searches for reply pulse-pairs (X-mode = 12- microsecond spacing) with the correct interval and reply pattern to its original interrogation pattern. The aircraft interrogator locks on to the DME ground station once it recognizes a particular reply pulse sequence has the same spacing as the original interrogation sequence. Once the receiver is locked on, it has a narrower window in which to look for the echoes and can retain lock. Note:  DME facilities identify themselves with a 1,350 Hz Morse Code three letter identity.  DME collocated with a VOR or ILS, will have the same identity code as the parent facility  DME identity is 1,350 Hz to differentiate itself from  The 1,020 Hz tone of the VOR or the ILS localizer. Diagram of DME ground equipment Airborne DME Equipment THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Note: 1. DME-equipped airplane uses the equipment to determine and display its distance from a land-based transponder 2. The ground stations are typically collocated with VORs 3. A low-power DME can be collocated with an ILS or MLS where it provides an accurate distance to touchdown, similar to that otherwise provided by ILS marker beacons A newer role for DMEs is DME/DME area navigation (RNAV). The accuracy of DME is generally superior relative to VOR. Navigation using two DMEs (using trilateration/distance) permits operations that navigating with VOR/DME (using azimuth/distance) does not. Advantages of DME Advantage of DME include:  Offers a reliable method for aircrafts to measure distance from ground stations.  Relatively simple to implement  Provides direct distance information. INSTRUMENT LANDING SYSTEM (ILS) The instrument landing system (ILS) is a precision radio navigation system that provides short-range guidance to aircraft to allow them approach a runway at night or in bad weather. In its original form, it allows an aircraft to approach until it is 200 feet (61 m) over the ground, within a 1⁄2 mile (800 m) of the runway. At that point the runway should be visible to the pilot; if it is not, they perform a missed approach. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Aircrafts being able to be brought this close to the runway dramatically increases the range of weather conditions in which a safe landing can be made. This condition has been improved by other versions of the system, or "categories", reducing the minimum distances further. ILS uses two directional radio signals in its operation on ground namely:  The localizer  The Glide Slope The two (2) transmitters are collocated with marker beacons or DME to provide distance from touch down ILS Localizer A localizer (LOC, or LLZ) is an antenna array located beyond the departure end of the runway. It generally consists of several pairs of directional antennas. They operate at 108 to 112 MHz radio frequency band. The purpose of the localizer in ILS operations is to provide horizontal guidance. Localizer operation The localizer provides lateral guidance to the runway centerline. The localizer antenna is normally placed centrally at the far end of the runway and consists of multiple antennas in an array. The system relies on the variation of the modulation of two signals across the entire width of the beam pattern. The system relies on the use of sidebands, secondary frequencies that are created when two different signals are mixed. ILS starts by mixing two modulating signals to the carrier, one at 90 Hz and another at 150.This combined signal, known as the CSB for "carrier and sidebands", is sent out evenly from an antenna array. The CSB is also sent into a circuit that suppresses the original carrier, leaving only the four sideband signals. This signal, known as SBO for "sidebands only", is also sent to the antenna array. Each individual antenna has a particular phase shift and power level applied only to the SBO signal such that the resulting signal is retarded 90 degrees on the left side of the runway and advanced 90 degrees on the right. Additionally, the 150 Hz signal is inverted on one side of the extract the original amplitude-modulated 90 and 150 Hz signals. These are then averaged to produce two direct signal providing the centre line. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Diagram of localizer array Glide Slope Instrument landing system glide slope (GS), is a system of vertical guidance embodied in the instrument landing system which indicates the vertical deviation of the aircraft from its optimum path of descent. A glide slope antenna is sited on a tower which is offset approximately 250 to 650' to one side of the runway centerline and approximately 750 to 1250' beyond the approach end of the runway, adjacent to the runway touchdown zone. It transmits in the 328 to 336 MHZ radio frequency band- Ultra High Frequency (UHF). Operation of Glide Slope The glideslope works in the same general fashion as the localizer and uses the same encoding, but is normally transmitted to produce a centerline at an angle of 3 degrees above horizontal. The signal is amplitude modulated with 90 and 150 Hz audio tones and transmitted on a carrier signal. The centre of the glide slope signal is arranged to define a glide path of approximately 3° above horizontal (ground level). THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The localizer (LOC) and glide slope (G/S) carrier frequencies are paired so that the navigation radio automatically tunes the G/S frequency which corresponds to the selected LOC frequency. The LOC signal is in the 110 MHz range while the G/S signal is in the 330 MHz range Diagram of a Glide Slope Equipment The ILS Airborne Receiver The airborne system can detect and process the signals coming from the ground. The signals provide a position fix which enables the aircraft to navigate to the runway threshold and at the runway centre line. The glide path scale is located to the right of the attitude sphere. On aircraft which have a mechanical gyro compass are both the localizer and glide path indicated as a vertical and a horizontal arrow in the compass as well. They are essentially read in the same way. Diagram of the ILS Airborne System THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY In the air, pilots attempt to man oeuvre the aircraft to keep the indicators centered while they approach the runway to the decision height. Marker beacons can be placed on the path to provide distance information as the approach proceeds. The beacons indicate how far the aircraft is from runway threshold. The marker beacons include:  Outer Marker (OM)- 7 Nautical miles with blue blinking light in the cockpit  Middle Marker (MM)- 0.8 Nautical miles with amber blinking light in the cockpit  Inner Marker (IM)- 200 feet with white blinking light in the cockpit Marker beacons are being replaced by distance measuring equipment (DME). Advantages of ILS in air navigation The ILS can give both horizontal and vertical guidance to a runway. It can be made so precise that pilots can use the system to land at an airport without even seeing the runway. ILS approach is considered a precision approach because it can provide both lateral and vertical guidance. MICROWAVE LANDING SYSTEM (MLS) The microwave landing system (MLS) is an all-weather, precision radio guidance system. The system was intended to be installed at large airports to assist aircraft in landing, including 'blind landings'. MLS enables an approaching aircraft to determine when it is aligned with the destination runway and on the correct glide path for a safe landing. MLS was intended to replace or supplement the instrument landing system (ILS). THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Principles of Operations MLS employs 5 GHz transmitters at the landing place. The transmitters operate on the passive electronically scanned arrays to send scanning beams towards approaching aircraft. The aircraft that enters the scanned volume uses a special receiver that calculates its position by measuring the arrival times of the beams. A passive electronically scanned array (PESA), also known as passive phased array, is an antenna in which the beam of radio waves can be electronically steered to point in different directions (that is, a phased array antenna), in which all the antenna elements are connected to a single transmitter and/or receiver. MLS has a number of operational advantages over ILS, including:  A wider selection of channels to avoid interference with nearby installations  Excellent performance in all weather  A small "footprint" at the airports- occupies less space in the airport  A wide vertical and horizontal "capture" angles- allows approaches from wider areas around the airport.  The antennas did not have to be placed at a specific location at the airport, and could "offset" their signals electronically. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Some MLS systems became operational in the 1990s, the widespread deployment envisioned by some aviation agencies never became a reality. There were two (2) main reasons for difficulty in implementation, namely:  Economic- while technically superior to ILS, MLS did not offer sufficiently greater capabilities to justify adding MLS receivers to aircraft equipage;  Potentially superior third system(GPS) -based systems, notably WAAS- allowed the expectation of a similar level of positioning with following advantages:  No equipment needed at the airport. GPS/WAAS dramatically lowers an airport's cost of implementing precision landing approaches, which is particularly important at small airports. The radio navigation systems (Navigation aids (NAVAIDs)) are very important in air navigation for operating through an airspace. The ground systems are generally being replaced by satellite based systems namely Global Navigation Satellite Systems (GNSS). SURVEILLANCE SYSTEMS IN AIR NAVIGATION The concept of surveillance in air navigation is to have a sensor (or a network of sensors) that detects aircraft, calculates their position, obtains other data (such as identification, level, etc.) and then presents it to the controller on a situation display. The main applications of surveillance in air navigation is in Area control and Approach control. It is also used in the tower or in uncontrolled airspace where Advisory Service, Flight Information Service or Alerting Service are provided. The surveillance chain includes:  Surveillance sensors (PSR, SSR)  Data processing system  Situation display system The most common sensors used today are: 1. Radar Sensors  Primary Surveillance Radar (PSR)  Secondary Surveillance Radar (SSR)  Mode ‘S’ Radars 2. Multilateration THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY RADAR Sensors Radar is an acronym for Radio Detecting And Ranging. It was initially developed and introduced during the Second World War. It implies the use of radio signals to detect an object and determine its distance (range) from the detector. In air navigation, Radar refers to electronic equipment that detect the presence of objects by using reflected electromagnetic energy. Electromagnetic energy (radio waves) travels through a vacuum (air is approximated to be a vacuum) at a constant speed, at approximately the speed of light, 300000 km/s. This constant speed allows the determination of the distance between the reflecting objects and the radar site by measuring the running time of the transmitted pulses. Under certain conditions, a radar system can measure the direction, altitude, distance, course, and speed of these objects. The frequency of electromagnetic energy used for radar remains unaffected by darkness and also enters fog and clouds. This allows radar systems to determine the position of aircraft, ships, or other obstacles that are invisible to the naked eye due to distance, darkness, or weather. In air navigation, radars are classified into:  Primary Surveillance Radars (PSR)  Secondary Surveillance Radars (SSR)  Conventional (CSSR)  Monopulse (MSSR)  Mode ‘S’ radars Primary Surveillance Radars (PSR) in Air Navigation (PSR) A primary surveillance radar is an equipment that emits radio wave pulses and detects aircraft based on the echoes received. The system utilizes the reflection property of radio waves. It works on the same principle as an echo. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The system is defined as “A surveillance radar system which uses reflected radio signals (ICAO Doc 4444 PANS-ATM) Similarly, in its most basic form, the electromagnetic energy radiated by a radar system strikes a reflective object (called a target) and is echoed back to a receiver. The system uses radio waves in the L- Band (microwave range) above 1,000 MHz. Components of a PSR The major components of a primary radar system are:  The transmitter  The Antenna  The Duplexer  The receiver  The signal processor  The signal display Transmitter  Creates the radio waves to be sent out  Modulates it to form the pulse train identical with the station carrier frequency  Releases the pulses through a path called a waveguide to the antenna Antenna  Radiates the radar pulse from the transmitter into the air  Collects (Receives) the reflected signal from the air back to the wave-guide. Duplexer  This is a switch that on the transmission/ reception path (attached to the waveguide)  Alternately connects the transmitter or receiver to the antenna.  Its protects the receiver from the high power output of the transmitter. Receiver  Receives the reflected signal THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   Amplifies the signal (signal received is very weak)  Demodulates the signal (removes the carrier signal)  Provides video signals for processing/display Signal Processor  Converts the received data (signals) to information that can be understood by a user (Air Traffic Controller) Indicator The primary function of the indicator is to provide an easily understandable visual display of the ranges and bearings of radar targets from which echoes are received.The electrical power for all the components is provided by a dedicated power supply which can also provide enough power for the cooling system. Diagram of a PSR Antenna PSR Operation The system operation is based on the echolocation principle. The radars are placed in a slanting position and made to rotate 360 degrees at the speed of 5-12 rpm. They send out electromagnetic signals (radio waves) as they rotate. Upon reaching the aircraft or any other radio reflective object, the signal is reflected and returned to the radar scanner for reception and processing. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The system is able to calculate the distance of the reflecting object and also the bearing from the station North alignment. Primary Radar calculates the position of aircraft using the following measurements, 1. the distance is calculated based on wave transmitted and received time. 2. the angle is calculated based on the radar antenna direction 3. the radial velocity is calculated is based Doppler effect algorithm Primary radars are said to be non- cooperative as they do not need an air borne equipment to operate- they detect any radio reflective material in the coverage area and display them in the ATC monitor. The advantages of the primary radar include:  No on-board equipment in the aircraft is necessary for detecting the target  Can be used to monitor the movement of vehicles on the ground. The disadvantages include:  Lack of positive identification  Lack of altitude identification  High power requirement  Reception from any radio reflective surface (clutter)  Can be affected by blind speed operations  Can be affected by ducting (anomalous propagation) Secondary Surveillance Radar (SSR) in Air Navigation Secondary surveillance radar (SSR) is a radar system used in air traffic control (ATC) that relies on targets equipped with a radar transponder that reply its interrogation signals. The system is composed of an interrogating equipment on ground and a responding equipment (transponder) in the aircraft. Secondary radar was developed during the Second World War to differentiate between friendly aircraft and ships hence the term Identification Friend or Foe (IFF). ICAO defines SSR as “A surveillance radar system which uses transmitters/receivers (interrogators) and transponders” (ICAO Doc 4444 PANS-ATM) The purpose of SSR is to improve the ability to detect and identify aircraft while automatically providing the Flight level (pressure altitude) of an aircraft. The components of an SSR system are: THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   SSR ground station (interrogator)/receiver  SSR ground data processor  SSR display  SSR airborne receiver/transmitter (transponder)  SSR airborne data processor  SSR airborne data display The SSR systems in Air Navigation are of two (2) types namely:  Conventional SSR (CSSR)  Mono-pulse SSR (MSSR) The two differ in their operation method. Operation of Conventional SSR (CSSR) SSR ground station (interrogator) transmits interrogation pulses on 1030 MHz. The transmission is continuous as the antenna rotates. Diagram of SSR ground antenna The pulses transmitted ask questions to be responded to by the airborne system based on the question asked namely:  Identity THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   Altitude  Any special message The interrogation pulses differ depending on the pulse width between pulses P1 and P3The different interrogation types are called interrogation modes. A third pulse P2 is found in the civil 3/A and mode C to cater for side lobe suppression (elimination of unwanted interrogations). SSR mode pulses Note: The ground antenna is highly directional but cannot be designed without side lobes. Aircraft could also detect interrogations coming from these side lobes and reply appropriately. The replies cannot be differentiated from the intended replies from the main beam and can give rise to a false aircraft indication at an erroneous bearing. To overcome this problem, the ground antenna is provided with a second, mainly omni-directional, beam with a gain which exceeds that of the side lobes but not that of the main beam. A third pulse, P2, is transmitted from this second beam 2 µs after P1. An aircraft detecting P2 stronger than P1 (therefore in the side lobe and at the incorrect main lobe bearing), does not reply. CSSR interrogation modes SSR Mode P1- P3 pulse spacing Application 1 3us Identity (military) THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY  2 5us Identity (military) 3/A 8us Identity (military/civil) B 17us Identity C 21us Altitude D 25us Undefined S 3.5us Multipurpose The aircraft equipment (transponder) will receive the interrogation pulses, process and respond as appropriate. A mode-A interrogation (one used in air navigation for identity) will elicits a 12-pulse reply, indicating an identity number associated with that aircraft. The 12 data pulses are bracketed by two framing pulses, F1 and F2. The X pulse is not used. A mode -C interrogation produces an 11-pulse response (pulse D1 is not used), indicating aircraft altitude as indicated by its altimeter in 100-foot increments. Note:  Mode B gave a similar response to mode A and was at one time used in Australia.  Mode D has never been used operationally. This reply is provided through a frequency of 1090 MHz providing aircraft information. The reply is displayed as a tagged icon on the controller's radar screen at the measured bearing and range. An aircraft without an operating transponder still may be observed by primary radar, but would be displayed to the controller without the benefit of SSR derived data. It is typically a requirement to have a working transponder in order to fly in controlled air space and many aircraft have a back-up transponder to ensure that condition is met. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The ground interrogator can be mounted on the same site with a primary radar. Diagram of an SSR Antenna collocated with a PSR Antenna Typically, two Mode A interrogations are followed by a Mode C interrogation. The reason for using Mode A more frequently is that the identity of the aircraft (the SSR code) is of greater importance to the controller. The level (altitude) received from the transponder is always in respect to standard pressure (1013.25 hPa, 29.92" Hg) regardless of the altimeter setting selected by the pilot. This is a mitigation against human error. As a result, when the controller observes that two aircraft are separated by 1000 feet, it means that this separation exists regardless of the altimeter settings of the two aircraft. Advantages of SSR operation.  Requires much less power to achieve the desired range, in comparison to PSR.  The transmitted signal only needs to reach the aircraft, while the PSR needs to emit a signal strong enough to reach the aircraft and travel back to the antenna.  The information provided is not limited to range and bearing from the antenna but also includes additional data based on the transponder mode of operation (A, C or S). THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   Targets are easier to distinguish due to the different SSR codes.  SSR is immune to clutter as it uses different frequencies for interrogation (1030 MHz) and replies (1090 MHz). Disadvantages of SSR operations  The SSR relies on the onboard equipment to discover aircraft.  In case of transponder failure, the SSR will receive no reply and will therefore not discover the target.  This is mitigated by combining the SSR with a PSR.  If proper signal processing is used, it is possible to continue to track an aircraft (and preserve correlation) even if the transponder has failed completely provided that reliable primary data is received.  Note that in this case level information will be less reliable and more frequent pilot reports will be necessary.  Sometimes two replies are received at the same time (if the slant range and the bearings of the aircraft the same).  This phenomenon is called "garbling" and may result either in: o The "detection" of a false (non-existing) aircraft o In a target not being detected o This is considered a false target by the radar  A phenomenon called FRUIT also occurs in SSR operations.  FRUIT (False Replies Unsynchronized In Time or False Replies Unsynchronized to Interrogator Transmissions).  This happens when the radar receives a reply from a transponder that has been interrogated by another radar.  SSRs operate on the same frequencies, it is not possible to detect that the reply is related to another radar's transmission. Moreover, as the time of the interrogation is not known, the range calculation will most likely be wrong.  A false target may appear on the situational display.  If another (valid) transponder reply is received at the same time, garbling could occur.  SSRs are vulnerable to antenna shadowing THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   The onboard antenna is shadowed by the aircraft fuselage, e.g. due to the bank angle).  This is mitigated by placing more than one antenna (usually two - one on top of the aircraft and one at the bottom). Garbling and FRUIT are aggravated by the need of "classic" SSRs to use several interrogations for proper azimuth determination and can be mitigated by: o Using an MSSR (Mono pulse Secondary Surveillance Radar). Mono Pulse Secondary Surveillance Radar (MSSR) System Mono pulse secondary surveillance radar (MSSR) is an advanced radar that uses a different beam pattern that provides more accurate azimuth determination. As a result, fewer interrogations are required to determine the azimuth. It uses additional encoding of the radio signal to provide accurate directional information. The name refers to its ability to extract range direction from a single pulse. Monopulse systems have been classified as:  Phase comparison monopulse  Amplitude monopulse MSSR Principal of Operation The system is intended to operate with just a single reply from an aircraft, a system known as monopulse. It relies heavenly on the main or “sum” beam of the SSR antenna to get the required information.To produce the sum beam the signal is distributed horizontally across the antenna aperture.  The antenna feed system is divided into two equal halves (sum and difference)  The two parts summed again to produce the original sum beam.  The two halves are also subtracted to produce a difference output. A signal arriving exactly normal, or boresight, to the antenna will produce a maximum output in the sum beam but a zero signal in the difference beam. A signal arriving away THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY from the boresight will produce a less signal in the sum beam and non-zero signal in the difference beam. The angle of arrival of the signal can be determined by measuring the ratio of the signals between the sum and difference beams. The ambiguity about boresight can be resolved as there is a 180° phase change in the difference signal either side of boresight. Bearing measurements can be made on a single pulse, hence monopulse, but accuracy can be improved by averaging measurements made on several or all of the pulses received in a reply from an aircraft. Diagram of a Monopulse Beam The system was developed to overcome two (2) common problems with conventional systems namely:  Garbling- when several aircrafts are in close proximity making the transponder replies overlap  FRUIT- Replies being received by interrogators who have not sent interrogation pulses. These problems are resolved by analysing the received signals using a computer and by transmitting from the radar at a much reduced rate (about one tenth of the previously used rate). As a result, Garbling and Fruit are reduced by about 90% while directional accuracy is tripled compared to conventional SSR. The improved accuracy of MSSR allows radar separation minima to be reduced by about one half - to 3 nm if the aircraft is within 40 nm of the antenna and 5 nm if more than 40nm from the radar antenna. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY Mode S Radars Mode S is a Secondary Surveillance Radar process that allows selective interrogation of aircraft according to the unique 24-bit address assigned to each aircraft. Recent developments have enhanced the value of Mode S by introducing Mode S EHS (Enhanced Surveillance). Mode S operates on the principle that interrogations are directed to a specific aircraft using that aircraft's unique address. This results in a single reply with aircraft range determined by the time taken to receive the reply and monopulse providing an accurate bearing measurement. In order to interrogate an aircraft its address must be known. To meet this requirement, the ground interrogator also broadcasts All-Call interrogations, which are in two forms A mode S interrogation comprises two 0.8 µs wide pulses, which are interpreted by a mode A & C transponder as coming from an antenna sidelobe and therefore a reply is not required. The following long P6 pulse is phase modulated with the first phase reversal, after 1.25 µs, synchronising the transponder's phase detector. Subsequent phase reversals indicate a data bit of 1, with no phase reversal indicating a bit of value 0. This form of modulation provides some resistance to corruption by a chance overlapping pulse from another ground interrogator. The interrogation may be short with P6 = 16.125 µs, mainly used to obtain a position update, or long, P6 = 30.25 µs, if an additional 56 data bits are included. The final 24 bits contain both the parity and address of the aircraft. On receiving an interrogation, an aircraft will decode the data and calculate the parity. If the remainder is not the address of the aircraft, then either the interrogation was not intended for it or it was corrupted. In either case it will not reply. If the ground station was expecting a reply and did not receive one, then it will re-interrogate. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The aircraft reply consists of a preamble of four pulses spaced so that they cannot be erroneously formed from overlapping mode A or C replies. The remaining pulses contain data using pulse position amplitude modulation.  Each 1 µs interval is divided into two parts.  When a 0.5 µs pulse occupies the first half and there is no pulse in the second half then a binary 1 is indicated.  When there no pulse in the first half and 0.5 µs pulse occupies the second half then a binary 0 is indicated.  The data is transmitted twice, the second time in inverted form. This format is very resistant to error due to a garbling reply from another aircraft. The reply also has parity and address in the final 24 bits. The ground station tracks the aircraft and uses the predicted position to indicate the range and bearing of the aircraft so it can interrogate again and get an update of its position. If it is expecting a reply and if it receives one, then it checks the remainder from the parity check against the address of the expected aircraft. If it is not the same then either it is the wrong aircraft and a re-interrogation is necessary, or the reply has been corrupted by interference by being garbled by another reply. The parity system has the power to correct errors as long as they do not exceed 24 µs, which embraces the duration of a mode A or C reply, the most expected source of interference in the early days of Mode S. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The pulses in the reply have:  Individual monopulse angle measurements available,  Signal strength measurements This can indicate bits that are inconsistent with the majority of the other bits, thereby indicating possible corruption. A test is made by inverting the state of some or all of these bits (a 0 changed to a 1 or vice versa) and if the parity check now succeeds the changes are made permanent and the reply accepted. If it fails then a re-interrogation is required Mode S- All Call Operation In one form, the Mode A/C/S All-Call looks like a conventional Mode A or C interrogation at first and a transponder will start the reply process on receipt of pulse P3. Mode S transponder will abort this procedure upon the detection of pulse P4, and instead respond with a short Mode S reply containing its 24- bit address. This form of All-Call interrogation is now not much used as it will continue to obtain replies from aircraft already known and give rise to unnecessary interference. The alternative form of All-Call uses short Mode S interrogation with a 16.125 µs data block. This can include an indication of the interrogator transmitting the All-Call with the request that if the aircraft has already replied to this interrogator then do not reply again as aircraft is already known and a reply unnecessary. THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY The Mode S interrogation can take three forms as indicated below Name Form Use Surveillance Short Position update Comm-A Long Contains 56 data bits Up to 16 long interrogations strung together to transmit up to 1280 Comm-C Long bits The Mode S reply can take three forms as indicated below Name Form Use Surveillance Short Position update Comm-B Long Contains 56 data bits Up to 16 long interrogations strung together to transmit up to 1280 Comm-D Long bits Advantages of Mode S Operation The following are the advantages of Mode S operation to air navigation in a state: THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   Improved situation awareness  A clearer air situation picture, enhanced tracking and access to pertinent information direct from the aircraft enables the controller to benefit from quicker and more accurate recognition of airborne events.  Progressive reduction of R/T workload per flight.  There is scope for R/T usage between controller and individual flight under service to be reduced following the progressive introduction of Mode S Enhanced Surveillance.  This applies in particular to the requirement for SSR code verification procedures and also where system enhancements and/or the display of downlink aircraft parameters obviate the need for certain voice communication exchanges  Safety enhancement.  Access by controllers to aircraft intent DAPs, such as selected altitude enables: o Cross-checking of climb/descent instructions o Helps the early identification of potential level bust incidents.  Increased target capacity.  Mode S radars are able to process many more aircraft tracks (approximately double the number) than conventional MSSR installations.  The 24 bits in each case is combined aircraft address and parity Multilatration (MLAT) Surveillance Operations in Air Navigation MLAT is a surveillance application that accurately establishes the position of transmissions, matches any identity data (octal code, aircraft address or flight identification) that is part of the transmission and sends it to the ATM system. It is a proven technology that has been in use for many decades in both navigation and surveillance applications. It is based on a methodology known as Time Difference of Arrival (TDOA) which can be utilized in one of two ways:  The signal from a mobile unit is measured at a number of known, fixed locations  The signals from a number of fixed locations are measured by a mobile receiver The TDOA of the signals at the receiver(s) allows the position of the mobile entity to be determined. An MLAT system consists of the following components: THE TRAINING ARM OF KENYA CIVIL AVIATION AUTHORITY   A transmitting subsystem that includes interrogation message generation and transmission function;  A receiving antenna array subsystem that receives the transmissions from the target and timestamps receipt at each antenna  A central processor that calculates and outputs the MLAT Note:  Interrogation transmitter ensures regularity of responses from the target aircraft  The target aircraft must have a subsystem that will respond to an interrogation (transponder) In response to an interrogation signal from one of the MLat sensors, the vehicle or aircraft transponder will transmit a reply that will be received and processed by all of the MLat sites. The variance of TDOA at the various ground sites will allow accurate determination of th