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Blog Archive ▼ 2009 2009 (2)  (2) ▼ January (2) January (2)

Mul Multil tilaterat aterat ion sy stem DVOR DME

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Sunday, January 4, 2009

Multilateration system Multilateration is a proven technology that has been in use for many decades. decad es. It was developed develop ed for military purposes to accuratel accurately y locate locate aircraft ai rcraft —many of  which did not wish to be “seen” — by using a method known as Time Difference of Arrival (TDOA). Multilateration Multil ateration employs employs a number of ground stations, s tations, which which are placed in strategic locations a round an airport, its local local terminal area or a wider area that covers the larger larger s urrounding airspace. airs pace. Furthermore, while while the radar and multilateration mult ilateration “targets” on a controll controller’s screen are identic i dentical al in appearance, the very hig h update rate of the multilaterationderived multilaterationderived targets makes them instantl ins tantly y recognizable recognizable by their smooth movement across the screen. A screen displaying multilateration information can be set to update as fast as every second, compared with the 4 - 12 second position “jumps” of the radar-derived targets. targets . These T hese units listen for “replies,” typ typicall ically y to interrogation signals transmitted from a local SSR or or a multilateration station. Since individual aircraft will be at different distances from each of the ground stations , their replies replies will will b be e received by each station at fractionally fractionally different times. Using advanced computer comput er processing processi ng techniques, these individual indivi dual time time difference diff erencess allow allow an aircraft’s ai rcraft’s position to be precisely calcul calculated. ated. Multilateration requires no additional avionics equipment, as it uses replies from Mode A, C and S transponders, transponders, as wel welll as military mi litary IFF an and d ADS-B tr tra ansponders. nsponders. “Multilateration offers ANSPs the possibility of providing a surveillance service at a potentially much lower cost, greater reliability and higher levels of  accuracy than conventional SSR.” Alexander ter Kuile CANSO Secretary General Multilateration ( MLA T ) MLAT ground stations receive replies from all transponder-equipped transponder-equippe d aircraft, including including legacy radar and ADS -B avionics, avi onics, and determine aircraft position based on the time difference of arrival (TDOA) of the replies.

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Posted by Jack at 5:56 AM   No comments:

Saturday, January 3, 2009

DVOR DME

Principle Princip le of Opera Operation tion VOR 2007.01.20 VOR is an abbreviation for "VHF Omnidirectional Radio Range", which implies that it operates in the VHF band. Adopted by ICAO as early as 1960, VOR has been the main short-range navigational aid for several years. Short range infers that ranges up to 200 NM can be expected. It is still the most commonly used short-range aid. As opposed to the NDB, which transmits a non-directional signal, the signal transmitted by the VOR contains directional information. The principle of operation is bearing measurement by phase comparison. This means that the transmitter on the ground produces and transmits a signal, or actually two separate signals, which make it possible for the receiver to determine its position in relation to the ground station by comparing the phases of these two signals. In theory, the VOR produces a number of tracks all originating at the transmitter. These tracks are called «radials» and are numbered from 1 to 360, expressed in degrees, or ° . The 360° radial is the track leaving the VOR station towards the Magnetic North, and if you continue with the the cardinal points, points, radial 090° points to the East, the 180° radial to the South and the 270° radial to the West, all in relation to the magnetic North. See Fig 1.1

  Before we look in detail at how the system works the following example illustrates the principle and should make it easier to understand. Think of a lighthouse at sea and imag ine the white light light rotating at a speed of one revolution per minute (60 seconds). Every time this white narrow beam passes through Magnetic North, a green omnidirectional om nidirectional light light flashes. Omnidirectional means that it can be seen from any position around the lighthouse. If we are situated somewhere in the vicinity of the light sources and are able to see them, we can measure meas ure the time interval interva l from the green light flash until we we dv or-dme.blogspot.in or-dme.blogspot.in

 

see the white light. The elapsed elapsed time is directly directly proportional to to our position line in rel relation ation to the lighthouse. The speed of 1 RPM corresponds to 6° per second, so if 30 seconds elapse between betw een the time we see s ee the green flash and the white rotating light, we are on the 180° radial, or directly south of the station (30 sec x 6°/sec = 180°).. This 180°) T his calcul calculation ation can be done from any position pos ition and the elapsed time is directl directly y proportional proportional to our angul ang ular ar position pos ition (radial). (ra dial). We W e coul could d name these light signals, calling the green one the Reference (REF) signal and the white beam the Variable Variable (VAR) signal. si gnal.

Ground Installation The VOR system operates on frequencies between 108 MHz and 117.95 MHz. Channel separation is 50 kHz and the signals have a horizontal polarisation. Frequencies between 108 MHz and 111.95 MHz are primarily used for the localiser part of the ILS but can be shared with short range VORs, or socalle cal led d terminal VORs. The VOR uses frequencies having an even decimal as the first digit after the last MHz digit, while localizers use odd decimals. When assigning a VOR to this part of the frequency band, it is an essential requirement that it does not interfere with an adjacent ILS channel. The frequencies from 112.00 to 117.95 117.9 5 M Hz are s olely olely used by VOR, both on odd and even ffrequenc requencies ies 108.10 Localizer 112.00 VOR 108.15 Local L ocalizer izer 112.15 VOR 108.20 T-VOR 112.10 VOR 108.25 T-VOR 108.30 Localizer 117.95 VOR 108.35 Localizer The ground equipment is set up on a fixed, surveyed site and consists of a transmitter driving a combined aerial system; one part producing the Reference (REF) signal, the other producing the Variable (VAR) signal. The REF signal is an omnidirectional continuous wave transmission on the carrier frequency of that particular VOR station. It carries a 9960 Hz subcarrier that is frequency modulated at 30 Hz. Since this is an omnidirectional transmission, the polar diagram of the REF signal is a circle. In the receiver, it is the 30Hz component of this signal that is used as a reference for measuring the phase difference. The variable signal (VAR) is transmitted from a n aerial that is effectively a loop. loop. This ‘loop’ produces produces a figure fi gure of 8 polar diagram, diagram , which is electronic electronicall ally y rotated at 30 revolutions per second. When the two signals (VAR & REF) are mixed together, the resul res ulting ting polar diag ram will be be a cardioid, but unlike the cardioid of the ADF, this does not have a «null» position. We call it a «limacon». It rotates at 30 revolutions per second, indicated with an arrow on fig.1.2 The rotation of the limacon limacon creates an effective ampl am plitude itude modul modulation ation of  30 Hz. The VOR receiver splits these two signals into the two original components. The two signals are processed through different channels and dv or-dme.blogspot.in or-dme.blogspot.in

 

the phase of the 30 Hz modulations of the fixed REF signal and the VAR signal are compared in a phase comparator. The phase difference between these two signals si gnals is i s directly directly proportional to to angular position pos ition with reference to the VOR station. As explained, magnetic North is the normal reference for the radials, so when 0° phase difference is detected, the receiver is on the 360° radial from the station. The phase difference and variable signal at the cardinal points. The description above is valid for the convential VOR, CVOR. The CVORs suffer from reflections from objects in the vicinity of the VOR site and it was found that errors due to this could have been reduced reduced if the horizontal antenna dimensions were increased. This was not practical to do and a new system had to be developed: the DOPPLER VOR, DVOR. The CVORs are now gradually being replaced by DVORs, that will be described in the next section.

Dopp Dop pler VOR (DVOR) The Doppler VOR is the second generation VOR, providing improved signal quality qual ity and accuracy. accuracy. T The he REF signal si gnal of the DVOR is amplitude amplitude modulated, modulated, while the VAR signal while s ignal is frequency frequency modul modulated. ated. This means that the modulations are opposite as compared to the conventional VORs. The frequency frequenc y modulated modulated si signal gnal is less less subject to interference than the amplitude amplitu de modulated modulated sig nal and therefore therefore the received signals si gnals provi provide de a more accurate bearing determination. The Doppler effect is created by letting the VAR signal be «electronically rotated», on the circular circular plac placed ed aerials, at a t a sspeed peed of 30 revolu rev olutions tions per second. W ith a diameter diam eter of the circle circle of 13.4 1 3.4 meters, m eters, the radial velocity of  the VAR signal will be 1264 m/s. This will create a Doppler shift, causing the frequency to increase as the signal is rotated towards the observer and reduce as it i t rotates away with with 30 full cycl cycles es of ffrequenc requency y vari variation ation per second. This results in an effective FM of 30 Hz. A receiver situated at some distance in the radiation field continuously monitors the transmitter. When certain prescribed deviations are exceeded, either the IDENT is taken off, or the complete transmitter is taken off the air. We come back to this in the section Limitations and accuracy. The VOR receiver does not know if it is receiving a signal from a CVOR or a DVOR and the pilot treats both types types in i n the sam same e way. The change of FM and AM for the REF and VAR signals, as compared to the CVOR, is compensated for by having the DVOR antenna pattern rotate tthe he opposite way, compared to the CVOR.

A bout of DVO DVOR/DME R/DME produ producers cers and their lis listed ted price pricess . Today few company manufacturing manufacturing DVOR D VOR and DME DM E in world. world. I introducing

 

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some price of their system from my own experiency. 1.NORTHROPGRUMMAN http://www.parkairsystems.com/  http://www.parkairsystems.com/  USD 520,000580,000* http://www.thalesatm-services.com/   USD 550000-650,000* 2.Thales http://www.thalesatm-services.com/ 3. INDRA INTERSCAN http://www.interscan.com.au/ http://www.interscan.com.au/   USD 380,000460,000* 4. Telematics Co(Korea) www.telematics.co.kr/   USD 320,000-400,000 5. FERNAU AVIONICS http://www.fernau.com/  http://www.fernau.com/  USD 430,000-500,000 6. ALCATEL Air Navigation Systems http://www.alcatel.com/  http://www.alcatel.com/  USD 480,000-580,000* * Some above price's including installation and site surveying cost of DVOR and counterpoise's cost. Counterpoise installation is very hard and too expensive.

DETERMINING THE INSTALLATION SITE The area in i n which a DVOR is to be installed installed is determined by the responsible Civil Aviation Authority according to the international air traffic regulations. The area is generally generally suff sufficientl iciently y large to all allow ow selection of a point with optimum topography and thus the optimum selection propagation conditions can be met. The ins tallation tallation is determined by means of a site survey at which a surveyor must always be present. ATM can provide an engineering consultant on site for this survey. When the installation site has been determined, precise bearings must be taken, either with reference to trigonometrical points or, if a satel sa tellite lite receiver is available, via GPS receiver for increased accuracy. Please see fig 1.3  

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Fig1.3

SITE PREPARA PREPARA TION When the exact ex act location location of the DVOR facility is determined by the the customer and/or the Navaid Nav aid S upplier, upplier, preliminary topographical and geotechnical surveys shall be conducted. A site survey will be performed by a civil engineer. These inves tigations s hall hall pr provide ovide information on exis existing ting ground conditions (s oil, water, etc.) etc.) to determine: — bearing capacity for the design of either high level foundations in the sand fill or deep deep piled foundations, — conductibility conductibility for the design des ign of the earthing network, network, — existing services (pipes, cables, technical trenches, etc.). The earth works shall allow the construction works. If necessary the earth works shall include the demolishing of existing constructions and foundations. Each site si te shall be connecte connected d to the road network of the airport. Light Li ght circulation circul ation only is expected durin during g theofnorma normal l operation. operat access roads shall resist to the loads 25 tons trucksion. andHowever, cranes. the The energy, tel telephone, ephone, multi-pair multi-pair cabl cables es and the connections of the interface points to the external networks networks shall be be assumed as sumed by others. FOUNDATION FOUN DATION DETAILS DETAI LS The overall dimensions of the DVOR counterpoise array and a general installation overview are given in figure.1.3 Details on the specific foundations are given in figure 1.3 The minimum deepness of each pier foundations must be at least 1.20 m under the ground These external dimensions are minimal, and the effective dimensions are to be computed by the Civil Civi l Engineer according to the nature of the soil soi l and the local local standard in force. The install ins tallation ation of the earthing system s ystem shall s hall start during the construction construction of foundations of antenna and shelter slabs.

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