Impact of Dynamic Scatterers upon Frequency- and Amplitude-Modulation (Tienda española)

A Theoretical and Practical Treatise in the Context of the Doppler-VOR and Wind Turbines

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Indice, PDF (160 KB)
Lectura de prueba, PDF (630 KB)

In 1959 Anderson et. al publish their paper „The CAA Doppler Omnirange”. In that contribution they present their analytically derived receiver‑model for quantifying the bearing error of the Doppler VOR (DVOR) due to multipath propagation. At that time this model exclusively serves for comparing the susceptibility of the DVOR with the one of its precursor which is the conventional VOR. For this purpose, they take the impact of a static omnidirectional scatterer solely upon one signal component into account, which is the frequency modulated one.

Due to the number of already installed wind turbines and especially due to the desire to install way more turbines, the signal integrity of the DVOR has become a very timely topic in Germany in the context of renewables energies.

In this dissertation Anderson’s basic generic model is both improved and substantially extended with respect to the impact of wind turbines upon the multipath signal.
In the first part of this work Anderson’s error model is quantitively expended with respect to the relative amplitude of the scattering path. Furthermore, the analytical model is fundamentally improved with respect to quality: For the first time the analytical model allows to take the dynamic effects of wind turbines into account, i.e. both Doppler shifts as well as an additional amplitude modulation due to the scattering object — namely the wind turbine. Additionally, this analysis is carried out for the DVOR’s reference provided by an amplitude modulated signal component, which has been completely neglected so far by the current state of the art.

These analytical models allow for extensive parameter studies, which are applicable e.g. for the validation of both numerical simulation tools as well as approaches by measurements.
In the second part of this work the dynamic impact of wind turbines upon the DVOR’s bearing intelligence is investigated by measurements. This is carried out in an environment scaled with a ratio of 1:144. It utilizes the equipment realized within the projects “Sk-ILS” and “min-Vor-Win” and expands it by inventing a procedure for crafting and electromagnetically characterizing voluminous scattering bodies. These allow for a systematic analysis of the impact of terrain topologies.

A variety of measurements and the corresponding fundamental analysis address: Doppler shifts and Doppler spectra depending on the orientation of the plane of rotation, the blades’ shape, revolutions per minute, and the position of the turbines as well as the amplitude and width of Doppler spectra.

Fundamental results of this work are e.g.: A 10 km safety‑radius of the DVOR’s protective area, up to now applied in Germany and as well recommended by the ICAO, can be considered way to restrictive. Furthermore, the receiver settings play a crucial role, when determining the bearing error. Thus, stating the latter makes it mandatory to state the receiver settings as well.

Rezension

To produce green energy efficiently, signal interferences by wind-turbines is a significant problem for aeronautical radio navigation systems. From the view point of safety in aircraft operations, interferences of electromagnetic waves should be paid attention to not only VOR but also other systems such as DME, radar, and communication systems. Therefore, this fundamental research has the potential to contribute to the entirety of the aeronautical industry – in terms of methods of operation, safety management, and radio systems deployment. The followings are my review comments.

In Chapter 1, background is described. The basic theory is indicated, and then the issues of wind-turbines are noted. VOR as well as Doppler VOR are the main focuses in this thesis. The author’s mention of the importance and strategy of today’s green energy produced using wind-turbines and the problems of electromagnetic waves caused by the blades is a nice point. Given that the safety of aircraft operation and the huge costs for wind-turbine deployment are strongly related with each other, the influences of signal interference on with (D)VOR should be investigated. This chapter notes the technical provisions, the present assessment for (D)VOR, and the environment in Germany, where the wind turbine and (D)VOR systems are located. The main point of the thesis is easily understood by the reader.

In Chapter 2, the principle of DVOR is explained. In Chapter 3, signal deterioration of FM-components by static scatterer is described. The formulation is well-known, but the way it is constructed helps readers to understand the principles, including scatter waves. The relationship between the receiver antenna (location and azimuth angles) and errors is shown. There is one mistake on line 7 of page 20 where Figure 10.2 is referred to as Figure 3.2. Considering the dynamic scatter, this chapter is fundamental and quite important for analyzing signal interference from wind-turbines.

Next, the author expands on the formulation for the static case to cover non-static situations in Chapter 4. A receiving antenna loaded on an aircraft obtains three main types of data: static scattered fields, doppler shifted scattered fields, and amplitude modulation from wind-turbines with moving blades. This expansion of the formulation is explained in detail through the procedures. The effects of doppler shift can be easily understood. In contrast to the FM-components discussed in Chapters 3 and 4, signal deterioration of AM-components caused by non-static scatterers is discussed in Chapter 5. Some relation can be confirmed from the results, which include receiver allocation (distance and angle), phase shift, and errors as parameters.

In Chapter 6 is an interesting element of the thesis. The method of assessment is an important factor for operators and developers. Assessment should be classified corresponding to the positional relation between the source(emitter), the receiver, and the scatterer. This means the areas are evaluated in precise, approximate and simple assessments. This thesis covers the approximation, but this case would be often used in real situations.

In Chapter 7, the measurement setup is described. A scaled model is employed. This measurement is conducted for the frequency of the ILS (Instrument Landing System). The operating frequency is 16 GHz, and a metallic sphere is selected as the scatterer. The results show that the analytical values and actual measurement values coincide well. The value of RCS (radar cross section) for the sphere is confirmed.
In chapter 8, the main measurement concept for (D)VOR is described. The schematic hardware setup used for the channel sounding is shown. The frequency of 15.9 GHz is selected – the actual (D)VOR frequency is approximately at 110 MHz. Other devises and apparatus are introduced here. The antennas used and their specifications are described in Chapter 9. A patch antenna and horn antennas are chosen in this thesis. In Chapter 10, the influences of terrain topology are evaluated. Scattering from terrain profiles is also an important factor to the total received fields. Therefore, the behavior of electromagnetic fields

affected terrain-roughness should be investigated for radio propagation. The author presents the differences from materials and the effects of scaled generic hillsides. I think the latter case especially is very interesting, and the thesis demonstrates diffracted fields from the hill. In some cases, wind-turbines are deployed on mountains to generate more electric power. Considering real signal environments, receivers on aircraft catch the direct path from the emitter including scattered waves from terrain and obstacles like wind-turbines. Consequently, the values shown in this chapter are a performance index of (D)VOR.

In Chapter 11, fabricated scale-model wind turbines are introduced referred to as K1 and K2. The scale is 1:144. The length of the blades is approximately 40 cm. In Chapter 12, the author discusses the effects of these scale-model wind-turbines. The main results are shown here. Based on these results, the author indicates the impact of wind-turbines on propagation channels. Measurements were performed at an open area test site. The angle between the emitter and the receiver was 45 degrees. The distance from the emitter to the scatterer was 21 m, which corresponds to 3 km in actual scale. The doppler spectrum could be detected in the experiment. In addition, the results is confirmed that the shape of blades (4 shapes were tested) affects the characteristics of the doppler spectrum. The different rotating angle of wind-turbines was also demonstrated. The effect of wind-turbines was shown to be maximized at half of the angle between the emitter and the receiver. The measurement scenario with three wind turbines located on the top of a hill model is interesting. In this scenario, the impact of the wind-turbines is relatively small in comparison to measurement without a hill model. Of course, the relationship between the height of the emitter and the scatterer affects the impact of wind-turbine. Finally, in Chapter 13, the measurement results for line of sight are shown. This is the case of wind-turbines located at the midpoint between the emitter and the receiver. It seems the shadow case of the wind-turbine or the transmitted case for the receiver. Through this measurement, we can understand the characteristics of radio waves at the back of blades. As we can imagine, the impact of the wind-turbine is maximum when the scatterer is at an angle of 0 or 180 degrees to the emitter. This measurement result is useful for the developers considering the wind-turbine placement.

In Chapter 14, the thesis is concluded. This research was carried out from both measurement and theoretical viewpoints. Topics ranging from basic mathematics to extended forms for the author’s purpose were addressed, and the measurement scenarios were well-considered to fit the real signal environments. Through the thesis, readers can clearly understand the social issues related to green energy for the aeronautical industry and the importance of assessment. The thesis is organized into sections such as introduction, AM-and FM- components in (D)VOR, general concept of the purpose, used components, results of doppler shift, and so on. I think that this result will help assist the ongoing introduction of green energy,

Junichi Honda, Tokyo (Japan)
Electric Navigation Research Institute (ENRI)
25th May 2022

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Im Hinblick auf die Beeinflussung von Funk- und Radarsignalen durch Objekte in ihrer Umgebung bestehen auch fast 150 Jahre nach der Konsolidierung der elektromagnetischen Feldtheorie durch James Maxwell noch viele Fragen. In den vergangenen Jahren haben sich derlei Fragen unter anderem auf dem Gebiet der Windenergieplanung ergeben. Von Interesse ist etwa, ob und in welchem Umfang Windenergieanlagen Signale von Funk- oder Radaranlagen negativ beeinflussen, beispielsweise ein Standortsignal derart verfälschen könnten, dass negative Auswirkungen auf die Flugsicherheit zu erwarten sind. Dass diese Fragen keineswegs nur akademischer Natur sind, zeigt eine Vielzahl an Genehmigungsanträgen für Windenergieprojekte, die mit der Begründung abgelehnt wurden, ein luftverkehrsrechtlich unzulässiges Störungspotenzial sei gegeben.

Intuitiv könnte man annehmen, dass ein Störungspotenzial selbstverständlich anzunehmen sei. Immerhin erreichen moderne Windenergieanlagen Nabenhöhen von über 150 m und Rotorlängen von über 60 m. Hinzu kommt ihre notwendige Eigenschaft, sich zu drehen. Das macht sie aus Sicht von Funk- oder Radarstellen auch zu dynamischen Reflektoren. Dadurch können Windenergieanlagen ein elektromagnetisches Signal theoretisch so beeinflussen, dass sich seine Lage im Frequenzband ändert, was die Funktion des Signals beeinträchtigen kann.

Klar ist, dass Genehmigungsanträge über Windenergieprojekte im Umfeld von Funk- und Radaranlagen nicht nach der Intuition der beteiligten öffentlichen Stellen entschieden werden dürfen. Vielmehr bedarf es wissenschaftlicher Untersuchungen, die zum einen Aufschluss über das theoretische Störpotenzial von Windenergieanlagen liefern, zum anderen ihre praktische Vorhersage im Rahmen von Planungsverfahren auf wissenschaftlich fundierte Art und Weise ermöglichen. Die Dissertation von Björn Neubauer baut auf dem Stand der wissenschaftlichen Erkenntnisse auf und trägt zu beiden Zielen bei.

Dr. Neubauer widmet sich im ersten Teil seiner Dissertation den theoretischen Grundlagen für die Berechnung von möglichen Einflüssen auf DVOR-Funknavigationsanlagen. Die Grundlage stellt ein in den 1950er Jahren von Anderson und Flint entwickeltes Modell dar. Das Modell beschreibt den Einfluss eines statischen Störobjektes auf die frequenzmodulierende (FM)-Signalkomponente und diente zum Vergleich des DVOR mit seinem technischen Vorläufer. Es wird heutzutage häufig in Simulationsstudien verwendet, um den aus Signalreflexionen resultierenden Winkelfehler abzuschätzen. Dabei bleibt die amplitudenmodulierte (AM-)Referenzphase eines DVOR-Signals auch in Simulationsstudien nach dem Stand der Technik unberücksichtigt.

Dr. Neubauers Beitrag liegt darin, dass er das Andern-Flint-Model zum einen um die dynamischen Einflüsse auf die FM-Signalkomponente erweitert hat und zum anderen die statischen als auch die dynamischen Einflüsse auf das AM-Referenzsignal analytisch beschreibt. Erkennt man an, dass für eine valide Abschätzung des Winkelfehlers immer beide Phasen heranzuziehen sind, stellt dies eine herausragende Leistung dar. Unter anderem wird es dadurch möglich, dynamischen Einflüsse, die durch die Rotordrehung entstehen, modellseitig zu berücksichtigten. Darüber hinaus zeigt er den Einfluss der gemäß den Vorgaben der Internationalen Zivilluftfahrt-Organisation (ICAO) anzuwendenden – bislang aber im Regelfall vernachlässigten – 95% Auftrittswahrscheinlichkeit auf die maximale Abweichung; und zwar für orbitale und radiale Flugtrajektorien.

Im zweiten Teil seiner Dissertation prüft Dr. Neubauer den dynamischen Einfluss der Rotordrehung in einem skalierten Windpark. Dabei handelt es sich um physische Modelle in Form verkleinerter Versionen einer DVOR-Funkstelle und mehrerer Windenergieanlagen. Dr. Neubauer untersucht die Abhängigkeit von Doppler-Verschiebungen des DVOR-Signals von verschiedenen Variablen wie der Position der Windenergieanlagen, der Ausrichtung und der Form der Rotoren und den Umdrehungen pro Minute. Seine Befunde lauten wie folgt: Erstens ist der Radius des in Deutschland verwendeten Anlagenschutzbereiches von 10 km zu restriktiv bemessen. Anlagenschutzbereiche stellen Worst-Case-Annahmen in dem Sinn dar, dass mögliche Störungen außerhalb der Bereiche ausgeschlossen werden. Eine Verringerung des Anlagenschutzbereiches ermöglicht es folglich, außerhalb liegende Windenergieprojekte ohne Einzelfallprüfung ihrer elektromagnetischen Eigenschaften zu genehmigen. Die restriktive Auslegung der Schutzbereiche rührte daher, dass die Auswirkungen der Rotordrehung auf das DVOR-Signal bislang unklar waren. Dr. Neubauer schließt diese Wissenslücke.

Festzuhalten ist, dass zu den dynamischen Einflüssen von Windenergieanlagen auf DVOR-Signale zentrale Fragen offen waren, zu deren Beantwortung Dr. Neubauer mit seiner Dissertation wesentliche Beiträge leistet. Diese betreffen insbesondere die analytische Abbildung dynamischer Einflüsse in einem mathematischen Modell und ihre empirische Überprüfung unter Laborbedingungen. Die Befunde der Dissertation sind in hohem Maße praxisrelevant. Erstens ermöglichen sie die Verbesserung der Qualität von Simulationsstudien im Hinblick auf das verwendete Modell des Empfängers. Zweitens zeigen die Befunde der Messungen, dass Anlagenschutzbereiche in Deutschland über viele Jahre hinweg zu restriktiv bemessen waren, und damit den Ausbau der Windenergie im Umfeld von DVOR in wissenschaftlich nicht begründeter Weise eingeschränkt haben.

Rezensiert von Dr. Neven Josipovic. Braunschweig, Juli 2022

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Mathematics is such a universal language that it can be applied to many diverse fields that seemingly have not much in common. For me seeing this book which is about wind turbines was a pleasant surprise. My field is digital communications and of course terms such as amplitude and frequency modulation and multipath feel as if they belong only to one field. Here is a book that successfully uses these mathematical concepts to tackle the impact of wind turbines on the VOR navigation signals. In addition to this theoretical approach, the impact of the wind turbines upon the bearing intelligence is quantified by measurements.
One of the main areas of concern is the effect of the wind turbines on VOR navigation signals and to this effect, the International Civil Aviation Organisation (ICAO) recommends Building Restricted Areas (BRA). The impact of wind turbines on signals is assumed to be mitigated by these requirements. This book examines a more nuanced analysis and how that may impact the modification and management of these restricted regions and in fact the management and assessment of the receiver-bearing errors.
VOR (VHF Omnidirectional Radio Range) is a ground-based radio navigation aid transmitting an omnidirectional signal that enables one to determine the user bearing in relation to the location of the beacon. DVOR (Doppler VOR) is the development of VOR, providing better accuracy. It provides bearing data up to a radius of 300 km.
The Neubauer book examines and extends the integrity analysis of the Doppler VOR’s frequency-modulated signal component due to a static omnidirectional scatterer as developed by the Anderson classical paper. The book moves the science forward by allowing for dynamic effects due to wind turbines by taking into account both the Doppler shifts and amplitude modulation that result from the scattering caused by the turbine blades. The work is validated by detailed simulation and makes recommendations on receiver error envelopes and settings. The extension covered herein takes into account amplitude modulation due to the scattering effect, which is done by the inclusion of a constant as well as a time-dependent Doppler shift.
Chapters two and three of the book go into explicit detail about the state of the art with illuminating graphs that show the impact of each parameter. An evaluation of the comparison with Anderson’s error factors is provided. Chapters 4 and 5 then cover the degradation of the FM component and the AM component by a stationary but non-static scatter. The discussion and the accompanying graphs are done in color so as to make the various components stand out.
In chapters 7 to 13 the scattering scenario is investigated by measurement. This starts in chapter 7 by providing a detailed and thorough introduction to the concept of scaled measurements. In part 8 the applied channel sounding and the utilized hardware setup are explained. Chapters 9 to 11 describe the design, realization, and characterization of the elements in the scattering scenario/measurement setup consisting of the antennae, wind turbines, as well as terrain topology. Parts 12 and 13 present the measurements consisting of triangular and line-of-sight alignments, respectively. The measurements show answers to several relevant questions, such as:
• Wind turbines have to be considerably closer than 3 km to the emitting Doppler¬VOR ground station in order to cause a relevant effect.
• In the line of sight scenario only wind turbines with generic blades — flat, rectangular, unproportionally large, and thus in turn absolutely unrealistic — cause an unacceptable bearing deviation with a conclusion that a minimum distance of 3 km preserves a decent safety margin.
• As forecasted by the analytical (treatment/equations), the amplitude-modulated reference part of the bearing intelligence, which has not received attention prior to this work, has to be taken into account. It is more susceptible to multipath scattering than the FM signal of the DVOR.
• In specific constellations, wind turbines may result in a Doppler shift. But due to their very low magnitude, they are not significant for the bearing intelligence.
Yes, the book treats a demanding topic and is heavy on math. But in contrast to typical publications on the subject, the author has made a real effort to make the topic accessible to the reader. The summary of this work is accompanied by a mind map, which further adds to the clarity.
This book is unique in its thorough treatise of this topic. It is a must-have for both the Wind turbine as well the VOR user and receiver designers. It will be a valuable addition to anyone’s library that is involved in this field.

Charan Langton,
- Consultant for communications systems, California, USA
- 20 years as engineer for Loral Space Systems In Palo Alto, CA