ARTES 5.1 Program ESTEC Contract No /12/NL/CLP

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1 ARTES 5.1 Program ESTEC Contract No /12/NL/CLP Aeronautical Satellite Communications Channel Characteristics Executive Summary Report Report Authors: M. Schwinzerl (JR), T. Jost (DLR), F. Pérez-Fontán (UVigo), A. Khan (INMARSAT) M. Schönhuber (JR), T. Pelzmann (JR), M. Richter (JR), G. Obertaxer (JR) W. Wang (DLR), M. Walter (DLR), B. Matuz (DLR), S. Dimitrov (DLR) S. Scalise (DLR), U. Fiebig (DLR) DIG AF Version 3 21/11/17 EUROPEAN SPACE AGENCY CONTRACT REPORT The work described in this report was done under ESA contract. Responsibility for the contents resides in the author or organisation that prepared it. The copyright in this document is vested in Joanneum Research. This document may only be reproduced in whole or in part, or stored in a retrieval system, or transmitted in any form, or by any means electronic, mechanical, photocopying or otherwise, either with the prior permission of Joanneum Research or in accordance with the terms of ESTEC Contract no

2 ESA STUDY CONTRACT REPORT SPECIMEN ESA Contract No: SUBJECT: Aeronautical Satellite Communications Channel Characteristics CONTRACTOR: Joanneum Research ESA CR( )No: No. of Volumes: 1 This is Volume No: 1 CONTRACTOR S REFERENCE: C _Exective_Summa ry_report_draft_v3 ABSTRACT: This document summarizes the rationale behind and the results of the activities, measurements and simulations performed during the AeroChannel project s work-packages. The work described in this report was done under ESA Contract. Responsibility for the contents resides in the author or organization that prepared it. Names of authors: M. Schwinzerl, T. Jost, F. Pérez-Fontán, A. Khan, M. Schönhuber, T. Pelzmann, M. Richter, G. Obertaxer, W. Wang, M. Walter, B. Matusz, S. Dimitrov, S. Scalise, U. Fiebig NAME OF ESA STUDY MANAGER: Nicolas Floury TEC-EEP, Wave Interaction and Propagation Section, Electromagnetics Division, Electrical Engineering Department ESA BUDGET HEADING: ARTES5.1

3 Version Date Author Versions / changes /07/15 M. Schwinzerl Initial Version /08/30 M. Schwinzerl First draft version /09/15 M. Schwinzerl, T.Jost /11/20 M. Schwinzerl T. Jost Revised and completed version Added Abstract, Section 3.1 Experimental database, expanded Section 4 Outlook & Conclusions Table of Contents ABSTRACT INTRODUCTION Motivation, Aim and Scope of the Activity Schedule of the Activity DESIGN AND EXECUTION OF EXPERIMENTS AND MEASUREMENTS Rationale for and Design of the Experiments Selection of Equipment Manufacturing, Acceptance Testing, Qualification Ground Based Experiments Airborne Experiments DATA ANALYSIS, SIMULATIONS AND EXEMPLARY RESULTS Experimental Database & Data Repository Ground-Based Surface Reflection Measurements GPS Signal Processing Ground-Based Antenna plus Platform Pattern Measurements Modelling Approach and Exemplary Simulation Results OUTLOOK AND CONCLUSION ACKNOWLEDGEMENTS BIBLIOGRAPHY i

4 Abstract Note: This abstract is reproduced from the Projects Final report for the sake of reference. Please refer to the document [1] itself for more details. The aim of the Aeronautical Satellite Communications Channel Characteristics (in short: AeroChannel) project was to improve the characterization of the aeronautical RF channel for receiving wide-band satellite transmission in (ideally: generic) airborne scenarios. These scenarios are intended to encompass different frequencies (i.e. L-Band and Ka-Band), different aircraft (small and large fixed wing aircrafts with extensions towards rotary wing aircrafts desired), realistic flight scenarios and realistic coverage of ground effects via multipath reception. The overall goal is the development of a software channel simulator capable of predicting the evolution of the channel characteristics for the full duration of a flight. Within this report, the sequence of activities and tasks performed during the run of the project are summarized and presented in chronological order. Beginning with a review of the baseline channel model and the identification of limitations of this model (summarized in the WP1000 Baseline Channel Model report), an updated channel model is proposed and implications resulting from the initially targeted parameters (like for example the carrier frequencies, frequency allocations in these bands, and interference and noise effects) are discussed. Then a set of measurements and experiments, both airborne and ground-based, is devised which provides the required input data to the proposed channel model (cf. the WP2000 Experimental Campaign Design Report). Additionally, a set of suitable aircraft is identified and a first iteration of identifying and formulating requirements on the equipment groups, measurements and experimental approach is performed. The critical design phase of the project (and the section of the report pertinent to it) is concluded by a detailed campaign planning effort (WP3100 Experimental Campaign Plan), a second and final iteration of the equipment selection process (WP3200 System Requirements and Design Report), the formulation of factory acceptance procedures (WP3200 Experimental System FAT Procedures) and a complementing set of acceptance procedures concerning the data evaluation software (WP3300 Experimental Data Analysis Software FAT Procedures & Report). Subsequent efforts to manufacture, test, and qualify the equipment for use in the experiments are documented and outlined with a special emphasis on the process of obtaining an airworthiness qualification and certification for the components used during the airborne experiments (cf. WP4000 Experimental System FAT Report). The report continues with an overview about the performed measurements and experiments during the course of the activities experimental execution work-package (i.e. WP5000 Experimental Campaign Report) and provides an accompanying overview about the structure and format of the data products gathered during the measurements (WP5000 Raw Data Report). Data evaluation and extraction and data analysis, including the approach taken for verifying the data with respect to completeness and correctness and the preparation of the data for further usage in the context of the channel model, are discussed in the section following section (i.e. WP6000 Channel Model Validation Results and Channel Model Report), together with an updated description of how the data ties into the channel modelling approach. At this stage, potential extensions and future refinements of the channel model are discussed and a lessons learned about the outcome of the experimental campaign is provided. Finally, the report is concluded by the user manual of the resulting software channel simulator (WP7000 Channel Model Software User Manual). 2

5 1. Introduction This executive summary report tries to summarize the motivation, the experimental and simulation approach and the execution of the activities within the ESA/ESTEC Contract No Aeronautical Satellite Communications Channel Characteristics (AeroChannel). A more detailed view onto aspects of this project can also be found in [2] [3] [4] and [5] Motivation, Aim and Scope of the Activity With the dispersion of modern smartphones, the demand on data rates has increased. Modern mobile communication standards like long term evolution (LTE), LTE Advanced or the upcoming fifth generation (5G) promise an increase in data rate to fulfill the rising requirements. Still, ground based systems are not able to reach every possible user like for example passengers in an aircraft. Examples for services targeting users in an airborne scenario range from internet supply to passengers, commercial data services for telemetry and aircraft management up to safety-of-life related functionality like data communications in aircraft traffic management (ATM) or aircraft positioning by global navigation satellite systems (GNSSs) in different phases of flight. Especially functionality concerning safety-of-life related wireless transmission links require high availability and continuity-of-service. Obviously, for an aircraft flying for example over the ocean, arctic ice-covered polar regions or other large non- or only very sparsely populated areas, data transmission via satellites is a more reliable and practical solution to meet this ends. Communications via satellites have been used for decades at different frequency bands like L- or C-band. Nevertheless, to reach data rates in the Gb/s range a larger bandwidth has to be used. Therefore, higher carrier frequencies like K-band between 18GHz and 27GHz become attractive [6] since sufficient bandwidth is available. Within the focus of scenarios of satellite-to-aircraft data transmission, three different effects can be observed resulting in fading and transmission loss [6] [7] [8]: Atmospheric effects in the ionosphere or troposphere, Local blocking, scattering and reflections from the structure of the aircraft itself Ground based originated scattering of the satellite signal. Figure 1: Schematic representation of the receiving situation in an airborne scenario The position and field of view of the receiving antenna(s) on the aircraft can further enhance or diminishe the relative importance of these effects.. Antennas with hemispheric patterns (which are commonly used in lower frequency bands such as L-band) are prone to picking up contributions from ground-reflections more easily than for example directive antennas used in higher bands such as Ka-band which are (in absence of pointing errors) usually directed towards the satellite. The smaller wavelength of signals in bands such Ka-band does, however, require describing description and characterization of the pertinent physical influences with a higher degree of precision and spatial resolution. 3

6 Other parameters influencing the satellite communication performance are the form, size, and material of the aircrafts fuselage itself (e.g. blocking caused by features of the aircraft such as helicopter rotors), the type of terrain at the ground-reflection zone below the aircrafts path, and the constellation between ground, aircraft and satellite (modified by e.g. the aircrafts attitude during maneuvering). Hence, the project s international team, consisting of researchers from JOANNEUM RESEARCH (Austria), the German Aerospace Centre DLR (Germany), the University of Vigo (Spain) and INMARSAT (United Kingdom), aimed at building a software simulator that is capable of describing the channel for (almost) generic combinations of satellite, aircraft, receiving antenna, and course over ground including: Specific geometrical aspects of the plane based on its 3D model, e.g. wings, wing-tips, rotors, empennage, etc. Specific patterns of the involved airborne antennae (including non-hemispheric patterns) Different frequency bands of interest Ground effects, especially over different types of terrain in combination with Various satellite elevations General flight movement patterns and maneuvering The aim to use realistic inputs for these quantities wherever possible (e.g. real-world aircraft data, nonidealized flight paths, realistic land cover and ground elevation data, patterns of realistic COTS receiving antenna) is intended to address the limitations and imperfections of the various other channel models from literature like [7] [8] [9]. Development of the software simulator has been accompanied by a series of experiments and measurements. The process of designing these measurements, building and qualifying the equipment, performing the experiments and bringing all together in a finalized software simulator is recapped and summarized in the following sections of this report Schedule of the Activity This summary executive report covers the achievements and results from all activities within the timespan of January 2013 to March 2017 according to the following schedule: WP Activity and Tasks From Until WP1000 Baseline channel model, Planning 2013/ /05 WP2000 Experimental planning, Selection of aircrafts, terrain, equipment, etc. 2013/ /08 WP3000 Critical System and Software design, Exploratory experiments 2013/ /02 WP4000 Equipment manufacturing, Qualification for airborne use, Acceptance Testing 2013/ /11 WP5000 Measurements under summer/autumn conditions 2014/ /12 Measurements under winter/spring conditions 2015/ /05 WP6000 Data evaluation, Channel model verification 2015/ /12 WP7000 Channel modelling, Software Simulator Development and Documentation 2016/ /03 Table 1: Overall schedule for the activity 2. Design and Execution of Experiments and Measurements 2.1. Rationale for and Design of the Experiments The general approach taken by the project team resembled a ladder-like structure: Results from experiments are analysed and verified using the results of earlier measurements. Given the requirement to have aircrafts, signals in different frequency bands, suitable receiving antennas for the signals and different types of terrain contributing specularly reflected components, the following experiments have been devised: A Measurement of the EM characteristics for each antenna in its frequency band 4

7 For each aircraft and for each frequency, a ground-based measurement of the resulting EM pattern for the antenna mounted on the aircraft For each frequency and each type of terrain, a measurement of the surface reflection properties of homogenous and idealized terrain For each aircraft, an in-situ airborne measurement of the received signals during full scenario flights over realistic terrain Characterization of the measurement environment with respect to interference and noise The interaction of these inputs in the resulting implementation of the channel simulator is depicted schematically in Figure 2 below: Figure 2: Graphical representation of the aeronautical channel model Selection of Carrier Frequencies, Bandwidths, and Signals; Geometric Constraints For this project, it had been decided to concentrate on two frequency domains: L-Band and Ka-band. In L-Band, GPS L1(CA) signals at a frequency of GHz with a bandwidth of ± 2 MHz can be used. GPS as a ubiquitous signal source with (in the context of the constraints of the Band) comparable high bandwidth can be used free of charge and a wide range of consumer off-the-shelf (COTS) hardware and equipment is available on the market. Additionally different satellite constellations (i.e. elevations above ground) can be studied simultaneously. For Ka-band, it was found to be out of reach performing airborne experiments due to a lack of (affordable) airborne-certified equipment (i.e. antenna plus pointing platform) usable on any of the four selected aircrafts (not to mention: all of them) and the luck of a matching, wide-band signal source. It was therefore decided to concentrate for the airborne measurements on L-Band but perform all preparatory ground based measurements in Ka-band as well. Regarding the study of the influence imposed by platform effects on the characteristics of the receiving antennas, a wider and less specific range of frequencies is required. This holds especially true in L-Band where the spatial resolution of the GPS signals intended for an airborne use is limited to approx. 300m, clearly too coarse to capture multi-path and local scattering effects originating at the aircrafts body itself. Moreover, the need to have a dynamic range of 25 db or more for the ground-based characterization also ruled out the use of GPS for these particular experiments. The measurements have therefore been conducted using the MEDAV RUSK Channel Sounder provided by DLR at a center frequency of 1.51 GHz with a bandwidth of ±60MHz. Note that the upper edge of these wide-band signals borders directly on the frequency band targeted by the airborne experiments, thus allowing a confident translation of results gathered during wide-band platform characteristics measurements to later activities. For Ka-band, a measurement at a center frequency of GHz had been selected for practical and frequency regulatory reasons. The signals in the experiments covering Ka-band are created by singlestep up- and subsequent single-stage down-mixing of the L-band signals provided by the Channel- Sounder, resulting in an effective measurement range of also ±60MHz, i.e to GHz. Using this number, the (theoretical) spatial resolution can be stated as approx. 2.5 m in both frequency domains, sufficient to resolve multi-path and similar local effects in careful designed and executed scenarios. 5

8 The spatial resolution given above gives rise to geometric constraints when designing the experiments. In the airborne experiments, a minimum altitude above the ground has to be kept in order to resolve groundreflected multi-path components. Given the minimum elevation of a visible satellite to be approx. 15, a minimum altitude H min of approx. 600 m has to be kept throughout the experiment. Consequently, at such an altitude, the horizontal displacement between the point right below the aircraft and the specular reflection point D max can extend to a distance of approx. 2.2 km (cf. Figure 3). Figure 3: Geometric constellation pertinent to sampling of ground-contributions during the airborne experiments for elevations below 45 (left) and above 45 (middle). The run-length difference Δpath between the reflected signal and the line-of-sight signal divided by the speed of light yields the delay of the specularly reflected component and determines the minimum required altitude above ground H as a function of the elevation of the satellite (right) For the ground-based ground reflection characterization experiments, a spatial resolution of approx. 2.5m results in the requirement to have RX and TX site elevated by m above the ground and displaced by approx m for grazing angles in the range of 4-5 (cf. Figure 4 below). Figure 4: Example geometric constellation for the ground-based surface reflection experiments at 5 grazing angle Selection of Target Aircrafts and Terrains In order to provide a higher degree of relevance of the gathered data and to ensure the ability of the resulting software simulator to handle different combinations of airborne platforms and antenna configurations, the experiments targeted four different aircraft types: one small and one large fixed wing aircraft (i.e. a Pilatus Porter PC-6 and Lockheed C130 Hercules, respectively) and one small and one large helicopter (Aérospatiale Alouette III and Sikorsky S-70 Black Hawk). All aircrafts used during the experiments have been provided and operated by the Austrian Armed Forces (AAF). The four aircrafts depicted in Figure 5 exhibit a considerable diversity with respect to their shape, size, geometric constellation, arrangement of engines and rotor configuration, etc., thus contributing to the stated goal of keeping the resulting simulator generic with respect to the used vehicles. Regarding terrain, a set of five land cover classes of interest had been selected: Water, Urban, Vegetation, Forest, and Mountains. Data from the Corine Land Cover (CLC) project had been used to identify regions of homogenous for the airborne measurements if taking the expected values for the horizontal displacement D into account. Seasonal changes in the effects of foliage, vegetation and snow/ice covers had been studied by performing measurements under both summer/autumn and winter/spring conditions. Based on the geometric constraints of the ground-based surface reflection experiment (i.e. distances of m, ability to deploy high cranes, plane surfaces, no obstacles), only Water, Vegetation (i.e. Gras), Asphalt (wet and dry) and Ice had been found to be feasible to study. 6

9 Figure 5: Impressions of the selected aircrafts for this project (starting from top left, then moving clockwise): Lockheed C130 Hercules, Sikorsky S-70 Black Hawk, Aérospatiale SA-316B Alouette III, Pilatus Porter PC Selection of Equipment Selection of Antenna Types and Aircraft Antenna Mounting Positions For L-band and GPS signals, COTS airborne dual polarized antenna had been used for mounting on the aircraft. For the wide-band ground-based measurements involving the channel sounder, a different antenna type had to be selected as the RF requirements concerning the cross-polar discrimination (XPD, 15 db) and the half-beam width (>120 ) could not simultaneously be satisfied with the requirements for the (airborne) GPS reception, including the routing of the signal cables. Figure 6: The wide-band L-band RX antenna for the ground-based experiments (left) and the airborne GPS RX antenna mounted on the same structure and position (right) Thanks to a custom design made by ANTCOM, both used antenna types are however mechanically compatible, thereby enabled mounting them using the same structures and therefore to carry the results from the platform measurements over to the evaluation of the flight experiments. For each of the selected aircraft types, a constellation of four antennas distributed along the fuselage had been found so that a) At least two sections along the aircrafts main axis (i.e. front/rear) could be sampled, b) Two sideway (i.e. canted under an angle of ) antennas had been used and 7

10 c) The same 1 antenna pos. could be used for both ground and airborne experiments Figure 7: Antenna positions on the Alouette III helicopter (left) and detail picture of the tilted antenna at position 2 (middle); detail about tilted antennas 2 and 3 on the C130 Hercules (right) Additionally, a downward looking antenna array consisting of 4 individual dual-polarized patch antennas, arranged in a 2x2 grid and provided by DLR, had been deployed for the Alouette III and the PC-6 aircrafts, thus enabling them to better sample ground contributions and effects. Figure 8: Downward facing antenna array mounting position at the PC-6 aircraft (left) and detail view of the underside of the PC-6 (middle); right: top-down view onto the antenna array. Note that the mounting structure depicted on the picture on the right has not been used during experiments Remark: Having two aircrafts equipped with the down-ward oriented antenna array requires and allows some refinement of the list of experiments previously outlined in section 2.1: Due to the difficulty of characterizing the pattern of the antenna array during the ground-based antenna + platform measurement activities, a dedicated measurement had been devised during which the aircraft passes several times in low altitude ( m) over a test sender emitting signals vertically For the antenna array, additional flights over selected terrain with lower velocity and flight patterns to ideally expose all four quadrants of the antenna array have been devised for the smaller two aircraft The full channel measurements however will be performed with all four aircrafts using typical and realistic flight maneuvers and cruising speeds These measurements have internally been called Experiments 3, 4 and 6, respectively and can be arranged in one single flight. For ground-based reflection experiments in L-band, a set of two helix antennas manufactured by Gigawave with a sufficient large bandwidth to emit and receive the wide-band signals at 1.51 GHz under a HPBW of (approx. 27 ). Cf. Figure 9 for reference. For Ka-band, the highly directed and actively pointed/steered antenna systems common in commercial aviation have been simulated using a dedicated, linear, dual-polarized and highly directed horn antenna provided by AInfo. This RX antenna had been mounted on a structure which allowed the free alignment and orientation of the RX antennae towards virtually any specific LOS orientation originating from the transmission site. 1 With small differences allowed because of slightly different wiring and cable routing constraints especially for the C!30 Hercules aircraft. 8

11 Figure 9: L-band helix antenna used during the ground-based experiments (left) and deployed during measurements at the river Danube (right) Figure 10: Ka-band dual-linear RX antenna plus the steerable mounting structure detail picture (left) and deployed on top of the S-70 helicopter with partially blocking rotor blade (middle) and on top of the PC-6 (right). Note the low-noise amplifiers attached directly to the RF terminals of the antenna. The same type of antenna (converted by the manufacturer using a polarization-shifting network to emit right-hand and left-hand circular polarized, i.e. RHCP and LHCP, signals) had also been used in the respective ground-reflection experiments in Ka-band. All used antennas have been selected to be passive so that the whole signal path can be measured and calibrated out of the gathered data. To this end, additional external low-noise amplifiers had been selected and applied as close to the mounting position of the antenna Ground-Based and Airborne Data Acquisition Equipment With four antennas mounted on top of each aircraft with two channels corresponding to RHCP and LHCP for each antenna, at least 8 channels had to be recorded simultaneously. For the two smaller types of aircraft, the Alouette 3 helicopter and the PC-6 plane, additional 8 channels originating from the downward facing antenna array have to be recorded during the airborne experiments, increasing the total number of channels used to 16 at maximum. In Ka-band, two channels (vertical and horizontal polarization) had to be captured simultaneously as well. For the ground-based experiments, data acquisition is handled by the MEDAV RUSK channel sounder provided by DLR which can receive and record 8 channels simultaneously. For Ka-band, a single-stage up-converter for transforming the L-band signals up into Ka-band and two synchronized single-stage down-converters and wide-band Ka-Band LNAs (not depicted) are added to the setup. Note that the input signals from L-Band have to be down-converted to the base-band of the data-grabber using so called GPS frontends, in this setup being implemented using 16 single-stage down-mixing blocks manufactured and integrated by Kuhne Electronics. Note that the frontends (or more specifically: the local oscillator feeding all down-mixing stages) and the data-grabber are synchronized using a common 10 MHz Rubidium frequency normal / atomic clock. 9

12 Figure 11: Block diagram for the wide-band ground-based experiments in L-Band (left) and Ka-Band (right) For the airborne experiments in L-Band at the GPS L1 frequency, a sixteen channel high-speed (50 MSamples/s per channel) data grabber based on a National Instruments NI 5751 A/D converter. Figure 12: Block diagram of the airborne data equipment setup for data grabbing GPS signals (left) based on the NI 5157 A/D Converter, the NI 7966R FPGA and the NI HDD8265 storage module depicted in a laboratory setup (right) In both setups, the signal levels of the input channels have to be brought to a range that can be sampled with sufficient dynamic range without overflowing the permissible input range to avoid damaging the data capturing equipment. For typical cable lengths in the airborne scenario (with a full-scale power level of +4 dbm in the base-band), this amounted to a required net gain of approx db for each channel, depending among others on the measured noise floor level. This has been achieved by carefully balancing amplifiers and attenuators in each channel and gave rise to an elaborate calibration and normalization procedure required for each experiment/flight. Position and Attitude Measurement Equipment Faithful representation of the geometric constellation between sender (either the TX antenna in the ground based experiments or the GPS satellites for airborne aspects of the experiments), the RX antennas and the ground/terrain layer (where involved) is crucial for making use of the measured information in the simulation. Figure 13: The aircraft s attitude can be described in terms of Euler angles roll, pitch and yaw, defined relative to the aircrafts main axis coordinate system (left). Position and attitude have been recorded using a Novatel IMU-IGM S1 tactical grade MEMS IMU (green), a Novatel ProPak 6 GNSS receiver (magenta) 10

13 For the airborne experiments, the aircrafts position and attitude (i.e. the Euler angles roll, pitch and yaw, cf. Figure 13) had been recorded with an Inertial Measurement Unit (IMU) and, in later experiments due to experienced problems with the reliability of the GPS reception, an additional GPS data logger. Due to constraints in the ability to mount additional antennae with good reception characteristics on the selected aircrafts fuselage, no dedicated GPS antennae had been used for supplying the receivers with GNSS information. Instead, the upward facing GPS antennae used for the data reception had been utilized using a splitter and additional amplifiers for providing a suitable signal and ensure proper separation of the measurement signal- and GNSS reception signal paths. The position of the IMU/GNNS receiver(s) relative to its respective GPS antennas (i.e. the lever arm), the orientation of the IMU s internal coordinate system relative to the aircrafts main axis and the positions of the upward and down-ward facing antennas relative to the aircrafts body had been measured using a tachymeter. The same procedure had also been repeated for the ground-based experiments, with a common set of reference points being used to translate the positions from both series of experiments into each other and to also reference the measured points of interest within a CAD drawing/3d model of the aircraft. Similarly, for the ground based surface reflection measurements, the geometry constellation depicted in Figure 4 had also been measured using reference points on the involved antenna and the elevated structures (e.g. cranes). Figure 14: Left: Tachymeter measurements for the upward facing antenna positions of the C130; Middle: Highlighting of the antennas corner points aiding the tachymeter measurements by increasing contrast (PC-6, downward facing antenna array); Right: One of the selected reference points on the S-70 s hull, easily identifiable also in a 3D model Auxiliary Measurement Equipment and Data Sources Additionally, two video cameras (one operator camera oriented sideways/in-flight direction to monitor the environment and immediate weather conditions, and another one oriented down-wards to document the terrain cover below the aircraft) had been during all airborne experiments. Moreover, meteorological data had been recorded by the central Austrian Weather Service (ZAMG) during all experiments. A standalone stationary GPS data logger had been used as a base station at starting point of the airborne experiments for providing reference information for calculating the position and attitude solution from IMU and GNSS receiver data. Figure 15: Impressions of the operator video camera mounted in the cockpit of the C130 (left) and the downward oriented video camera attached to the boarding aid on the S-70 (middle); Right: Interference & Noise measurement using the hemispheric wide-band antenna. 11

14 Interference and Noise have been recorded in L-band using a Rhode & Schwartz FSP3 Spectrum Analyzer and a hemispheric antenna Manufacturing, Acceptance Testing, Qualification The manufacturing of the equipment into blocks and structures ready for deployment in both the groundbased and airborne experimental settings took longer than expected during the initial planning of the activity. Several problems not initially foreseen occurred (e.g. problems with the IMU and GNSS reception, difficulties with the video signal recording, vibration induced power supply instabilities, etc.) up until the early flights in WP5000, requiring changes to the setup and ongoing efforts to improve on the experimental readiness. Because of different (and ultimately: incompatible) trade-offs imposed by the inner dimensions of the smaller of the two aircrafts, two different airborne rack structures had been built in the end, one for the Alouette III and one for the PC-6, the latter of which was used for the experiments involving the S-70 and C130 as well. Amplifiers Amplifiers I/O Box for Data Grabber Video Recorders (obsoleted) GPS Frontends Atomic Clock DC/DC Power Supply Data Grabber Storage Array (HDD) AC Power Inverter Figure 16: Overview about the two racks constructed for the airborne experiments. Left: Front view of the rack and equipment for the PC-6. Middle: Same view onto the rack constructed for the Alouette III. Right: rear view onto the PC-6 rack, exposing the power supply. The video recorders had been obsoleted by a change in the cameras used and were therefore not present in later flights (but retained as a backup option) One problem concerned the intermittent reception of interferences in the signals captured by the data grabber, which happened only while airborne and which, despite best efforts to apply filtering and shielding to the wiring of the power supply and the RF chain, persisted throughout the measurements Ground Based Experiments Ground Based Antenna + Platform Pattern Measurements Based on the characterization of the RX antennas in an anechoic chamber, wide-band measurements of the antenna patterns modified by the platform on which the antennas are mounted (i.e. the aircrafts body) have been performed similar to the sketch presented in Figure 17. For L-Band and each aircraft, a series of measurements sampling the pattern for all four dual-channel RX antennas simultaneously while varying elevation and azimuth over a series of values and while varying the rotor position of the helicopters and the propeller state for the PC-6 in discrete steps 2 Dedicated interference and noise measurements in Ka-band had been considered but were deemed not practical due to the requirement to use directed antennas. 12

15 Figure 18 presents the variation of these parameters (at a given elevation) using the S-70: Figure 17: Left: Side-view onto the setup for the ground-based antenna plus platform pattern measurement, showing two different elevations; Middle: Top view of the same setup, showing two different azimuth angles; Right: View from the TX positon along the line-of-sight onto the C130 during the actual experiment in L-band. Figure 18: Left: For each aircraft (here: the S-70) and for each of the azimuth positions a) e), a whole series of elevations had been measured, incl. a variation of the rotor position f) for each elevation/azimuth combination. Left: Turning the aircrafts on the spot had not been possible, thus the TX antenna had to be aimed at the same section of the aircraft for each azimuth/elevation combination (here: the center of the Alouette s main rotor) Similar series of azimuth/elevation and rotor/propeller position positions had also been recorded in Kaband with the main difference being, by virtue of the directed RX and TX antenna, that both the TX and the RX antenna had been aligned towards each other. Line-Of-Sight (LOS) TX Antenna RX Antenna Figure 19: Left: Series of azimuths and rotor positions used throughout the Ka-band measurements with RX antenna cone visualized as blue and TX antenna half-power cone pictured in red. Right: View from the perspective of the RX antenna towards the TX antenna mounted on top of the elevated platform Note that, due to the different behavior of directed antennas in Ka-band, in general viewer azimuth and elevation constellations have been measured if compared to L-band, as the measurements had a focus on verifying specific blocking/scattering scenarios. Ground Based Surface Reflection Measurements over Homogeneous Terrains The measurements concerning the reflectivity of water under small grazing angles had been performed at the river Danube near the city of Tulln, using two 40 m cranes facing each other on opposite banks of the river at the maximum distance possible. 13

16 RX Position TX Position Figure 20: Impressions from the ground-reflection measurement near river Danube. Left: Top-down view onto the constellation from Google Earth. Middle: Crane with the RX equipment, as seen from the ground near the TX position across the river. Right: Crane with TX equipment seen from the same vantage point The same cranes had been used in a subsequent series of measurements at the military airbase in Langenlebarn covering first vegetation (i.e. the grass covered areas around the runway) and then dry and wet asphalt/concrete (i.e. the tarmac runway itself). For the measurement of ice/snow, some compromises had to be made as several limiting factors (availability of snowy/icy conditions, availability and lead-times of cranes, ability to operate cranes under slippery/frozen or dewing ground conditions, access to the site even with no skaters or other activities on the ice, remoteness of location to obtain a frequency permit, etc.) were difficult to fulfill simultaneously. TX Pos Vegetation RX Pos Asphalt TX Pos Asphalt TX Pos Vegetation RX Pos Vegetation RX Pos Asphalt Figure 21: Ground-based surface reflection measurements of vegetation (grass) and asphalt. Left: top-down view onto the measurement for the two series of measurements according to Google Earth; sequentially, the measurement started with the vegetation measurement and each crane moved then to the position designated for the asphalt measurement. Middle: View onto the TX crane from the RX position for the vegetation measurement. Right: View onto the RX crane from the TX crane; note that the wetting of the runway by the fire brigade can be seen here. By increasing the elevation angle slightly to a value of approx. 6.2, the distance between RX and TX could be reduced to approx. 420 m so that two smaller cranes (24 m and 12m) could be used near Lake Putterersee in Aigen im Ennstal. 14

17 TX Position TX Position RX Position Figure 22: Constellation for the ice surface reflection measurements at Lake Putterersee. Left: The top-down view of the setup, exposing a distance between TX and RX of approx. 420 m. Right: View from the RX crane over the frozen surface of the lake towards the TX position. Due to the small hill on the opposite site of the Lake, a much smaller crane could be used; otherwise, the measurement would not have been possible under such conditions Terrain Location LOS Distance TX to RX Grazing Angle Water (River) Tulln / Danube 782 m 5.3 Vegetation (Grass) Langenlebarn (Airbase) 850 m 4.7 Concrete (Dry) Langenlebarn (Airbase, runway) 852 m 4.8 Concrete (Wet) Langenlebarn (Airbase, runway) 852 m 4.8 Ice (Frozen Lake) Aigen im Ennstal / Putterersee 423 m 6.2 Table 2: Summary about the ground-based ground reflection measurements performed in this project 2.5. Airborne Experiments The different measurements introduced earlier flights over a test emitter to better characterize the down-ward looking antenna array (Exp. 3), flights with adapted flight dynamic over selected terrains to better characterize the terrain from an airborne perspective (Exp. 4) and full channel experiments under realistic flight dynamics (Exp. 6) had been performed with all four aircraft. Aircraft Season Exp. 3. Exp. 4. Exp. 6 PC-6 Autumn Alouette III Autumn/Winter Alouette III Winter Alouette III Winter PC-6 Winter C-130 Winter/Spring PC-6 Spring S-70 Spring/ Table 3: Overview about the performed measurement flights Note that the entries in Table 3 refer to individual flights. Some experiments have been, for organizational and economic reasons, been performed sequentially during one flight. Regarding seasonal coverage, the 15

18 first flight with the Alouette III occurred in very early December but under meteorological and environmental conditions that are more representative of autumn than winter. The flight involving the C130 was scheduled in the mid of March and stretched over pre-alpine regions (still covered with sizeable patches of snow and ice) and lower regions with already beginning vegetation and foliage. The spring measurements involving the PC-6 and S-70 occurred in late April where full foliage and vegetation had already developed. The flight paths followed during these experiments share the following objectives: All flights had been conducted as closed-circuit flights, i.e. airport of depart and landing have been the same. The extent of the circuit was determined by the endurance of the aircraft and (in the case of the C130) the availability of the plane for measurements. The courses were designed to sample as large and homogenous patches of terrain as possible while keeping the minimum required altitude over ground (600 m) and flight regulatory constraints in mind. This is especially true concerning the flights over the urban regions of Linz and Vienna where the altitude was determined by the air traffic control operators in these regions It was ensured that regions of interest were sampled using two different aircraft. I.e. the section containing the Lakes and mountains from the C130 flight (Figure 23) was also covered by the Alouette III helicopter earlier during winter while the trajectories of the PC-6 and S-70 were mostly congruent, covering therefore the same areas. Problems with equipment and especially influence by interferences prevented the full evaluation of all performed flights. The two presented flights with the C130 and the PC-6 and the final flight with the Alouette III helicopter under wintry conditions however are fairly complete and provide a good base for further analysis and evaluation. Figure 23: Left: Trajectory of the PC-6 during the last flight in spring, covering among other features the city of Vienna and Lake Neusiedlersee. Right: Trajectory of the C130 during its flight from the city of Linz over Lakes Traunsee and Attersee, across the mountains and back to Linz. 3. Data Analysis, Simulations and Exemplary Results 3.1. Experimental Database & Data Repository All experimental and processed data products originating from this activity are collected in an exhaustive experimental database which is part of the deliverables of this activity. The database contains channel sounder data from the ground based experiments, data logger data from the airborne GPS acquisition 16

19 equipment, Tachymetric measurements, IMU and GPS receiver binary logs and calculated trajectories, meteorological data, CAD models for the involved aircrafts, photo documentation about the experiments, video recordings of the operator and down-ward oriented cameras on-board during the airborne experiments, interference and noise measurement data and the antenna patterns. The data repository is organized primarily by type, with overlays bundling the data products together per date, per experiment or per target (i.e. the aircraft and the primary terrains covered). Please consult the accompanying database manual [10] for further reference about the raw data, the data formats (including reference C-header files and structures for accessing the binary data format used by the channel sounder and the data grabber) and an overview about auxiliary software used for accessing the data. Figure 24: View on to the per-experiment overlay onto the raw experimental data for the PC-6 run on 2015/04/ Ground-Based Surface Reflection Measurements For the snapshots transmitted and recorded by the channel sounder, both the line-of-sight and the reflected component had been received simultaneously. To discriminate and estimate the amplitudes of both paths, a space-alternating generalized expectation maximization (SAGE) [11] algorithm had, in combination with linear regression for improved delay- and a least-square approach for improved amplitudes estimation, been employed. The captured amplitude time-series in the two frequency domains L- and K-Band show behavior that is in-line with the expected characteristics of the studied terrains: The water measurements exhibit smaller variations in L-Band and considerable variations in Ka-Band (Figure 25). These short-term varying fading effects are absent from the simultaneously captured LOS signal and also from the reflection signal of other terrains (including the measurements over the frozen lake, cf. Figure 26), hence these variations are most likely being caused by the motion of ripples and waves on the river. L-Band Comparably Small Variations at L-Band Ka-Band Larger Variations due to waves on the water surface Figure 25: Amplitude time-series data for water reflection measurements in L-band (left) and Ka-band (right) 17

20 L-Band Ka-Band Figure 26: Amplitude time-series ice reflection measurements in L-band (left) and Ka-band (right) A comparison of the calculated reflection coefficients Γ for both frequencies reveal that surfaces of comparable roughness (i.e. in terms of the Rayleigh criterion) exhibit similar values for the surface reflection coefficients (cf. Table 4). Terrain Reflection Coefficient Γ L-Band (RHCP) Ka-Band (RHCP) Water (River) Vegetation (Grass) Concrete (Dry) Concrete (Wet) Ice Table 4: Resulting reflection coefficients for both frequency domains and all studied terrains Both vegetation (Grass) and Water feature large differences between the frequencies explainable by the scale of variations in the height (e.g. the ripples and waves on the river and the sub-structure of the grass itself). The notable differences for ice could be explained by the reflections for the two different signals at different layers of the surface, i.e. Ka-band signals at the ice-surface and L-band at the water horizon below the ice. No conclusive proof of this hypothesis was developed in the project, however GPS Signal Processing Exemplarily, results originating from the last flight with the PC-6 near Langenlebarn and Vienna will be presented in this section. The duration of this flight was approx. 105 min and led over forests, farmland, sub-urban and urban areas and water bodies like Lake Neusiedlersee (cf. Figure 23 for details). The logical structure for evaluating the gathered raw data and extract amplitudes estimates is depicted in the left detail of Figure 27 for one given satellite at the time. Due to the large number of simultaneously visible satellites (cf. right detail of Figure 27) a very diverse selection of elevation angles and constellations within a single flight can be sampled. 18

21 Figure 27: Left: Processing and evaluation chain for obtaining estimates for the signal amplitudes from the up-ward and down-ward oriented antennas and the associated raw data and inputs. Right: Evolution of the elevation value for the (theoretically) visible satellites (i.e. elevation 0 ) over the course of the whole measurement with the PC-6 Combining time-discretized position and attitude information, satellite ephemerid data, digital elevation map information providing the terrains height profile and surface cover information provided by the 2012 CLC dataset (the latter two sampled on a common 25x25 m grid) allows the calculation of the specular reflection point for every point along the aircrafts trajectory and every visible GPS satellite. Moreover, periods when the reflection point remained steadily within a region with a homogeneous terrain could be identified for further study and analysis. Figure 28: Left: Land cover information provided by the CLC 2012 dataset with all surface types displayed. Middle: The same region displayed with homogenized terrain types (water, urban, rock, grass, and forest) and superimposed trajectory (black) and specular reflection points for one satellite over water. Right: Graphical representation of the digital elevation map (DEM) of the same region with grayscales representing different elevations above the WGS84 ellipsoid ranging from 160 m (black) to 590 m (white); Territory belonging to the Slovak Republic is not included here Moreover, the estimated delay of the signals and the estimated amplitude of the received signals of the upwards- and down-wards facing antennas allow comparison to theoretically calculated and predicted values (Figure 29) and the extraction of reflection coefficient which can be compared with expected values again depending on the terrain type (Figure 30). Figure 29: Left: Visualization of the signal delay estimates for the different kind of antennas. While the four upwards ( ) directed antennas show a very similar delay estimate denoted as direct signal, the estimated signal of the downward ( ) directed signal displayed as green line has an offset that fits well to the theoretical calculation displayed as dashed black curve. Right: Averaged estimated amplitudes for the upwards and the downwards directed antennas. 19

22 Projection of the Trajectory Specular Reflection on Lake-Surface Trajectories formed by the time-evolution of the specular refl. points calculated for each satellite Figure 30: Visualization of one segment of the PC-6 flight with the projection of the aircrafts trajectory and the calculated position of the satellites specular reflection points, evolving over time because of the movement of the plane and the satellite depicted. The colors of the reflection points encode the estimated reflection factor for both LHCP (left) and RHCP (right) signals. Note the highlighted surface reflection over the small water body in LHCP 3.4. Ground-Based Antenna plus Platform Pattern Measurements For evaluating the results from the ground-based antenna-pattern measurements during EM simulations, RX and TX sides had been reversed as seen in Figure 31 below. Observing the signal levels while taking the constellations (i.e. azimuth of the aircraft, elevation of the measurement TX antenna relative to the antennas on the aircraft, distance) into account, the resulting data is consistent with the effects expected due to the influence of the fuselage of the aircraft. Figure 31 below present these measured results as a series of lines with markers over the range of the covered azimuths and elevations for the co- and cross-polarizations, using one of the antennas of the PC-6 as an example. Figure 31: Schematic illustration of the measurement and simulation configurations in L-band (left) for one of the four antennas and Ka-band for both of the targeted antenna positions at the PC-6 (right) Comparing the results from measurement with EM simulations performed using FEKO in a hybrid method of moments (MoM) and physical optics (PO) approach based on a realistic model of the aircraft s fuselage (with simulated results being drawn in solid bold lines). Figure 32: Comparison of the measured signal levels in db for antenna position 1 at the PC-6 over the range of all measured azimuths (denoted by color and line type) and all elevations in both co- (left) and cross-polarization (middle). Right: Comparison between the simulated antenna pattern (blue) and the pattern measured in the anechoic chamber (green) over the zenith angles covered during the measurement. 20

23 Using the simulation, a modified antenna pattern (Figure 32, right) can be derived which subsequently has been used as input for the channel simulation. Similar evaluations and analysis has also been performed for Ka-band Modelling Approach and Exemplary Simulation Results In addition to the inputs and results discussed so far (i.e. the modified antenna pattern, the signals evaluated from the airborne GPS measurements, the estimation of the ground-reflection characteristics, land-cover information and elevation model of the terrain, and the satellite constellation), a model for the aircraft movement is needed. It is dominated by the deterministic flight trajectory and geometric constellation, superimposed with a random processes describing variations caused by vibrations (i.e. due to wind, engines etc.) and jitter in the attitude/flight geometry. The parameters of the stochastic model enhancing the aircraft movement are determined by an autoregressive process (AR), i.e. by a Gaussian distributed noise process filtered with an all-pole filter of order P. Data from flights with the VFW614 experimental aircraft owned by DLR and the flight with the Pilatus Porter PC-6 had been used for fitting the parameters. The simulator yields time-series for the LOS and the reflected component, an exemplary result for the PC-6 flight is presented below in Figure 33: Figure 33: Example results originating from the channel simulator, depicting the direct Line-Of-Sight component (left) and the reflected ground-component (right) for a portion of the PC-6 flight. 4. Outlook and Conclusion As described in this document, dedicated measurements were conducted to analyze each propagation effect of the satellite-to-aircraft channel. Using Figure 1 as reference, the impact of the aircraft on the receiving antenna pattern has been analysed, a corresponding random flight movement due to vibrations is derived, and the ground scattering component has been evaluated for low elevation angles and for higher elevations based on measurements. The channel model introduced in this activity took the different propagation effects into account by utilizing geometrical calculations as well as randomness. For the final satellite-to-aircraft model a two path model is proposed with a direct and a ground reflected path. While the direct path amplitude can be calculated taking free space propagation, the amplitude of the ground reflection uses a stochastic process for the Fresnel reflection coefficient that is characterized using measurement data. The influence of the aircraft on the receiving antenna pattern is taken into account by direct application of a pre-calculated modified antenna pattern that includes the antenna pattern of the receive antenna as well as its platform. Here electromagnetic tools like FEKO have to be used prior to the usage of the channel model. Using the pre-calculated receiving pattern, flight manoeuvres like curved approaches that may produce variations on both incoming signals (direct and ground reflection) due to fuselage blockage at large roll 21

24 angels may be simulated. An additional random flight component derived from flight attitude measurements is applied to simulate vibrations of the aircraft during flight. These variations induce slight variations of the incoming angle of arrival resulting in amplitude changes of the direct component due to the receiving antenna pattern. The impact of the random flight movement on the randomness of the ground reflection for a straight flight is negligible compared to the stochastic process of the Fresnel reflection coefficient. Therefore, the developed channel model is suitable for satellite-to-aircraft wave propagation simulations for broadband signals at L-band frequencies. It allows simulating arbitrary aircrafts and antenna placements if pre-calculated modified antenna patterns are available. Furthermore, arbitrary flight trajectories and dual polarization receive antennas are supported. The developed channel simulator comprises a solid and robust base for further studies both within the scope of the data gathered during the experimental activity as well as for future and not yet studied scenarios. Extensions and refinements to the simulator already anticipated include Aircraft / Antenna Placement, allowing the study of other fixed-wing aircrafts by use of presimulated antenna plus platform patterns as well as rotary-wing aircrafts. In the latter case, the time-variant behavior of the rotor would have to be considered with some measurement in this direction already being available courtesy of this activity. A first analysis on the periodic blockage of rotor blades on the direct path is shown in Figure 34 below, revealing the time-variation of the underlying platform: Figure 34: Prompt correlator output for PRN 3 of a GPS signal recorded with a receiving antenna directed towards the sky. As visible in the figure, the rotor blocks block the direct signal periodically creating a diffraction pattern with a period of ~57 ms corresponding to ~350 rpm. Different carrier frequencies require characterization of the ground-components in these frequencies. The results from this activity provide already indicative values suitable for at least a worst-case approximation for relevant other frequencies. Usage of directive antennas, including the treatment of pointing errors and the influence of flight dynamics on these errors. Simulation of signals with larger bandwidths would require the characterization of the RX antennas and could also mandate the treatment of ground originated multipath at different delays, i.e. a delay spread of the ground surface reflection. 22