Chapter 13,4
Module 13 – Aircraft aerodynamics, structures and systems
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Module 13 – Aircraft aerodynamics, structures and systems
Comunication and navigation frequencies
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Module 13 – Aircraft aerodynamics, structures and systems
• The main characteristics of an antenna are:
1. The directivity 2. The gain
3. The polarization
4. The opening and the polarization diagram 5. The efficiency
6. The characteristic impedance 7. The length of the antenna.
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• The directivity of an aerial is the capacity of an antenna to irradiate or to pick up signals in a specific direction. The directivity of an
aerial, in a particular direction, is defined as the ratio between the intensity of the radiation, sent in this direction, and the total power, irradiated in all directions
• The gain of an antenna provides information about the capacity of transmission and receipt of the analyzed antenna, comparing it with an omni-directional aerial. The gain is expressed in dB (is a logarithmic scale)
• The efficiency of an antenna is defined as the ratio between the irradiated power and the input power accepted by the feeding cable of the antenna
Antennas
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• The radome is a structure used to protect the antennas from the atmospheric phenomena, such as the wind, the rain, the ice, etc.
Radome
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Module 13 – Aircraft aerodynamics, structures and systems
The transmission line is a circuit that permits the energy transfer between the generator and the antenna.
There are many different types of transmission lines:
• Bifilar
• Coaxial
Fundamentals of transmission lines
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Module 13 – Aircraft aerodynamics, structures and systems
The most common types of modulation, used in the aeronautical field, are:
• AM (Amplitude Modulation)
• FM (Frequency Modulation)
• Pulse modulation
The AM modulates the amplitude of the carrier wave, in a proportional way to the amplitude of the modulating signal. The frequency of the carrier wave is the same of that of the modulated signal.
The FM modulates the frequency of the carrier wave, in a proportional way to the amplitude of the modulating signal.
The main types of pulse modulation are PAM, PWM and PCM.
Modulation
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Module 13 – Aircraft aerodynamics, structures and systems
Modulation
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Module 13 – Aircraft aerodynamics, structures and systems
• In the aeronautical field, the HF communications between 3 to 30 MHz are used for long range communications, such as the oceanic communications
• This radio can operate in 3 different modes, selectable through a specific knob: AM, USB, e LSB
HF communication system
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Module 13 – Aircraft aerodynamics, structures and systems
• The ADF receiver receives and processes the signal from the selected radio station (NDB)
• The ADF measures the angle between the longitudinal axis of the aircraft and the line that connects the aircraft and the NDB station.
This angle is called relative bearing
• The transmission of waves is subjected to the variation of the height of the ionosphere
NDB and ADF
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Module 13 – Aircraft aerodynamics, structures and systems
• The DME (Distance Measuring Equipment) is a navigational radio-aid that provides the aircraft slant distance from a ground station.
• DME operate from 960 Mhz to 1215 Mhz.
• The DME is based on the direct wave propagation. The maximum real range of the DME is about 200-300 NM. The accuracy of the DME decreases with increase of range
DME
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• The display indicates the distance between the aircraft and the station, measured by the system, the time necessary to cover this distance, and the detected speed of the aircraft.
• The cockpit instrument shows the distance, travelled by the
emitted signal, and so it is the slant distance between the aircraft and the station
DME
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Module 13 – Aircraft aerodynamics, structures and systems
• The Instrument Landing System (ILS) is the primary precision approach facility for the civil aviation
• ILS system comprises three different elements
1. A localizer: provides lateral steering signals for front course approaches to the runway
2. • A glide slope: provides vertical steering signals for landing in one direction on the runway
3. • Two or three radio markers beacons with a vertical transmission, called outer, inner and middle markers: provide spot checks of position at predetermined distances from the threshold of the runway.
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The glide slope receiver is essentially a UHF receiver in the frequency band 328.6 to 335.4 MHz with 150 kHz spacing between channels.
The signal of the glide slope is composed by two signals:
• One signal modulated at 90 Hz
• One signal modulated at 150 Hz.
ILS – Glide slope
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Module 13 – Aircraft aerodynamics, structures and systems
• The localizer and the glide slope signals can be divided into two
ideal lobes, one modulated at 150 Hz and the other modulated at 90 Hz. The course signal is obtained when two signals are received with equal intensity. The 150 Hz modulated signal prevails on one side of the runway centerline (blue area), while the 90 Hz modulated signal prevails on the other side (yellow area).
ILS – Localizer
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• The operational frequency of the system is selected through the same selector that is used for the VOR.
When it tunes the localizer frequency, the system automatically sets the corresponding glide slope frequency.
ILS cockpit indicator
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Module 13 – Aircraft aerodynamics, structures and systems
• The ILS critical area is an area of defined dimensions, identified around the localizer and the glide slope antennas, where vehicles, including aircraft, are excluded during the operation of the system.
This area protects the functioning of the ILS from unacceptable disturbances caused by the presence of vehicle and aircraft.
• Instead, the ILS sensitive area extends beyond the critical one. In this sector the movements or the parking of aircraft are controlled, in order to prevent that they interfere with the system operation.
ILS: critical and sensitive areas
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• The purpose of the markers is to provide distance information, while the aircraft is doing the approach procedure.
• All markers emits a signal at the operative frequency of 75 Mhz. The signals of the each marker differ each other due to the different modulation
Marker beacons
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Module 13 – Aircraft aerodynamics, structures and systems
• The hyperbolic systems of radio-navigation are all navigational systems that use the geometric proprieties of the hyperbole in order to calculate the aircraft position
• In order to guarantee a correct calculation of the aircraft position, the main characteristic of all hyperbolic navigational system is the synchronization of the different ground station.
The main hyperbolic navigation systems are:
1. • The Loran
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Loran C
• The functioning of the LORAN system is based on a series of chains of ground stations that emit a signal which is then processed by the airborne equipment of aircraft
• The LORAN uses 27 chains of stations. Each chain is made of a main station, called master, and of a variable number of secondary
stations, called slave. The minimum number of the slave for each chain is two, while the maximum number is four.
Loran
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Module 13 – Aircraft aerodynamics, structures and systems
• The Decca was a hyperbolic radio-navigational system, initially used in the Northern Europe during the Second World War. This system transmitted continuous radio waves at low frequencies (LF).
• In order to determine the aircraft position, the Decca used a
comparison of the received signals’ phases, similarly to the OMEGA system
Decca
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Module 13 – Aircraft aerodynamics, structures and systems
• The Doppler effects is an apparent variation of the frequency of the radio waves, due to the relative motion of the source of the waves in relation to an observer
• Apparent increase in frequency: when the transmitter moves towards the receiver
• Apparent decrease in frequency: when the transmitter moves away from the receiver
Doppler effect
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Module 13 – Aircraft aerodynamics, structures and systems
The main task of the FMS is that to assist the pilot in the flight
management, doing, in an automatic and optimal way, many activities, which otherwise he must do manually:
• To integrate and to manage the information provided by all used navigational systems in all flight phases
• To calculate in real time the aircraft performances, in terms of ground speed, fuel consumption, endurance. So it permits to reduce the
operative costs of the flight mission
• To manage in an interactive way the flight plan, according to the information provided by the airborne systems and ATC
• To manage the autopilot system, in coupling with the data of the
flight plan, in order to follow the calculated and planned route in a fully automatic way
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• The GPS (Global Positioning System) is a global navigational
satellite system, which provides the aircraft position in every point of the Earth
• The dialogue between the satellites and the ground stations happens on 2 UHF frequencies:
1. The 2227.5 MHz, used to send the signals from ground towards satellites
2. The 1783.74 MHz, used to receive in the stations the signals transmitted by the satellites
GPS
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Module 13 – Aircraft aerodynamics, structures and systems
• The Local Area Augmentation System (LAAS) is an augmentation system of the GPS, based on real-time differential correction of the GPS signal
• The information provided by the LAAS system is used during the approach and landing phases, because the precision reached by the LAAS systems is about 1 m.
LAAS
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Module 13 – Aircraft aerodynamics, structures and systems
• The Global Navigation Satellite System (GNSS) is a project in via of realization that should join the operation of all the navigation
satellite systems, such as the American system (GPS), the Russian GLONASS (Global Navigation Satellite System), and the European system (Galileo), in order to permit the true global navigation.
• This big system should permit the navigation in any point of the Earth, and in any flight phase, through a unique system
GNSS
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Module 13 – Aircraft aerodynamics, structures and systems
• The Primary Surveillance Radar (PSR) is the first instrument for the exercise of the Air Traffic Control (ATC).
• The PSR operates receiving the signal reflected by the aircraft.
• The PSR has the advantage to detect and to determine the position of every not cooperative target that reflects the radio signals.
• The PSR isn’t able to identify any aircraft.
PSR
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Module 13 – Aircraft aerodynamics, structures and systems
• The Secondary Surveillance Radar (SSR) is the main instrument for the exercise of the Air Traffic Control (ATC).
• Unlike the PSR, the SSR requests the active collaboration of the aircraft, which must receive the signal and respond with another one.
• The SSR interrogations are sent in the form of a group of 3 pulses, called P1, P2 and P3. The spacing between the P1 and P2 is constant and it measures 2 microseconds.
SSR
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Module 13 – Aircraft aerodynamics, structures and systems
• The transponder emits 3 pulses: P1; P2; P3
• The spacing between P1 and P2 is 2 micro seconds.
• The spacing between P1 and P3 pulses, transmitted by the radar antenna, is set at a value of response of the transponder:
1. The mode A with a spacing of 8 microseconds 2. The mode C with a spacing of 21 microseconds
SSR and transponder
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Module 13 – Aircraft aerodynamics, structures and systems
• The TCAS (Traffic Collision Avoidance System) is a system that operates in connection with a normal transponder, informing the flight crew about the presence of other aircraft in the surrounding airspace only if it is equipped with a transponder, and regarding time and distance of possible collision
• The TCAS transceiver of an aircraft periodically interrogates the transponder of the other aircraft, in order to identify the presence of the airplane and to recognize the characteristics. This aircraft, in situation of possible collision, is called intruder.
TCAS
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Module 13 – Aircraft aerodynamics, structures and systems
• In case of possible collision, the TCAS emits two different signals:
1. The TA alarm, generated by all TCAS generations, provides to pilots the direction of arrival and the relative distance of the intruder. This type of alarm will be generated on cockpit displays, in the case in which the estimated collision point is between 20 e 48 seconds, in accordance to the speed and altitude of the aircraft.
2. The RA alarm is generated when the intruder is at about 15-35 seconds form the hypothetical collision point, according to the altitude of the aircraft
3. TCAS II: it can provide the Resolution Advisory (RA) for the horizontal and vertical plane
TCAS
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Module 13 – Aircraft aerodynamics, structures and systems
• A weather radar is a type of radar used to locate precipitation, to calculate its motion, to estimate its type (rain, snow, hail, etc)
• Check the aircraft position in relation to the ground
• The weather radar of the aircraft sends some directional pulses in the band of microwaves. The frequency commonly used is the 9375 MHz.
• It is important to remember that the Clear Air Turbulence (CAT), that is very dangerous for the flight safety, cannot be detected by the weather radar, because it isn’t associated with any
meteorological phenomena.
• It mainly identifies the cumulonimbus
• Another function of this instrument is ground mapping
• The pencil-shaped beam is used for weather scope
• It doesn’t measure the height of the elements of the underneath ground
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• The weather radar antenna can move up or down from its neutral position of about 10°
• The precipitations are represented with colors, for example high intensity are RED color, heavy in MAGENTA and black if the intensity of precipitation is less than 0.7 mm/h
Weather radar
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Module 13 – Aircraft aerodynamics, structures and systems
• The radio altimeter measures the vertical distance between the aircraft and the ground, with the scope to provide to the pilot an information about the underneath terrain. The radio altimeter is used in the approach phase
• The radio altimeter compares the frequency of the received signal with the frequency of the transmitted signal, because this
difference is proportional to the time and the distance travelled by the emitted signal with the frequency between 4200 and 4400 Mhz.
Radio altimeter
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Module 13 – Aircraft aerodynamics, structures and systems
• The ELT (Emergency Locator Transmitter) is an emergency self-contained and self-powered radio transmitter, designed to transmit a signal on the international emergency frequency in conjunction with the satellites,
• It is installed near the tail of the aircraft
• The ELT automatically activates when an aircraft impact happens or by a remote switch in the cockpit.
ELT
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