Communication, Navigation and Surveillance (CNS) technologies are key enablers for future safe operation of drones in urban environments. However, the design of navigation technologies for these new applications is more challenging compared to e.g., civil aviation. On the one hand, the use cases and operations in urban environments are expected to have stringent requirements in terms of accuracy, integrity, continuity and availability. On the other hand, airborne sensors may not be based on high-quality equipment as in civil aviation and solutions need to rely on tighter multisensor solutions, whose safety is difficult to assess. In this work, we first provide some initial navigation requirements related to precision approach operations based on recently proposed vertiport designs. Then, we provide an overview of a possible multisensor navigation architecture solution able to support these types of operations and we comment on the challenges of each of the subsystems. Finally, initial proof of concept for some navigation sensor subsystems is presented based on flight trials performed during the German Aerospace Center (DLR) project HorizonUAM.

In this paper, we evaluate the performance of the dual-frequency airborne ionospheric gradient monitor proposed for dual-frequency multi-constellation (DFMC) Ground Based Augmentation Systems (GBAS) at different distances from the airport. We use two types of thresholds for this assessment: (i) a constant threshold derived from operational requirements, and (ii) a dynamic threshold that increases with the distance from the airport. Increasing the threshold allows more ionospheric error within the position solution, but also enables the use of the primary single-frequency modes without the need to switch to the ionosphere-free (Ifree) solution for a longer period, which generally degrades the performance because it combines the noise and multipath of two frequencies. Furthermore, we compare the performance of the two potential architectures for DFMC GBAS: (i) the so-called GAST F architecture, which is based on single-frequency 100 seconds smoothing, and (ii) the GAST X architecture, which is based on divergence-free (Dfree) smoothing with variable and potentially longer smoothing time constants. Results with both simulated and real data show that the use of a variable threshold significantly reduces the probability of excluding satellites and switching to the Ifree mode for both GAST F and X, thereby increasing the availability of GBAS.

The future architecture for Dual-Frequency Multi-Constellation Ground-Based Augmentation Systems (DFMC GBAS), known as GBAS Approach Service Type (GAST) E, is designed to rely on measurements from two frequency bands, specifically L1/E1 and L5/E5a. However, unintentional Radio Frequency Interference (RFI) could affect one of the frequencies, typically L1/E1, potentially leaving GBAS with just a single operating frequency. In such a scenario, GAST E cannot function as intended, necessitating the definition of single-frequency fallback modes, either on L1/E1 or L5/E5a, to maintain CAT III service. Therefore, the definition of a GAST D-like mode based on L5/E5a becomes important. However, the integrity monitors designed for GAST D in current standards have only been validated for L1 and GPS and are known to be sensitive and trigger excessive false alarms when the ionosphere is very active. Therefore, in this paper, we evaluate the performance of a GAST D equivalent mode implemented for L5/E5a. The purpose of this study is to assess if this fallback mode can support CAT III operations, or whether the integrity monitors trigger excessive false alarms and consequently regular switches to a CAT I fallback mode can be expected. For these studies, we implement and adapt the main existing GAST D monitors for anomalous ionospheric gradient mitigation to operate in the L5/E5a frequency band. First, we assess the performance of these monitors under nominal conditions using real data collected by the Multipath Limiting Antennas (MLAs) installed at Tenerife Norte Airport and a user ground receiver located within the airport’s protected area. Following the assessment under nominal conditions, we adapt the thresholds of these monitors and evaluate their performance using real data during scintillation events. Results show the need to adapt the monitors for L5/E5a and the suitability of the adapted thresholds to detect high ionospheric activity.

The project City-ATM, launched by the German Aerospace Center DLR in 2018, aims to integrate new airspace users, such as unmanned aerial vehicles, into uncontrolled airspace. An air traffic management and traffic flow control concept were developed. The phase 1 demonstration in 2019 showed a bridge inspection featuring, amongst other elements, the flight of several drones in a limited area, beyond visual line of sight operation, strategic flight planning considering strategic geo-fences, and tactical conflict detection. To enrich the base concept from phase 1 with more U-Space services, the concept has been extended for phase 2 by dynamic geo-fencing for the tactical avoidance of danger spots. This article describes how dynamic geo-fences can be modeled around a hazard area, how the dynamic opening and closure of these fences can be distributed to relevant systems, and how the fences are considered by on-ground and already in air vehicles. The complete concept has been implemented and tested at the National Experimental Test Center in Cochstedt. Finally, the execution of successful flight trials has been described.

This paper describes a concept for local GNSS (Global Navigation Satellite System) augmentation derived from the established Ground Based Augmentation System (GBAS) in civil aviation. The aim of this concept is to provide reliable and accurate GNSS measurements for integration into a redundant, safe and reliable integrated navigation architecture tailored to serve Urban Air Mobility (UAM). The proposed concept for local GNSS augmentation addresses the specific challenges of UAM, in particular for take-off and landing operations, and ensures safe separation between UAM vehicles en route between vertiports. By using less expensive hardware compared to the traditional GBAS, the concept aims to make the integration into future urban airspace easier and more cost-effective, both in terms of ground infrastructure demands and on-board navigation hardware. In addition to the high-level system concept and considerations, we present an initial nominal performance assessment of local augmentation using lower-cost airborne and ground hardware. This assessment is based on actual UAV (Unmanned Aerial Vehicle) flight trials conducted in different urban scenarios, as well as long-term rooftop measurements.

This work proposes a new method for detecting ionospheric activity in the context of future dual-frequency Ground Based Augmentation Systems (GBAS). We utilize measurements available to an airborne GBAS user to derive a probability for large anomalous ionospheric gradients currently present between the aircraft and the local GBAS station. For this purpose, two metrics in satellite range domain are introduced for activity detection: the first is based on the total ionospheric delay difference between ground and air and the second one uses a cumulative sum (CUSUM) based monitoring approach of the rate of change of ionospheric delay difference estimates. After introducing the new concept for this ionospheric activity monitoring, we explain the design parameters that influence the sensitivity of the test and how they can be adjusted. Based on a large simulation dataset, we evaluate the detection capability of the new approach and subsequently the potential to reduce the ionospheric protection levels under nominal conditions by ensuring the absence of harmful ionospheric disturbances. Finally, the monitoring is applied to a measurement dataset within an experimental GBAS facility to evaluate the potential to improve monitor performance and thus improve nominal availability in the presence of significant local multipath. Overall, the results indicate the potential to improve system availability, especially in situations where few satellites are available, such as when using a single constellation, while being able to detect ionospheric activity before the onset of harmful errors for an airborne GBAS user on approach.

Certified Ground Based Augmentation Systems (GBAS) as of today operate using only GPS signals in the L1 frequency band to enable precision approach guidance for landing aircraft. Ionospheric activity (i

Large ionospheric gradients acting between a Ground Based Augmentation System (GBAS) reference station and an aircraft on approach could lead to hazardous position errors if undetected. Current GBAS stations provide solutions against this threat that rely on the use of "worst-case" conservative threat models, which could limit the availability of the system. This paper presents a methodology capable of detecting ionospheric gradients in real time and estimating the actual threat model parameters based on a network of dual-frequency and multi-constellation GNSS monitoring stations. First, we evaluate the performance of our algorithm with synthetic gradients that are simulated over the nominal measurements recorded by a reference network in Alaska. Afterwards, we also assess it with one real ionospheric gradient measured by the same network. Results with both simulated gradients and a real gradient show the potential to support GBAS by detecting and estimating these gradients instead of always using "worst-case" models.

The ground-based augmentation system (GBAS) is the cornerstone for enabling automated landings without the instrument landing system (ILS). Currently, GBAS is evolving to GBAS approach service type-D (GAST-D) for category III landings. This development toward GBAS GAST-D extends GBAS via the use of multiple frequencies (L1/L2 and L5) and the use of multiple global navigation satellite system constellations. GBAS requires correction data to be broadcast to aircraft, which is currently handled via the VHF data broadcast (VDB) datalink. However, VDB has several known shortcomings: 1) low throughput, 2) small area of operation, and 3) no cyber-security measures. In this article, we propose the use of the L-band digital aeronautical communications system (LDACS) for broadcasting GBAS correction data to address these shortcomings. In flight experiments conducted in 2019, we set up an experimental GBAS installation using LDACS. Broadcast data are secured using the timed efficient stream loss-tolerant authentication (TESLA) broadcast authentication protocol. Our results indicate that cryptographically secured GBAS data via LDACS can provide GAST-C and GAST-D services with high availability if cryptographic parameters are chosen appropriately.

The increasing number of modernized GNSS signals and the availability of multi-constellation receivers are crucial for improvements of both precision and robustness of GNSS based positioning. However, the abundance of GNSS observations is not always useable as applications, using differential positioning or other techniques, may have limitations with respect to computational resources or communication bandwidth for reference data, and therefore require a qualified selection of a subset of observations for positioning. This paper is based on the work conducted in the project PREParE SHIPS funded by the European Union Agency for the Space Programme (EUSPA) on the specific application of Maritime Navigation using Network Real Time Kinematic (NRTK) and will focus on the satellite selection algorithms of the Prepare Ships dissemination solution. This study is motivated by data rate requirements and restrictions of the VDES dissemination solution developed in Prepare Ships. The restricted data rate for dissemination of RTK observations via VDES implies the need for a qualified pre-selection of satellite subsets to match the available bandwidth and the requirements of the positioning system. For this, multiple algorithms have been developed and tested in static and dynamic scenarios. Optimization techniques for height (for vertical position), two and three dimensions were examined. Different weighting schemes were used. During the evolution of the satellite selection study, it was concluded that it is necessary to retain satellites with the highest elevation as this will empirically improve integer ambiguity resolution for position fixing. Also fixing a minimum number of satellites for each constellation was required to enable a fair weightage to the different constellations used. Such algorithms should prove to be very useful for research on various Network RTK applications which require/prefer limited bandwidth such as for cadastral surveying and mapping, for airborne geo-referencing of aerial mapping data using Unmanned Aerial Vehicles (UAV) and on the road and sea for positioning and navigation of automated transport. Additionally, these algorithms could also be extended to consider satellite visibility in e.g. urban areas (i.e. urban canyons) by inclusion of true surface information for more robust GNSS positioning in automated transport applications [1]. This could either be for pre-evaluation or for dynamically considering spatial information. While this work is a part of PREParE SHIPS, it is also motivated by a more general applicability of the algorithms presented for other similar applications. RTK correction dissemination with limited bandwidth requirements is very promising for RTK research and therefore this study on optimized selection of satellite subsets is of vital importance and could tap multiple opportunities of huge potential such as those involving NRTK or combination of Precise Point Positioning with RTK.

The VHF Data Broadcast (VDB) data link is responsible for transmitting Ground Based Augmentation System (GBAS) corrections from the GBAS ground station to the aircraft. Thus, it is a major bottleneck for the evolution and security of GBAS. It provides limited bandwidth, range, only line-of-sight capabilities, and no cyber-security protections for the transmitted data. Overcoming these constraints is required for the future and calls for an alternative data link for GBAS. A promising candidate is the L-band Digital Aeronautical Communications System (LDACS). First demonstrations of secure GBAS over LDACS used the Timed Efficient Stream Loss-tolerant Authentication (TESLA) for broadcast authentication of GBAS data. In flight trials, the concept and support of TESLA secured GBAS via LDACS for GBAS services, supporting category II/III precision approach capabilities, were proven. In this paper, different ways are investigated to further optimize latency and security data overhead for an optimized transmission of TESLA secured GBAS packets via LDACS. Initial evaluations show how promising the different options are, especially in respect of a reduced latency of 55.45 ms compared to previous 632.98 ms. Further, it is shown how the developed concept for secure GBAS can also be applied to secure general broadcast applications over LDACS.

L'article presente le projet ERSAT GGC et les techniques de detection des effets locaux sur les GNSS developpees pour l'introduction du GNSS dans le systeme ERTMS/ETCS

HorizonUAM is a DLR project to research the vision of urban air mobility. One important topic is the safety and security aspect. This paper discusses the safety and security considerations in the topic areas of: safe autonomy, reliable multisensor navigation, robust and efficient communication, U-space and safe air traffic, and finally cyber-physical safety and security. As a basis for future discussion, the challenges and gaps are identified, furthermore the research in the context of the project is briefly outlined. From a safety perspective, the missing pilot of an autonomous vehicle is equivalent to a missing fallback layer, which has to be mitigated. Additionally, trust needs to be established for new techniques, specifically machine learning, which has recently gained massive interest. To solve this, verification, standardization, and regulation aspects of autonomy have to be addressed. The research is therefore targeted towards assessing and increasing the safety of autonomous operations. Moreover, a multi-sensor navigation system is necessary to achieve required safety levels in challenging environments, while meeting the cost and weight constraints. In order to ensure the integrity of the navigation system, an innovative integrity monitoring architecture for the multi-sensor solution will be investigated in the UAM context. Furthermore, many flying vehicles will need to share information in a reliable and trustworthy manner. Therefore, a robust and efficient communication system is specifically designed to meet the requirements of the future urban air mobility in order to mitigate midair collisions. To achieve safe air traffic, U-space services would have to be established. The project will specifically address gaps in the proposed services and identify additional required information and services. As a last aspect, the cyber physical safety and security has to be considered. With highly connected systems, attacks can happen over the Internet from virtually anywhere. Therefore, it is required to establish a holistic approach for cyber-physical safety and security.

In this paper we present a modular framework to classify railway track areas regarding the expected presence of local GNSS threats. This information might be critical for a safe signalling operation, for example to determine where virtual balises could be placed safely. We show first how different GNSS threats can be detected using dedicated detection algorithms and how these individual detection results can be then transformed from time to the track domain. An overall decision logic is subsequently used to identify an area as suitable or unsuitable for GNSS usage by combining all available GNSS data collected over the same track area. Finally, the framework implementation is evaluated with railway data obtained during a measurement campaign in Sardinia, Italy in 2019. Even though developed in the railway context, the presented framework architecture and methodology may be also considered to perform similar classification tasks for other means of transport.

Ground transportation systems demand accurate and robust localization functions. Satellite navigation is considered a key element in those systems, but its position determination can be highly corrupted in urban environments because of the presence of reflected signals (i.e. multipath). This paper deals with the detection of multipath in the code measurements of GNSS receivers for mobile users in urban scenarios. First, we discuss the different alternatives and limitations to properly isolate multipath autonomously at the receiver based on Code-Minus-Carrier (CMC) techniques in challenging GNSS applications. We then propose a practical methodology to design a suitable multipath detector based on the time difference of CMC. All the analysis and evaluations are supported with real measurements collected in Railway scenarios.

Satellite navigation plays a critical role in new railway signaling and train control systems. It will allow higher levels of automation at a reduce cost in installation and maintenance as compared to current infrastructure-based technologies. However, in order to be integrated in railway systems, GNSS must guarantee the stringent railway safety requirements. GNSS safety has been extensively quantified for the aviation community, however, in the railway environment the probability of local GNSS hazards like interferences is larger due to its operation in a non-restricted space. Railway GNSS receivers must be therefore equipped with mechanisms to protect the position determination from being corrupted by undesired interferences (jamming or spoofing) without being aware of it. In this paper and based on Commercial-Off-The-Shelf (COTS) hardware we implemented interference detection functions suitable for railway localization. We detail the algorithm design as well as the necessary calibration procedure to determine the nominal signal model. The performance of the interference detection is analyzed with an experimental setup on a commercial train during the measurement campaign organized for the EU project ERSAT-GGC. Figures about the detection capability and detected events along the selected line are finally provided.

As of today, all Ground Based Augmentation Systems operate using only the navigation signals from GPS (and Glonass for a number of stations in Russia) in the L1 band. Ionospheric activity, especially in equatorial and polar regions, limits the availability of the service in those regions. Therefore, an extension to dual-frequency and multi-constellation (DFMC) techniques is currently being developed. In previous work we presented an initial concept for ionospheric monitoring and preliminary evaluations using the L1/L2 frequency combination of GPS. In this paper we present an analysis of the monitoring threshold depending on the number of satellites and constellations used. Furthermore, we discuss the potential issue of multiple satellites being affected by a gradient simultaneously which causes the monitoring to become quite sensitive in the original monitoring concept. Next, we consider the impact of ground and airborne noise and multipath on the monitoring performance using latest results of the effort of developing the respective models. Finally, we show the performance of the monitor in real-world operations using flight test data from flight trials with our Airbus A320 and our experimental dual-frequency multi-constellation GBAS ground station with the currently available Galileo satellites broadcasting signals on E1 and E5a and the GPS Block IIF satellites that broadcast signals on L1 and L5.

This is the deliverable D4.5 Process Execution Report of the ERSAT GGC project. This document reports the execution of the track area classification process developed during the work package WP4 Track Survey and Track Classification along with the main outcomes and results obtained during this process. The goal of WP4 is to set up a standard measurement and classification system that is able to provide a prediction of the expected presence of GNSS hazard causes and degraded performance to be expected at a given track area, in order to enable the evaluation if the specific environment is suitable or not to place virtual balises or on the contrary should be discarded beforehand. In this document, we detail the results of applying the classification standard process, by means of the developed software Toolset to the measurement data collected in the three measurements campaigns during ERSAT GGC project. This execution report addressed the main important aspects that need to be carried out for the correct application of the classification process and the results obtained with the data collected.