Satellite Selection Methodology for Horizontal Navigation and Integrity Algorithms

With the new upcoming GNSS constellation in the future it might no longer be possible to use all satellites in view for navigation due to limited tracking channels. This is in particular true in the context of Advanced Receiver Autonomous Integrity Monitoring (ARAIM), where the use of dual frequency is favorable to mitigate ionospheric disturbances. To address the issues of limited channels we propose two different satellites selection strategies adapted for Horizontal ARAIM in this paper. First a bare geometric approach which comes with almost no additional computation effort at the cost of less stable results. And second a heuristic optimization which improves selection results significantly while adding additional computational effort. Both approaches are compared to brute force selected best sets in terms of resulting protection levels, computational cost and achieved ARAIM availability. Results show the general applicability of both presented selection methods in Horizontal ARAIM. Using limited sets instead of all satellites in view can still provide global availability. Depending on the method more or less satellites are necessary to ensure sufficiently small and stable protection levels

Detection of GNSS multipath with time-differenced code-minus-carrier for land-based applications

Ground transportation systems demand accurate and robust localizationfunctions. 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.This work has been funded by the European GSA H2020 project ERSAT-GGC. The authors would like to thank all the partners of the ERSAT-GGC consortium.Peer ReviewedPostprint (published version

Satellite selection in the context of an operational GBAS

When incorporating multiple constellations into future ground based augmentation systems (GBAS), a problem with limited VDB (VHF data broadcast) capacity might arise. Furthermore, the number of airborne receiver tracking channels could be insufficient to use all visible satellites. One way to cope with these issues is to perform a satellite selection to limit the number of used satellites with minor impact on performance. This paper investigates different factors that constrain the approach of simply selecting "the best set in every epoch" and shows how to overcome some limitations. These constraints include limitations in satellite visibility, loss of satellites during approach (i.e. in curves), and convergence times in the airborne processing until satellites are usable. Various protection level simulations are performed to show the influence of the named factors on the nominal performance. Taking into account all these contextual influences, results show satellite selection is still applicable in GBAS ground stations

A secure broadcast service for LDACS with an application to secure GBAS

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.Postprint (published version

Vertiport Navigation Requirements and Multisensor Architecture Considerations for Urban Air Mobility

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

A Secure Broadcast Service for LDACS with Application to Secure GBAS

The VHF Data Broadcast (VDB) data link, responsible for transmitting Ground Based Augmentation System (GBAS) corrections from the GBAS ground station to the aircraft, is one 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. Hence the use of an alternative data link for GBAS, overcoming these constraints, is desirable. The L-band Digital Aeronautical Communications System (LDACS) has been demonstrated to overcome aforementioned issues. The first demonstration 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 GAST-D services, supporting category II/III precision approach capabilities, was demonstrated. In this work, different ways are investigated to further optimize latency and security data overhead for an optimized transmission of TESLA secure GBAS packets via LDACS. Initial evaluation show how promising the different options are. Further it is shown how the developed concept for secure GBAS can also be applied to generalized secure broadcast over LDACS

Network‐based ionospheric gradient monitoring to support GBAS

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

Future GBAS Processing - Do we need an ionosphere-free mode?

The Ground Based Augmentation System (GBAS) is a landing system for aircraft. It consists of carefully sited reference receivers at an airport, generating corrections for the navigation signals from Global Navigation Satellite Systems (GNSS). Along with the corrections, integrity parameters are generated and transmitted to arriving aircraft that allow the aviation users to bound their residual position errors after applying the corrections. Currently, corrections are generated for the GPS constellation and the L1 frequency. However, with the ongoing buildup of the European Galileo, the Chinese BeiDou and the modernized Russian Glonass the number of available GNSS constellations is increasing. This provides the opportunity to design systems more robust against disturbances, such as ionospheric scintillation effects, through a larger number of available ranging sources. Furthermore, all Galileo and the latest generation of GPS satellites feature signals in the L5 band that can be used by aviation. Therefore, it is possible to apply dual frequency techniques for mitigation of the ionospheric gradient threat. This paper discusses the advantages and disadvantages of using an ionosphere free combination of the signals for positioning and if such a mode for a future generation of GBAS should be developed