1. What is the difference among Guidance, Navigation and Control functions?GUIDANCE is related to the definition of nominal flight plan to execute a flight mission; guidance instrumentation is centered on Flight Control Computer; it includes 3D/4D definition of routes or waypoints.NAVIGATION is the function that estimates the current aircraft dynamics. The aircraft is modelled as a rigid body hence its dynamics is given by estimating its position, velocity, and attitude (9 DOF status). Instruments used to estimates navigation status are called navigation instruments.CONTROL function permits to generate commands on flight control surfaces and throttle in order to keep deviation from nominal mission within acceptable level of error. Instruments used for this function are called control instruments.

2. What are the duties and the functions of Air Traffic Control?Duties:- Prevent in-flight collision by exploiting procedures and surveillance;- Maintain safe aircraft separation;- Direct air traffic in order to accelerate operation and keep timetables;- Provide information to pilots to improve situational awareness (Flight Information Service – FIS);- Manage emergency and Search and Rescue in case of accident.Functions:- Communications: Send orders to traffic in order to keep safe separation and avoid potentially dangerous conditions, such as severe bad weather; Receive reports from pilots about current onboard status; -Navigation: Monitor position of aircraft with respect to assigned routes and boundaries for each mission phase;- Surveillance: Monitor vertical, lateral, and longitudinal separation among aircraft.

3. What is basic «T»?Requirements for the certification of aircraft are contained in the Federal Aviation Regulations (FARs). Many sections of FAR Part 25 specify what equipment is required on transport category aircraft and where the equipment is to be placed.FAR Part 25, section 1303 gives the layout requirements of basic flight instruments. The arrangement of flight displays describes a «T» configuration, which is found in the older analog cockpits. In the modern glass cockpits these functions have been integrated into one display called the Primary Flight Display (PFD).In the upper row/line we have, from the left to the right, the Airspeed Indicator, the Attitude Indicator and the Altitude Indicator. In the second row/line, below the Attitude Indicator, we have the Direction Indicator.

4. What are the components of EFIS in glass cockpit?Thanks to the availability of new technologies, such as high performance microprocessors, microcontrollers, high performance electric motors, deterministic data buses, Fly-by-wire architecture and novel display technologies, like Cathode Ray Tube (CRT), Liquid Crystal Display (LCD), Light Emitting Diode (LED) and Touch Display, we were able to design and implement the Electronic Flight Instrument System (EFIS), defined as glass cockpit. EFIS is composed of:Primary Flight Display where I find information about the essential instrument, like the “basic T”;Multi-Function Display/Navigation Display with all the information about routes, track, fixed point, airports, ATC structures and Air Navigation Aids.Engine Indications and Crew Alerting System (EICAS)/Electronic Centralized Aircraft Monitoring (ECAM) to show to pilots and crew members all the information about the engines and every malfunction at an onboard system.Input System composed of Keyboard Trackball and/or Touch Screen.EFIS uses Fly-by-wire technology and it has a Control Display Unit to enter extra data.

5. What are the types of HUD?HUD: Head Up DisplayCollimating HUD: The lens system generates a collimated image of display symbology reflected to the eye via a flat combiner. The instantaneous field of view is limited by the size of the collimating lens and the pilot has to move

around the eyebox to see the total field of view.The total field of view (TFoV) of any HUD is the total angle subtended by the display symbology seen with head movement from any location.The instantaneous field of view (IFoV) of a refractive HUD is the display field of view seen from one head position using one eye and can be simply derived from the size of the collimator exit lens as seen by the observer, reflected in the combine.If the pilot’s head moves forwards towards the combine, the instantaneous field of view will increase. Similarly, if the pilot’s head moves laterally, angles further off-axis will be seen.The pilot sees two portholes from one head position, one with each eye. This describes the binocular field of view (BFoV).It is possible to increase the instantaneous field of view in the vertical axis by adding a second combiner parallel to and vertically above the first.

Pupil forming HUD: The lens relay system generates an intermediate image of the display symbology, which is then collimated by reflection from the curved combiner. The instantaneous field of view is larger, defined by the size of the curved combiner. It is highly desirable to offer a larger field of view than that obtainable from a collimating HUD to support more aggressive maneuvering and to give pilots the sensation of flying under visual (VMC) conditions at night using a projected FLIR image.It is possible to place the collimator closer to the pilot and therefore achieve a larger IFoV without infringing the ejection line using the optical principle of reflection.If an object is placed within the focal length of a concave mirror, the rays of light are reflected by the mirror towards the observer, who sees a magnified virtual image of the object.

Pupil Forming HUD is more efficient than Collimating HUD because it allows for a large instantaneous field-of-view (IFoV) and it does not require the pilot to move the head.HUD represents Primary Flight Data, Navigation Symbology, Air to Surface Weapon Aiming (continuously computed impact point CCIP, bomb fall line and safe-pass height marker for the fragmentation zone of the bomb) and Air to Air Weapon Aiming (continuously computed impact line CCIL, ranging circle of the target aircraft LCOS, target designator box and firing solution indicators).

6. What are the optical configuration of HMD?HMD: Helmet Mounted Display- Image source. Usually a high brightness, high resolution miniature CRT (though early sights used a small LED matrix).

Emerging technologies are AMLCD (transmissive and reflective), electro-luminescent (EL), organic LED (OLED) and low-power laser scanning directly into the eye-ball.- Relay and fold mirrors. The relay is a group of mirrors, lenses and prisms, which translate the image from the source into the correct place to be projected into the pilots’ line of sight by the combiner.- Combiner. The combiner projects the image from the relay lens into the pilots’ line of sight. It may be flat or have optical power. It is semi-transparent to the outside world, but reflective to the specific wavelength of light from the image source.HMD Principle. The field of view of the HUD is very limited when compared with the aperture of a modern radar (120°) and the acquisition cone of a modern air-to-air missile (90°). HMD overcome this limitation producing a display for each eye, combined with accurate head tracking to present a stereoscopic full colour image in any direction.Operational possibilities. Off-boresight target cueing by directing the pilot/weapon aimer to look in the direction of potential targets or threats detected by on-board sensors and/or advised by data link from cooperating aircraft and ground stations. Designation “off-boresight” one the target has been recognized. Weapon release can be commanded without having to maneuver the aircraft. Handover is facilitated between crew members and between cooperating aircraft flying the same mission.HMD System Components. Basic helmet assembly with communications equipment, oxygen mask, retractable sun visor and head protection during ejection. Display module assembly with dual image intensifiers, dual CRT displays, optical assemblies, a dual visor system, a helmet tracker receiver, autobrilliance sensor, battery pack and umbilical cable. Helmet electronics unit including system interfaces, processing, display drive and helmet tracking. Boresight reticle unit. Quick release connector cable assembly.Head tracker. To be effective as a sight, is required to have an accurate knowledge of the head pointing angle. HT types are Optical Head Tracker and Electromagnetic Head Tracker.JSF HMD. It is a binocular day/night Helmet Mounted Display and no Head Up Display. It provides targeting information and sufficient primary flight information. The optics is a pupilforming arrangement with the relayed image collimated and introduced into the pilot’s sightline by a diffractive holographic semi-transparent curved combiner introduced into the visor. The image source is a high resolution transmissive LCD, illuminated with a bright LED backlight.

7. What are the configurations of onboard data buses?Data buses perform data exchange among sensors, CPUs, displays and actuators. They are essential in a Fly-by-wire architecture, so we need to increase reliability by redundancy and function separation. We could calculate the data rate and the latency of a specific data bus thanks to his deterministic capability.Distributed Analogue Architecture: The major units are interconnected by hardwiring. No data buses are employed. We have a huge amount of aircraft wiring. The system is extremely difficult to modify if change is necessary (ex. Boeing 707 and DC 9).Distributed Digital Architecture: Half-duplex (unidirectional) digital data buses (ARINC 429 and Tornado serial) allowed important system data to be passed in digital form between the major processing centers on the aircraft. Slow for today standards. Digital data buses gave navigation and weapon aiming systems several performance improvements.Federated Digital Architecture: Relied principally upon the availability of the extremely widely used MIL-STD-1553B data bus. It uses dedicated 1553B-interfaced Line Replaceable Units (LRUs) and subsystems. The wide availability of so much system data means that significant advances could be made in the displays and other aircraft systems such as utilities or aircraft systems where avionics technology had not previously been applied. ARINC 629 is a 2 Mbit/s system that uses a collision avoidance protocol providing each terminal with its own time slot during.Integrated Modular Architecture: The system is conceived using standard building blocks that may be used throughout the aircraft level system. Common processor modules, common memory and where possible common input/output modules offer the means of rapidly conceiving and constructing quite extensive system architecture.

8. What are the Bus Architectures?Point-to-point: Each units has a dedicated connection for all other units forming the bus; it’s a channel that connects two components with two lines (input and output).Linear: each device is connected one after the other in a sequential chain. The entire network relies on each connection and network speed is reduced as more devices are added.Star: it’s a type of channel in which there is a central hub connected with all the devices, and each device is only connected by mean of a dedicated line to/from the hub. The hub can work as a “data traffic regulator” giving priority

to those who need.Ring: Similar to linear but it has no termination (the first and last device are connected).

9. What are the main specification of MIL 1553, ARINC 429, and ARINC 629?

ARINC 429. Data transfer by means of a single twisted shielded pair of wires. Each component that need to talk to other components has a wire going to the other components from its transmitter. Each component that needs to receive data from other components has a wire from that component to its receiver. “Broadcast mode”: receivers are not required to respond that data has been received. Each transmitter able to send to up to 20 receivers. Data encoded in binary. Data words composed of 32 bits, including word type and parity bit. Relatively low speed, high reliability data transfer (high speed 100 Kbit/s, low speed 12-14.5 Kbit/s). Unidirectional, Asynchronous, serial data transfer. Data types: Discrete (Boolean), Numeric, Alphabetical, Graphic Symbols.MIL-STD 1553B. Relatively high speed (1Mbit/s) high reliability data transfer. Single (redundant) pathway to connect each bus element (component) to the bus controller terminal. All equipment connected to the bus has access to information sent by means of the bus. Data transfer by means of a single twisted shielded pair of wires. It can handle up to 31 terminals per data bus. Bidirectional (half-duplex) asynchronous serial data transfer. Data flow controlled by a bus controller. Redundant controllers optional. Overhead penalty for multiple terminal and controller checking, testing and switching functions. It can send up to 32 block of Data Word, and the Data Word size is of 20bits. Transmission accuracy determined by a single parity bit. Synchronization by a three-bit synchronizing field.ARINC 629. High speed data transfer (2 Mbit/s). Shielded or unshielded twisted pair or fiber optic mode. It can connect up to 120 terminals. Linear topology with the implementation of carrier-sense multiple access/collision avoidance protocol -> multitransmitter implementation. Orderly transfer of data ensured by the protocol rather than a bus controller. Special current-mode coupling devices direct connected to bus. Self-monitored terminals to reduce the risk of bus failure. Thanks to its modularity, you can easily add and delete terminals without affecting overall bus functioning. Compatibility with ARINC 429.

10. What are the advantages of STANAG 3910?The evolution of STANAG 3910 was motivated by a desire to increase the data rate above the 1 Mbit/s rate provided by MIL-STD-1553B.The high-speed fiber-optic data terminals pass data at 20 Mbit/s and are connected using a star coupler. Control is exercised by MIL-STD-1553B using electrical connections. It allows for transmitting data output from radars and IR cameras. The ability to transfer messages of up to 132 blocks of 32 words (a total of 1496 data words) is a huge advance over the 32 word blocks permissible in 1553. A total of 31 nodes (terminals) may be addressed, which is the same as 1553.JIAWG Architecture. Radio-frequency (RF) apertures and sensor front ends associated with electrooptic (EO), missile warning, radar, electronic warfare/electronic support measures (EW/ESM), and communications, navigation and identification (CNI) systems. A fiber-optic switched network handling incoming preprocessed sensor data. Integrated avionic racks encompassing signal and data processing and interconnected using switched networks, parallel buses and serial buses. A fiber-optic switched network handling video data destined for displays. Aircraft and weapon

systems. A high-speed data bus (HSDB) (fiber-optic bus) interconnecting the avionics major systems to the integrated cabinets.

11. What are the 9 terms that define the navigation status of an aircraft?Navigation is the function that estimates the current aircraft dynamics (aircraft as a rigid body): estimation of position, velocity and attitude (9 Degree of Freedom status).Three terms for Position. The position of an aircraft is the localization of its center of mass in the Earth Frame.- φ - Geodetic latitude;- λ - Longitude;- h - Geodetic altitude above spheroid.Three terms for Velocity. Aircraft velocity is given by geographic frame (NRF) components of relative speed between BRF origin and ECEF frame.- VN - North component of Earth-relative Velocity;- VE - East component of Earth-relative Velocity;- VD - Down component of Earth-relative Velocity.Three terms for Attitude. Attitude is the orientation of BRF with respect to NRF (Euler Angles).- γ , ψ – Heading: angle of BRF X axis w.r.t NRF X axis;- β , θ – Pitch: angle of BRF Y axis w.r.t NRF Y axis;- α , φ - Roll angle of BRF Z axis w.r.t NRF Z axis.ECEF: Earth-Centered Earth-FixedBRF: Body Reference FrameNRF: Navigation Reference Frame/Local Reference Frame.

12. What are the Mechanization Models of inertial Navigation?Stabilized platform: Accelerometers are installed on a stabilized platform that keeps the orientation of their input axes fixed in the Locally Level Frame during navigation; this way the local level gyro orientation minimizes g-sensitive errors; the local level stabilization minimizes gyro rotation rate requirements. They are Rate Integrating Gyro because the measure directly the angle and the local level accelerometer outputs are directly in form needed for navigation calculation, with simplified computer requirements. On the other hand, the reliability/cost of gimbal assembly depends on torque motors, bearings, slip rings, resolvers and high-power drive electronics. It can’t give rate signals for output to other aircraft systems and the body acceleration is not directly available.Strapdown: Inertial sensors are mounted directly to vehicle frame, with lower cost, reduced onboard resources consumption and higher reliability. This way the platform can directly measure the body accelerations using direction cosine matrix (DCM) relating body coordinates, local level navigation coordinates and strapdown body mounted gyro outputs. An additional computer throughout for strapdown algorithms is not a problem for modern day microprocessors. This system needs higher performance requirements, like high rotation rates for the gyros and non-cancelling bias error effects for the accelerometers; it is more difficult to calibrate the sensors.

13. What are the main error sources of inertial sensors?Fixed bias: this is the constant term of the error and there is even if the acceleration is 0. It can have a slow drift with time that is mainly related to the effect of temperature.Scale factor error: this is the error in the determination of the linear term of sensor input-output transfer function. We measure it in parts per million (PPM).Random noise: this is the random term determined by Brownian motion of electrons. The motion of the electrons causes an oscillation in the measurement with an average that is 0.Bias repeatability (Turn-on to Turn-on Bias): For each powerup of the IMU the initial bias is different.Bias stability (In-run Bias): While the IMU is powered on the initial bias changes over time. This change in bias is related to temperature, time and/or mechanical stress on the system.Sensor non-orthogonality (misalignment): The three gyroscopes and three accelerometers are mounted orthogonal to each other. The mounting, however, have errors and so are not perfectly 90 degrees. This lead to a correlation between sensors.G dependency (Acceleration effect): A change in bias depending on how the sensor experiences acceleration.Timing errors (latency): The difference between the time the Inertial Measurement Unit measures motion and the

time that external sources like GNSS measure the same motion. Integrated INS equipment corrects for this difference in measurement time automatically and removes this error from the output measurements and final navigation solution.

14. What are the common forms of accelerometer?Force feedback pendulous accelerometer: It works by balancing the displacement of a mass installed on the tip of a cantilever. The acceleration is proportional to the electromagnetic torque needed to balance the system. When the pendulous arm moves closer to an excitation coil, the pick off measure the position of the pendulous and apply an electromagnetic force in the restoring coil to reposition the pendulous. Input range up to +/- 100g, scale factor stability 0,1%, fixed bias up to 0,01g.Surface acoustic wave accelerometer: It works by estimating the bending frequency drift determined on a cantilever by the stress introduced with acceleration. The saw resonators make the cantilever vibrate at its specific frequency. When we have a force along the input axis, the cantilever changes its frequency. The thermic dilatation of the cantilever changes the resonation frequency, but we can measure this dilatation. This instrument has a very small bias. Input range +/- 100g, scale factor stability up to 0,5%, fixed bias up to 0,5 milli-g.Both systems have miniaturized MEMS version.

15. What are the applications of symmetric gyro?Vertical Gyro – Artificial Horizon Directional Gyro – Gyrocompass

16. What is the principle of operation of conventional SDOF gyro?SDOF gyro: Single Degree of Freedom gyro.It has a second order linear system dynamical model: −kϑ−b ϑ̇=Jξ ϑ̈−I 3Ω3Ψ̇Used as Rate Gyro (RG) to measure rotation rates (spring effect prevails);- Gimbaled Inertial Navigation Units;- Strapdown Inertial Navigation Units;Used As Rate Integrating Gyro (RIG) to measure angles (dumper effect prevails);- Missile guidance.

17. What is the principle of operation of Coriolis Gyros?It is a two-degree of freedom harmonic oscillator.The mass performs free oscillation along driven axis (horizontal).If an attitude rate is provided as input along the axis orthogonal to the plane of the figure, the mass starts to oscillate along sense axis (vertical) because Coriolis force introduce a forced oscillating motion.The amplitude of the motion is proportional to the attitude rate provided as input.Two main models exist: Tuned fork gyro and Circular perimeter vibrating gyros. Both have MEMS (Microelectromechanical system) version.

18. What is the principle of operation of Optical Gyros?It is the Sagnac Interferometer. It estimates the phase or frequency shift between two laser beams that are travelling in a closed path. This shift is proportional to input attitude rate. Two main models are available:- Fiber Optic Gyro that estimates the phase shift between two beams travelling in a fiber optic path;

- Laser gyro that estimates the frequency shift between two beams travelling in a laser cavity.Laser Gyro is the most accurate system.

19. What is the most important term for assessing the performance of gyros?It is the bias drift (bias deviation over the time from the starting bias).If it is better than 1deg/hour, the gyro is capable to measure the Earth Rate, thus it is capable of autonomous heading initialization; These gyros are called «Tactical Grade» gyros;If the bias drift is better than 0.01 deg/hour the gyro is called «Navigation grade gyro» because it is capable to perform autonomous inertial navigation for several minutes.

20. Describe the process of inertial navigation.In the strapdown form it performs the following iterative sequence:- Determination of attitude by integrating gyro measurement;- Determination of transport and Earth rate terms;- Determination of velocity by integrating local level components of accelerations (needs compensation of gravity, transport and Earth rate);- Determination of position by integrating velocity.It need external sources for two reasons:- Perform initialization of navigation status;- Perform aiding to compensate navigation drift.

21. What are the segments of a GNSS system?The Global Navigation Satellite System is a system that uses satellites to provide global geospatial positioning.It is composed of three segments:The Control Segment. The Master Control Stations (Colorado Springs and Cape Canaveral as backup) check the status of the satellites and the correct position, together with the 4 Monitor Stations (along the equator), and order to the satellites to move if they have to correct the orbit. They compensate satellite motion in space and time drift. Every station can track up to 11 satellites. The system has a certain number of ground antennas to track and communicate with the satellites.The Space Segment. There are 36 satellites on 6 orbital planes. Each circular orbit has 20200 km radius and 55° inclination relative to the equator, with a 12 h orbital period. They assure worldwide positioning service below polar circles. Each satellite has 4 atomic clocks and provides ranging pulses.The User Segment. Every receiver installed in a transport platform that need to perform navigation. The receiver calculates the range between the satellite and it with the Time of Arrival (TOA) of the ranging pulse. The satellites transmit at frequencies L1: 1575,42 MHz (mandatory) and L2: 1227,60 MHz (optional used to increase accuracy only for military use). Signal is digitally speared by means of unique pseudorandom sequences. On each carrier wavelength two modes are available:- Standard accuracy (10m) C/A mode with 1 ms pseudorandom chip rate at 1,023 MHz;- Military accuracy (1m) P mode with 1-week pseudorandom (Montecarlo sequence) chip rate at 10,23 MHz.The satellite message has 2 other information:- Almanac, so that you know from which satellite you can receive information in that moment and where they are in that moment.- Ephemeris, that are information transmitted from a specific satellite and give to the receiver the possibility to calculate the position with accuracy thanks to the precise position of the satellite contained in the ephemeris.

22. What are the error sources of GNSS systems?Common mode errors:Selective availability: intentional error of GPS signals.Atmospheric errors- Ionosphere: it is the biggest GPS Receivers’ error and it is determined by the presence of ions in the Ionosphere.- Troposphere: caused by the changing humidity, temperature and atmospheric pressure in the troposphere.- Satellite clock error: caused by the satellite oscillator not being synchronized to the GPS time.- Ephemeris error: user error on the satellite position determination.Non-common mode errors:Receiver time shift: receiver clock have 1 micro second drift time measurement per minutes (it is a very large error

and it is compensated by adding a further equation to solve position).Receiver noise: it depends on the receiver thermal internal noise.Multipath: The presence of signal’s secondary path affects the signal’s direct path (signals reflection).Commonality condition:Receivers and ground station are closer than 50 km.Receivers are connected to the same set of satellites.

23. Describe the determination of position by exploiting GNSS systemsTo determinate Latitude, Longitude and Altitude, GPS uses Pseudo-Range Equations, which estimates User position from the Satellite’s point of view.The basic measurement of GNSS system is a range to a set of satellites with known position in the ECEF (Earth-Centered Earth-Fixed) frame (WGS 84 Earth model).Dilution of Precision is the term used to specify the additional multiplicative effect of navigation satellite geometry on positional measurement precision.

24. What are the typical solutions adopted to reduce the error of GNSS?Use of multiple frequencies to remove Ionospheric error. For conventional GPS systems this function is reserved for military use, since the use of L2 frequency needs the capability to recognize the epoch of a 1 week long pseudorandom sequence. Only military receivers have information to perform this function.Use of differential correction, i.e. a fixed station estimates position or range error and transmits it to moving rovers. This technique has large aeronautical applications by exploiting Wide Area Augmentation Systems, i.e. geosynchronous satellite that relay differential correction at continental level on the same frequencies of GPS system.

25. Define the integrity of a navigation system and discuss how GPS receiver performs integrity.Integrity is the capability to detect a non-nominal output by a navigation system within a fixed time to alert that depends on the flight mission phase. For GPS receiver, integrity is provided by means of the Receiver Autonomous Integrity Monitoring technique, i.e. the receiver performs the fix with redundant number of satellites in order to identify the satellite with anomalies. Aeronautical certified receivers must be RAIM (Receiver Autonomous Integrity Monitoring) enabled.

26. Describe the types of terrestrial systems and the terms measured.According to the method to assess position fix, we have 5 different types of terrestrial systems: - ρ - θ range-bearing (1 station).- θ - θ bearing-bearing (2 stations).- ρ - ρ range-range (3 stations).- hyperbolic (3 stations): it is based on the difference in timing between the reception of two signals. Hyperbolic navigation is a class of navigation systems based on the difference in timing between the reception of two signals. This timing reveals the difference in distance from the receiver to the two stations. Plotting all of the potential locations of the receiver for the measured delay produces an hyperbolic line on a chart. Doing another measurement and looking for the intersection of the two hyperbolic lines we are able to determine the aircraft position.- pseudoranging.Range is the distance between the aircraft and the station.Bearing is the angle between current aircraft radial to a station and magnetic North direction. It is positive for counter clockwise directions.

27. Describe the principle of operation of ADF/NDB.The Non-Directional Beacon is a Ground Station Transmitter, at a known location, used as a Terrestrial Radio Navigation System. It works at medium frequencies, between 200 and 400 KHz, so it can be received at great distance and at low altitude because it follows the Earth surface curvature. It is called Non-Directional because it emits a constant signal in every direction, that is why is also called Omni-directional Beacon.NDB estimates the Heading to station angle, which is the heading that an aircraft needs to add to its Compass Heading

in order to get to the NDB Station. Its Receiver is composed of two antennas: one is Omni-directional and the other is Directional Rotating. The Directional Rotating antenna keeps on rotating until it catches di NDB signal. Once it catches the signal, the Directional Rotating antenna stops and keeps on pointing the NDB Station. This Bearing is displayed on a Bearing Indicator that looks like a compass card with a needle superimposed. The position of an NDB on the ADF is expressed in degrees with respect to the station; once the radio-assistance frequency has been selected, the pilot receives the name of the radio-assistance in Morse code.

28. Describe the principle of operation of VOR.VHF Omni Range is used to estimate bearing without compass. VOR is a Ground Station Antenna composed of two antennas: one is a fixed Omni-directional antenna and the other is a 360° rotating Directional antenna. The Omni-directional signal is exploited in order to permit the On-board Receiver to synchronize to the VOR Station. The principle of operations is based on calculating the time difference between the moment that the Directional signal passes North direction and the moment that the Directional signal passes the aircraft.It’s necessary to understand that- The Omni-directional signal maintains constant phase in time.- Meanwhile the Directional rotating signal changes its phase in time as it rotates 360°. We get the difference in time by means of the difference in phase between the Omni-directional signal and the Directional signal when it touches the aircraft. Knowing the Directional signal angular velocity, it is possible to deduce difference in angle between North direction and aircraft, by multiplying the angular velocity times the difference on time.

29. Describe the principle of operation of DME.Distance Measuring Equipment (DME) it’s an On-board Equipment, whose transmitter emits a signal to the Ground Station, called Interrogation Signal. It is possible to estimate the distance between the Ground Station and the aircraft by measuring the two-way time of travel. In fact, the On-board Transmitter transmits the signal as interrogation and it is relayed by the Ground Station to the aircraft. The distance can be computed as the signal travels at the speed of light.It is necessary to consider that the Ground Distance between the aircraft and the Ground Station is not the same as the DME Distance, estimated along the Line of Sight.Searching modeWhen the aircraft tunes a DME Station, the On-board Receiver enters into Searching Mode. The Searching Mode is the phase in which the On-board Receiver recognize the Ground Station reply to its first Interrogation Signal. The On-board Transmitter starts sending Interrogation Signals to the Ground Receiver with a frequency of 8 times per second. It keeps doing that until the On-board Receiver doesn’t recognize the Ground Station’s reply to the On-board Transmitter Interrogation. The recognition of the On-board Transmitter Interrogation is obtained by comparison of multiple listening. When the On-board Receiver recognizes the On-board Transmitter’ signal, the Tracking Phase starts. This phase can take up to 30 seconds.TrackingOnce the replay is sent back to the On-board Transmitter, the Ground Receiver is capable of focusing to the Replay in order to estimate the distance.

30. Describe the principle of operation of Instrument Landing System.ILS is a radionavigation system which provides aircraft with horizontal and vertical guidance just before and during landing and, at certain fixed points, indicates the distance to the reference point of landing. It enables pilots to conduct an instrument approach to landing if they are unable to establish visual contact with the runway.The ILS system on the ground is made up of three devicesLocalizer. It provides guidance on lateral displacement with respect runway centreline. It is formed by an array of antennas located from 220 m to 300 m far from the runway head.Glideslope. It provides guidance on vertical displacement with respect to ideal landing plane that forms an angle of 2°-3° with respect to horizontal runway plane.No. 2 or 3 marker beacons to assess the transition at fixed distances from the head of the runway (7.5 – 13 km, 1000 – 2000 m, and 75m).The on-board instrument has a dial with two cross-shaped indicators, three lights with acoustic device and a panel to select the ILS frequency of the desired track.The vertical hand of the instrument swings to the right and left and is called the Localizer. It represents the centreline

of the track; if the hand moves to the right, it means that the track is more to the right of the aircraft and vice versa; the pilot must therefore "follow" the hand and try to keep it as close to the center as possible.The horizontal hand swings up and down, is called Glide slope and represents the correct glide angle; the pilot follows the lancet as for the localizer and must make sure that it remains as central as possible. If both hands are perfectly in the middle, it means that the aircraft is following the correct descent path.

31. Describe the principle of operation of Radar Doppler Altimeter.Radar Doppler takes advantages from the Doppler Effect, which is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is used to estimates the aircraft ground speed. The integration of the Inertial Unit Measurement information and the Ground Speed gives the Geographic Components of the Ground Speed. It has the advantages that it gives a direct estimate of the ground speed (even for very slow speed), it is autonomous (it doesn’t need any ground station), it is all weather and it can also be used as a high-accuracy Ground Altimeter by means of a Triangular Frequency Modulation.

32. Describe the principle of operation of Air Data Systems.Sensors such as pitot static system, gyroscopes, thermo couples, accelerometers and GPS can estimate total pressure, static pressure, indicated air temperature, angles.The Air Data Computer (ADC) computes these inputs and transforms them into output displayed on the on-board displays. Hence, an Air Data Computer (ADC) is a computer that, rather than individual instruments, can determine by itself:- True and Calibrated airspeed (TAS e CAS);- Mach Number;- Pressure Altitude;- Altitude Rate/Vertical Speed;- Static Air Temperature;- AoA and Angle of Sideslip.

33. Describe the determination of barometric altitude by air data system.Barometric altitude can be defined as the altitude indicated when the altimeter is set on a pressure basic value and calculated according to the standard atmosphere laws.Combining Stevino’s law and the Ideal Gases Equation it’s possible to determine barometric altitude.Stevino’s law: dP=−ρ gdhGas equation: P=ρRT

34. Describe the components of ATCRBS system.The air traffic control radar beacon system (ATCRBS) is a system used in air traffic control (ATC) to enhance surveillance radar monitoring and separation of air traffic. The ATCRBS components are:- Primary Surveillance Radar: it is used to get updated estimates of range and azimut, i.e. horizontal position, of each aircraft that flies within a control region;- Secondary Surveillance Radar: it is used to perform identification of aircraft equipped with transponders. It provides Identification Friend or Foe (IFF) service;- Transponder: it is a transceivers that returns identification pulses and other information as a reply of Secondary Radar interrogations.In brief, it consists of a rotating ground antenna and transponders in aircraft. The ground antenna sweeps a narrow vertical beam of microwaves around the airspace. When the beam strikes an aircraft, the transponder transmits a return signal back giving information such as the flight number designation and altitude of the aircraft.

35. Describe the principle of operation of Primary Radars.Primary Radar is a pulsed radar. It is a conventional radar sensor that illuminates a large portion of space with an electromagnetic wave and receives back the reflected waver from targets within that space. It quickly scans 360° around the site, giving distance and radial speed of the target with good precision. It shows bright (or coloured) blips on the radar screen produced by the radio frequency energy reflection from the target’s “skin”. The main advantage of the PSR is that no on-board equipment in the aircraft is needed to detect the target, so it can be used to monitor the movement of vehicles. Radar outputs are reported on dedicated monitors called PPI (Plane Position Indicator).

36. Describe principle of operation of Secondary Surveillance Radars.Secondary Surveillance Radars are used to perform aircraft identification. They send interrogation pulses similar to DME and wait for transponders answers. In fact, they relies on targets equipped with a radar transponder, that replies to each interrogation signal by transmitting a response containing encoded data. SSR is based on the military identification friend or foe (IFF) technology.

37. Describe the form of reply for Mode A/C ATCRBS transponders.The interrogation is made of three pulses. The interrogation mode is defined by the interval time between the first and the third pulse. The second pulse is used for sidelobe suppression.Mode A has the minimum reply. It outputs just four-digit octal aircraft identification code that is the squawk code, as set by the pilot.Mode C replay includes also information about barometric altitude.A Squawk code is assigned by controllers each time an aircraft enters a control region.

38. Describe the form of reply for Mode S transponders.It allows for selective interrogation of transponders.It provides a message that includes several information including aircraft identification (squawk and ICAO 24 bit unique aircraft address), barometric altitude, GPS derived position and ground speed, data figures of merit.It is compatible with modes A/C.It is a primary component of TCAS and ADS-B.It provides a sort of data link among ground stations and traffic.Its radio output is better than other modes. It is more resistant to interference. Selective mode helps to avoid interference among replies.

39. Describe the principle of operation of ADS-B.Automatic Dependent Surveillance Broadcast (ADS-B) is a surveillance system onboard aircraft that periodically transmits its state vector estimates. The state vector estimates are derived from navigation avionics and transmitted via common communication channel. These indicate that ADS-B is highly dependent on the navigation and communication systems. In addition, the system requires ground stations to retrieve the broadcast information.It periodically broadcasts its position and other information without knowing the recipients and without expecting acknowledgements as the system only supports one-way broadcasts. The system is automatic in the sense that it does not require external intervention to transmit the information. It is characterized as dependent due to its dependence on aircraft navigation avionics to obtain the surveillance information. ADS-B is a cooperative system, because it requires common equipage for aircraft, or vehicles on the airport surface to exchange information. It provides aircraft state information such as horizontal position, altitude, vector, velocity and trajectory intent information.Additional information are provided by:- Traffic Information Service-Broadcast (TIS-B): it includes information about aircrafts that are not equipped with ADS-B. This information is generated by ground radars.- Flight Information Service-Broadcast (FIS-B): it includes weather information, Notice To Airman (NOTAM) and Automatic Terminal Information Service (ATIS).

40. Describe the principle of operation of TCAS.A Traffic Collision Avoidance System is an aircraft collision avoidance system designed to reduce the incidence of mid-air collisions between aircraft. It monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder, independent of air traffic control, and warns pilots of the presence of other transponder-equipped aircraft which may present a threat of mid-air collision.TCAS is based on secondary surveillance radar (SSR) transponder signals, but operates independently of ground-based equipment to provide advice to the pilot on potential conflicting aircraft.TCAS uses a directional antenna that is mounted on the top of the aircraft to receive range and altitude data on targets above the aircraft and to transmit interrogations at varying power levels in each of four 90° azimuth segments. Two antennas are also required for the Mode S transponder. One antenna is mounted on the top of the aircraft while the other is mounted on the bottom.The pilot can select three modes of TCAS operation:- Stand-by: Power is applied to the TCAS Processor and the mode S transponder, but TCAS does not issue any interrogations and the transponder will reply to only discrete interrogations.

- Traffic Advisory Only (TA-Only): The mode S transponder is fully operational. TCAS will operate normally and issue the appropriate interrogations and perform all tracking functions. However, TCAS will only issue traffic advisories (TA), and the resolution advisories (RA) will be inhibited.- Automatic: The mode S transponder is fully operational. TCAS will operate normally and issue the appropriate interrogations and perform all tracking functions. TCAS will issue traffic advisories (TA) and resolution advisories (RA), when appropriate.

41. Describe the principle of operation of EGPWS.An Enhance Ground Proximity Warning System is a system designed to alert pilots if their aircraft is in immediate danger of flying into the ground or an obstacle.It works by performing integrated estimates of local altimetry as detected by several sensors (ADS, GPS, ILS, and Radar Altimeter).The system monitors an aircraft's height above ground as determined by a radar altimeter. A computer then keeps track of these readings, calculates trends, and will warn the flight crew with visual and audio messages if the aircraft is in certain defined flying configurations ("modes"). The modes are:- Mode 1. Excessive descent rate.- Mode 2. Excessive terrain closure rate.- Mode 3. Altitude loss after take off or with a high power setting.- Mode 4. Unsafe terrain clearance.- Mode 5. Excessive deviation below glideslope.- Mode 6. Excessively steep bank angle.- Mode 7. Windshear protection.