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Operating remotely piloted aircraft is not imaginable without a continuous data exchange between the air vehicle and the remote pilot. This data exchange requires reliable data links. One approach for such a data link discussed in the community is a terrestrial system deployed in C-band. A good knowledge of the physical conditions of the communication channel, in this case the air-ground/ground-air channel, is indispensable for the development of wireless data links. Therefore we carried out a 50MHz bandwidth channel sounding campaign with a terrestrial transmitter and an airborne receiver. In this paper we give a detailed description of our campaign setup and the processing of the collected data. The campaign covered several flight scenarios, such as take-off, taxiing, and multiple en-route maneuvers. We furthermore present results on the received power and the amplitude distribution of the first resolvable path of the received signal for the different flight scenarios. We observed significant drops in reception power during certain maneuvers that need to be considered in the design process of a data link for unmanned aviation. Additionally, we show that the amplitude distribution follows the distributions commonly used in statistical channel modeling of wireless channels to some extent. We finally present parameter sets for multiple flight scenarios for scaling the amplitude distributions to allow a statistical channel modeling of the reception power of the first resolvable signal path.

In civil aviation, air-traffic management and with that the underlying communication, navigation, and surveillance (CNS) infrastructure is currently being modernized. To test improved or newly developed terrestrial CNS systems without costly implementations and measurements, computer simulations using an AG channel model are crucial. In this dissertation we propose an AG channel model for the L-band which unlike previous proposals - fully captures the statistical non-stationary nature of the channel: we are now able to test the full range of terrestrial CNS systems. We propose a geometry-based stochastic channel model (GBSCM) architecture for the AG channel. In the GBSCM, geometrical model elements represent the different propagation effects, e.g., reflections off buildings. The model elements and their properties are environment specific and are characterized by statistical distributions: the channel model can be adapted to different environments by choosing the respective statistical distributions. We conducted two L-band flight trial campaigns in a regional airport environment to investigate the AG channel. Throughout this dissertation we use the collected data to parameterize the channel model architecture. To characterize the geometrical model elements for the parameterization, multipath components (MPCs) in the measured signal need to be detected and their parameters have to be estimated: we propose an algorithm combining super-resolution parameter estimation with a tracking filter. Based on the estimated MPC parameters we propose localizing the reflectors causing these MPCs using a Bayesian method. The presented algorithms may not only be used in AG channel sounding but in any scenario where an accurate MPC parameter tracking and reflector localization is required. Applying the algorithm to the measurement data demonstrates the non-stationarity of the AG channel. The results also allow us to parameterize the channel model to a regional airport environment. To assess the channel model quality the parameterization is successfully validated against measurement data. In consequence, our newly developed channel model is of high value for the computer-based performance analysis of CNS systems: the performance of new or improved CNS systems can now be realistically tested using computer simulations.

In recent years, there has been a dramatic increase in the use of unmanned aerial vehicles (UAVs), particularly for small UAVs, due to their affordable prices, wide availability, and relative ease of operability. Existing and future applications of UAVs include remote surveillance and monitoring, relief operations, package delivery, and communication backhaul infrastructure. Additionally, UAVs are envisioned as an important component of 5G wireless technology and beyond. The unique application scenarios for UAVs necessitate accurate air-to-ground (AG) propagation channel models for designing and evaluating UAV communication links for control/non-payload as well as payload data transmissions. These AG propagation models have not been investigated in detail, relative to terrestrial propagation models. In this paper, a comprehensive survey is provided on available AG channel measurement campaigns, large and small scale fading channel models, their limitations, and future research directions for UAV communication scenarios.

In this contribution, we present a method to analyze the effectiveness of multipath mitigation by extended averaging in the context of terrestrial radionavigation. We derive the requirements of extended averaging on the geometry between a ground station and an aircraft. Using that knowledge we are able to analyze the effectiveness of extended averaging for real world flight trajectories.

The paper investigates the variation of the received line-of-sight (LoS) power due to ground multipath propagation for the L-band air to ground radio channel. Within, both theoretical as well as results taken from flight trial measurements are presented. In theory, ground multipath propagation leads to periodic amplification and attenuation of the received LoS power. Measurement data presented within the paper confirms that in general, the variation can be well modeled by introducing an additional ground multipath component. Nevertheless, in reality strong signal variations do not appear as often as expected, i.e. the reflection off the ground is strongly attenuated. Two main reasons can be identified attenuation. First, ground multipath propagation can be blocked by nearby buildings, terrain features or vegetation. Second, a rough ground surfaces causes the incident radio waves to be reflected in all directions. This effect is called scattering and results in only a small portion of the power being received at the aircraft.

In recent years the German Aerospace Center (DLR) has been actively working on a proposal to deploy an L-band Digital Aeronautical Communication System type 1 (LDACS1) for future Alternative Positioning, Navigation, and Timing (APNT) services. In 2012 a flight measurement campaign has been performed to validate LDACS1-based navigational functionality. The results indicated a strong influence of multipath propagation on ranging performance. To better characterize multipath environment for future ground-based APNT services a second measurement has been performed in November 2013. This paper outlines the November 2013 measurement campaign and provides corresponding range estimation results that make use Bayesian filtering methods and Doppler smoothing to mitigate multipath and improve range estimation.

Over the course of the last years, civil aeronautics have been undergoing a perpetual innovation: the way we communicate, navigate, and survey the airspace is currently being redefined. In the past, pilots mainly relied on DME (distance measuring equipment) and VOR (VHF omnidirectional radio range) for navigation. Compared to state-of-the-art navigation aids these systems offer only a limited performance while being spectrally inefficient. Therefore, it is planned in the future to rely on global navigation satellite systems (GNSS) for navigation. GNSS offers a highly superior navigation performance as compared to that obtained with legacy DME/VOR infrastructure. To guarantee the required degree of integrity, GNSS systems will be accompanied with ground or satellite based augmentation systems (G/SBAS). However, an increased use of GNSS for aviation brings new challenges. Especially integrity, continuity, and availability of navigational information are of exceptional importance in a safety-of-life environment. Due to the low power levels received from in-orbit satellites, GNSS signals are susceptible interferences, both intentional and unintentional. Hence a ground based navigational backup system, referred to as alternative positioning, navigation and timing (APNT), needs to be employed. These systems should be used when GNSS services become temporarily unavailable. Different proposals for APNT are currently being discussed. One approach is based on the legacy system DME. The major advantage of DME is that it allows reusing the existing infrastructure; in fact, DMEs can be currently used as an APNT system. However, a large drawback of this solution is the continued use of old, spectrally-inefficient technology. But, what is more important, is that once the transition to GNSS-based air traffic management has been realized, a DME-based APNT solution will not be able to support the same level of navigational information in terms of accuracy or precision. Thus, alternative APNT approaches that provide accurate positioning, while at the same time allowing for a “gentle” phasing out of DMEs, are needed. One such example is the future communication system LDACS1, which signals can be also used for ranging and positioning. Hereby, communication infrastructure can also be employed for navigation purposes. Independently of the underlying technology, the majority of the APNT proposals are ground-based systems with planned allocation in the L-band. If, however, the future APNT systems are to deliver highly accurate positioning information, it is crucial to understand the characteristics of the ground-to-air (G2A) L-band wireless propagation channel in more detail. Yet this remains a challenging task as channel models for L-band G2A useable for testing and validation of range estimators are sparse. The existing models are extensions of classical statistical channel models developed for wireless communication applications. Thus, dependable simulations of the navigation performance of the proposed systems are hard to realize. To address this challenge, DLR has conducted in 2013 an extensive channel measurement campaign in the L-band. To this end, the setup consisted of a ground-based transmitter located on top of a building in an airport, and a receiver in a research aircraft Dassault Falcon 20E . For both the transmitter and receiver commercial L-band antennas were employed. The measurement bandwidth was set to 10MHz with a center frequency at the lower end of the L-band at 965 MHz; the transmit power was set to 39 dBm. Different flight patterns were tested in order to allow an extensive investigation of the channel characteristics. These include flights at different altitudes and transmitter-receiver separations, approach on the airport, take-off and landing. First preliminary analysis of the recorded data identifies an interesting characteristic of the L-band G2A channel relevant for navigation applications: for all altitudes multiple propagation paths can be observed. This effect is especially profound at low altitudes. The multipath is prone to occur in close vicinity to the transmitter, but excess delays sometimes reaching up to a few kilometers. The additional propagation paths are also strongly correlated in Doppler domain. As the result, for ranging and positioning applications such multipaths cause a strong interference to the direct path, significantly biasing the output of the correlator tuned to the line of sight (LOS) path. Furthermore, the spatial proximity of the different propagation paths complicates their separation and detection. This makes the compensation of the multipath effects a very challenging task. Especially for systems with low bandwidths, such as DME or LDACS1 that are considered for APNT services, multiple propagation paths represent a significant error source. Data collected in previous flight trials show that errors in the region of hundreds of meters are possible when ranging with a low bandwidth system in strong multipath environment. In the final paper we will discuss the above mentioned problems in more details. We will begin by outlining the goals of the channel measurement campaigns and explain the measurement setup used in our measurements. The latter includes a short description of both the hardware and software employed during the campaign as well as the applied algorithms. Then, we will concentrate on the analysis of the measurement data. Hereby a special focus is placed on the multipath characterization of the G2A channel using a novel superresolution multipath estimation algorithm. The paper is concluded with a discussion of the obtained results and outline of the future investigation directions.

The growth of the civil air traffic in recent years has made a more efficient management of the air space necessary. This requires both a higher degree of positioning performance and a higher communication capacity. Therefore, the communication, navigation, and surveillance (CNS) infrastructure for civil aviation is currently undergoing a major modernization process. In communications, the current analog voice radio system will have to be substituted by more efficient digital systems for two reasons. First, to keep pace with the steady growth of civil air traffic. Second, to enable modern air traffic management (ATM) concepts which require data rather than voice exchange. One of the most promising candidates for the future ATM data link is the L-band digital aeronautical communication system – type 1 (LDACS1) [1]. LDACS1 is largely based on 4th generation telecommunication technology and employs orthogonal frequency-division multiplex (OFDM) as modulation. Compared to the current analog systems, it offers a vastly increased capacity, scalability, and efficiency. As for communications also for navigation, a paradigm shift is taking place currently. In the past, pilots had to rely solely on DME (distance measuring equipment) and VOR (VHF omnidirectional radio range) for means of navigation. Compared to state-of-the-art navigation aids, these systems offer only a limited performance while being spectrally inefficient. Therefore, in the future aircraft will increasingly rely on GNSS (global navigation satellite systems) offering a highly superior navigation performance compared to DME/VOR. To guarantee the required degree of integrity, the GNSS systems will be accompanied with ground or satellite based augmentation systems (G/SBAS). However, an increased use of GNSS for aviation brings new challenges. Especially, integrity, continuity, and availability of navigational information are of exceptional importance in a safety-of-life environment. Due to the low power levels received from distant satellites, navigation is susceptible to intentional or unintentional interference. Hence a parallel backup navigational infrastructure referred to as alternative positioning, navigation and timing (APNT), needs to be employed. This system are used when GNSS services are temporary unavailable. Currently, different approaches to design a backup system for GNSS exist. Most proposals rely on an increased use of the DME technology. However, this exhibits two main drawbacks: First, the required infrastructure extension is costly. Second, precious spectrum is assign to an old and spectrally inefficient technology. This assignment would last for decades and avoid sustainable use of the L-band for modern CNS approaches which are imperatively needed for future aviation. To support sustainable spectrum usage, we propose to consider the future communication system LDACS1 as APNT solution. To verify the practical ability of LDACS1 to act as APNT system for GNSS backup, DLR has carried out a measurement campaign in November 2012. The goal of the campaign is to examine whether LDACS1 is capable to deliver ranging/positioning results with sufficient accuracy as required for APNT. For the measurements, four LDACS1 ground stations were set up in a quadrangular shape about 40 kilometers apart from each other and used as transmitters. A Dassault Falcon 20 aircraft with respective onboard equipment served as receiver. Several patterns were flown within the test area at several altitudes. This allows an evaluation of the systems performance for various real world geometries. According to the Cramer-Rao lower bound the nominal LDACS1 transmit power 43 dBm should deliver reliable and precise navigation for the aircraft. Nevertheless in any real measurement different error sources exist. Therefore, the main question of the campaign is, how much the performance is degraded due to those various error sources. Firstly, the requirement of synchronization accuracy in the range of nanoseconds in order to avoid concealment of measurement errors put hard constraints on the synchronization setup of the four ground stations. Thus, the station setup included oven-controlled Rubidium clocks at the stations as well as dual frequency GNSS timing receivers, the latter serving to monitor those clocks. The setup of each station was calibrated individually to mitigate the effect of hardware biases affecting synchronization performance. Application of a common-view time transfer technique independent of the GNSS receiver's position, velocity, and time (PVT) solution allows an analysis of the clock biases and drifts up to nanosecond level. Secondly, the typical challenges known from GNSS systems exist, e.g. multipath resolution or interference from other systems like the onboard DME. In particular, the LDACS1 bandwidth of 500 kHz makes the resolution of close multi-paths challenging. The analysis so far gives very promising results. With the four stations visible from the aircraft the LDACS1 system offers a performance considerably better than DME systems in almost all cases. However, under certain conditions, and without applying appropriate countermeasures, the performance may be degraded, e.g. in cases of strong interference or strong multipath situations. The final paper starts with a brief review of the LDACS1 system. This is followed by a description of the measurement campaign setup, including the specific challenges which had to be solved to conduct the campaign. Hereby, a detailed analysis of the different error sources and their influence on the performance of the measurement system is given. This includes errors due to clock synchronization and unknown hardware delays. In addition, the error sources which would impair the ranging performance also in an operational system are assessed, like Doppler effects, multipath propagation, and interference from onboard systems. Taking those error sources into consideration information on both positioning precision as well as integrity is presented. This includes a discussion of the employed algorithms and methods. The paper concludes with an outlook on future work to be conducted in the field of LDACS1 navigation.

The communication, navigation and surveillance (CNS) structure in the civil aviation sector is currently undergoing a major modernization process: A large number of old, analog systems have either already reached their capacity limit or are expected to do so in the near future. As for communication aspects, the current analog voice radio system will have to be substituted by a more efficient system to be able to keep pace with the current growth of the civil air traffic. One of the most promising candidates for the future air traffic management (ATM) data link is the L-band digital aeronautical communication system – type 1 (LDACS1) [1]. LDACS1 is largely based on 4th generation telecommunication technology and employs orthogonal frequency-division multiplex (OFDM) as modulation. Compared to the current analogue systems, it offers a vastly increased capacity, scalability, and efficiency. In the sector of aircraft navigation, currently also a change of paradigm is happening. In the past, pilots had to rely on DME (distance measuring equipment) and VOR (VHF omnidirectional radio range). Compared to state-of-the-art navigation aids, these systems offer only a poor performance while being spectrally inefficient. Therefore, in the future the aircraft will increasingly rely on GNSS (global navigation satellite systems) offering a highly superior navigation performance compared to the legacy systems. To guarantee the required degree of integrity, the GNSS systems will be accompanied with a ground or satellite based augmentation system (G/SBAS). However, an increased use of GNSS brings new challenges with regard to integrity, continuity and availability of the navigational information. Due to the low power levels received from a distant satellite, navigation is susceptible to intentional or unintentional interference. Hence a parallel backup navigational infrastructure, referred to as alternative positioning, navigation and timing (APNT), needs to be employed. This system can be used when GNSS services are temporary unavailable. Currently, different approaches to design a backup system for GNSS exist. Most proposals rely on an increased use of the DME technology. However, this exhibits different drawbacks: First of all, a costly extension of the infrastructure is required. Secondly, the spectrally inefficient DME system would be expanded and use additional spectrum resources needed for the deployment of the required new communication system in the L-band. Therefore, the approach of using the future communication system LDACS1 for navigation is evaluated in this paper. As shown in [2], positioning with an OFDM system is generally possible with high precision. Navigation using LDACS1 has been proposed in [3], where the theoretically possible bounds for precision of range measurements are assessed. To verify the practical ability of LDACS1 to act as an APNT system for GNSS backup, DLR has carried out a measurement campaign in November 2012. Four LDACS1 ground stations were set up in a rectangular shape about 40 kilometers apart from each other. For the measurement a Dassault Falcon 20 aircraft with an onboard receiver flew several patterns within the test area at various altitudes. According to the theoretical limits for transmission at a reduced power of 39 dBm, LDACS1 should deliver reliable and precise navigation for aircraft between the stations. Typical challenges for navigation with LDACS1 are similar to those of GNSS systems, e.g. multipath resolution or interference of other systems. In particular, the LDACS1 bandwidth of 500 kHz makes the resolution of close multi-paths troublesome. A major challenge during the campaign was the synchronization of the ground stations. A synchronization accuracy in the range of nanoseconds was desired to avoid concealment of measurement errors by synchronization errors. To make statements about the impact on positioning precision, both synchronization error and variance have to be assessed for every point in time during the campaign. For this purpose, oven-controlled Rubidium clocks at the stations were monitored in a calibrated setup by high precision dual frequency GNSS timing receivers. The calibrated setup is required to mitigate the effect of hardware biases affecting the synchronization performance. This approach allows an analysis of the clock biases and drifts up to nanosecond level by the application of a common-view time transfer technique. It is independent from the GNSS receivers’ position, velocity, and time (PVT) solutions, which merely served as backup [4]. A first analysis of the data indicates that under good conditions the LDACS1 system offers a performance considerably better than DME systems. However, the performance may be degraded in cases of strong interference by other systems or strong multipath situations. The final paper starts with a brief description of the LDACS1 standard. It is followed by a detailed description of the measurement campaign setup, including the specific challenges which had to be solved to conduct the campaign. This is followed by an assessment of the performance of the ground station synchronization, and the presentation of first results for the pseudo range generation and position determination. Therefore an elaborate description and discussion of the employed algorithms is necessary. The paper concludes with an outlook on the future work to be conducted in the field of LDACS1 navigation.

The future L-band Digital Aeronautical Communication System LDACS1 shows a very promising performance as a communication service. The system offers the possibility to also provide a navigation service with minimal adaptation. This paper investigates the level of performance achieved by an LDACS1 based navigation service. The results are outstanding and show that the stringent requirements of RNP0.1 can be reached in a large service volume. Two key parameters have a large impact on the performance of the navigation solution: the density of stations used as ranging sources and the time synchronization accuracy of these stations. The sensitivity of the performance to the second parameter has been investigated in this paper. It is shown that the time synchronization has significant impact on the achievable positioning performance only if the error is larger than 100 ns (one standard deviation). This is a very large synchronization error that can be easily reduced to 20 ns using existing low cost atomic clocks. The simulation results are very promising and open the doors to further investigations including flight experiments (planed in 2012), in particular the investigation of high performance and low cost system architectures that keep or improve the level of safety.