Navigation Principles PDF

Navigation Principles Latitude Latitude is measured as the angular distance in degrees north or south of the equator. The North and South Pole are located at 90 degrees north and south latitude, respectively. Latitude lines are parallel to each other, so they are also called parallels or parallels o...

Navigation Principles Latitude Latitude is measured as the angular distance in degrees north or south of the equator. The North and South Pole are located at 90 degrees north and south latitude, respectively. Latitude lines are parallel to each other, so they are also called parallels or parallels of latitude. Lines of latitude 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 275 of 356 Longitude The zero circle is the line which passes through Greenwich, England, and is called the Prime Meridian, or the zero-longitude line. The distance between longitude lines is greatest at the equator. Longitude lines are numbered west and east of the Prime Meridian at 0 degrees longitude. Lines of longitude – Prime Meridian at Greenwich The International Date Line (IDL) is an imaginary line defining one calendar day from another. It is on the exact opposite side of the Earth from the Prime Meridian, at 180 degrees. This date line does not follow a straight line, instead deviating around some island territories. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 276 of 356 Plotting a Position Because the Earth is round, we use an angular distance rather than a straight distance to measure from a point of origin (which would usually be (0, 0) on a flat graph). On the Earth, the origin or (0, 0) point is the centre of the Earth. A location on the Earth’s surface can be plotted by measuring two angles, perpendicular to each other, and with respect to the centre of the Earth. The angular measurements are known as latitude and longitude. Both latitude and longitude are measured in degrees. The point where the prime meridian and the equator cross each other is 0 degrees latitude and 0 degrees longitude. Any other location is identified with reference to this point. Plotting a position As an example, the position P on the illustration is located 40 degrees above the horizontal and 60 degrees to the left of a defined reference position. Any angular position above the equator is designated north, and any angle below the equator is designated south. Likewise, a position to the west of the prime meridian and up to 180 degrees, the IDL, is designated west, and east is for any position up to 180 degrees east of the prime meridian. The correct identification for point P is 40 degrees north latitude and 60 degrees west longitude. A position on the earth can be pinpointed more accurately by resolving degrees into minutes and seconds. Each degree can be divided into sixty minutes, and each minute can be divided into sixty seconds. Brisbane is situated at 27 degrees, 28 minutes, 14 seconds south latitude, and 153 degrees, 2 minutes, 45 seconds east longitude. In traditional format this is written as 27° 28' 14” S, 153° 2' 45” E. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 277 of 356 Nautical Mile A nautical mile is a unit of measurement defined in 1929 by the First International Extraordinary Hydrographic Conference in Monaco as precisely 1852 metres (6076.1 ft; 1.1508 mi). Historically, it was defined as one minute of latitude, which is equivalent to one sixtieth of a degree of latitude. Today, it is a non-SI unit which has a continued use in both air and marine navigation and for the definition of territorial waters. One tenth of a nautical mile is a cable length. The geographical mile is the length of one minute of longitude along the Equator, about 1855 m on the WGS 84 ellipsoid. The Nautical Mile 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 278 of 356 Knots The derived unit of speed is the knot, defined as one nautical mile per hour. Measurement of speed Compass Direction To navigate from a known position to a selected position, it is necessary to use a compass to give lateral direction. A compass is referenced to the earth’s magnetic field or magnetic north, the poles of which are in a different position to the geographical poles. Geographic north is known as true north. When navigating, it is important to distinguish between these two references. Magnetic compass 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 279 of 356 Aviation Navigation Pilotage In its simplest form, air navigation is the guiding of an aircraft by observing the relative position of landmarks. This means the pilot finds his way by following rivers, towns, roads, railroads and other geographical features or outstanding objects. The pilot compares his sightings with symbols on aeronautical charts in order to navigate to his destination. This method has some obvious disadvantages. Poor visibility caused by inclement weather can prevent a pilot from seeing the needed landmarks and cause the pilot to become disoriented and navigate off course. Pilotage 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 280 of 356 Dead Reckoning After the invention of the compass, mariners could begin reckoning their approximate position by using their speed, time underway and direction to pinpoint their location. This deduced reckoning or dead reckoning wasn't very accurate, but it formed the basis of navigation. Air navigation using dead reckoning determines and records times, distances, directions and speeds to calculate an aircraft’s position. Navigation problems are associated with dead reckoning, for example, if wind velocity and direction are unknown or incorrectly recorded and applied, then the aircraft will be off course. Errors build upon errors, meaning that the greater the elapsed time, the greater the compounding error. The method used to reduce this error is to obtain “fixes” from known references and update the aircraft’s position at various times throughout the flight. Dead reckoning 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 281 of 356 Inertial Navigation System Introduction to Inertial Navigation System An Inertial Navigation System (INS) is a totally self-contained and independent dead-reckoning system. It requires no facilities external to the aircraft in the form of ground transmitters or satellites to make it work. This is different to radio navigation systems, including the Global Positioning Systems (GPS), which are termed dependent navigation systems, as they need ground equipment or satellites to work. Given its starting position, an INS keeps track of all movements in all directions and calculates the aircraft's flight position in relation to that point. Inertial navigation system 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 282 of 356 Accelerometer A device used to measure acceleration is called an accelerometer. Newton’s First Law of Motion can be written as so: A body at rest tends to remain at rest and a body in motion tends to remain in motion in a straight line unless acted upon by an external force. Newton’s Second Law of Motion can be written as so: The acceleration of a body is directly proportional to the force causing it and inversely proportional to the mass of the body. For example, the vehicle below accelerates from rest to 100 km/h and then cruises at that speed for a short period before stopping. When the vehicle is accelerating, the pendulum will swing backwards. Pendulum under acceleration However, during cruise at 100 km/h, the pendulum will hang vertically as velocity is constant and acceleration is zero. When the brakes are applied, the pendulum swings forward, and the harder the brakes are applied, the further forward it will swing. Finally, when the vehicle is stationary, the pendulum hangs vertical again. Pendulum under deceleration By noting pendulum direction, we have acceleration or deceleration, and by measuring the pendulum’s displacement from the vertical position, we have its magnitude. An accelerometer is sensitive along a single axis. This axis of maximum sensitivity is known as the sensitive axis of the accelerometer. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 283 of 356 Electronic Integrator If the mathematical process of integration is applied to acceleration, then the result obtained is velocity. If we integrate velocity, we can now calculate distance. Integration principle These calculations may be performed electronically using an integrator circuit. Stable Platform By measuring the acceleration about the aircraft’s three axes (longitudinal, lateral and vertical) and by integrating these measurements, we can now calculate aircraft velocity (speed, direction and heading), and if we have a starting reference (latitude/longitude), position. Aircraft axes 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 284 of 356 For the accelerometers to remain accurately aligned to the aircraft axes and perpendicular to gravity regardless of the attitude or heading of the aircraft, the accelerometers are mounted on a gyrostabilised platform. This stabilised platform is positioned by servomotor systems and supported by a gimbal system, giving the platform freedom in pitch and roll. Two rate gyroscopes are mounted on the platform. Any displacement of the platform away from the reference, or null, position is sensed by the gyroscope, which provides an electrical signal proportional to the displacement. This signal is applied to a torque motor which is used to drive the platform back to the reference position. INS stable platform 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 285 of 356 Inertial Navigation System Components A single INS consists of the following: An Inertial Navigation Unit (INU) A Control Display Unit (CDU) A Mode Selector Unit (MSU) A Battery Unit (BU). Inertial Navigation Unit Inertial navigation unit 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 286 of 356 Inertial navigation unit The Inertial Navigation Unit (INU) contains the stabilized platform (gyroscopes, servomotors), integrators, power supplies and processing circuits to perform navigational computations. The INU is a critically aligned component which is hard mounted to the airframe. During transit, the unit must be handled with extreme care. Control Display Unit Control display unit 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 287 of 356 The Control Display Unit (CDU) provides the user interface. The display is used to show present position and other navigational functions as determined by the selector switch. Fault codes, alignment progress and BITE information is also displayed. A keypad is used to enter initial position and enroute waypoints. In aircraft with a Flight Management Systems (FMS), a dedicated control display unit may not be used, as INU data would likely be entered and displayed through the FMS system. Mode Select Unit Mode Select Unit The MSU selects the operational mode and provides an indication to show the platform is aligned and ready for navigation. Battery Unit Battery unit The battery unit provides an uninterruptible power supply, which is necessary during power supply switching or for a thirty-minute period in the event of a total power failure. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 288 of 356 During normal operation, the BU is on standby and is maintained at full charge by a trickle charge supply from the INU. Battery size and capacity varies between installations. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 289 of 356 Inertial Reference System Introduction to Inertial Reference System Although an INS is a very accurate and reliable navigation reference, innovations in laser detecting and computation power has led to the introduction of similar systems which use the term Inertial Reference System (IRS) instead of INS. Laser ring gyro An IRS is known as a strapdown system. Instead of having a physical gyro-stabilised platform, it now uses software-generated stabilised platforms. Strapdown draws its name from the fact that the accelerometers and the ring laser gyros are fixed to the frame of the unit; the accelerometer outputs are modified electronically to compensate for attitude changes. The navigational outputs are the same as for an INS and can be distributed over a digital data bus to navigation computers, flight control computers and multi-function displays. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 290 of 356 Ring Laser Gyro Principles LASER is an acronym for Light Amplification by Stimulated Emission of Radiation and was first discovered in 1960. A laser produces light which is termed coherent: all light produced is of specific wavelength and the waves are in phase. Laser light Doppler Ring laser gyro operation 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 291 of 356 The above image depicts a merry-go-round platform which is stationary, with two people walking around it from the same starting point. One walks clockwise and the other anticlockwise, and both can walk entirely around the merry-go-round back to their starting point by walking 100 steps. However, if the platform is rotated slowly clockwise, the person walking with the platform would need to take shorter steps to complete the journey in the same number of steps. Conversely, the person walking against the direction of rotation would need to take longer steps to complete the journey in 100 steps (like walking on an escalator: more steps to achieve the same distance). A similar phenomenon takes place in our laser gyro. Laser ring gyros are constructed so that two laser beams are reflected around a triangle, causing the light to travel in an enclosed loop. The light travels in both directions at the same time, so we have a clockwise beam and a counter-clockwise beam. The laser gyro detects motion by measuring the difference in beam frequencies If the gyro is turned clockwise (CW), the beam completes the journey in a shorter time. In order to complete the journey in the same number of cycles, the beam wavelength must be compressed, that is, the frequency must be increased. Conversely, the counter-clockwise (CCW) beam wavelength must increase (frequency decreased). This apparent change in frequency is termed the Doppler effect. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 292 of 356 Ring Laser Gyro A mixture of helium and neon is used in ring laser gyros. This gas mixture is held at low pressure inside a sealed tube exposed to an anode and cathode plate. When a high voltage, about 3000 volts, is applied across these plates, the gases ionise, producing the required coherent light output. Two laser beams are reflected around a triangle, causing the light to travel in an enclosed loop. The light travels in both directions at the same time, so we have a clockwise beam and a counterclockwise beam. Mirrors are installed at each corner. One mirror is fixed, one mirror can be adjusted by a servo motor and is used to tune the path length and the third mirror is partially transparent, allowing some laser light to reach the two photocell detectors mounted behind the mirror. Laser gyro The beams are directed to a pair of photocell detectors through the partially transparent mirror, where the frequency (wavelength) difference is quantified as an interference pattern. These gyros accurately sense the Earth’s rotation rate of 15 degrees per hour, which causes a laser beam frequency change of only 4 Hz. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 293 of 356 Inertial Reference Units The IRS operation is like the INS in that a mode selector unit and control display unit provide a means of user interfacing with the inertial reference unit. Three ring laser gyros and three accelerometers, each aligned perpendicular to each other, provide sensing input to the computer section. The inertial reference unit is hard mounted to the airframe and critically aligned with it. Aviation Australia IRS block diagram The IRS and INU system outputs can replace a lot of the equipment used in conventionally equipped aircraft, including: Vertical axis gyroscope Rate gyroscopes Remote-sensing compass systems, including a directional gyro, flux valve and amplifiers Instantaneous vertical speed and altitude. Many strapdown IRUs contain an embedded Global Positioning System (GPS) receiver module with the following advantages: A GPS provides a means of inflight alignment, removing the need for the aircraft to be held stationary for up to 5 minutes. The IRU provides a seamless fill-in for GPS outages resulting from jamming and obscuration caused by maneuvering. The IRU provides a means of smoothing the noisy velocity outputs from the GPS and a continuous high-bandwidth measurement of position and velocity. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 294 of 356 Radio Navigation Introduction to Radio Navigation Radio navigation provides the pilot with position information from ground stations located worldwide. Almost all flights, whether cross-country or local, use radio navigation equipment in some way as a primary or secondary navigation aid. Radio navigation aids enable the pilot to navigate in all weather conditions to a given location with a high degree of accuracy and safety. Different radio navigation systems use different frequency bands, broadly based on the age of the system and whether the navigation requirement is long range or short range. Older systems and longrange navigation systems use the lower frequency bands. Radio navigation frequency bands 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 295 of 356 Automatic Direction Finder Introduction to Automatic Direction Finder An Automatic Direction Finder (ADF) operates in the lower frequency bands between 190 and 1800 kHz, which produce ground waves. This frequency range is not limited to line of sight and will function over reasonably long distances. In most cases, the ADF system uses the network of radio beacons designed expressly for aircraft navigation which are in the 190 to 400 kHz range. These beacons transmit a non-directional signal and are called Non-Directional Beacons (NDB). The signal is modulated with a three-letter Morse code signal which enables identification of the NDB being received. An ADF system is also capable of detecting the AM commercial radio broadcast band, which has a frequency band of 840 to 1750 kHz. For operation the ADF systems relies on two aircraft antennas: loop and sense. Loop and sense antennas 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 296 of 356 Loop Antenna In the diagram below, the receiving antenna, or loop, is turned, and the transmitting antenna is fixed. The magnitude of voltage induced into the loop antenna is shown below. When maximum voltage is induced into the loop, this is represented by the highest value on the polar diagram. Loop antenna signal strength If we accept that the direction to the station is 90° to that shown, using a control system, we drive the loop antenna either mechanically or electronically for a null. The direction of the incoming transmission can then be located. The problem is there are two nulls providing two directions to the station. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 297 of 356 Sense Antenna Polar Diagram The sense antenna is an omnidirectional (receives equally in all directions) antenna and can be mounted either on top of or underneath the aircraft. The induced voltage is of a uniform magnitude and fixed polarity regardless of the direction to the transmitter. The polar diagram is therefore represented by a circle. Sense antenna polar diagram 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 298 of 356 Direction Finding Cardioid Diagram When the loop and sense signals are electronically combined, the resulting Cardioid antenna pattern produces a single null (as shown below). Resulting cardioid pattern The ADF system uses a stationary sense antenna and a rotating loop antenna to determine the direction to a ground transmitter. This can be established without the ambiguity of two null points detected by a loop antenna alone. Whenever the loop antenna is pointing to the ground station, the signal will drop, and the aircraft system controlling the loop antenna will identify the bearing to the ground station. The bearing can then be displayed on a suitable indicator. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 299 of 356 Resulting cardioid pattern Automatic Direction Finder System The ADF system consists of the following components: Antenna system (loop and sense) Receiver Control unit Indicator. ADF block diagram 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 300 of 356 Antenna ADF systems have two antennas. Older ADF systems have two separate antennae, while modern systems have a combined loop and sense antenna. Modern loop antennas are rotated electronically, not physically. The ADF antenna is usually located aft of the main wing and can be either on the bottom or the top of the aircraft and is a small flat antenna without moving parts. Combined ADF antenna Receiver ADF receiver 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 301 of 356 The receiver contains all necessary circuitry to: Process the incoming radio signals from the sense and loop antennas Determine the direction of the beacon Produce the necessary control voltages to drive the bearing indicator. Control Unit The control unit allows frequency selection and volume control. Although there may be a dedicated ADF control unit incorporated in the aircraft, it is not uncommon for communications radio control panels to incorporate the ADF controls. ADF control unit Indicator ADF bearing information is displayed on a Radio Magnetic Indicator (RMI). This indicator houses a compass card driven by the remote compass and pointers, which can be positioned either by ADF or VOR (VHF Omni-Range) navigation systems. Radio magnetic indicator 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 302 of 356 Automatic Direction Finder Operation Modern aircraft automatic direction finding (ADF) systems employ a receiver which carries out the direction-finding function automatically and displays the direction to the transmitting station on an indicator on the instrument panel. ADF block diagram The pilot selects an ADF ground station to obtain a bearing to it by tuning to the ADF station broadcast frequency. This is normally done using an aircraft communication radio, or there may be a dedicated ADF receiver incorporated into the aircraft. Either way, ADF is selected (thus selecting the ADF antenna as the receiver) and the appropriate frequency is tuned into the receiver. The pilot can tune to an AM radio station, in which case they will be able to listen to the radio, and the HSI or RMI will display the bearing to the radio station transmitter. Normally a pilot will tune to the frequency of a non-directional beacon (NDB) located at an airfield to obtain bearing to the airfield. In this case they will only hear a Morse code signal identifying the NDB transmitter selected. If the pilot tunes to an AM radio station as an ADF reference, the reception will be poor compared to simply receiving the AM signal through the normal AM radio antenna (ADF antennae are not designed for maximum reception efficiency; they are designed for direction-finding). Homing The ADF has automatic direction-seeking qualities which result in the bearing indicator always pointing to the station to which it is tuned. The easiest and perhaps the most common method of using ADF is to “home” to the station. Since the ADF pointer always points to the station, the pilot can simply head the airplane so that the ADF pointer points straight up on the HSI. The station will be directly ahead of the airplane. Since there is almost always some wind at altitude the pilot must also allow for drift. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 303 of 356 System Errors in Automatic Direction Finder Night Effect The ratio of the intensity of indirect to direct waves in the total received signal determines the liability of error of the radio compass. As the strength of the indirect waves is far greater at night, errors then are more common and of greater magnitude: this is called ‘night effect.’ Often this effect is more pronounced within an hour of sunrise or sunset, when the changes in the state of ionisation of the upper atmosphere are particularly violent. The nighttime range of an NDB is only dependable over distances where the ground wave transmission predominates, which is approximately 60 miles over land and 100 miles over sea under reasonable propagating conditions. As the distance increases, the ratio of indirect to direct waves will increase, and bearing indications will become erratic. Treat NDB reception with caution beyond these ranges. At the best, the night effect causes minor deviations in bearing readings; at the worst, the ADF pointer goes round in circles. Coastal Refraction The useful range of an NDB is influenced by the type of terrain over which the radio wave travels. It is greatest over the sea and least over sandy or mountainous country, and an NDB with a daylight range of 600 miles over the sea may only have a range of little more than 100 miles over unfavourable types of land. Therefore, when an NDB is located on the coastline, its range in different directions can be expected to vary considerably. Mountain Effect Sometimes an effect similar to the night effect is obtained in mountainous areas, where the energy received from an NDB consists of two or more waves, one of them direct and others by reflection from the mountains. Bearing indications are found to change rapidly until the affected area is passed. Station Interference This occurs when other stations on or close to the ADF frequency are received. This may occur when tuned to a broadcast station or when a high-power station is close in frequency to the frequency being received. The only method of overcoming this problem is in the design of the receiver, which is designed with a high adjacent channel rejection ratio. Static Interference A thunderstorm generates a tremendous amount of radio frequency energy, and when the aircraft is near to a storm centre, the radio compass may indicate the direction of the storm and not that of the NDB to which it is tuned. Therefore, when flying in the vicinity of a thunderstorm, the accuracy of the bearing indications should be checked by other means whenever possible. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 304 of 356 VHF Omni-Range VOR Principle of Operation A VHF Omni-Range (VOR) system uses ground-based radio beacons that transmit a signal which contains precise azimuth information in the VHF range 108 to 118 MHz. VOR reception is strictly line-of-sight. The VOR ground station transmits two signals for navigation. The reference signal is of constant phase in all directions, while the variable signal varies in phase relative to the constant signal. For example, at magnetic north, both signals are in phase, but at magnetic south, the variable signal lags the reference signal by 180 degrees. The VOR receiver senses the phase difference between the two signals and indicates the direction of the VOR transmitter. VOR bearing calculation The reference and variable signals are both modulated on single carrier frequency: one as Amplitude Modulation (AM), the other as Frequency Modulation (FM). The station also transmits a Morse code identification signal. The name omni-range leads one to believe that range to a station is provided. The inclusion of the word range is an unfortunate choice of title because a VOR transmitter provides only bearing to a station, not range. The signals transmitted produce bearings which locate the aircraft direction from the station. These bearings may be visualized as lines radiating from the beacon like spokes of a wheel. These bearings are generally limited to one-degree increments and are known as radials. A radial is identified by its magnetic bearing outbound from the VOR beacon. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 305 of 356 Upon reception of the VOR signal, an aircraft can locate itself on a particular radial. VOR antenna Here, the aircraft indication would be 315° (radial from the VOR). It should be noted that its position on a radial is independent of aircraft heading, which in this example is 350°. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 306 of 356 Aircraft VOR System Components The airborne portion of the VOR system consists of: An antenna A receiver A control unit Indicators. VOR system Antenna The VOR antenna has an omnidirectional reception pattern (equal reception in all directions) and can receive RF signals in the 108 to 118 MHz range. The antenna is usually located on the vertical fin of the aircraft. VOR/ILS antennae 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 307 of 356 Receiver The receiver contains all the circuits necessary to receive, decode and provide outputs of aircraft bearing information from the received VOR signal. In addition, the receiver contains self-monitoring circuits which validate the signals received and the accuracy of the bearing information sent to the indicator. Most commercial aircraft carry two complete VOR systems. VOR receiver Control Unit The control unit provides off to on control and frequency selection for the VOR system operation. Many control units also incorporate the VHF communications receiver controls. When selecting navigation frequencies within the 108 to 118 MHz range, the controller may also automatically tune to paired Distance Measuring Equipment (DME) or glide slope Instrument Landing System (ILS) channels. These pairings are detailed in ICAO Schedules. VOR control unit 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 308 of 356 Indicators VOR information can be presented on either one or two displays. Relative bearing to the station can be viewed on a Radio Magnetic Indicator (RMI). This instrument is also able to display bearing information from the ADF, and its operation and presentation is the same. The pilot needs to switch between ADF and VOR as necessary. This display mode is termed “automatic” VOR Automatic VOR presentation VOR information can also be displayed on a Horizontal Situation Indicator (HSI) or similarly named indicator. In this case, the pilot can select a specific radial along which they wish to fly or intersect. VOR operation in this instance is termed “manual” VOR. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 309 of 356 Manual VOR presentation 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 310 of 356 Distance Measuring Equipment Introduction to Distance Measuring Equipment Distance Measuring Equipment (DME) is an electronic device that measures slant range from a ground-based DME station. As DMEs are used as locators during landing and takeoff, generally the altitude is small, which implies the slant range shown below approximates the ground distance. DME slant range Principles of Operation DME equipment works on the principle of timing the delay between transmitting a UHF signal to the ground-based DME station and receiving a reply. The longer the delay, the further you are from the station. The DME ground station can respond to interrogation signals from a number of aircraft. DME operation 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 311 of 356 The process starts when the airborne transmitter sends an interrogation signal. This signal is received by the ground station and re-transmitted back to the aircraft in the same format after a delay of 50 microseconds. The airborne receiver can discriminate its own signal from others by matching the received signal against its own previously transmitted random signal. Once matched, the receivers range computer applies the elapsed time (from transmission to reception of the ground station pulses) and solves the equation: Elapsed T ime − 50μS Distance in N M = 12.359μS Note Radio waves take 12.359 microseconds to travel one nautical mile and return, and 50 microseconds is the time taken for the ground station to respond to the interrogation. The calculated distance is then sent to the DME indicator or HSI for display. Distance Measuring Equipment System Components The airborne components of the DME system are: Antenna Transmitter/receiver Controller Indicator. DME block diagram 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 312 of 356 Antenna A single UHF omnidirectional antenna is used for both transmission and reception of DME signals. The antenna is located on the lower surface of the fuselage. DME antenna Transmitter/Receiver The DME transmitter/receiver may also be referred to as the RT unit, or interrogator unit. The transmitter section contains all the necessary circuits to generate, amplify and transmit the interrogating signal. The receiver section contains the circuits required to receive, amplify, decode and calculate the required distance. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 313 of 356 DME Controller The VOR control unit provides the necessary control, tuning and switching circuits for the DME system. As the VOR and DME frequencies are paired, selection of the appropriate VOR frequency will automatically tune DME to the matching frequency. DME controller 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 314 of 356 DME Indicator The distance indicator shows aircraft slant distance from the tuned DME ground station expressed in nautical miles. The indicator will also display a flag, or other warning, if the system is malfunctioning or not locked on to a reply signal. Some indicators also display computed ground speed and the time to reach the ground station (Time to Station or TTS). The computed ground speed and TTS are accurate only if the aircraft is flying directly to or from the ground station. DME distance indications may be included on a Horizontal Situation Indicator (HSI). DME indication DME indication 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 315 of 356 Electromagnetic Radiation Hazards As this system incorporates a transmitter, the following warning applies. Over exposure to radiation may cause serious health problems. You must be aware of the safe working distances near operating antennas. RF burns or electric shock may result from touching the antennas or metallic objects in the radiation field near antennas. RF radiation can be dangerous to personnel and equipment. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 316 of 356 Instrument Landing System Introduction to Instrument Landing System To guide an aircraft from several kilometres out to a runway threshold, a system providing lateral and vertical guidance is used. The lateral guidance portion is called the localizer, and the vertical guidance part is called the glideslope. In addition to the localiser and glideslope signals, there are marker transmitters which provide pilots with an indication of their approximate position at intervals along the approach path to the airport’s runway. This total system using ground based and airborne equipment is called the Instrument Landing System (ILS). Instrument landing system 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 317 of 356 Instrument Landing System Ground Components The ground-based equipment consists of the following: A glideslope (GS) transmitter and antenna located near the runway threshold A localiser (LOC) transmitter and antenna located at the far end of the runway A group of three marker beacons at designated intervals along the runway approach path. Glideslope The glideslope provides vertical guidance to the aircraft during the ILS approach. The standard glideslope path is 3° downhill to the end of the runway. The glideslope is normally usable to 10 NM. The glideslope signal channels are between 328 and 335 MHz in the UHF band. The glideslope signal is radiated to produce two intersecting lobes, one above the other. The upper lobe is modulated by a 90 Hz signal, the lower lobe by a 150 Hz signal. When the aircraft is on the centreline, the two signals are equal. This occurs at approximately 3° above the horizontal. This line of equal modulation defines the glideslope approach path. If the aircraft is too high, the 90 Hz signal will predominate, and if it is too low the 150 Hz signal will predominate. Glideslope ground station 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 318 of 356 Localiser The localiser signal provides lateral information to guide the aircraft to the centreline of the runway. It is like a VOR signal, except it provides directional information for only a single course: the runway heading. The localiser antenna is located at the far end of the runway. The localiser signal is normally usable 18 NM from the field. The Morse code Identification of the localiser consists of a three-letter identifier preceded by the letter I. Localiser and glideslope frequencies are paired so that selecting the localiser frequency automatically selects the glideslope frequency. The localiser transmitter operates in the VHF band between 108.1 and 111.95 Megahertz. The localiser signal is like the glideslope, except the 90 and 150 Hz signal radiated lobes are side by side and directed along the centre of the extended line of the runway. The line of equal modulation defines the centreline of the runway approach path. If the aircraft is left of the centreline, the 90 Hz signal will predominate. If it is right of the centreline, the 150 Hz signal will predominate. Localiser ground station 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 319 of 356 Marker Beacons The three ILS marker beacons operate on a fixed frequency of 75 MHz radiated in a narrow fanshaped vertical pattern. This information is presented to the pilot by audio and visual cues by illuminating a lamp and emitting an audio signal which corresponds to the marker being overflown, thus providing pilots with an indication of distance to touchdown. The marker beacons are arranged as follows: The Outer Marker (OM) is approximately 8 km from the runway threshold. Its signal is modulated with a 400 Hz tone of dashes, at a rate of two per second. The Middle Marker (MM) is approximately 1 km from the runway threshold. Its signal is modulated with a 1300 Hz tone of dots and dashes. The Inner Marker (IM) is located approximately 150 metres from the runway threshold. Its signal is modulated with a 3000 Hz tone of dots, at a rate of 6 per second. The inner marker is used only for category II operations. The IM is sometimes called the airways, fan or “Z” marker. There are no inner markers left in Australia. Notice that the sound gets quicker and the tone higher as the aircraft moves towards the airport: first, dashes; then, dots and dashes; finally, just dots. The OM normally indicates where an aircraft intercepts the glide path at the published altitude. On a glide path at the MM, an aircraft will be approximately 200 feet above the runway. The IM is the decision height point for a category II approach. Marker beacons 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 320 of 356 In summary, the complete ground-based ILS installation consists of the following: A glideslope transmitter which produces a radio beam providing vertical guidance to the runway threshold A localiser transmitter which produces a radio beam providing lateral guidance to the runway threshold Marker beacons which produce vertical, fan-shaped radio beams, providing an indication of position along the glide path. Instrument landing system ground installation 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 321 of 356 Instrument Landing System Airborne Equipment The airborne equipment portion of the ILS system consists of the following: VHF localiser receiver Glideslope receiver Marker beacon receiver Control unit Antennae An indicator displaying localiser and glideslope deviation Marker beacon indicators. Instrument landing system airborne components 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 322 of 356 Receiver The localiser and glideslope receivers are usually contained in the one radio unit. This receiver contains all necessary circuits for receiving, decoding, and generating the localiser and glideslope signals to drive indicating circuits. The receiver also contains the self-testing and self-monitoring circuits that ensure the reliability of the decoded signals sent to the indicator. The marker beacon receiver contains the circuits necessary to receive a modulated carrier signal and convert it to audio and visual output to indicate passage over one of three marker beacons. Control Unit As the frequencies used by the localiser system lie between 108.1 and 111.95 MHz and are interleaved with VOR channels, the VHF Navigation controller is used to control channel selections. When the controller selects a VHF localiser frequency, it automatically selects the paired UHF glideslope frequency according to the ICAO schedule. VHF NAV control unit 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 323 of 356 Antenna – Localiser The antenna is omnidirectional (equal reception in all directions), operating between 108–112 MHz (VHF). Localiser antenna This antenna may be shared with the VOR system, as the reception frequencies are shared. Antenna – Glideslope The glideslope operates in the UHF band (329–335 MHz range), so the antenna is much smaller than the VOR antennas. Glideslope antenna 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 324 of 356 Antenna – Marker Beacon The marker beacon receiver uses an antenna operating at 75 MHz and is usually fitted to the bottom of the aircraft. Marker beacon antenna Instrument Landing System Indicators ILS information can be displayed on several different types of indicators. Course Deviation Indicator Course Deviation Indicator (CDI) 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 325 of 356 These indicators include Horizontal Situation Indicators (HSIs), Attitude Director Indicators (ADIs) and electronic versions EADI/ EHSI and Head-up Guidance Systems (HGSs). These types of indicators show lateral deviation (localiser) and vertical deviation (glideslope) with respect to an aircraft symbol in the middle of the indicator. Warning flags are displayed whenever the information supplying that function becomes unreliable. Electronic Attitude Director Indicator Electronic Attitude Director Indicator (EADI) EHSI 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 326 of 356 Marker Beacon Indicators A marker beacon indicator simply consists of three annunciators (or lights) which indicate the marker beacon over which the aircraft is passing. These three annunciators are usually colour coded as follows: Blue — outer marker (O) Amber — middle marker (M) White — inner marker (I), sometimes marked with an “A” for airways marker. Marker Beacon Indicators Instrument Landing System Operation Some ILS receivers are part of the VOR and communications receiver, as shown in the block diagram below. The VOR/LOC converter in the nav/comm receiver and the glideslope receiver respectively develop localizer and glideslope deviation and flag signals. The marker receiver, and its controls and lamps, provides an audio signal to the nav/comm receiver. The indicator shows localizer deviation (vertical bar) and warning flags and glideslope deviation (horizontal bar) and warning flags. Instrument landing system block diagram 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 327 of 356 Global Positioning System Introduction to Global Positioning System Global Positioning System (GPS) (Navstar) was first developed and funded by the United States Department of Defense for military use. Once commissioned, the system became available for civilian use, but the military system is more accurate. Navstar global positioning system satellite The system is based on a constellation of 24 satellites orbiting the Earth at an altitude of approximately 20 200 km. Out of the 24 satellites, only 21 are used; the other three are maintained as spares. The satellites are placed in six orbital planes (four satellites in each plane). The satellites continuously transmit a UHF band radio signal with satellite identification data and time signal. Each of these satellites houses an atomic clock and circles the globe, making two complete rotations every day. The orbits are arranged so that at anytime, anywhere on Earth, there are at least four satellites visible in the sky. As of August 10, 2018, there were a total of 31 operational satellites in the GPS constellation, not including the decommissioned, on-orbit spares. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 328 of 356 GPS satellite network 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 329 of 356 GPS Position Fixing Each of the GPS satellites transmits a signal which travels as a spherical front away from the satellite. If you know you are 10 miles from satellite A in the sky, you could be anywhere on the surface of a huge, imaginary sphere with a 10-mile radius. If you also know you are 15 miles from satellite B, you can overlap the first sphere with another, larger sphere. The spheres intersect in a perfect circle. If you know the distance to a third satellite, you get a third sphere, which intersects with this circle at two points. A fourth sphere, when drawn, will determine exactly your location. The Earth itself can act as a fourth sphere, and as only one of the two possible points will be on the surface of the planet, you can eliminate the one in space. Receivers look to four or more satellites to improve accuracy and provide precise altitude information. Three-dimensional trilateration principle 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 330 of 356 Three-dimensional trilateration principle In order to calculate and position the spheres, the GPS relies on two facts: The speed of light (and radio waves) is a constant at 300 000 000 metres per second (or 186 000 miles per second). The location of at least four satellites or three satellites plus the receiver’s altitude. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 331 of 356 Distance Measurement Using the time taken for the radio signal to travel from the satellite to the receiver and the speed of light, a calculation provides the exact distance from the satellite. Each satellite emits a coded signal which contains the exact timing of the signal emission to Earth. This signal is extremely accurate due to the use of three atomic clocks in each satellite. To measure the time taken accurately would require an atomic clock in the receiver. If our receivers needed atomic clocks (which cost upwards of $50 000 to $100 000), no one could afford it. Luckily, the designers of GPS devised a clever solution that allows the use of much less accurate clocks in receivers. If our receiver’s clocks were perfect, then all our satellite ranges would intersect at a single point (which is our position). But with an inaccurate receiver clock, all measurements will not intersect. This original range measurement is known as the pseudo range. Since any offset of the receiver’s clock from universal time will affect all four of our measurements, the receiver looks for a correction factor that it can subtract from all its timing measurements that would cause them to intersect at a single point. One consequence of this principle is that a GPS receiver needs to have at least four channels to perform this calculation. We generally think of GPS as a navigation or positioning resource, but the fact that every GPS receiver is synchronized to universal time (from the satellites) makes it the most widely available source of accurate time. This time is known as UTC (Universal Time Coordinated). Satellite Locations Encoded with the time code transmitted from each satellite is essential information like its position (termed ephemeris) and the position of all satellites within the constellation (termed almanac). 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 332 of 356 Satellite Transmissions Every satellite transmits coded signals on two frequencies. This allows two levels of accuracy to determine the pseudo ranges. The L1 or Coarse/Acquisition (C/A) code is transmitted on 1575.42 MHz. The L2 or Precise (P) code is transmitted at 1227.60 MHz. The C/A code is available for civilian applications and allows a precision from 100 metres to a few metres. The P code is encrypted to deny access to anyone other than military users. When P and C/A signals are used, a position precision up to seven times better than just the C/A code is obtained. GPS systems to suffer from a few errors which affect accuracy of the position fix: Satellite position errors Atmospheric errors Multipath errors Satellite geometry. Satellite Position Error The position of each satellite is vital to the operation of the aircraft GPS receiver. Satellites orbits deviate slightly due to factors like the gravitational effect of the moon and the effect of solar wind. The altitudes, positions and speeds of each satellite are constantly monitored, and each satellite is commanded by the ground control station to transmit corrected position information in addition to the timing data. Correcting for satellite position 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 333 of 356 Atmospheric Errors The transmissions from the satellites can be slowed during passage through the ionosphere and troposphere. Signal deviations are random, so some inaccuracies are inevitable. Atmospheric errors Satellites which are low on the horizon are unsuitable for GPS purposes, as the atmospheric errors are magnified. Typically, satellites which are below the 7.5 degrees horizon are rejected by the receiver. Mask angle 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 334 of 356 Multipath Error Satellite signals can be reflected from mountains, buildings and other objects. The system must discriminate between the original signal and reflected signals. Bogus signals take longer to reach the receiver and can interfere with the original signal, which may result in position error. Multipath error, called ghosting, has a similar effect to that experienced on older analogue TV reception. Satellite Geometry The more spread out the satellites are, the more accurate the position fix. Bunched satellites produce a degraded position fix. A good quality GPS receiver will take into account the relative position of satellites (GDOP – Geographic Dilution of Precision). Satellite geometry 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 335 of 356 Differential GPS Differential GPS is a refinement of the standard GPS which eliminates most of the errors and greatly increases its accuracy (less than 1 metre). The GPS signals are checked at monitoring stations on the ground, with the resulting corrections and integrity data then transmitted to aircraft receivers. There two systems currently employed: Satellite-Based Augmentation Systems (SBAS) Ground-Based Augmentation Systems (GBAS). Satellite-Based Augmentation System The GPS signals are checked at monitoring stations on the ground, with corrections and integrity data uplinked to satellites for downlink to aircraft receivers. Satellite Geometry 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 336 of 356 Ground-Based Augmentation Systems A ground station at the airport transmits locally-relevant corrections and integrity data to aircraft using a VHF band radio transmission. Differential GPS 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 337 of 356 GPS Receivers Basic GPS receivers provide present position in the form of latitude and longitude. The information is used in the flight management system to calculate navigation parameters and provide heading commands. GPS receivers can use a self-contained display where the aircraft’s position and a trace of its path are overlaid on a moving map. Maps are stored in the receiver’s memory or are available from plug-in map cartridges. GPS Multi-Function Control Display Unit The GPS receiver can also provide several pieces of valuable information: Distance travelled Travel time Current speed Average speed A trace of the aircraft’s path across the map The estimated time of arrival at the destination GPS altitude (above mean sea level). 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 338 of 356 GPS Airborne Components There are various configurations of aircraft GPS systems: Simple general aviation units consisting of a combined panel mounted antenna/receiver/controller. Remotely mounted receiver providing data to the Flight Management System (FMS)/ Multifunction Control Display Unit (MCDU). The system that uses a remotely mounted receiver coupled to a dedicated MCDU consists of the following components: Antenna and pre-amplifier unit Receiver/processor unit Multifunction Control Display Unit (MCDU) Data loader. Aviation Australia GPS block diagram 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 339 of 356 GPS installation Antenna System Due to the high frequency (UHF) and low signal power (remember the nearest satellite is 20 000 km away), the GPS antenna is quite small and incorporates an integral preamp. GPS reception relies on an antenna system which has line-of-sight access to the satellites. Aircraft have the GPS antenna on the forward upper fuselage so that satellites are not shielded by the tail and antenna cable runs are kept to a minimum. GPS antenna installation 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 340 of 356 Receiver The GPS receiver processor is a rack-mounted unit that processes the incoming satellite radio signals and calculates present position, groundspeed, estimated time to waypoint and the aircraft’s path. The unit has a power supply, memory (to store satellite constellation and navigational data) and an input/output interface to connect with the flight management computer and other aircraft systems. GPS receiver Multifunction Control Display Unit The MCDU provides flight crews with the controls and displays necessary to: Turn the system on and off Display information such as present position, groundspeed, estimated time to waypoint and the aircraft’s path Access navigation database information such as waypoints, radio beacon location and frequencies, airport information and geographical information Create or edit flight plans Warn of GPS malfunctions. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 341 of 356 Data Loader Unit Updated navigation information can be loaded into the GPS memory via the data loader. This information would include a worldwide information base of waypoints, airways, airports, runways, VOR beacons and arrival and departure procedures. Generally, this information base is updated every 28 days by maintenance personnel. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 342 of 356 Radio/Radar Altimeter Introduction to Radio/Radar Altimeter Barometric altimeters measure altitude by reference to the local air static pressure based on standard barometric pressures. The barometric system of altitude measurement has limitations, particularly when the reference is incorrectly set, as the indicator does not provide pilots with a distance from the aircraft to the surface of the Earth. For example, an altimeter might read 3000 feet (with the reference set at local sea level), but this is relatively worthless if it is operating in a mountainous area where the peaks vary from 2500 to 3500 feet. Radio altimeters use radio waves to measure the height of the aircraft above the local terrain. Barometric and radar altimeters 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 343 of 356 Altimeter operation There are two basic methods used for the measurement of height: Pulsed systems Frequency Modulated Continuous Wave systems (FMCW) Pulsed systems are often referred to as radar altimeters, while the FMCW systems are termed radio altimeters. Radar and radio altimeters are both usually simply referred to as RADALT systems. RADALT systems are completely self-contained within the aircraft, that is, they need no ground-based facilities for operation. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 344 of 356 Radar Altimeter Basic Principle RADALT systems determine height by measuring the time delay between the transmission of signal and its reception back at the aircraft after reflection from the surface of the Earth. Two antennas are used: one dedicated for transmission and the other for reception. Some systems use a single antenna which transmit and receive simultaneously. Radar altimeter antennas The altitude range of RADALT systems is typically 2500 feet, with output power levels ranging from 5 to 100 milliwatts depending on type and manufacturer. Both RADALT systems operate on a frequency of 4.3 GHz. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 345 of 356 Components of a RADALT system Although the pulsed and FMCW systems operate on different principles, the components used in each system appear similar. Both systems use: A system transmitting antenna A system receiving antenna Transmitter/receiver An indicator. Antenna The RADALT antennas are flush mounted along the aircraft centreline on the underside of the fuselage. It is important that they are not affected by projections from the aircraft such as landing gear and flaps and should not be painted. Correct orientation of the antenna is essential due to the antennas producing and receiving a directional radiated beam. Generally, arrows marked on the antenna show the correct orientation. RADALT antenna 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 346 of 356 Transmitter/Receiver Most systems in use today employ a combined receiver/transmitter-computer assembly. Typically, these are rack-mounted units. When in the transmit mode, the receiver is disabled so as not to pick up transmissions direct from the transmitter. If radar altitude is lost for a short period of time, the receiver-transmitter will continue to search for a period before energising a fail flag on the indicator. RADALT R/T 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 347 of 356 Indicators Like most modern avionics systems, there are several ways of displaying radio altitude, including: Attitude direction indicator (ADI or EADI) Dedicated radio altimeter. RADALT indicators Most indicators incorporate a decision height (DH) annunciator which can be selected by the pilot. This annunciator can be positioned by the pilot, and if the selected height is breached, the pilot will receive an altitude warning. The RADALT system power on/off and initiate built-in test switches are typically incorporated on the indicator. The needle is masked when the system is turned off and will mask if the RADALT fails inflight or loses height reference, e.g., the aircraft flies too high. Electromagnetic Radiation Hazards As this system incorporates a transmitter, the following warning applies. Overexposure to radiation may cause serious health problems. You must be aware of the safe working distances near operating antennas. RF burns or electric shock may result from touching the antennas or metallic objects in the radiation field near antennas. RF radiation can be dangerous to personnel and equipment. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 348 of 356 Ground Proximity Warning System Introduction to Ground Proximity Warning System The Ground Proximity Warning System (GPWS) monitors the data received from a variety of aircraft systems. Using these inputs, the computer calculates the likelihood of the aircraft crashing and provides an appropriate advisory message or warning to the flight crews. The primary system input is from the radio altimeter system. By monitoring radio altitude and, more importantly, rates of change of radio altitude, the system can compute predictions on the likelihood of the aircraft impacting with the ground. Modes of Operation Because the likelihood of crashing varies with the aircraft configuration and the stage of flight, there are six modes of GPWS operation which can be summarised as follows: Mode 1: excessive sink rate Mode 2: excessive ground closure rate Mode 3: descent after take-off Mode 4: proximity to the ground with landing gear in the up position or the flaps not in the landing position Mode 5: descent below glideslope Mode 6: descent below the decision height selected on the RADALT. Radar altimeter operation modes 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 349 of 356 Ground Proximity Warning System Function Two warning lamps located on each side of the instrument panel directly in front of pilot and co-pilot. A single amber GPWS INOP warning lamp is located on the instrument panel. This will illuminate if the system fails. Messages from the GPWS are classified as either advisory or warning. The advisory message simply alerts the crew to the situation. If corrective action is not taken and the situation becomes dangerous, a more urgent warning will be given. For example, Mode 1 provides an advisory “sink rate - sink rate” aural message if the aircraft loses altitude when close to the ground. If the flight crews fail to take appropriate action, the message is upgraded to the more urgent warning of “whoop whoop pull up whoop whoop pull up.” All warning messages and most advisory messages consist of a combined visual annunciator and aural announcement. Ground proximity warning system warning indicators 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 350 of 356 Ground Proximity Warning System Operation The major part of the GPWS is the computer. This is a single black box unit which interfaces with the aircraft systems and provides outputs to the warning lamps and the audio system. The GPWS computer receives information from the following systems: Radio altitude from the RADALT system Landing gear position Flap position Glideslope deviation from the VHF navigation system Mach (airspeed) data from the Central Air Data Computer (CADC) Barometric rate of change of altitude from the CADC. This is a block diagram of a typical GPWS aircraft installation. The computer is the central component, and it receives input from a variety of systems. Ground proximity warning system warning indicators The computer calculates the likelihood of danger based on the aircraft system inputs and: Generates the synthesised voice messages Activates the caution or warning lamps. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 351 of 356 Enhanced Ground Proximity Warning System Enhanced ground proximity warning system (EGPWS) has recently become available and incorporates a worldwide terrain database and GPS inputs. These features allow the system to monitor the aircraft’s position relative to the surrounding terrain and provide much greater warning times of impending ground contact. Model chart of mountainous zone on a flight route Geographical data displayed on the monitor in the cockpit 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 352 of 356 Area Navigation Introduction to Area Navigation To ensure aircraft maintained accurate positional awareness, VOR/DME navigation and other ground-based stations were used by aircraft to fly from point to point. An aircraft would take-off and fly outbound on a VOR radial until it intersected the VOR radial of the next VOR station, where it would fly inbound on a selected radial. After passing over that station, it would again fly outbound on a VOR radial which would aim the aircraft at the next VOR station on the flight plan. Area Navigation or Random Navigation (RNAV) allows the pilot to fly direct to a destination without the need to fly directly over VOR stations or other ground-based facilities. This is a far more efficient method of navigating as it allows a much more direct route from origin to destination, reducing flight time and conserving fuel. Benefits of area navigation (RNAV) Some of the available systems which support RNAV include Global Positioning Systems (GPS), Inertial Navigation Systems (INS) and VOR/DME stations. Using Aviation RNAV procedures, a waypoint (or point in space) is established using a navigation system entered into the navigation computer which then directs the aircraft to fly-over or fly-by waypoints to navigate. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 353 of 356 Fly-by waypoints 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 354 of 356 Area Navigation Computer Older RNAV computers are referred to by several names: RNAV transceiver Course-line Computer (CLC) Track-Line Computer (TLC). The unit is panel mounted and incorporates a keypad which relevant navigation information and waypoints can be entered. Area navigation (RNAV) system The block diagram below refers to a ten-waypoint RNAV computer. The waypoint parameters may be entered from a keyboard on the front panel or from a portable magnetic card reader. The unit can be used for frequency management of both VHF communication and navigation (VOR/DME) frequencies. The complete RNAV system consists of the navigation computer, a combined VHF navigation/communication transceiver, a DME transponder, Electronic Chart Display and Information System (ECDI) and an encoding altimeter. A card reader allows updating of the navigation data base. Bearing and distance to the active waypoint are found by solving first the slant range triangle, then the RNAV triangle. The Electronic Course Deviation Indicator (ECDI) displays bearing and distance information and provides flight guidance to the pilot. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 355 of 356 Area navigation (RNAV) block diagram An aircraft with an INS and or a FMS is not likely to have a separate RNAV computer, as this function would be performed internally by the INS and or FMS. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems CASA Part Part 66 - Training Materials Only Page 356 of 356