modern threats to precision approach and landing - the ...modern threats to precision approach and...

Paper ISPA2004 (Intern. Symposium on Precision Approach and Landing), Munich 10/2004

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Modern Threats to Precision Approach and Landing - The A380 andWindgenerators and their Adequate Numerical Analysis

Gerhard Greving

NAVCOM ConsultZiegelstr. 43D-71672 Marbach/[email protected]://www.navcom.de

ABSTRACTClassical and modern navigation, landingand radar systems rely on the radio trans-mission and reception. Relevant objects inthe radiation field can harm the intendedcharacteristics of these systems. Modernstate-of-the-art simulation can predict in anincreasing number of complicated cases theelectrical performance in the presence ofthese objects. Countermeasures can be de-signed from this knowledge.This paper deals with the "threat" (potentiallyun-acceptable distortions) on these systemsby the forthcoming new large aircraft A380and by the windgenerators which are con-structed in an increasing number sometimesclose to the systems. The mathematical andnumerical analyses are outlined and someresults are presented. It is in particular em-phasized to apply three-dimensional andsophisticated state-of-the-art methods whichare adapted to the three-dimensional char-acteristics of the objects in contrast to inade-quately simple methods.3D-modeling examples for the A380 andwindgenerators and some principle resultsare presented.

INTRODUCTIONAlmost all classic and modern navigationlanding and radar systems rely on radiotransmission and reception. In a clean en-vironment these systems may work prettywell, but the real life is different. More andmore complex distortion and interferenceproblems for navaids, landing and radarsystems are encountered today (Fig. 1).These so-called “problems” are caused bymajor objects around and in the vicinity ofthese systems, creating additional reflectionsand scattering signals (“multipath signals”)by the principally unavoidable illumination of

these objects by the systems themselves.These objects can be terminals, hangars,large buildings, windgenerators and powerlines as well as the aircraft itself.A reliable prediction of the potential “threat”,i.e. the unacceptable effects on the systemsin question is required in advance before theobjects are built or before the objects appear.This task can be solved today by systemsimulations using state of the art numericalmethods. Quite a number of publicationshave been released by the author in the past/2-12/ on the subject of numerical systemsimulations. This paper highlights two typesof objects somewhat more fully, namely theA380 and the windgenerators (Fig. 2). Thispaper is not intended to present rules anddefinitions for dimensions of safeguardingareas or safety distances.

MODELING AND SYSTEM SIMULATIONThe modeling and simulation process (Fig. 3and 4) must accomplish the following basictasks� Sufficiently realistic modeling of the object

having in mind that the subsequentsimulation is treating the model and notthe reality. The form, the shape and thematerials of the object as well as the ex-citing field above ground have to bemodeled sufficiently. The basic way ofmodeling depends also on the numericalmethod used in the next steps.

� Detailed modeling of the system in ques-tion which is generating the undistortedsignal.

� Simulations of the reflection and scatter-ing process by the application of ade-quately state-of-the-art numerical me-thods. The most accurate method shouldbe generally used; approximations maybe used only if the results are sufficientlyaccurate.


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� Evaluation of the decisive system pa-rameter which the aircraft uses for navi-gation or landing.

The two objects of this paper (Fig. 2; A380,windgenerator) are highly three-dimensionaland therefore a two-dimensional approachand model is neither sufficient nor "state-of-the-art". It is obvious that a three-dimen-sional approach is not only necessary toapply, but also requires a lot more modelingand computation work. No trade-off is ac-ceptable between the accuracy and the effortto be invested due to the safety issue in-volved. A simple 2D-treatment which re-duces a 3D-A380-aircraft to one rectangularplate in the most extreme simplification, orseveral composed rectangular flat plates,can be up to many orders of magnitudefaster, but can also yield wrong and unpre-dictable results /12/. Such kind of simpleapproaches cannot be cured by "somemeasurements" at a few points under someconditions.The simulation of the system must take intoaccount all relevant details which affects theso-called "system-parameter" of that particu-lar system, i.e. the DDM (Difference of Depthof Modulation) for the ILS (Instrument Land-ing System) or the "bearing error" for theVOR/DVOR (Very high frequency Omnidi-rectional Range; Doppler-VOR). These de-tails comprise as an example (see for moredetails Fig. 3)� the correct geometrical and electrical set-

ting and numerical installation of the ac-tual system in the pre-processing section

� the signal processing, the filtering, thesampling, and the receiving antennas inthe post-processing section.

Other field quantities (e.g. "field distortions")cannot describe in general sufficiently thesystem effects. "Field distortions" are neces-sary effects for system distortions, but arenot a sufficient parameter to quantify thesystem distortions.The verification of the correctness and thereliability of the system results is a particularchallenging task as discussed in /12/. A sin-gle or “some measurements” are not suffi-cient. Each result has to be verified in princi-ple.

Fig. 4 shows the overall flow-chart of theapplied IHSS (Integrated Hybrid SystemSimulation). The best suited numericalmethod is taken for the particular problem,i.e. the A380 and the windgenerators. In

certain cases cross checks can be made forapproval by comparing the results of thepreferred approximative IPO-method (im-proved physical optics) with the results fromthe rigorous MoM- or ML-FMM-methods(method of moments, multi-level fast mul-tipole method). The latter family cannot beapplied efficiently for large aircraft andwindgenerators due to the "exploding" stor-age requirements and/or the excessive com-puter time for systematic simulations.Moreover, the ML-FMM has the generalproblem of a questionable convergence ofthe iterative solution of the integral equation.Cases have been experienced where thedefined convergence criterion, e.g. 10-3, hasnot been achieved after 500 iterations for anaircraft.The GTD/UTD method is not the preferrednumerical method for the discussed applica-tions and three-dimensional curved objectsdue to the general caustic problem and thegenerally unavoidable discontinuities of thesolution, which results in problematic discon-tinuities in the DDM-results for ILS.

For both cases (A380, windgenerator) thestructure is subdivided into a large number ofmetallic triangles (Fig. 7 and 10) where thereal exciting field is applied.Worst case principles may be applied as anexample for the dielectric blades by assumingthe metal material.

Great care and knowhow has to be appliedwhen carrying out these sophisticated meth-ods and interpreting the results in each case,because each of the methods can fail incertain situations /12/. Conclusions on thebasis of incorrect results can yield a waste ofmoney or can be the reason for hardly ac-ceptable, in fact unnecessary consequences,such as the closure of a taxiway for A380taxiing /12/.

PRACTICAL PROBLEMSThe A380 on Airports and ILSThe future A380 is currently the largest civil-ian aircraft (Fig. 5,7) which will appear insome years on the airports. Compared to theother large aircraft, this aircraft has a maxi-mum height of the tail-fin of 24.1m. Due tothe horizontal polarization of the ILS-fields thehigher parts of the aircraft may have strongereffects compared to the lower parts.However that does not mean that the tail-fin


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can describe the total aircraft sufficiently.Generally speaking, the larger and higherthe aircraft, the larger the distortions for theILS-subsystems. Therefore, the largest cur-rently existing (military) aircraft (AN225; Fig.5) will not have necessarily the largest DDM-distortions.The A380 will be by nature on an airport inseveral operational phases and by that inquite a number of relevant positions andorientations (Fig. 6), such as :� landing, rolling out, taxiing, parking,

starting� on parallel or sometimes inclined taxi-

ways in relation to the Localizer and/or tothe glidepath

� rolling off after landing� rolling on for starting� crossing the runway on taxiways in dif-

ferent angles� taxiing behind the Localizer and/or be-

hind the glidepath� starting and flying over the Localizer

while not precisely above the runwaycenterline

The international specification ICAO Annex10 defines the DDM-tolerance limits for eachoperational category. The signal in spacemust meet these specifications when a lan-ding aircraft is using ILS. The provider hasto guarantee the compliance with thesespecifications and has to take measures forthat.As a matter of practical handling this basictask is met by the following safeguardingzones and lines� critical areas (forbidden to enter for all

vehicles and aircraft; technically speak-ing small objects may enter in certain ar-eas except the nearfield monitor area. Averification is recommended by adequatemethods.)

� sensitive areas (controlled access pos-sible; e.g. for not too large vehicles andsmall aircraft; verification recommended)

� holding lines .These zones and lines have to be redefinedfor the A380 aircraft. The sizes of thesezones or the distances of the holding lines tocenterline or on the taxiways depend on anumber of factors which have to be takeninto account, such as� type of system (single/dual frequency;

installation of out-of-phase clearance(/1/)

� type and characteristics of antennas(medium, wide aperture, capture ratio,pattern shape, sidelobe suppression etc.)

� existing distortions by stationary objects(hangars etc.)

� structural details of the layout of the air-port (e.g. length of the runway)

� topological details of the runway or air-port, such as humped runways

� operational category CATI-III� type of the aircraft� single or groups of aircraft (e.g. when

queuing for takeoff)� type of receiving antenna in the aircraft or

used for ground measurements.The setting and installation of the ILS-anten-nas as well as the receiving antennas dohave a great impact on the results. Simpleunrealistic dipoles, adapted antennas (e.g.R&S HE108) or 3 element Yagis will showvery much different DDM-results /1,6,9/ andwould result in quite different safeguardingareas or holding lines. It is problematic tocompare measurements results for verifica-tion purposes and also to compare differentresults of simulations when the boundaryconditions and the underlying numericalmethods are not sufficiently known.

The minimum separation between successivelanding aircraft is also a function of thepreceding rolling-off aircraft or of the startingaircraft. It is a well-known effect that largeDDM-oscillations occur when an aircraft islifting off and flying over the ILS-Localizerwhich serves the signal for the next aircraft inthe landing sequence. Unlocks or discon-nects of the autopilot may occur.It is obvious that only a large number of tedi-ous simulations for the particular case and ata given airport for a certain ILS can yield therequired results. The providers and in par-ticular the airports are highly interested tohave the minimum size of the safeguardingareas without increasing the risk for unspeci-fied ILS-signals and reducing the safety.

The Figures 8 and 9 show two examples ofsimulations from a methodological point ofview� A380 on a parallel taxiway for a medium

aperture Localizer antenna. The nose ofthe A380 is assumed to be in the xy-co-ordinates, the axis is parallel to centerline.Filtered DDM-data are presented. DDM-isolines are marked. From such resultsthe lateral extension of the safeguarding


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areas can be defined taking into accountthe locus of the maximum DDM and therelated maximum tolerance limits at thispoint. Holding points result from similar simula-tions where the aircraft is inclined ac-cording to the angle of the rollon taxi-ways.It is noted that the safeguarding areas("critical, sensitive areas") cannot be de-fined on the basis of the maximum DDM-limits. Some margin for existing distor-tions, stationary objects and superposingaircraft has to be provided. Also, theDDM-distortions increase drasticallywhen the aircraft is positioned in an in-clined orientation to the centerline. Thisis operationally the case when the air-craft is turning towards the runway on therollon taxiway.

� A380 rolling off on a high speed taxiway.Time dependant DDM-data are pre-sented. From these results the longitudi-nal extension of the critical area can bedefined.

In both cases large DDM-distortions are en-countered under the given circumstances.

The windgenerator and navaids systemsWindgenerators (WG, "windmills", "windtur-bines") are constructed more and more inmajor quantities as a single installation or inlarge arrays ("windparks"). Often these ob-jects are close to navigation stations or in thecoverage volume of radars of various types.The advanced analysis of the effects ofthese WG on the navigation and radar sys-tems is of increasing interest. The differentnature and function of the navigation sys-tems and radar suggest that the simulationalso must be quite different. However, theintroduced IHSS (Figure 4) and its imple-mented features allow the adapted analysis.Extensions in the pre-processing part and inthe final post-processing part had to be inte-grated. This is especially true for the analysisof the Doppler-shift characteristics of a turn-ing windgenerator (Fig. 10 and 13). TheDoppler shifted scattered fields may haveadverse effects on the VOR/DVOR-systembecause these systems evaluate 30Hz am-plitude and 30Hz frequency modulations.This frequency can easily be produced bythe fast turning blades (Fig. 10 right) even atthe VHF ILS/VOR-carrier frequency of about110MHz.

The WG are highly 3D-structures and needan equivalent modeling (Figure 13). Typicallythe shaft is a shaped metal tube or stronglyreinforced concrete. The cover of the gen-erator house and the rotor blades are usuallymade of glass-fibre material. The bladeshave an integrated metallic lightning protec-tion system. However, the total structure hasbeen modeled for the "worst case" to be fullymetallic, i.e. by a large number of metallictriangles. This takes into account envi-ronmental conditions. In principle the mod-eling strategy is identical for the A380 and thewindgenerators.For the VOR/DVOR systems the scatteredfield components are superposed and proc-essed appropriately, yielding the decisivesystem parameter, i.e. the bearing error. Thefield distortions of the VOR/DVOR-field initself are not a measure for the bearing errorsand system distortions. Fig. 11 shows suchkind of field distortions on a horizontal planein 3D-representation. The largest fielddistortions are behind or beyond thewindgenerator, but the bearing errors areminimum in this region. Potential "shadowingeffects" are negligible for realistic distances ofthe windgenerators to the VOR/DVOR-station. The acceptable bearing errors aredefined in ICAO Annex 10 and in the flightinspection manual DOC 8071. The bearingerrors have to be simulated at the lowestheight of the coverage volume defined foreach VOR/DVOR. Figure 12 shows anexample where the VOR/DVOR bearingerrors have been calculated on a horizontalplane (100km*100km) at a height of 3300ftMSL for a large windgenerator.In this 2D-result for a defined height, all radi-als in that height are contained up to about50km. It can be clearly seen that the maxi-mum bearing errors of the DVOR are muchsmaller compared to the VOR.

Doppler spectrum

The scattered Doppler-frequencies dependon the radial velocity of the scattering objects(Fig. 10 right). This fact has been used todefine a method for the calculation of theDoppler-spectrum of the reflected/scatteredfield.In a side study it has been evaluated that thebearing error of a 30Hz Doppler-shiftedcomponent is larger by factors compared to anon-shifted component. By that, the su-perposed amplitude of the 30Hz shifted


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component at some field point yields thepotential bearing error.It can be easily understood that for the turn-ing blades, the Doppler spectrum must besymmetrical in principle around the un-shifted zero line caused by the stationarysubparts of the windgenerator, i.e. the shaftand the machine house.Fig. 13 shows such an example of the Dop-pler amplitude spectrum of a windgeneratorin some geometrical configuration. Thespectrum is calculated and plotted for sev-eral angular positions of the blades. It canbe seen and understood that the spectralamplitudes for a certain frequency are timeperiodic and time dependant. The ampli-tudes are small compared to the static zero-line. The results so far indicate that thisDoppler-spectrum does not seem to be aproblem for the VOR/DVOR .

CONCLUSIONLarge objects close to navigation, landingand radar systems can distort the electricalcharacteristics of these systems. The newlarge aircraft A380 will appear relatively soonon the airports. An increasing number ofwindgenerators are constructed often closeto these systems.The numerical 3D-treatment using "state-of-the-art-principles" of these objects has beenoutlined and contrasted to simplified 2D-approaches.A380 aircraft will be present in many differ-ent positions, in many orientations and op-erational phases on the airports. Safe-guarding areas ("critical and sensitive ar-eas", holding lines) have to be defined andinstalled to protect the ILS. The case of the"parallel taxiway" is a relatively simple casein terms of the amplitudes of the DDM-dis-tortions. Some principle results for the A380have been presented, one for a parallel taxi-way and one for the dynamic rolloff case. Asexpected, these results and further resultsshow that the ILS-distortions by the A380 areremarkably larger than for the B747. Bysystematic simulations for the individualsituation on a given airport the adapted andminimized safeguarding areas can be de-termined.The numerical treatment of the windgenera-tor with respect to a VOR/DVOR navigationsystem has been outlined. Some principleresults have been presented, showing thebearing errors for a large windgenerator on a

horizontal plane in some minimum coverageheight. The Doppler spectrum of thewindgenerator has been discussed with re-spect to the VOR/DVOR. A numerical resulthas been presented showing the time variantspectrum of small amplitudes for severalblade positions.

REFERENCESNot all references are explicitly cited in this paper. Thereferences are sequenced according to the time ofappearance./1/ GREVING G. ILS CATIII site problems - A newverified system solution, 7th Intern. flight inspectionsymposium, London 1992, p.224-236/2/ GREVING G. Computer aided site analysis and siteadapted installation - Efficient commissioning of landingsystems, 8th IFIS International flight inspectionsymposium, Denver USA, June 1994/3/ GREVING G. Numerical system-simulations in-cluding antennas and propagation exemplified for aradio navigation system, AEÜ, Inter, J. of Electronicsand Communications, Vol. 54 (2000) No.3, pp. 183-189/4/ GREVING G. Hybrid-Methods in Antennas and3D-Scattering for Navaids and Radar System Simu-lations; Antenna and Propagation Conference AP2000,April 2000, Davos Switzerland/5/ GREVING G. Recent Advances and New Resultsof Numerical Simulations for Navaids and LandingSystems, 11th IFIS International Flight InspectionConference, Santiago Chile, June 2000/6/ G. GREVING, N. SPOHNHEIMER Problems andSolutions for ILS Category III Airborne and GroundMeasurements - European and US Views and Per-spectives, 11th IFIS International Flight InspectionSymposium, Chile 2000, Proc. pp. 51-62/7/ GREVING G. Application of Modern NumericalMethods for Navaids and Landing Systems - Theoryand Results, ISPA 2000, International Symposium onPrecision Approach and Landing, DGON, Munich, Proc.pp.49-61/8/ GREVING G. Latest Advances and Results ofComplex Numerical Simulations for Navaids andLanding Systems, 12th IFIS International Flight In-spection Symposium, Rome/Italy 2002, Proc. pp. 152-162/9/ G. GREVING, N. SPOHNHEIMER Problems andSolutions for Navaids Airborne and Ground Measure-ments – Focus on receiver Sampling and TCH; 12th

IFIS International Flight Inspection Symposium,Rome/Italy 2002, Proc. pp. 90-99/10/ G. GREVING, H. WIPF Flight Inspection Aircraftin Multipath Environment; 12th IFIS International FlightInspection Symposium, Rome/Italy 2002 Proc. pp. 198-207/11/ GREVING G. Status and experiences of advan-ced threedimensional system simulations for navaidsand radar, Aviation World Conference, Kiew/Ukraine,September 2003, pp. 5.1-5.5/12/ GREVING G. Advanced Numerical SystemSimulations for Navaids and Surveillance Radar - TheVerification Problem, 13th IFIS, June 2004, Mont-real/Canada, pp.173-186


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Fig. 1: Sketch of an airport, humped runway, the subsystems of an ILS and MLS, some dis-torting objects and a landing aircraft

Fig. 2: The large A380 aircraft and windgenerators pose a potential threat to all introducedclassic and modern navigation, landing and radar systems

CL

hybrid_11.dsf

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buildings, hangars, cranes,aircraft, tanks, towers, fences, high voltage lines, ...

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humped runway

not scaled

layout, system position, safe-guarding areas, holding points,grading, earth movement, ...

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A380


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Fig. 3: General process flow and aspects for the modeling and the numerical treatment of theA380 and the windgenerator

Fig. 4: Process flow of the IHSS (Integrated Hybrid System Simulation)

System results, System parameter

GO/GTD/UTD

System Post-processingFiltering, sampling, RX-antenna, ...

Multilayer, Greensnow,rain,glas, ...

PEhumped runways, ...

PO, IPO, EPOmedium objects, aircraft, windmills, ...

Hybrid combinations

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Details

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Annex 10; DDM, bearing error, mod%, PFE,CMN ,range error, monopulse error, false target, ...

data basesA/C,cranes,materials,...

design,installationproposals, actions

The real life problem, System + Environment - in advanceairport, enroute; ILS-LOC/GP, VOR/DVOR, MLS, DME/TACAN, ASR,SSR, weather radar, comm ...

Superpositions

System pre-processingLanding Systems,Navaids, RadarModeling

Numerical MethodsAntennas,scattering objectsground effects,wave propagation

System post-processingSystem parameter

convergence?

"State-of-the-art" system analysis

reality model

"tool", math engine

best adapted for the model/realityleast approximationsmost advancedbest approved/verified highest accuracy promisingno compromise for speedlatest scientific results

3Dnumerical method

MoM, ML-FMM, GTD/UTD, PE, PO/IPO,...

IHSSIntegrated Hybrid System Simulation

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Height=18.2m

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Length=84m

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Comparison of SizesAN225 / A380 / B747

Numerical 3D-models

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Fig. 5: Numerical 3D-models of 3 large aircraft AN225, A380-800, B747-400; size comparison

Fig. 6: The A380 aircraft on airports (runways, taxiways, landing, starting) with regard to ILS(Localizer LOC, glidepath GP)

GP

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Fig. 7: Numerical 3D-model of the aircraft A380-800 consisting of 37054 triangular metallicpatches at 110MHz (ILS Localizer). A reduced number of triangles is displayed.

Fig. 8: DDM-distortions on the runway for CATIII applications of an A380 on parallel taxiway;medium aperture dual frequency Localizer; Filtered data; R&S HE108 RX antenna

Basic Dimensions:length = 78.9m span = 79.8m height = 24.1m37054 triangular patches (110MHz)

Analysis by the IPO-method improved and extended PO : (modified) basic PO-currents + rim currents + Fock currents + shadowing effects

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Fig. 9: Time dependant dynamic DDM-distortions by an A380 when rolling on a fast-roll-offtaxiway; The landing aircraft is on the glidepath and moving also.

Fig. 10: Windgenerators of the type Vestas V80 (left) and Enercon E66; 3D-models for the nu-merical evaluation consisting of a very large number of triangular patches; some details of theDoppler-shift problem (right)

wind3dpatch.dsf 03/03

6119 Patches / 110MHz ILS/VOR499291 Patches / 1030MHz SSRca. 4Mio Patches / 3GHz PSR

shaftconcretemetal tube or lattice(shaped)

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Fig. 11: Distorted field of a DVOR on a horizontal plane caused by a nearby windgenerator

Fig. 13: Simulated Dopplerspectrum for VOR/DVOR-frequency of the rotating blades in diffe-rent angular positions

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Fig. 12: VOR/DVOR bearing errors caused by a windgenerator type Enercon E70; identicalgeometrical configuration. Some used radials are marked. Note the different color coding of thebearing errors

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Fehler / Grad

DVOR Antenne:Reflektordurchmesser: 30mReflektorhöhe: 4.4m über BodenAntenne: 1.2m über Reflektor

Freq.: 111.2MHzDVOR bei (X/Y): 0 / 0WKA bei (X/Y): 228m / 1351mWKA (E70) Nabenhöhe: 115mAzimutaler Drehwinkel der WKA: 309°(Bild rechts oben)

DVOR

WKA

0.2°

116.9°

159.1°

Y / m

X/m

13401360

210

220

230

240

250

Enercon E70Draufsicht (Drehwinkel: 309°)

Y

Z

200 250 3000

20

40

60

80

100

120

140

Enercon E70

Y / km

X/k

m

-50-40-30-20-10010203040-50

-40

-30

-20

-10

0

10

20

30

40

5094210.40.20.10

11.8.04allersb_vor7d

301.7°

Fehler / Grad

VOR Antenne:Loop 1 bei z=3.60m U=1Vej0°

Loop 2 bei z=5.11m U=0.71Ve-j15°

Freq.: 111.2MHzVOR bei (X/Y): 0 / 0WKA bei (X/Y): 228m / 1351mWKA (E70) Nabenhöhe: 115mAzimutaler Drehwinkel der WKA: 309°(Bild rechts oben)

WKA

0.2 °

116.9°

159.1 °

Y / m

X/m

13401360

210

220

230

240

250

Enercon E70Draufsicht (Drehwinkel: 309°)

Y

Z

200 250 30 00

20

40

60

80

100

120

140

Enercon E70

VOR station - bearing errors by E70 ispa04vordvor.dsf 10/04 horizontal plane 100km*100km, 3300ftMSL

DVOR station - bearing errors by E70 horizontal plane 100km*100km, 3300ftMSL

VOR

WG azimuth angle 309°

max bearing error 10°

WG azimuth angle 309°

max bearing error 0.72°

modern threats to precision approach and landing - the ...modern threats to precision approach and...

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