A Doppler VHF Omnidirectional Radio range (DVOR) radiates an omnidirectional signal that is used to generate a ground clutter map of reflective objects in this Passive Bistatic Radar (PBR) application. The derivation of the clutter map and its RCS values are discussed in this paper. The data are finally used as preload parameters to forecast the degradation of the VOR angular bearing when used for aircraft navigation. A small aircraft carrying a VHF sensor performs an orbit flight around the transmitter. Two signal properties are analyzed: Doppler shifts from the direct to the scattered signal deliver the difference of incident angles for all scatterers. Secondly, the DVOR inherently transmits a signal that has a directional-sensitive frequency modulation component which means that the PBR transmitter is partially cooperative. A Matched Filter is used to derive that directional information from the same scatterers, which then allows their localization by means of a cross bearing. Since the system is strictly linear, specific RCS values can be assigned to reflective objects on ground.

The radar echo of a large wind turbine (WT) is investigated in the C band: An unmanned aerial system (UAS) is used to perform a reflectivity measurement across the flight altitude using a passive bistatic radar (PBR) constellation of a non-cooperative precipitation radar transmitter in horizontal polarization. This is done at various distances to check if far field conditions apply to derive a radar cross section (RCS). As a fully independent method, a monostatic FMCW radar is installed on ground at certain distances to the WT. The results of both methods are compared against each other regarding the applicability of the RCS.

The impact of scattering objects such as wind turbines (WTs) and other buildings on the VOR has been addressed in various publications. Nevertheless, a practically feasible possibility to predict especially the Doppler VOR bearing error during the building’s planning phase is challenging, since a electromagnetic model of that facility must replicate its radiation with both CSB and SBO antennas in a complex scenario that is large w.r.t. the wavelength. A new time domain method will be introduced to address this topic for dynamically disturbed environments. Separating its frequency modulated (FM) and amplitude modulated (AM) content, the indicated bearing is calculated subsequently by determining the corresponding phase difference. Within the research project WERAN, an UAS-based (here: octocopter) platform with dedicated antennas, receivers and adapted signal processing was developed. Using this device, it is now possible to measure separately small phase errors on the 30Hz FM and AM signal components. According to the prediction, bearing errors due to existing wind farms are now traceable at specific positions in the airspace. Hence, the numerical results can be validated by in situ measurements of real DVOR signals-in-space in the environment of real WTs. The presentation will introduce the numerical method and the measurement platform and give an overview on the validation cases.

In this paper, we describe measurement results of the signal-in-space of very high frequency (VHF) omnidirectional range (VOR) facilities. In aviation VOR are used to display the current course of the aircraft in the cockpit. To understand the influence of wind turbines (WT) on the signal integrity of terrestrial navigation and radar signals, the signal content and its changes, respectively, must be investigated. So far, only numerical simulations have been carried out on the frequency-modulation (FM) part of the Doppler-VOR (DVOR) signal to estimate the influence of WT on DVOR. Up to now, the amplitude-modulated (AM) part of the DVOR was not assessed at all. In 2016, we presented an unmanned aerial system (UAS) as a carrier for state-ofthe-art radio-frequency (RF) measurement instrumentation (Schrader et al., 2016a, c; Bredemeyer et al., 2016), to measure and to record the true signal-in-space (both FM and AM signal) during the flight. The signal-in-space (which refers to time-resolved signal content and field strength, respectively) is measured and sampled without loss of information and, furthermore, synchronously stored with time stamp and with precise position in space, where the measurements were taken.

The disturbing effects of scattering objects such as wind turbines (WTs) and other buildings on the flight navigation aid Doppler very high frequency omnidirectional radio range (DVOR) have been addressed in previous publications. Nevertheless, a practically feasible possibility to precisely and consistently predict the DVOR bearing deviation before the buildings’ construction is still missing. In this contribution, a method based on numerical simulations is presented which allows calculating the DVOR receiver's antenna voltage in the time domain for dynamically disturbed environments. Separating its frequency modulated and amplitude modulated content, the indicated DVOR bearing is calculated subsequently by determining the corresponding phase difference. Finally, the calculation results are validated by in situ measurements of DVOR signal disturbances in the environment of eight WTs.

The Netherlands Organisation for Applied Scientific Research TNO is regularly tasked to perform windfarm-radar impact studies. The requests most often come from windfarm operators in Europe, but studies for an operator based in Australia have also been performed. Referring to terminology introduced by Eurocontrol’s wind-turbine task force (WTTF), both simple engineering assessments (SEAs) and detailed engineering assessments (DEAs) are conducted. Certain civilian or military ANSPs (air navigation service providers) do, however, not follow Eurocontrol’s guideline, in which case a DEA is conducted, regardless of the distance between the radar system and the windfarm. The analyses may involve both primary and secondary radar systems. The main ingredients of TNO assessments are briefly sketched. Subsequently, the judgement process, i.e., the criterion upon which the verdict on windfarm construction is based, is discussed. It is observed that ANSP’s do not always utilize a well-defined criterion, which gives rise to subjectivity.

A passive bistatic radar (PBR) constellation of a radar transmitter, wind turbines as targets and an unmanned aerial system (UAS) carrying the receiver is used for scattering measurements and discrimination between single wind turbine (WT) echoes. Since the geometry is fully known, the radar echo gained from the WT is related to its reflectivity across the flight altitude of the UAS. These results are compared against computer simulations of the same scenario.

In the project WERAN it is also aimed to determine the interaction of wind turbines (WT) with radar signals. The evaluate the strength of single a single WT reflecting radar transmissions, measurement campaigns with multicopters (UAS) have been carried out at various radar sites in the L (Air Defense), S (AD and ATC) and C band (Precipitation radar). The presentation focusses on radar echoes received at different UAS flight altitudes and distances to the scatterers. It is useful to analyse the radar return both in the time and frequency domain to detect static and time variant signal components. Some results will be shown and discussed. In the follow-up project WERAN plus there are some additional airborne vehicles in use to carry the measurement equipment. Some ideas of what can be expected from measurements at higher altitudes and moving at horizontal speed will be shared.

Flight inspection of radio-based terrestrial navigation systems is crucial for sufficient accuracy. Especially in proximity of wind turbines (WT), which have arisen in recent years due to government subsidies for renewable energies, flight inspections must be carried out. This causes problems that hinder the usage of conventional aircraft: Even rotorcraft (e.g. helicopters) require a certain amount of clearance to WTs and cannot measure within wind farms. To solve this problem, unmanned aerial systems (UAS), such as multicopters, are a promising approach. Beside the solution of the clearance problem, they also provide further benefits in terms of cost effectiveness and flexibility; but on the downside they have limited payload. Since commercial off-the-shelf (COTS) test receivers exceed the UAS's loading capabilities, we developed a weight- and size-optimized test receiver that can compete with full-compliance measurement COTS devices. This has been achieved by focusing on the demands regarding carrier frequency and bandwidth of the system under test (SUT). In this paper we describe the RF front end for the reception of the forward scatter emitted by the airport surveillance radar (ASR) and show the first measurement results.

We describe the development of a system for measurements of electromagnetic field strength distributions and on-site antenna calibrations based on an unmanned aerial system (UAS). The commercially available octocopter was improved by a state-of-the-art GNSS navigation system and a shielding against electromagnetic harsh environments. We have designed, built and tested the FPGA-based data logging hardware as well as several RF frontends and antennas to be mounted on the UAS. We show first applications of our UAS measuring the interaction of wind turbines and terrestrial navigation systems such as DVOR and radar systems used for air traffic surveillance.

Field strength or signal-in-space (SIS) measurements have been performed by using manned helicopters, aircrafts or from ground level using extendable masts. With the availability of unmanned aerial systems (UAS) such as multicopters a new versatile platform for SIS measurements is deployable. Larger types show up to eight individually driven electric motors and controllers (therefore called octocopter). They provide the ability to fly along predefined traces, to hover at waypoints and to initiate other actions when those have been reached. They provide self-levelling and stabilisation and moreover, they may gear at a point of interest regardless of their actual position, e.g. during their flight around a tower. Their payload mainly depends on the platform size and allows integration of complex measurement equipment. Upgrading their navigation capabilities including state-of-the-art global navigation satellite system (GNSS) and ground station transmitter (real-time kinematic – RTK) enables precise localisation of the UAS. For operation in electromagnetic harsh environments a shielding can be considered and integrated into the concept. This paper describes concept and design of an octocopter and its instrumentation, along with applications in recent projects, in which we measure and validate terrestrial navigation systems applied in air traffic and the weather forecast services. Among those are instrumentation landing systems (ILS), VHF omnidirectional radio ranges (VOR), airport traffic and weather radars as well as military surveillance radars, and UHF wind profilers. Especially to investigate the possible interaction of VORs and radars with single wind turbines (WT) or wind power plants has become a major request of economy, military and politics. Here, UAS can be deployed to deliver measurement data investigating this interaction. Once developed and setup to a certain extent, UAS are easy and cost-efficient to operate. Nonetheless, due to their compact size, UAS will have rather low interaction with the electromagnetic field to be measured compared to the operation of manned helicopters.

An unmanned aerial system (UAS)-based measurement process to supplement conventional flight inspection of terrestrial navigation aids is described. In contrast to typical flight inspection with an aircraft, the platform allows quasistationary hovering in critical areas with extended observation times, without using expensive manned helicopters to carry measurement equipment and antennas. A microcopter carries the payload which consists of a short linear antenna and a highly miniaturized, FPGA-based large bandwidth receiving/recording system. In contrast to conventional methods, the raw band pass signal-in-space covering the complete channel bandwidth is sampled at a high data rate, and is directly recorded without any preprocessing whatsoever. This preserves maximum opportunities for any signal post-processing to extract all essential parameters of interest. Among typical flight guidance parameters such as DDM, the nature of scatterers can be shown in the time and frequency domains. All data is synchronized in time with the flight vector gained from an advanced on-board position system. The paper describes experiences gained with the system, and provides first measurement results obtained from ILS localizer and VOR facilities. INTRODUCTION Absolute field strength and signal-in-space (SIS) measurements have been performed by using manned helicopters, aircraft or ground vehicles with extendable masts where necessary. Also helium-filled balloons and blimps have been used in the past. Drawbacks of their operation are high costs, fast movement (no repetitive measurement samples can be taken at the same spot), limited maneuverability, long setup time, or, in case of a mast, limited air space to be covered. Now, with the availability of unmanned aerial systems (UAS) such as microcopters a versatile and comparatively cost-effective platform can be deployed for such purposes. Fields of application such as aerial photography, infrared spectroscopy and thermometry, surveillance, inspection and service, surveying, etc. are partly already firmly established with many small companies offering these services. However, these platforms also offer several features that drastically improve the effectiveness of SIS measurements. Here, the UAS is used for precision electromagnetic field and signal measurements of CNS facilities defined in ICAO Annex 10 [1]. This is a task which cannot be assessed sufficiently by conventional flight inspection (FI), and is beyond of the procedures defined in DOC8071 [2]. In the current WERAN project (German abbreviation for “Measuring the potential interaction of wind turbines with terrestrial navigation and radar systems” – [3] – supported by the Federal Ministry of Economic Affairs and Energy on the basis of a decision by the German Bundestag [grant: 0325644A]), the potential interaction between wind turbines and terrestrial navigation / radar systems is investigated. The frequencies of interest span across the rather wide frequency range of 200kHz to 5GHz. The measurements presented in this paper were performed employing the measurement platform developed in the WERAN project. DESIGN OF A MEASUREMENT SYSTEM Among the numerous design requirements for the microcopter (here: octocopter) designated “PTBee” (cp. Fig. 1, 2) a few stand out: smooth pre-planned and highly precise operation, also in adverse (e.g. windy) weather conditions Electromagnetic Compatibility (EMC) issues necessity to calibrate the receiving antenna factor various electrical requirements quick exchange of batteries Electromagnetic Compatibility (EMC) issues were of particular importance. On the one hand, motor speed controller switching and high motor lead and battery currents have the potential to cause internal EMC problems such as affecting the magnetic sensor controlling the yaw angle, but also to cause interference with the fields to be measured, especially at frequencies up to a few hundred MHz. On the other hand, it is intended to fly in electromagnetically saturated environments, e.g. at airports or in the vicinity of a radar. Therefore, shielding against high power RF external sources is a basic requirement to protect the UAS itself. This shielding will also reduce inherent emissions from the UAS. Since payload weight and dimension is limited on all flying platforms, the shielding has to be of light weight. To reach the defined safety limits, all electronic instrumentation need to be encapsulated by the shielding, including motor controllers, flight and navigation controllers, data sampling/storage unit and batteries. To prevent overheating of the now internal electronics, a mesh shielding was designed that allows for some air flow. Another advantage of the mesh shielding is that the internal barometric pressure sensor of the UAS contributing to the height information relative to the ground level is not affected. Fig. 1: Mechanical setup of the UAS with the

The steadily increasing air traffic in particular on transoceanic routes requires extending controlled airspace to those regions not yet covered by ground based surveillance. In this paper the authors present the world's first in-orbit demonstration of a space based ADS-B system, hosted on the ESA satellite PROBA-V.

We present a traceable calibration of a specially designed horizontally polarised reference antenna with an omnidirectional pattern in the E-plane for the frequency range between 105 MHz and 120 MHz. This antenna is used as a validation tool for absolute field strength measurements at the localizer transmitter of an instrument landing system (ILS) at airports and is carried by a helicopter. We investigate whether we can treat it as a dipole-like antenna in the calibration setup despite its disk-shape body. We also investigate the suitability of an anechoic chamber for antenna calibration though it was not designed for that purpose. The measurements are based on scattering parameters (S-parameters) which we apply in the 3-antenna-method (TAM or 3-AM) to obtain the antenna gain and the antenna factor, respectively. An uncertainty budget for the antenna gain calibration is derived. We also report on the first practical application of the calibrated reference antenna.

A helicopter-based measurement process to complement conventional flight inspection of terrestrial navigation aids is described. As opposed to the rapid penetration of areas of interest with a fixed-wing flight inspection aircraft the platform is suspended in a stationary hover in critical areas thus providing an increased observation time at a its quasi-stationary position. A reference antenna with an antenna factor (AF) traceable to national calibration standards and therefore to the International System of Units (SI) is used to measure the true field strength of the electromagnetic far field. The hovering helicopter carries the autonomous payload on its external load hook which consists of the reference antenna and the receiving/recording system. In contrast to conventional methods, the raw bandpass signal-inspace covering the complete channel bandwidth is sampled at a high data rate and is directly recorded without any preprocessing. This grants a

A new method is proposed to measure the ATC/ADR radar signal-in-space to validate the significance of results gained from electromagnetic wave solvers in context with ATC/ASR and ADR. The most effective method consisting of a CW signal to be transmitted from a hovering platform was developed step-by-step, also taking into account specific advantages and disadvantages. In the near future there will be a strong need for expertises in context with wind turbines potentially degrading radar performance. It is therefore indispensable to strengthen the confidence in the applicability of the chosen wave propagation tools.

ILS receivers used for conventional flight inspection purposes are normally not dedicated measurement equipment. Some parts of a receiver are sensitive to dynamic signal changes caused by multipath propagation effects, e.g. those components which apply timedependent parameters such as gain control or filters. Measurement results may suffer from degradation effects and tend to be unreproducible. Real conditions of a multipath-affected RF environment cannot be obtained and interpreted from the DDM curve alone but must be derived from the complex bandpass signal of a receiver being placed in that RF environment. Useful information about the scatterer such as exposure time and reflection coefficient can then be derived from both time and frequency domain analysis. Having this information, it is possible to synthesize an RF signal close to real conditions. Since conventional ILS generators can only produce static DDM without reflections they are unable to generate complex signals. Hence, an experimental generator was designed to generate an arbitrary ILS signal on the carrier frequency containing dynamic multipath effects. The real reflective scenario of an A380 aircraft at Frankfurt airport was taken as an example to reproduce a corresponding RF signal that was then fed into a state-ofthe-art ILS measurement receiver to evaluate and to optimize its performance under these conditions. INTRODUCTION In the past ILS receivers were developed mainly to provide instruments aboard an aircraft that are able to generate the necessary flight guidance information for the pilot during landing. These devices used to have an analogue or semi-analogue (digital signal processing only in the baseband) receiver concept. However, the analogue components like diode demodulators or crystal filters on the intermediate frequency (IF) often cause problems due to non-linearities and temperature drift. Additionally, parts of these receivers such as filtering and AGC are sensitive to fast dynamic signal changes, caused for example by multipath effects. Normally, flight inspection today is still performed with ILS receivers designed for flight guidance, although they are clearly not dedicated measurement equipment and lack the required accuracy of all necessary parameters (e.g. RF power) and suffer from the effects mentioned above. The main objective of this paper is to describe a suitable method for improving even dedicated ILS measurement receivers by using the scientific findings obtained in the RF reflective scenario. The new ILS/VOR Analyzer R&S®EVS300 from Rohde&Schwarz was selected as the device under test.