Radionavigation basics

Transcrição

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Freitag, 06. November 2009 um 19:36 - Aktualisiert Samstag, 07. November 2009
Basic Radio Propagation Theory Definitions Radio Waves Radio waves travel at the speed of light, which is approximately 300 000km/s or 162 000 NM/s Cycle A complete series of values of a periodical process Hertz One Hertz is one cycle per second Polarisation The polarisation of an electromagnetic wave describes the orientation of the plane of the oscillation of the electrical component of the wave with regard to its direction of propagation Frequency The number of cycles occurring in one second in a radio wave expressed in Hertz (Hz) Wavelength The physical distance travelled by a radio wave during one cycle of transmission The relationship between wavelength and frequency is: 1 / 34
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wavelength (?) = speed of light (c) or ?(meters) = 300 000 Frequency (f) kHz Amplitude The maximum deflection in an oscillation or wave Phase The fraction of one wavelength expressed in degrees from 000° to 360° Phase Shift The angular difference between the corresponding points of two cycles of equal wavelength, which is measurable in degrees Bands The bands of the frequency spectrum for electromagnetic waves are: Very Low Frequency (VLF) 3 - 30 kHz Low Frequency (LF) 30 - 300 kHz Medium frequency (MF) 300 - 3000 kHz High frequency (HF) 3 - 30 MHz Very high frequency (VHF) 30 - 300 MHz Ultra high frequency (UHF) 300 - 3000 MHz Super high frequency (SHF) 3 - 30 GHz Extremely high frequency (EHF) 30 - 300 GHz Classification of Radio Signals A radio signal may be classified by three symbols in accordance with the ITU radio regulation vol.1: e.g .A1A - First symbol indicates the type of modulation of the main carrier 2 / 34
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- Second symbol indicates the nature of the signal modulating the main carrier - Third symbol indicates the nature of the information to be transmitted Radio Aids Ground D/F VDF information is divided into the following classes according to ICAO Annex 10: Class A. Accurate to within ± 2° Class B. Accurate to within ± 5° Class C. Accurate to within ± 10° Class D. Accurate to less than class C Coverage and range Following formula can be used to calculate the range: 1,23 x ?transmitter height in feet + 1,23 x ?receiver height in feet NDB / ADF Introduction NDB - Non Directional Beacon is the ground part of the system 3 / 34
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ADF - Automatic Direction Finder is the airborne part of the system NDB operate in the LF and MF frequency bands. The frequency band assigned to aeronautical NDB’s according to ICAO annex 10 is 190 – 1750 kHz The non directional beacon, short NDB, transmits a low to medium frequency non directional signal. Depending on their position and transmitter power output they are used for enroute navigation, to mark mandatory reporting points, as radio beacons for holding patterns and for instrument approaches. A locator beacon is a LF/MF NDB used as an aid to final approach usually with a range, according to ICAO annex 10, of 10-25 NM. All NDB’s operate in the frequency band between 200 and 526.5 kHz and they transmit a continue carrier with 1020 Hz modulation. NDB’s used for enroute navigation have a three letter identifier which they transmit 2 times per minute. Depending on their transmitter power output they have a range between 25 and 150 nautical miles. NDB’s normally used in conjunction with an instrument landing system are called locators and they have a two letter identifier which they transmit 6 times per 4 / 34
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minute. These locator NDB’s have a range between 10 and 25 nautical miles. The system consists of a 30 to 50 meter high antenna tower or a T antenna, the transmitter and the standby transmitter station, an external and an emergency power supply and a performance monitoring system. This monitoring system will alarm the responsible air traffic control station as soon as the transmitter power falls below 50%, the identifier is no longer transmitted or the performance monitoring system is defect. The range of the transmitted signal mostly depends on transmitter power and the antenna used. For example a transmitter with 2000 watts will have a range of about 100 nautical miles and a transmitter of 25 watts will have a range of about 15 nautical miles. The range further depends on atmospheric conditions. For these reasons the system should only be used for navigational purposes within the range published in the AIP. 5 / 34
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ADF The automatic direction finder, short ADF, automatically points into the direction of the station it is tuned to. The ADF system consists of the antenna system, made up of a loop antenna and a sense antenna, the ADF receiver, the ADF control panel and the ADF navigational display. The ADF antenna system consists of the loop antenna that is normally mounted on the fuselage top or bottom and a sense antenna. On modern system both are combined in one ADF
antenna. The purpose of the loop antenna is to determine the angle relative to a station. The position of the plane of the loop determines the induced voltage in the sides of the loop. Maximum voltage is induced when the loop is parallel to the wave travel and therefore maximum signal strength is received through the antenna in this position. A sense antenna is also needed for the operation of the ADF system because the loop antenna can sense direction but it can not determine if the station is to the left, to the right, ahead or behind of the aircraft. 6 / 34
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The signals of both antennas are combined to determine the relative bearing to the station. The ADF receiver works between 200 to 1600 KHz. This allows for reception of NDB’s and also broadcasting stations transmitting in within this frequency range.
A wide range of ADF systems is available and the design varies from model to model but all systems basically have the same control switches such as the function switch to select different modes of operation, the volume control knob and the frequency selector. The different selectable modes are the off position where the system is turned off, the ADF position where the system determines the bearing to the station, the ANT position where the system only uses the sense antenna and in this position the ADF is used as a receiver without direction finding capability. 7 / 34
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The BFO position, meaning ‘beat frequency oscillator’, to receive signals of the modulation type A1A, which is an unmodulated carrier wave interrupted in the rhythm of the identifier. By selecting BFO a signal produced inside the receiver is added to the incoming signal. By combining these signals a tone is produced that can be heard. Some ADF systems also contain a test function to check the ADF for proper operation. When the test function is actuated the ADF needle should move to a relative bearing of 90 degrees. As soon as the test function is deactivated the ADF needle must return immediately to its original indication. We divide between three ADF navigational displays: - The relative bearing indicator, RBI, - the moving dial indicator, MDI and - the radio magnetic indicator, RMI. The relative bearing indicator has a fixed 360 degree scale with the zero degree mark or so called lubber line aligned with the aircraft longitudinal axis. The ADF needle installed in the indicator always points to the tuned NDB and the angle between the lubber line and the needle indication is called the relative bearing which is 90 degrees in this case. By adding the aircraft heading to the relative bearing we receive the QDM which is the magnetic bearing to the station. By subtracting 180 degrees from this value we receive the QDR which is the 8 / 34
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magnetic bearing from the station. This calculation has to be repeated constantly to determine the exact position of the aircraft relative to the NDB. On the moving dial indicator the scale can be manually adjusted to the aircraft heading. Used in this way the needle tip will automatically indicate the QDM and needle end will indicate the QDR. When changing to a new aircraft heading the scale of the moving dial indicator again should be manually adjusted to the new heading to receive the correct QDM and QDR.
The radio magnetic indicator, RMI, is coupled to the directional gyro system and automatically displays present aircraft heading. Therefore the needle tip will always display the QDM and the needle end will always display the QDR, independent of aircraft heading and position relative to the NDB. 9 / 34
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Most RMI’s have to pointer needles for two navigational systems and on many RMI’s the navigation system for each needle can be selected to either ADF or VOR.
In general the ADF has an accuracy of plus minus 5 degrees over its full range of indication. This error is mainly caused by the instrument and the quadrantal error. The quadrantal error is caused by deflection of the radio waves by different metal parts of the aircraft and it varies with the direction of the aircraft relative to the NDB. By equipment compensation this error can usually be reduced to about plus minus 2 degrees. ADF indications are subject to disturbances that may result in erroneous bearing information. An aircraft flying in the vicinity of mountains for example will receive signals reflected from the mountains and this can lead to a false indication. This phenomena is called the mountain effect. NDB radio waves first travelling over land and than over water are subject to a similar phenomena than light shining from air into water. 10 / 34
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The different conductivity of the medias will cause a change in direction of the radio waves. The amount of change depends on the angle with which the radio waves cross the coastline. This effect is called the coastal refraction. At altitudes above 6000 feet the coastal refraction can be considered to be insignificant. Weather phenomena such as precipitation static and lightning can have a great influence on ADF indication. They actually work similar to small radio stations that disturb reception in the long to medium wave range. Hereby NDB reception can be disturbed so badly that the ADF indication can not be used at all. At night the ADF indication can also be influenced by the reception of the sky waves of distant stations that are reflected by the Ionosphere. Most disturbances that have an effect on ADF indication usually also affect the station identification. A bad, noisy or even erroneous identification may be heard when an erratic or false bearing is displayed. Because there is no warning flag on the ADF to warn the pilot when no or erroneous signal is received, the pilot should monitor the NDB’s identification. The range of the NDB is reduced at night, when interaction occurs between the ground wave and the sky wave. The accuracy the pilot has to fly the required bearing in order to be considered 11 / 34
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established during approach according to ICAO DOC 8168 is within ± 5°. VOR and Doppler-VOR including the use of the radio magnetic indicator Today the VHF omni-directional range or VOR is the most commonly used short and medium range navigation aid. It is used for cross country navigation, holding patterns, terminal navigation and also for approach purposes. The frequency band allocated to VOR according to ICAO annex 10 is VHF and the frequencies used are 108,0 – 117,975 MHz. Frequencies in the allocated VOR range with the first decimal place an odd number, are used by ILS. The following types of VOR are in operation: - En-route VOR for use by IFR traffic - Conventional VOR (CVOR) a first generation VOR station emitting signals by means of a rotating antenna - Doppler VOR (DVOR) a second generation VOR station emitting signals by means of a fixed antenna utilising the Doppler principle - Terminal VOR (TVOR) a station with a shorter range used as part of the approach and departure structure at major airports - Test VOR (VOT) a VOR station emitting a signal to test VOR indicators in an aircraft The VOR ground station consists of a special small building that contains: - a primary transmitter, 12 / 34
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- a standby transmitter that will automatically start to transmit in case of a primary transmitter failure, - the omnidirectional antenna which also forms the roof of the building, - the revolving directional antenna, - the normal power supply and - an emergency power supply. The VOR works in the frequency range of 108.00 to 117,95 MHz. A high frequency generator inside the transmitter produces a carrier wave within that frequency band. The signal sent to the omnidirectional antenna is then amplitude modulated with a secondary frequency of 9960 Hz and a frequency modulation of 30 Hz whereas the signal sent to the revolving directional antenna is only frequency modulated at 30 Hz. The working principle of the VOR can be explained as follows: A light signal is transmitted in all directions every 60 seconds and a rotating beacon that performs a full rotation every 60 seconds transmits a second light signal. When the rotating beacon rotates through magnetic north it is in phase with the omnidirectional light and therefore both lights can be seen at the same time. An observer can now measure the time difference between the first omnidirectional light signal and the second rotating light beam and calculate the magnetic direction from the light source. This is done by dividing the time with the time the rotating beacon needs to complete one rotation and then multiplying it by 360. The elapsed time therefore corresponds to a certain angle on the compass rose. 13 / 34
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Related to the VOR this angle measurement is done by checking the amount of phase shift between the omnidirectional reference phase transmitted by the omnidirectional antenna and the variable phase that is transmitted by the rotating directional antenna which turns at a rate of 30 revolutions per second. The amount of phase shift determines the angle relative to magnetic north and the phase shift at magnetic north is
zero. The VOR therefore delivers tracks around 360 degrees and beside the 3 letter VOR Morse code identifier that is transmitted at 1020 Hz modulation it can also be used for voice transmissions such as ATIS or VOLMET. The doppler VOR station consists of a round center antenna that transmits a omni directional carrier signal to all sides and 39 round antennas that transmit an omni directional secondary wave in a clockwise sequence. These antennas are arranged in a circle with a diameter of 13.5 meters and all antennas are mounted on a reflector with a diameter of 30 to 40 meters whereas the transmitter station is constructed below the reflector. The doppler VOR uses the doppler effect, named after its discoverer Christian Doppler, to produce a signal that can be processed by the aircraft VOR equipment. This signal is produced by the 39 antennas that transmit a signal in a sequence that revolves with 30 Hz, which means 30 times per second, around the center 14 / 34
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antenna. Due to the diameter of 13.5 meters a rotational speed of Mach 4 would be required and therefore this rotation is simulated by switching the antennas on and off in sequence. This produces a phase shift between the signals which can then be registered by the aircraft VOR receiver and converted into radial information for navigation purposes. The VOR test facility, VOT, is used to check the operational status and accuracy of the VOR receiver while on the ground where the VOT is located. When the VOT frequency is tuned in the indicator should be centred with an omni bearing selection of 0 and the to/from indicator should indicate from or the omni bearing selector should read 180 with the to/from indicator showing to. In case of the RMI the needle should show 180 degrees. The VOR aircraft receiver system consists of the VOR antenna, the control panel, the VOR receiver and the indicator. A VOR antenna must be horizontally polarized and possess good radiation characteristics around 360 degrees. Suitable antennas are the V-Dipole antenna, which is normally mounted on the aircraft fuselage the Balanced Loop antenna, which is normally mounted on the vertical stabilizer for antenna amplification and the balanced slot antenna which is normally also mounted on the
vertical stabilizer for antenna amplification. The VOR receiver works in the frequency range between 108.00 and 117.95 MHz with a frequency spacing of 50 kHz and it is normally installed in the avionics compartment. It also contains the ILS module for localizer and glide slope reception. 15 / 34
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To calculate the range, following formula can be used: 1,23 x ?transmitter height in feet + 1,23 x ?receiver height in feet The accuracy the pilot has to fly the required bearing in order to be considered established on a VOR track when flying approach procedures according to ICAO DOC 8168 is within half full scale deflection of the required track. DME - Distance Measuring Equipment A DME operates in the UHF band between 960 – 1215 MHz according to ICAO annex 10. The system comprises of two basic components: - the aircraft component, the interrogator; and - the ground component, the transponder The distance measuring equipment, short DME, is used to measure the line of sight, also called slant range, distance of the aircraft to the selected station. This is done by sending out a paired pulse at a specific spacing, the interrogation, which will be received by the DME ground station. The ground station, the transponder, will then respond by also transmitting a paired pulse on a different frequency but at the same pulse spacing. This difference between transmitted and received frequency is always +/- 63 Hz. By measuring the time required by the signals to complete the round trip the slant range distance can be calculated. 16 / 34
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The DME ground station is normally associated with navigation facilities such as VOR, ILS or localizers, micro wave landing systems, VORTAC and TACAN. Only in rare instances stand alone DME stations can be found. The DME ground station consists of the antenna, the receiver and the transmitter. The antenna is used for reception and transmission of the signals and because the DME works in the UHF frequency band and on the line of sight principle the area around the antenna has to free of obstacles and objects can cause reflection or block the signals. The receiver will receive the signals sent out from an aircraft. The DME signals are impulses that are pulse position modulated. Two pulses are generated and sent out within a relatively short time. These two pulses are called a pulse pair. The time difference between the pulses and the operating frequency are the call sign of the particular ground station called. The time difference between the pulses pair is the call sign of the particular airborne station and it is different on every aircraft DME station. The transmitter will then transmit signals that are modulated in the same way as the incoming signals. These signals can be identified by the aircraft DME station and by measuring the time difference between transmitted and incoming signal the distance will be calculated. The DME ground station can handle 100 aircraft at the same time and should there be more than that, only the 100 nearest will receive a response. 17 / 34
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The aircraft DME system consists of: - the DME antenna, normally mounted on the fuselage bottom side, - the receiver/transmitter, - the DME controls that are normally limited to DME hold, NAV 1 and NAV 2 modes and - the DME indicator with modes such as distance, ground speed and time to station. DME ground stations are normally located together with VOR and ILS systems and work in the frequency range of 960 to 1215 MHz. Reliable signals may be received up to a line of sight distance of 199 nautical miles with an accuracy better than
0.5 NM or 3%, whichever is greater. When identifying a DME station co-located with a VOR station, the identification signal with the higher tone frequency is the DME which idents approximately every 30 seconds. In certain flight attitudes such as high climb and descent angle or a steep bank the communication between the aircraft and the DME station may be interrupted. The velocity memory will then continue to provide an indication, but if this interruption takes longer than 10 seconds a new search will be started. The aircraft DME and the ATC transponder systems transmit signals in the same frequency range. Therefore the two systems are interlocked to prevent them from transmitting or receiving at the same time. 18 / 34
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The error of the DME according to ICAO annex 10 should not exceed ± 0,25 NM + 1.25% of the distance
measured. ILS - Instrument Landing System The instrument landing system, ILS, is designed to provide precision horizontal and vertical approach guidance during all weather conditions. It contains a localizer system that provides the pilot with course guidance to the runway centreline, a glide slope system that provides the pilot with navigation information for descent to the lowest authorized decision height and the marker beacons that indicate positions along the ILS path. The localiser antenna should be located on the extension of the runway centre line at the stop end. The glidepath antenna should be located 300 metres beyond the runway threshold, laterally displaced approximately 120 metres to the side of the runway centre line. Marker beacons produce radiation patterns to indicate predetermined distances from the threshold along the ILS glidepath. However, marker beacons are sometimes replaced by a DME paired with the LLZ frequency. The ILS frequency assigned band 108,0 – 111,975 MHz; only frequencies with the first decimal odd are ILS
frequencies. The LLZ operates in the VHF band 108,0 – 111,975 MHz according to ICAO Annex 10. 19 / 34
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The GP operates in the UHF band 328,6 – 335,4 MHz according to ICAO Annex 10. The localizer, LLZ, system contains - the antenna system that is normally made up of12 dipole antennas that are positioned 300 to 900 m from the runway end, - a transmitter for redundancy constructed as a dual transmitter station, that operates in the frequency range of 108.1 to 111.9 MHz with a channel spacing of 200 KHz and - a performance monitoring system that continuously checks the course, modulation, transmitter power and identification for proper function and will inform the responsible air traffic control office immediately if something does not work correctly. The purpose of the localizer is to show the pilot the correct course for the final approach. The antennas transmit two signals that cross each other in the horizontal. The left signal is modulated at 90 Hz and the right signal is modulated at 150 Hz. The line of points where the signal strength of the 90 and 150 Hz signal are equal is the final approach course. The localizer identification is a three letter code that is transmitted in international morse code. In case of a VHF communication failure on board of an aircraft there also exists the possibility to transmit voice communication over the localizer frequency to inform the pilot about weather condition and runway condition or other important information. 20 / 34
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The normal localizer system is very vulnerable against deflection by obstacle such as hills, buildings or even vehicles. To minimize the course disturbance by such objects a dual frequency localizer was developed. The dual localizer has two antenna systems, the directional antenna which provides the primary course guidance and the clearance antenna which provides coverage of the other required areas and transmits at a frequency shifted by 9 KHz. Due to the clearance antenna the energy transmitted by the directional antenna can be bundled up much tighter. In fact the directional antenna transmits 90% of its energy in a sector of plus minus 7 degrees and outside a sector of plus minus 10 degrees with means no transmitted energy can be registered. The normal range of localizer system in europe is 25 nautical miles at an angle of plus minus 10 degrees and 17 nautical miles at an angle of plus minus 35 degrees. The range of localizers in the USA is 18 nautical miles at an angle of plus minus 10 degrees and 10 nautical miles
at an angle of plus minus 35 degrees. Some airports with instrument landing systems also have a localizer back course installed which provides course guidance for missed approaches and instrument departures. The localizer back course can also be used for instrument approaches but the pilot has to be aware that in this situation the 90 Hz modulation field will be on the right and the 150 Hz modulation field will be on the left side. 21 / 34
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This is important for instrument setup and interpretation. The ILS glide path system consists of a special antenna system that is normally positioned about 240 to 480 meters from the approach end of the runway and about 120 to 180 meters left or right of the runway centreline. The glide path transmitter which transmits in the range of 328.6 to 335.4 MHz, this frequency is linked to the localizer frequency. The glide path transmitter is normally constructed as a dual system for redundancy, and a performance monitoring system that continuously checks the glide path angle, beam concentration and transmitter power for proper function and will inform the responsible air traffic control office immediately if something does not work correctly. The glide path, or also called glide slope system provides vertical guidance during the approach. The glide path antenna transmits two signals whereas the upper signal is modulated at 90 Hz and the lower signal
is modulated at 150 Hz. The glide path projection angle is normally adjusted to 3 degrees above horizontal and it provides guidance down to a specific altitude, the so called decision height which is normally 200 feet for CAT 1 approaches. At this decision height the pilot must decide to land or to perform a missed approach, depending if he has the runway in sight or not. The glide path is normally usable up to a minimum range of 10 nautical miles and 22 / 34
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the normal glide slope angle is 3 degree above the horizontal. However, at some locations the system has been certified for use at distances that exceed this range and glide path angle. The transmitted glide path beam is 1.4 degrees wide. Pilots must be alert when approaching the glide path because outside of the normal limits and at angles considerable greater than the published angle false course indication and reverse
sensing may occur. Marker beacons transmit their signals straight up into the air. They are used to mark certain distances to the runway, to reporting points or to be able to know the exact crossing time over a VOR or NDB station. By construction and type of use we divide between Z or so called zero markers and fan markers. The zero marker has a round transmission footprint that is transmitted straight upward. It is normally used with VOR and NDB navigational facilities to mark the exact time of crossing and to reduce the effect of the cone of silence. The zero marker transmits at a frequency of 75 MHz and with a power of 10 watts which assures reception up to 6000 feet. The z marker can be identified by a continuous 3000 Hz tone. Today the zero marker is not very common anymore. The fan markers transmit an elliptical signal or a bone shaped signal and we divide between airway and ILS markers. The airway marker transmits at a frequency of 75 MHz and with a power of 100 Watts. Due to this high transmission power it can be received at a maximum altitude of 20 000 feet. The airway marker can be identified by one or two letters in 23 / 34
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international morse code modulated at 3000 Hz. The ILS markers provide information about the distance from the runway. An ILS normally has two, but a maximum of three markers installed. These markers are the outer marker, the
middle marker and the inner marker. The outer marker is normally located 3.5 to 6 NM from the approach end of the runway -ideal would be 3.9 NM. It transmits at a power of up to 10 W which gives the signal a range of up to 6000 ft and its identifier consists of
two dashes per second. The middle marker is normally located 1050 m plus minus 150 m from the approach end of the runway with 75 m maximum offset from the extended runway centreline. In bad weather conditions this arrangement should advise the pilot to check for visual approach aids such as VASIS, PAPI or APP lights when passing the middle marker in bad weather conditions. It transmits at a power of up to 10 W which gives the signal a range of up to 6000 ft and its identifier consists of alternating two dashes and two dots per second. The inner marker is normally located 75 to 450 m from the approach end of the runway with 30 m maximum offset from the extended runway centreline. In bad weather conditions this arrangement should advise the pilot that he is approaching the runway threshold. It transmits at a power output of up to 10 W which gives the signal a range of up to 6000 ft and its identifier consists of 6 dots per second. 24 / 34
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Summary frequency, modulation and identification assigned to all marker beacons according to ICAO Annex 10: All marker beacons operate on 75 MHz carrier frequency Modulation frequencies are: - outer marker 400 Hz - middle marker 1300 Hz - inner marker 3000 Hz Identification is continuous modulation of the audio frequency as follows: outer marker 2 dashes per second continuously middle marker a continuous series of alternate dots (6 per sec) and dashes (2 per sec) inner marker 6 dots per second continuously According to ICAO DOC 8168, the final approach area contains a fix or facility that permits verification of the ILS glidepath/altimeter relationship. The outer marker or DME is usually used for
this purpose The ILS performance monitoring system checks all ILS components for proper function and will alert the applicable air traffic control station in case of a failure. The localizer is constantly checked for bundling, course, modulation, identification and power of the transmitted signal. Should any of these items malfunction or be outside of the tolerances, a warning light will immediately inform the applicable air traffic control station. The glide path is constantly checked for bundling, angle above the horizontal and power of the transmitted signal. Should any of these items malfunction or be outside of the tolerances a warning light will immediately inform the applicable air traffic control station. The marker beacons are constantly checked for modulation, identification and 25 / 34
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transmitter power. Should any of these items malfunction or the transmitter power be below 50 % a warning light will immediately inform the applicable air traffic control station. The aircraft instrument landing system consists of components for localizer, glide path and marker beacon reception, control and indication. The components for the localizer are the VHF NAV antennas, the VHF NAV receiver, the VHF NAV control panel and the indication instrument in the cockpit with a vertical needle. The components for the glide path are the glide path antenna, the glide path receiver which is normally integrated into the VHF NAV receiver and glide path indication with a horizontal needle. The components for the marker are the marker antenna, the marker receiver, the marker control panel and the marker indication lamps. The localizer is received via the same reception system as the VOR. A switching unit is inside the VHF NAV receiver which activates the localizer receiver as soon as one of the 40 ILS channels between 108.10 and 111.95 MHz is selected. If the selected localizer can be received with sufficient signal strength the localizer warning flag will go off and the course deviation indicator, short CDI, will move, depending on the position relative to the localizer. The working principle of the localizer receiver is quite simple. The receiver constantly looks for 90 and 150 Hz modulation and compares the strength of the two signals. If the strength of the 150 Hz modulation signal is greater than the 90 Hz modulation signal, the CDI will be deflected to the left showing the aircraft to the 26 / 34
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right of centreline and vice versa whereas the amount of deflection depends on the amount of course deviation from the centreline. If the signal strength of both signals is the same, the aircraft is on course and the CDI is entered. A full deflection of the CDI from center to the left or right on the localizer is 2.5 degrees and not 10 degrees as on the VOR and this further means that one dot on the HSI will only be 1.25 degrees on the localizer. During a normal ILS front course approach the HSI needle works as a command instrument. This means that when left of course the HSI needle will command a right turn to return on the ILS centreline and when right of course it will command a left turn. The same principle is true when flying outbound or performing a missed approach on the localizer back course. During a localizer back course approach the HSI will work in the same way as long as the front course is selected. If the back course is selected, the HSI needle will indicate the position of the aircraft relative to the ILS centreline. The HSI needle now works as an indication instrument and no longer as a
command instrument. It therefore is very important that the pilot be aware of this reverse sensing and that he acts accordingly. The glide path works on the same basic principle as the localizer but the glide path frequency can not be selected individually. The 40 glide path frequencies between 329.15 and 335.00 MHz are fixed to 27 / 34
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certain localizer frequencies and when one of these is selected also the applicable glide path frequency is called up. Then, provided the glide path is working properly and can be received with sufficient strength, the glide slope warning flag will disappear and the glide path indicator will come into view. A full deflection of the glide slope indicator needle from center position to full up or full down is 0.5 degrees. This means that one dot on the scale corresponds to 0.25 degree on the glide path. The glide slope indicator works as a command instrument and should the aircraft get below the glide slope, it will command a reduction in rate of descent and if the aircraft gets above the glide
slope, it will command an increase in rate of descent. Full scale deflection of the CDI needle corresponds to approximately 2,5° displacement from the ILS centre line. Full scale deflection on the GP corresponds to approximately 0,7° from the ILS GP centre line The accuracy the pilot has to fly the ILS localiser to be considered established on an ILS track is within half full scale deflection of the required track. The aircraft has to be established within half scale deflection of the LLZ before starting descent on the GP. The pilot has to fly the ILS GP to a maximum of half scale fly-up deflection of the GP in order to stay in protected
airspace. If a pilot deviates by more than half scale deflection on the LLZ or by more than half course fly-up deflection on the GP, an immediate missed approach should be executed, because obstacle clearance may no longer be guaranteed. 28 / 34
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To be able to compute the required rate of descent before performing an ILS approach rate of descent charts are published. For example, the required rate of descent on a standard 3 degree glide path with a speed of 130 kts is 690 ft per minute. The rate of descent for a 3° glide path angle given the groundspeed of the aircraft can be calculated using the following formula: Rate of descent (ROD) in ft/min = groundspeed in kt x 10 2 The rate of descent for any glidepath angle can be calculated with following formula: ROD ft/min = Speed factor (SF) x glidepath angle x 100 The aircraft marker beacon system consists of a receiver which is tuned to 75 MHz with three channels for 400, 1300 and 3000 Hz modulation frequency and the marker beacon antenna which is mounted on the bottom side of the aircraft fuselage. It further contains the on/off switch, the marker sensitivity selector which is used to select high or low system sensitivity and should be in low position for an ILS approach and the three marker beacon indication lights. When the aircrafts overflys the outer marker the blue light comes on and the outer marker identifier can be heard. When the aircraft overflys the middle marker the yellow light comes on and the middle marker identifier can be heard. When the aircraft overflys the inner marker the white light comes on and the identifier for the inner marker can be heard. 29 / 34
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The white light will also come on when crossing an airway or a zero marker. Pilots must be aware that disturbances to ILS localizer and glide slope may occur when vehicles or aircraft are operated near the localizer or glide slope antenna. Therefore ILS critical areas are
established near the antennas. At weather conditions with a ceiling less than 800 ft and/or a visibility below 2 miles no vehicles or aircraft is allowed within the localizer critical area except for aircraft landing, exiting the runway, departing or performing a missed approach when an approaching aircraft is between the ILS final approach fix and the airport. At weather conditions with a ceiling less than 200 ft and/or a visibility below 2000 ft RVR no vehicle or aircraft operation is authorized inside the localizer critical area when an aircraft is middle marker inbound on the ILS. Further no vehicle or aircraft operation is authorized in the glide slope critical area when an aircraft is between the ILS final approach fix and the airport. If weather conditions are less than 800 ft ceiling and/or 2 miles visibility, no aircraft holding below 5000 ft are allowed between the outer marker and the airport. The ILS critical Area is an area of defined dimensions about the LLZ and GP antennas where vehicles, including aircraft, are excluded during all ILS operations. The ILS sensitive area is an area extending beyond the critical area where the parking and/or movement of vehicles, including aircraft, is controlled to prevent 30 / 34
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the possibility of unacceptable interference to the ILS signal during ILS operations. MLS - Microwave Landing Systems According to ICAO annex 10, MLS is a precision approach and landing system operating in the S band on 200 channels assigned on the frequencies between 5031,0 – 5090,7 MHz with 300 kHz spacing. MLS is a time division multiplex system operating on a common frequency with time synchronisation of the transmission from ground equipment serving a particular runway, assuring interference-free operation. The micro wave landing system, MLS, was designed to provide precision navigation guidance for alignment and descent of aircraft on approach to a runway. In the future it will eventually replace the ILS as the standard landing system. The MLS is capable of providing precision three dimensional navigation guidance with the possibility to perform curved and segmented approaches, selectable glide path angles and to establish boundaries to ensure clearance from obstruction in the terminal area. The approach azimuth coverage of the MLS is at least 40 degrees to either side of the runway laterally and in elevation up to an angle of 15 degrees and at least 20 000 feet. The minimum range of the approach azimuth is at least 20 nautical miles. 31 / 34
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The back azimuth coverage is also at least 40 degrees to either side of the runway laterally and in elevation up to an angle of 15 degrees, but the minimum range coverage is only 7 nautical miles whereas the actual coverage is normally the same. Future goals are a coverage of 360 degrees laterally and 30 degrees in elevation. The micro wave landing system may be divided into five functions: The approach azimuth, the back azimuth, the approach elevation, range information and data communication. The azimuth and elevation stations transmit angle and data on one of 200 channels within the frequency range of 5031 to 5091 MHz. MLS identification is a four letter designation starting with the letter M and it is transmitted by the approach and back azimuth transmitter in international morse code 6 times per minute. The approach azimuth station is normally located about 1000 feet beyond the stop end of the runway and the elevation station is normally located 400 feet from the side of the runway between the runway threshold and touchdown zone. The back azimuth transmitter is normally located about 1000 feet in front of the approach end of the runway. The MLS precision distance measuring equipment works the same as the DME, and the MLS DME channel is paired with the azimuth and elevation channel. Range information is provided with an accuracy of about 100 feet. 32 / 34
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The data communication can include both basic and auxiliary data. All MLS facilities transmit basic data that consists of the station identifier, exact location of transmitting stations, ground equipment performance level and the DME channel and status. Auxiliary data that might be transmitted is data such as 3D location of MLS equipment, waypoint coordinates, runway condition and weather reports. With this time difference between the two impulses the angle relative to the airport can be calculated for the azimuth and the elevation. The aircraft has to be established within half scale deflection of the azimuth before descent is initiated on MLS elevation angle. The accuracy the pilot has to fly the MLS azimuth in order to be considered established on an MLS track according to ICAO DOC 8168 is within half full scale deflection of the required track. The accuracy the pilot has to fly the MLS elevation angle in order to stay in protected airspace according to ICAO DOC 8168 is maximum half scale fly-up deflection of MLS elevation angle. If the pilot deviate more than half scale deflection on the azimuth or more than half course fly up deflection on the MLS elevation angle, an immediately missed approach should be executed, as obstacle clearance may not exist. 33 / 34
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According to ICAO DOC 8168, the final approach area contains a fix or facility that permits verification of the MLS elevation angle/altimeter relationship. The outer marker or DME is usually used for this purpose. Range and accuracy can be affected by MLS multipath interference, which can be created due to reflections from large reflecting objects within the MLS coverage area. The MLS critical area is an area of defined dimensions about the localiser and glidepath antennas where vehicles, including aircraft, are excluded during all MLS operations. The MLS sensitive area is an area extending beyond the critical area where the parking and/or movement of vehicles, including aircraft, is controlled to prevent the possibility of unacceptable interference to the MLS signal during MLS operations. 34 / 34

Radionavigation basics

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