Chapter 2 Aircraft Communication Systems PDF

Official (Open) Chapter 2 Aircraft Communication Systems page 1 of 75 Effective date 14 Oct 2024 SEG Official (Open) Chap 2 : Aircraft Communication Systems Topics 2.1 Pri...

Official (Open) Chapter 2 Aircraft Communication Systems page 1 of 75 Effective date 14 Oct 2024 SEG Official (Open) Chap 2 : Aircraft Communication Systems Topics 2.1 Principles of Radio Communications 2.2 Very High Frequency (VHF) Communication System 2.3 High Frequency (HF) Communication System 2.4 Satellite Communication System 2.5 Intercommunication and Passenger Address Systems 2.6 Inflight Entertainment (IFE) Systems page 2 of 75 Effective date 14 Oct 2024 SEG Official (Open) Chap 2.1: Principles of Radio Communications Topics Characteristics of Electromagnetic wave Radio wave propagation mechanism Modes of radio wave propagation HF radio communication VHF/UHF radio communication Path loss models Communication Radio System Design Terminology Regulatory and Advisory agencies page 3 of 75 Effective date 14 Oct 2024 SEG Official (Open) Characteristics of Electromagnetic wave Information to be transmitted must be converted into an electrical signal that is compatible with the medium. This is called an Electromagnetic wave, or in short EM wave. This electrical signal consists of both Electric (E) and Magnetic (M) fields. This is called an Electromagnetic wave, or in short, EM wave. Radio Frequency (RF) wave is part of the EM wave. When the electric field E and the magnetic field H are transverse to the direction of propagation, the EM wave is known as a Transverse Electromagnetic Wave (TEM). y Direction of E H propagation H z E x TEM wave showing E-field and H-field page 4 of 75 Effective date 14 Oct 2024 SEG Official (Open) Characteristics of Electromagnetic wave EM waves can travel through long distances, through guided medium such as transmission wires or unguided medium such as the free space or vacuum. In radio communication, it is the radiated magnetic field that cuts the conductor of a remote antenna that generates back the signal. Electromagnetic Rx Tx wave (thru air) page 5 of 75 Effective date 14 Oct 2024 SEG Official (Open) Characteristics of Electromagnetic wave Polarization Polarization of an EM wave refers to the orientation of the E-field. Two common types of polarization are: Vertical Polarization Horizontal Polarization When the E field is in the vertical direction, the EM wave is known as a vertically polarized EM wave. A vertical antenna will produce a vertically polarized wave which will require a vertical antenna to receive it. Receiving Antenna Transmitting E field (Vertical) Antenna (Vertical) Direction of propagation Vertical polarization page 6 of 75 Effective date 14 Oct 2024 SEG Official (Open) Characteristics of Electromagnetic wave When the E field is in the horizontal direction, the EM wave is known as horizontal polarization. A horizontal antenna will produce a horizontal polarized wave. This effect can be seen on any aircraft. VHF communication frequencies are vertically polarized, and the aircraft antenna are vertical. Navigation frequencies are horizontal, and the antenna are horizontal, often in a “V” shape on the fin of the aeroplane. Transmitting Antenna (Horizontal) Receiving Antenna (Horizontal) E field Horizontal polarization page 7 of 75 Effective date 14 Oct 2024 SEG Official (Open) Characteristics of Electromagnetic wave Wavelength and frequency The wavelength  of an electromagnetic wave is defined as:  where  = wavelength (period) = = velocity of EM wave f f = frequency of EM wave A wavelength is the distance traveled by the EM wave in a time duration of 1 period of the signal. The velocity of propagation for the EM waves through free space or vacuum is a constant and is equal to 3 x 108 m/s. It is usually denoted by the letter ‘c’. When the same EM wave propagates through any other transmission media, its velocity will be lower than c. Frequency of the EM wave is inversely proportional to the 1 wavelength, f  i.e higher frequency signals have  shorter wavelength. page 8 of 75 Effective date 14 Oct 2024 SEG Official (Open) Characteristics of Electromagnetic wave Example 1. A radio signal has a wavelength of 3.5cm. What is its frequency? c 3 x108 f = =  3.5 x10 − 2 = 8.57GHz. 2. A radio signal has a frequency of 118MHz? What is its wavelength? c 3 x108  = = f 118 x106 = 2.54m. 3. A weather radar station uses a frequency of 9375MHz. What is the wavelength of the signal? c 3 x108  = = f 9375 x10 6 = 3.2cm. Size of order of raindrops page 9 of 75 Effective date 14 Oct 2024 SEG Official (Open) Characteristics of Electromagnetic wave Wavelength Electromagnetic spectrum 30 Hz 107 m 300 Hz ELF 106 m The entire range of VF Legend: 3 kHz 105 m frequencies that is occupied VLF 104 m Radio waves: by the electromagnetic waves 30 kHz LF EHF = Extremely high frequency is referred to as the 300 kHz 103m (Microwaves) electromagnetic spectrum. 3 MHz MF 102 m SHF = Super high frequency HF (Microwaves) 30 MHz 10 m UHF = Ultrahigh frequency 300 MHz VHF 1m VHF = Very high frequency The spectrum is divided into HF = High frequency 3 GHz UHF 10-1 m many subsections or bands, MF = Medium frequency each with some technical 30 GHz SHF 10-2 m LF = Low frequency EHF VLF = Very low frequency characteristics and 300 GHz 10-3 m VF = Voice frequency Millimeter waves peculiarities unique to that 10-4 m ELF = Extremely low frequency band and each allocated to 10-5 m different commercial, 10-6 m Frequency aviation and military 0.7 x 10-6 m Visible Light applications. 0.4 x 10-6 m etc. cosmic rays, gamma rays, X-rays, page 10 of 75 Effective date 14 Oct 2024 SEG Official (Open) Characteristics of Electromagnetic wave Electromagnetic spectrum Allocation of frequencies is made by world-wide agreements under the control of the International Telecommunications Union (ITU). Representatives from different countries meet at regular ITU conference to decide what services should be allowed to use which frequencies. Usually, some government regulatory bodies regulate the use of each of these frequency spectrums and the power levels to use. These are necessary so as not to cause interference to each other’s transmission. page 11 of 75 Effective date 14 Oct 2024 SEG Official (Open) Characteristics of Electromagnetic wave EM radio spectrum for aviation Many airborne navigation and communication systems uses radio wave propagation, and the frequencies of these systems span from very low frequencies to the microwave region. Aviation-use frequencies are spread throughout the entire frequency spectrum and are chosen for a required characteristics of the frequency. The very low frequencies are required for long distance navigation because of their predictable propagation characteristics. The low frequencies [190kHz to 1750kHz] of the Non-directional beacons (NDB) are chosen for their predictable propagation characteristics at short distances. The frequencies in the HF [3MHz to 30MHz] region are chosen for their long-distance propagation. The frequencies in the VHF [30MHz to 300MHz] and UHF [300MHz to 3GHz] region are chosen for their line-of-sight (LOS) propagation and relative freedom from atmospheric noise. The very high microwave frequencies [~10GHz] used by weather radar at the top of the EM spectrum are necessary because the very short wavelengths are easily reflected from raindrops in storm clouds. page 12 of 75 Effective date 14 Oct 2024 SEG Official (Open) Characteristics of Electromagnetic wave 3kHz Omega(10-13kHz) 30kHz 100kHz LORAN-C 300kHz 200kHz-500kHz Non-directional beacon 550kHz-1.6MHHz Standard broadcast 1.8-1.9MHz LORAN-A 3MHz 3-30MHz HF SSB 30MHz 75MHz Marker beacon 108-136MHz VHF Nav & Comm 300MHz 329-335MHz Glide Slope 960-1215MHz Distance Measuring Equipment 3GHz 1030,1090MHz Air Traffic Control Radar 4300MHz Radar Altimeter 5030-5090MHz Microwave Landing System 9375MHz Weather Radar 30GHz EM radio spectrum for aviation Source: Practical Aircraft Electronic Systems page 13 of 75 Effective date 14 Oct 2024 SEG Official (Open) Radio wave propagation mechanisms Electromagnetic signals, after leaving the transmitting antenna, will experience the following propagation mechanisms: Reflection ▪ When the EM signal impinges on an object whose dimension is very large compared to the wavelength of the EM signal. ▪ Reflections occurs from the surface of the object such as earth, building, etc. Refraction ▪ Bending of EM wave when the it passes from one density medium to another. Diffraction ▪ Bending of the EM signal around the sharp objects. Scattering ▪ When the EM wave encounters a rough surface or small objects, such as dust particles, leaves and raindrops, whose dimensions are small compared to the wavelength of the EM signal, it will be diffused or scattered in all directions. These propagation mechanisms are present to a certain extent in the different modes of radio wave propagation to be discussed later. page 14 of 75 Effective date 14 Oct 2024 SEG Official (Open) Radio wave propagation mechanisms wave is refracted Angle of refraction (or bent) Media #2 (denser) Angle of Media #1 incidence Reflected wave incident wave Angle of reflection Radio wave can be reflected or refracted when it encounter a boundary of 2 dissimilar medium page 15 of 75 Effective date 14 Oct 2024 SEG Official (Open) Radio wave propagation mechanisms Reflection Radio waves may be reflected from various substances or objects they meet during propagation if the dimension of the object is very large compared to the wavelength of the radio wave. The amount of reflection depends on the reflecting material. The surface of the Earth itself is a fairly good reflector. When radio waves are reflected, a phase shift will normally occur. ▪ Radio waves that keep their phase relationships (in-phase) after reflection normally produce a stronger signal at the receiving site. ▪ Those that received out of phase produce a weaker signal - fading. In the extreme, the signals could just cancel each other. page 16 of 75 Effective date 14 Oct 2024 SEG Official (Open) Radio wave propagation mechanisms R1 Incident wavefronts are in phase R2 Tx Earth’s Surface Rx Received signal = R1 + R2. Note their phase changes. In extreme cases, the phase changes are Phase Shift of Reflected Radio Waves exactly 180 degrees out of phase resulting in total cancellation of the received signal. page 17 of 75 Effective date 14 Oct 2024 SEG Official (Open) Radio wave propagation mechanisms Refraction The bending of the radio waves as it move from one medium into another medium due to the radio waves traveling with a different velocity of propagation in different medium. As the wavefront enters the dense layer of electrically charged ions, the part of the wavefront that enters the new medium first travels faster than the part of the wavefront that have not yet entered the new medium. This abrupt increase in velocity of the upper part of the wavefront causes the wave to bend back towards the Earth. Radio waves passing through the atmosphere are affected by factors, such as temperature, pressure, humidity, and density. These factors can cause the radio waves to be refracted. Examples where refraction of radio wave occurs are the ionosphere and the troposphere. page 18 of 75 Effective date 14 Oct 2024 SEG Official (Open) Radio wave propagation mechanisms Wavefront of the signal that first Maximum electron density enter the medium 2 now travel (radio signal travel faster) faster and hence bend the wave Travelling faster ➔ greater distance travelled Medium #2 (denser) Medium #1 Travelling slower ➔ lesser distance travelled Refraction of EM Wave Decreasing Ionization ionosphere Max. Ionization Increasing Ionization Effects of Ionospheric Density on Radio Waves page 19 of 75 Effective date 14 Oct 2024 SEG Official (Open) Radio wave propagation mechanisms Diffraction When radio waves meet a sharp obstacle, they have a natural tendency to bend around the tip of the obstacle. This results in a change of direction of part of the wave energy from the normal line-of-sight path. Because of this change, it is possible to receive energy around the edges of an obstacle as shown or at some distances below the highest point of an obstruction in the shadow zone. Although diffracted radio frequency energy is weaker, it can still be detected by a suitable receiver. The principal effect of diffraction extends the radio range beyond the visible horizon – radio horizon. page 20 of 75 Effective date 14 Oct 2024 SEG Official (Open) Radio wave propagation mechanisms Direct wave Weak diffracted wave Diffraction around a sharp object page 21 of 75 Effective date 14 Oct 2024 SEG Official (Open) Radio wave propagation mechanisms Scattering When the radio waves meet a rough surface or very small objects whose dimensions are comparable to the radio wave such dust particles, leaves or raindrops, it will tend to be reflected into many directions (scattered). This scattering effect is employed by weather radar to detect the presence of rain nearby. The effect is best observed at around 10GHz. page 22 of 75 Effective date 14 Oct 2024 SEG Official (Open) Radio wave propagation mechanisms satcom Ionosphere Scattered waves Rain cloud 10GHz HF SHF Weather radar VHF VHF VHF Scenario showing various propagation effects page 23 of 75 Effective date 14 Oct 2024 SEG Official (Open) Modes of radio wave propagation In radio communication, EM waves can be propagated in several ways depending on: The type of system The environment EM waves will travel in a straight line. Paths can be altered by the earth and its atmosphere. Different ways of EM wave propagation: Space waves → Direct wave (line-of-sight) & ground-reflected waves. Ground waves → surface waves. Tropospheric propagation → makes use refraction in the troposphere. Sky waves → makes use of refraction in the ionosphere. page 24 of 75 Effective date 14 Oct 2024 SEG Official (Open) Modes of radio wave propagation Satcom Earth’s Atmosphere Ionosphere Tropospheric Propagation Direct LOS wave space wave = Direct wave + ground reflected wave Transmit Receive Antenna Antenna Surface wave Earth’s Surface page 25 of 75 Effective date 14 Oct 2024 SEG Official (Open) Modes of radio wave propagation Ground wave propagation A Ground wave is an EM wave that travels along the surface of the earth from the transmitter to the receiver. As such, they are sometimes also referred to as surface waves. As the radio waves travel over hills or buildings, they are diffracted. This tends to make the wave follow the curvature of the earth. Ground waves remain relatively unaffected by changes in the earth's atmosphere since they are affected only by the electrical characteristics of the earth's surface - conductivity and dielectric constant of the earth. Ground waves must be vertically polarized. The electric field in a horizontally polarized wave would be parallel to the earth’s surface, and such waves would be short circuited by the conductivity of the earth’s surface. With Ground waves, the changing electric field will induce voltages in the earth’s surface, which will cause current to flow in the earth’s surface (energy will be expended). Earth surface also has resistance and dielectric losses. Therefore, Ground waves are attenuated as they propagate, and hence only short distance transmission is achieved through the use of Ground waves. page 26 of 75 Effective date 14 Oct 2024 SEG Official (Open) Modes of radio wave propagation Ground wave propagates best over a surface that is a good conductor, such as salt water, and poorly over dry desert areas. At frequencies below 1.5 MHz (MF radio frequency band), Ground waves provide the best coverage. This is because Ground losses increase rapidly with increasing frequency. Ground wave is commonly used for ship-to-ship and ship-to-shore communications, for radio navigation, and for maritime mobile communications. page 27 of 75 Effective date 14 Oct 2024 SEG Official (Open) Modes of radio wave propagation Advantages of Ground wave Given enough transmit power, Ground waves can be used to communicate between any 2 locations in the world. Ground waves are relatively unaffected by changing atmospheric conditions. Disadvantages of Ground wave Ground waves require a relatively high transmission power. Since Ground waves are limited to VLF, LF, and MF frequencies, they require large antenna. Ground losses vary considerably with surface material. page 28 of 75 Effective date 14 Oct 2024 SEG Official (Open) Modes of radio wave propagation Sky Wave Propagation Sky wave is the most frequently used method for long distance transmission. Sky waves are radiated in a direction towards the sky at a relatively large angle with reference to the earth. The radio waves are refracted back to earth by the ionosphere. Sky waves are much affected by changing conditions in the ionosphere as it depends on the ionosphere to refract the signal back to earth. Sky waves propagation depends on: Time of the day Locations of transmitter and receiver Sunspot activity(when sunspot activity are low, frequencies above 20 MHz are unusable because the E & F layers are too weakly ionised to refract signals back to the earth) Sudden ionospheric disturbances due to solar flares causing most HF signals to be absorbed at the D layer Sky waves are used for HF communications. page 29 of 75 Effective date 14 Oct 2024 SEG Official (Open) Modes of radio wave propagation Advantages of Sky wave Satellite and terrestrial communications are vulnerable to physical destruction and ECM attack (electronics countermeasure) during the war. High frequency radio transmission offers the advantage of long-distance communication and has a higher survival rate in harsh environments. The use of suitable man-pack High Frequency radio solved the communication problem in thick jungle, mountainous terrain and in environment where Line-Of- Sight transmission are often not possible. Disadvantages of Sky wave Quality and reliability of high frequency radio wave propagation is dependent on frequency, times of the day, seasons and degree of ionospheric disturbances.  In other words, one frequency that may be propagating well during certain periods of the day or night but may be poor or cannot be used during other periods of the day or night. page 30 of 75 Effective date 14 Oct 2024 SEG Official (Open) Modes of radio wave propagation Ionosphere The Ionosphere is divided into four distinct regions of 300km D Layer 200km E Layer 125km F1 Layer D E F 1 F2 75km F2 Layer It ranges from 48 km up to 400 km. These three layers vary in location and in ionization level with the time of the day. They also fluctuate in a cyclical pattern throughout the year The ionosphere refracts radio waves of specific frequencies, primarily HF (3 –30 MHz “shortwave”). It is this refraction of radio energy that makes worldwide HF radio communication possible without the aid of satellites. Each layer represent an increase in ion density on the one below it. Thus, the lowest layer, the D layer, is the weakest and disappear at night. page 31 of 75 Effective date 14 Oct 2024 SEG Official (Open) Modes of radio wave propagation Not all the layers are present all the time and their height and density vary considerably with the time of the day, and to a lesser extent, season. The ionosphere is weaker at night than during the day because the ionising solar radiation is not present. The D layer absorbs lower HF The two F layers combine frequencies and allows higher into one F layer at night frequencies to pass through it. The E layer refracts radio signals and caused them to skip back to Earth. The F-layer is the most common mode of propagation for HF skywaves. D layer disappears at night page 32 of 75 Effective date 14 Oct 2024 SEG Official (Open) Modes of radio wave propagation Effects of the Ionosphere on the Sky Wave The ability of the ionosphere to return a radio wave to the earth depends on the ionosphere’s refraction property. Refraction of the radio wave depends on ionization density, the frequency of the radio wave and the angle of the transmission. The higher the ionization density, the greater will be the refraction. In general, the ionization density increases from the D-layer to the F2 layer. Beyond that it decreases again. Therefore, there is less refraction at the D-layer than F2 –layer and this means the skip distance is lesser at the D-layer and longer at the F2 –layer. Critical frequency Lower frequency signals are more easily refracted back as compared to higher frequency signal. Higher frequency radio wave will travel further up into the ionosphere and if refracted back, the distance covered will be longer. If the frequency is sufficiently high, the wave will penetrate all layers of the ionosphere and continue out to space (signal lost). The highest frequency of the radio wave when transmitted vertically and such that the radio wave will be refracted sufficiently to turn back to earth is called the critical frequency. Critical frequency depends on the ionization density and therefore varies with the time of the day. page 33 of 75 Effective date 14 Oct 2024 SEG Official (Open) Modes of radio wave propagation Critical Angle The angle of radiation is important in determining whether a particular frequency will be returned to earth by refraction from the ionosphere. The least angle from the vertical direction at which a radio wave of a specific frequency can be propagated and still be refracted from the ionosphere is called the critical angle at that particular frequency. For each frequency, there is a critical angle. If a signal of this frequency is beam at a smaller angle than the critical angle, then refraction will not occur. Skip Distance The minimum distance from the transmitter, when the signal is at the critical angle, to where the sky wave can be returned to earth (receiver). The maximum skip distance occurs when the signal leaves the earth at a tangent and is restricted by the curvature of the earth. This is a theoretical limit as terrain will affect actual distance. Max skip dis tan ce = 143x ht of ionoshere (km) nautical miles = 264.8 x ht of ionoshere (km) km page 34 of 75 Effective date 14 Oct 2024 SEG Official (Open) Modes of radio wave propagation Freq B Freq C Beam at less than critical angle (signal lost) ionosphere Critical angle Freq B For the freq B Freq A ∡  Angle of radiation Freq C> B > A Rx Skip distance page 35 of 75 Effective date 14 Oct 2024 SEG Official (Open) Modes of radio wave propagation Blind Zone Also called the Skip zone. The area which is outside the coverage area of either the Ground waves or normal ionospheric sky waves. If a radio receiver is located in the blind zone, it will receive no signal or very weak signal. page 36 of 75 Effective date 14 Oct 2024 SEG Official (Open) Modes of radio wave propagation Example 1. What is the maximum theoretical skip distance of a HF wave refracted from the E- layer? E-layer is about 125km high. 𝑀𝑎𝑥 𝑠𝑘𝑖𝑝 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 = 143𝑥 ℎ𝑡 𝑜𝑓 𝑖𝑜𝑛𝑜𝑠ℎ𝑒𝑟𝑒 (𝑘𝑚) 𝑛𝑎𝑢𝑡𝑖𝑐𝑎𝑙 𝑚𝑖𝑙𝑒𝑠 = 143 125 = 1598 𝑛𝑎𝑢𝑡𝑖𝑐𝑎𝑙 𝑚𝑖𝑙𝑒𝑠 = 1598𝑥1.852 = 2961 𝑘𝑚 page 37 of 75 Effective date 14 Oct 2024 SEG Official (Open) HF(High Frequency) Radio Communications HF Communications For long distance communication using sky wave, it would appear that low frequencies are the most suitable as they refract best. However, there are several disadvantages: As the frequencies are low, very large antennas would be required which make them uneconomical. The low frequencies suffer most from ionospheric attenuation and noise static. The HF band is a reasonable alternative offering predictable skywave propagation with less attenuation and static and lower costs. The HF frequencies used in aviation range from 2MHz to 22MHz. The choice of frequency for the range and conditions can be quite important. Attenuation and static must be minimized by keeping the frequency as high as possible and the receiver must be kept out of the skip zone. page 38 of 75 Effective date 14 Oct 2024 SEG Official (Open) HF Radio Communications Selection of a suitable operating frequency is very important to maintain reliable HF communications. For successful communications between any two specified locations at any given time of the day, there is a: Maximum Usable Frequency (MUF), Lowest Usable Frequency (LUF) and an Optimum Working Frequency (OWF) Maximum Usable Frequency As discussed earlier, the higher the frequency of a radio wave, the lower the rate of refraction by an ionized layer. Therefore, for a given angle of incidence and time of day, there is a maximum frequency that the skywave is returned back to earth. This frequency is known as the Maximum Usable Frequency (MUF). The use of an established MUF does not guarantee successful communications between a transmitter and a receiver as the constantly changing conditions in the ionosphere will result in slight variations in the skip distance which would move the receiver into and out of the skip zone constantly interrupting the signal. page 39 of 75 Effective date 14 Oct 2024 SEG Official (Open) HF Radio Communications It is often a good idea to use a frequency that is closer to the MUF, to reduce attenuation and increase clarity as lower frequency may be more prone to absorption. Waves at frequencies above the MUF are normally refracted so slowly that they return to Earth beyond the desired location or penetrate through the ionosphere and are lost. The relationship between MUF fmuf , critical frequency fcrit and angle of incidence  is given by: f f muf = crit sin  >MUF ionosphere fcrit fmuf >MUF ∡ Rx Transmitter page 40 of 75 Effective date 14 Oct 2024 SEG Official (Open) HF Radio Communications Lowest Usable Frequency (LUF) Similarly, frequencies lower than MUF will also be received in the same position, but ionospheric attenuation and static will increase to the point where the signal is inaudible. This is the Lowest Usable frequency (LUF). ▪ So, a wave whose frequency is below the established LUF is refracted back to Earth at a shorter distance than desired and will not be picked up by the receiver. ▪ Also, as the frequency of a radio wave is lowered, absorption of the radio wave increases. A wave whose frequency is too low is absorbed to such an extent that it is too weak for reception, even if it reaches the receiver. page 41 of 75 Effective date 14 Oct 2024 SEG Official (Open) HF Radio Communications ionosphere LUF ∡ < LUF Receiver Transmitter Refraction of the Radio Wave whose frequency is < LUF will “fall short” of the target receiver page 42 of 75 Effective date 14 Oct 2024 SEG Official (Open) HF Radio Communications Optimum Working Frequency Neither the MUF nor the LUF is a practical operating frequency. ▪ While radio waves at the LUF can be refracted back to Earth at the desired location, the signal-to-noise ratio is still much lower than at the higher frequencies (ionosphere absorb more lower frequency energy) ▪ Operating at or near the MUF can result in frequent signal dropouts when ionospheric variations alter the length of the transmission path. The most practical operating frequency is one that can avoid the problems of absorption but not so high as to result in the adverse effects of rapid changes in the ionosphere. The frequency that meets the above criteria is known as the Optimum Working Frequency fowf fowf = 0.85fmuf page 43 of 75 Effective date 14 Oct 2024 SEG Official (Open) HF Radio Communications In summary, the following factors will affect the range and hence the quality of HF transmission. Transmit power Frequency Time of the day Disturbances in the ionosphere HF signals are usually Single-Side Band with suppressed carrier (SSBSC) to reduce the bandwidth and keep the required transmission power low. HF prediction software: grafex. page 44 of 75 Effective date 14 Oct 2024 SEG Official (Open) VHF/UHF Radio Communications Space Wave Propagation There are two types of space waves : Direct Wave and Ground Reflected Waves Direct wave ▪ The primary path of space wave. Signal travels in a straight line (Line of Sight, LOS) from transmitter to receiver. ▪ The LOS distance (optical distance) is the physical distance seen between the transmit and receive sites. It is affected by the antenna height and the curvature of the earth. ▪ Radio Horizon is the distance “seen by the radio waves” and is much longer than the optical distance because of refraction in the troposphere. Ground reflected wave ▪ The wave that reaches the receiving antenna after it has been reflected off from the earth's surface. ▪ Signal fading and/or distortion will result if the direct wave and the Ground reflected waves are not in phase – ghost images. Space waves are used for VHF frequencies and above. page 45 of 75 Effective date 14 Oct 2024 SEG Official (Open) VHF/UHF Radio Communications Direct wave Space wave =Direct wave + Ground reflected wave page 46 of 75 Effective date 14 Oct 2024 SEG Official (Open) VHF/UHF Radio Communications Radio horizon Radio wave path (refraction from troposphere) Transmit Receive Antenna Direct LOS path (optical distance) Antenna dt hr ht dr Radio Horizon, d Empirical Formula for Radio Horizon d is: dt = 4 h t , dr = 4 hr d = dt + dr where dt and dr = distance in km d = 4 ht + 4 hr and ht and hr = height in metre The radio horizon d is normally one-third farther than the optical distance. page 47 of 75 Effective date 14 Oct 2024 SEG Official (Open) VHF/UHF Radio Communications Example 1. Two aircraft are flying at a height of 10000m. The 2 aircrafts are communicating via VHF. Calculate the theoretical maximum communication range between the 2 aircrafts ? 𝑅𝑎𝑑𝑖𝑜 ℎ𝑜𝑟𝑖𝑧𝑜𝑛 = 4 ℎ𝑡 (𝑚) + 4 ℎ𝑟 (𝑚) 𝑘𝑚 = 2𝑥4 10000 = 800 𝑘𝑚. Note: The actual range will depend on several factors such as the Transmitter (Tx) power, the Receiver (Rx) sensitivity, terrain and weather conditions. page 48 of 75 Effective date 14 Oct 2024 SEG Official (Open) VHF/UHF Radio Communications Example An aircraft flying at 3000m is communicating with a ground station at the horizon on a VHF frequency. Estimate the maximum distance of the ground station from the aircraft that can maintain a communication link. We use half the radio horizon, since the ground station is at the horizon. 𝑴𝒂𝒙. 𝒅𝒊𝒔𝒕𝒂𝒏𝒄𝒆 (𝑑) = 4 ℎ𝑡 (𝑚) 𝑘𝑚 3000m = 4 3000 = 219 𝑘𝑚. d Ground station page 49 of 75 Effective date 14 Oct 2024 SEG Official (Open) VHF/UHF Radio Communications VHF/UHF communications uses mainly space wave and hence are restricted to Line of Sight (LOS) propagation path. Above UHF band, atmospheric attenuation becomes unacceptably high, thereby reducing transmission range. Therefore civil agencies uses the VHF band from 118MHz to 137MHz which gives minimal attenuation and negligible interference from static noise. VHF channel spacing is currently 25kHz in some airspaces with 8.33kHz spacing introduced in European states. The signals are amplitude modulated, Double-Sideband Full Carrier (DSBFC). As mentioned earlier, direct wave and ground reflected wave can move in and out of phase at the receiver, causing the signal amplitude to increase or decrease, an effect known as fading. page 50 of 75 Effective date 14 Oct 2024 SEG Official (Open) Path Loss Models Propagation Path Loss As the signal travels from the transmitter to the distant receiver, there is a decrease in the signal strength over the distance which is known as Path loss. It depends on many factors: Frequency Antenna heights Distance between antennas Terrain Weather Two path loss models that are suitable for use in VHF/UHF communications in an avionic environment are: Free space loss model Plane earth model Besides path loss, the received signal also suffers from multipath fading. At the receiver, besides the direct signal, there could be multiple copies of the transmitted signal being received. These signals may be in phase or out of phase with each other and when added together, give rise to rapid fluctuations of signal strength. page 51 of 75 Effective date 14 Oct 2024 SEG Official (Open) Path Loss Models Propagation path Multipath fading loss dips dB Signal Strength Tx distance Rx Propagation Path Loss page 52 of 75 Effective date 14 Oct 2024 SEG Official (Open) Path Loss Models Free space model The free space propagation model is used to predict signal strength when the transmitter and receiver have a clear, unobstructed line of sight path between them. This is an Ideal Model. Rx Tx power:PR power:PT d Antenna gain:GR Antenna gain:GT where d = distance in km between Tx and Rx Free space loss L (dB ) = 32.4 + 20 log dkm + 20 log fMHz f = frequency in MHz L = path loss The formula shows that the path loss increases with distance d2. page 53 of 75 Effective date 14 Oct 2024 SEG Official (Open) Path Loss Models Practical path loss should also consider the following: Antenna heights Terrain environment – mountainous, flat land, etc Weather conditions page 54 of 75 Effective date 14 Oct 2024 SEG Official (Open) Path Loss Models Plane Earth Model VHF, UHF and microwave signals normally propagate by space wave. Space wave consists of the direct wave and the ground reflected wave. Reflection from the ground will cause energy loss such that its attenuation will be higher than the free space loss. Direct wave Tx Rx ground-reflected wave At UHF and above, ground-reflection losses are greatly reduced by using highly directive antennas. Path loss is given: where L (dB) = 120 + 40 log dkm - 20 log[hr(m) ht(m)] d = distance in km ht,hr = antenna hts in m. The plane earth propagation loss is independent of frequency. page 55 of 75 Effective date 14 Oct 2024 SEG Official (Open) Path Loss Models The Plane earth model implies that the path loss is independent of frequency. This is not strictly true based on empirical results. A model proposed by Egli based on statistical analysis of measured data in a VHF/UHF environment shows that path loss is dependent on distance d as well as the frequency f. where f is in MHz L(dB) = 88 + 20 log fMHz - 20 log [ht(m) hr(m)] + 40 log dkm ht,hr = antenna hts in m. This formula works best for For 1< d< 50km and 40MHz < f < 1GHz page 56 of 75 Effective date 14 Oct 2024 SEG Official (Open) Path Loss Models Fade Margin A fade margin is usually provided for in path loss calculations to compensate for fading effects. It relates to the reliability of the radio link and is particularly critical for data applications where the corruption of a single bit may disrupt communications. With nominal path loss, the reliability is only 50%. To increase the reliability of the link, we must take into consideration a fade margin such that the received carrier level exceeds the receiver threshold ( sensitivity ). For short-term fading or multipath fading, the Rayleigh distribution is typically assumed. The fade margin can be determined with the help of a chart as shown below: page 57 of 75 Effective date 14 Oct 2024 SEG Official (Open) Path Loss Models -10 Received Signal referred to normal (dB) 0 +10 +20 +30 +40 +50 90.00 80.00 70.00 50.00 30.00 10.00 99.99 99.90 99.80 99.00 98.00 95.00 5.00 2.00 1.00 0.50 0.20 0.10 0.05 Percent of time that signal will exceed design value Q5.1 – Rayleigh Distribution Chart Fade MarginFigure based on Rayleigh Scattering Example: For 99% reliability, about 18 dB of Fade Margin is required page 58 of 75 Effective date 14 Oct 2024 SEG Official (Open) Communication Radio System Design In communication radio system design, it is important to understand the radio environment and determine the system parameters to ensure a successful communication link. The main parameters which must be considered : Radio line of sight ( LOS ) Tx output power Rx sensitivity Tx and Rx antenna gain Tx and Rx line losses ( branching and feeder losses, VSWR losses ) Path loss page 59 of 75 Effective date 14 Oct 2024 SEG Official (Open) Communication Radio System Design Example A tower of height 30m is sending a signal to an aircraft flying at a height of 1500m. Determine the maximum transmission distance, given that: Frequency of operation: 118 MHz Tower Tx output power, PT: 1 W  30 dBm Aircraft Rx sensitivity, PR: 1 V  -107 dBm ( assuming 50  system )   antenna gain, GT and GR : 1 dBi each Tower Line losses, LT : 5 dB Path loss : L GR = 1dBi Path Loss L PR= -107dBm GT = 1dBi PT= 30dBm LT =5dB Ht =1500m Ht =30m Tower Tx page 60 of 75 Effective date 14 Oct 2024 SEG Official (Open) Communication Radio System Design System equation : PT + GT - LT - L + GR = PR For 50% reliability, total path loss = L For 99% reliability, total path loss = L + Fade Margin From the chart, Fade Margin required is 18 dB for 99% reliability -10 Received Signal referred to normal (dB) 0 +10 +20 +30 +40 +50 90.00 80.00 70.00 50.00 30.00 10.00 99.99 99.90 99.80 99.00 98.00 95.00 5.00 2.00 1.00 0.50 0.20 0.10 0.05 Percent of time that signal will exceed design value Figure Q5.1 – Rayleigh Distribution Chart Rayleigh distribution chart page 61 of 75 Effective date 14 Oct 2024 SEG Official (Open) Communication Radio System Design For 99 % reliability Fade Margin (FM) = 18dB. We have PT + GT - LT – (L+ FM) + GR = PR L + Fade Margin = PT - PR + GT + GR - LT = 30 - (-107) + 2x1 - 5 = 134 dB L = 134 - 18 = 116 dB. We can assume free space loss here. Path loss L (dB) = 32.4 + 20 log dkm + 20 log fMHz 116 = 32.4 + 20log dkm + 20log118 dkm = 128.3km. We use half the radio horizon, since the ground station is at the horizon. Max distance = 4 ℎ𝑡 (𝑚) 𝑘𝑚 = 4 1500 = 155.0 𝑘𝑚. For successful communication link, transmission distance should be less than maximum radio distance. page 62 of 75 Effective date 14 Oct 2024 SEG Official (Open) Terminology As mentioned earlier, ground maps are essential for navigation. These maps or charts, used for air navigation are overlaid with a coordinate grid showing the local meridians of longitude and parallels of latitude. In aviation locations are generally defined in terms of latitude and longitude while distances are measured in nautical miles. Chart directions are referenced to true north, but unfortunately -the compass- aligns itself with the magnetic north. The angle between true north and magnetic north is called magnetic variation and is shown on all aeronautical charts. These values are updated regularly to account for time changes. 1. Latitude and Longitude Parallels of latitude are imaginary circles drawn around the Earth starting from the equator and reducing in circumference towards the poles. Parallels are identified by the angle which they subtend with the centre of the Earth and whether they lie north or south of the equator. The angle is measured in degrees, minutes and seconds. The equator has a latitude of 00, the North Pole has a latitude of 900N, the South Pole has a latitude of 900S. The equator is a great circle in that it is formed by a plane that passes through the Earth’s centre, thus bisecting the Earth. page 63 of 75 Effective date 14 Oct 2024 SEG Official (Open) Terminology page 64 of 75 Effective date 14 Oct 2024 SEG Official (Open) Terminology Meridians of longitude are half great circles, perpendicular to the equator, that extend from pole to pole. The meridians are identified by the angle which they subtend with the centre of the Earth and are measured East or West from the prime meridian. The angle is measured in degrees, minutes and seconds. The prime meridian is 00 longitude and passes through Greenwich, England and subsequent meridians are identified as 0East or 0West (maximum 1800) from the prime meridian. 2. Nautical mile 1 nautical mile is defined as exactly 1852 metres or 6076.1ft. It is roughly the distance between the 1 minute of arc of longitude at the equator. For measuring distance at a different latitude other than the equator, the formula is: D = distance in nautical miles at latitude  D =  cos   = distance in minutes of longitude at equator  = angle of latitude Incidentally, 1knot is the speed of 1 nautical mile/h. page 65 of 75 Effective date 14 Oct 2024 SEG Official (Open) Terminology Example 1. 2 locations on earth are at the same latitude of 300N but separated by 100 longitude. Calculate the distance between the 2 locations in nautical miles. 𝐷 = 𝜃 cos 𝜑 𝑛𝑎𝑢𝑡𝑖𝑐𝑎𝑙 𝑚𝑖𝑙𝑒𝑠 = (10 𝑥 60) cos 𝟑𝟎𝒐 = 519.6𝑛𝑚. page 66 of 75 Effective date 14 Oct 2024 SEG Official (Open) Terminology 3. Air Navigation Maps A map intended for air or marine navigation is a chart and the chart graticules are latitude and longitude, with the meridians more or less vertical on the sheet. As the Earth is a 3-dimensional sphere, the image of the surface of the sphere need to be mapped onto a flat 2-dimensional chart without overly distorting the represented areas. The most common method for aeronautical charts is “Lambert’s conformal conic” projection. Directions on air navigation charts are always expressed as the angular distance from the North Pole - true north-.in degrees from 00 at north clockwise. For example, the direction due East from a particular location is 0900. These directions may be described as bearings, headings, courses or tracks depending on the application. Directions are usually associated with distances expressed in nautical miles. Thus, the bearing and distance of a location 55nm due east would be expressed as bearing 0900/55. For the purpose of aerial navigation, the shape of the Earth is defined by a particular model known as the World Geodetic System 1984 (WGS84) which provides the horizontal datum for the chart coordinate systems. page 67 of 75 Effective date 14 Oct 2024 SEG Official (Open) Terminology Thus, a chart system is built on 3 basics which is defined as: ▪ The projection employed – “Lambert’s conformal conic” for air navigation. ▪ The coordinate system – Latitude and Longitude for air navigation. ▪ The geodetic datum – WGS84 is the standard horizontal datum for most aeronautical charts. When using GPS ensure that these 3 formats have been correctly selected. page 68 of 75 Effective date 14 Oct 2024 SEG Official (Open) G1 stop here Terminology World Aeronautical Charts These Australian charts are part of the ICAO 1:1,000,000 international series. Designed for pre-flight planning as well as pilotage, these charts are constructed using Lambert's conformal conic projection and conform to ICAO specifications. This chart is amended every 3-5 years. page 69 of 75 Effective date 14 Oct 2024 SEG Official (Open) Regulatory and Advisory Agencies Pilots must be licensed, and aircraft certified in all countries throughout the world. Pilots must be examined for their aviation knowledge and demonstrate their flying skills to a government-appointed examiner. Aircraft also must be certified as air-worthy and demonstrate its ability to fly. To ensure ongoing safety, both aircraft and pilot must be periodically examined and tested. The navigational aids and communication equipment on board the aircraft must also be designed, licensed and produced according to specifications and standards that are laid out in the world by several agencies dedicated to the production of a universal system that will allow international flight with a common set of airborne equipment. The standardization of aviation industry is divided into 2 types of organizations: regulatory and advisory. Only governments can maintain regulatory organizations to mandate aviation standards. Since there is no world government, there are only advisory agencies for international standardization and control. page 70 of 75 Effective date 14 Oct 2024 SEG Official (Open) Regulatory and Advisory Agencies International Civil Aviation Organization (ICAO) This is an agency of the United Nations headquartered in Montreal, Canada that fosters the planning and development of international air transport to ensure safe and orderly growth. It defines and publishes standards for navigation procedures and for equipment used in international travel. Most of the world’s countries are members of ICAO and they agree to implement recommendations given by the organization. Some navigation aids standardized by ICAO are VHF Omnirange, DME, and the air traffic control radar beacon system (ATCRBS). ICAO also specifies the signal formats, equipment performance levels and assigned frequencies for navigational aids. page 71 of 75 Effective date 14 Oct 2024 SEG Official (Open) Regulatory and Advisory Agencies Federal Aviation Administration (FAA) It is an agency of the US Department of Transportation with authority to regulate and oversee all aspects of civil aviation in the U.S. The FAA publishes rules and regulations (FARs) that covers rules of flying, requirements for pilots, certification of aircraft, maintenance of airways, etc. It also publishes technical standards for avionics known as Technical Standard Orders (TSO) which covers not only electronic equipment, but other aircraft systems, such as safety systems, wheels and tyres, etc. When equipment has been tested to TSO standards and has been accepted by the FAA, it is said that the equipment has been TSOed. FAA also provides the majority of navigation ground systems, as well as flight assistance and airport tower operations. It also investigates aircraft accidents and or incidents and publishes the investigation findings and other information for pilots and offer courses on air safety. In Singapore, the organization responsible all aviation matters is the Civil Aviation Authority of Singapore(CAAS). What does CAAS do? page 72 of 75 Effective date 14 Oct 2024 SEG Official (Open) Regulatory and Advisory Agencies CAAS is responsible for: Regulatory and Advisory services in Singapore, Air services development in Singapore, Airport management and development and Airspace management and organization page 73 of 75 Effective date 14 Oct 2024 SEG Official (Open) Regulatory and Advisory Agencies Radio Technical Commission for Aeronautics (RTCA Inc) It is headquartered in Washington, DC and is comprised of government and industry interest groups. Industry interest groups include manufacturers and avionic users. Government interest groups include the FAA and military users. Civilian users are represented by pilot’s associations, airlines and other groups. Its role is to define and set forth standards for avionics equipment. The specifications are often accepted by FAA which publishes the documents as a TSO. RTCA also works closely with many foreign government during the developmental phase of a specification, particularly with the European Organization for Civil Aviation Electronics (EUROCAE). page 74 of 75 Effective date 14 Oct 2024 SEG Official (Open) Regulatory and Advisory Agencies Aeronautical Radio Incorporated (ARINC) It is a non-profit corporation supported by revenues received for communication services supplied. It was formed and is supported by the scheduled airlines in North and South America and provides communications networks, both radio and telephone, for the airlines in the United States. ARINC has standardized the interconnection systems used in the avionics of aircraft so that equipment of several manufacturers can be interchanged. ARINC standards have been widely adopted by airline industry. page 75 of 75 Effective date 14 Oct 2024 SEG