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Ionospheric anomalies, like large ionospheric gradients, might produce a difference between the ionospheric error experienced by the Ground Based Augmentation System (GBAS) reference station and the aircraft on approach. This ionospheric delay difference could lead to hazardous position errors if undetected. For that reason, the GBAS Approach Service Types (GAST) C and D provide solutions against this threat, but the methods employed still face challenges by limiting the availability in certain cases, especially in regions with severe ionospheric conditions. This issue is caused by the use of very conservative ionospheric threat models derived based on the worst-ever-experienced ionospheric gradients in the relevant region. However, these worst-case gradients occur very rarely. Therefore, this paper proposes a methodology capable of detecting ionospheric gradients in real-time and estimating their parameters in near real-time by using a wide area network of dual-frequency and multi-constellation GNSS monitoring stations. Hence, the GBAS stations could use this information to update the threat model currently applied in their algorithms, which would result in an improvement of the GBAS availability in regions where it is degraded. The detection and estimation algorithm is initially theoretically explained. Then, the performance of this algorithm is evaluated with simulated gradients and with a real gradient, utilizing for both the real measurements recorded by a reference network in Alaska. The synthetic gradients are simulated over the nominal real measurements from this network and all the gradient parameters are modified within their ranges in the already existing threat models. In this way, we assess the performance of our algorithm by comparing the differences between the known simulated gradient parameters and the parameters estimated by our algorithm. Additionally, we also evaluate our algorithm with one real ionospheric gradient measured by the same network in Alaska to study the differences between using simulated gradients and real gradients. Results with both simulated gradients and the real gradient show the potential of our methodology to support GBAS stations by detecting and estimating the ionospheric gradients instead of using worst case models all the time.

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.

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.

L'article présente le projet ERSAT GGC et les techniques de détection des effets locaux sur les GNSS développées pour l'introduction du GNSS dans le système ERTMS/ETCS

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.

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.

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.

This paper presents the performance analysis of signals from the Galileo satellites in the E1 and E5a frequency bands and GPS L5 signals as measured by DLR’s experimental ground-based augmentation system. The results show that the raw noise and multipath level of Galileo signals and of the GPS L5 signals are smaller than that of GPS L1. The new signals are also less sensitive to the choice of carrier-smoothing time constant. Furthermore, the inter-frequency biases that affect dual-frequency processing are investigated. These biases differ between satellites and depend on satellite and receiver hardware, but they can be determined a priori. With known receiver and antenna configurations, it is possible to correct for these biases. A residual uncertainty associated with the bias correction has to be taken into account. This can be modeled as part of the ground and airborne bounding standard deviations (spr_gnd and spr_air) used in GBAS processing.

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.

This document is the deliverable D4.2-Technical Specification of the Survey Toolset where the outcome of the work performed in WP4.2 of the ERSAT GGC project is described. This document follows up the work performed in WP4.1 and reported in the deliverable D4.1, extending the ongoing general definition of the track area classification process. The goal of WP4.2 is the specification and development of the toolset for the survey and classification of track areas, giving the system overview and providing the different interacting blocks as the measurement hardware system, the processing software toolbox and the final classification data. Regarding this D4.2, the functional and technical specifications of the toolset are here provided with the description of the interfaces between the different detection techniques and decision logic blocks. Outcome of D4.3 will be the prototype implementation and installation. The document also includes the specification of the measurement system and hardware to be used in the survey process so that the necessary data for the classification techniques can be collected in the railway scenario. Finally, the specification of the output data - the track area classification results - is also analysed and provided. The specification of the survey measurement system and software toolset addressed in the current document will support the next project activities related to the development of the software tools for the classification of track areas in WP4.2 and the execution of measurement campaigns in WP4.3.

This document is the deliverable D4.3-Prototype Implementation of the Survey Toolset, which provides a high level description of the Toolset Prototype. This document is released in a pre-final version, where the toolset implementation has been done using demo data coming from RFI-ASTS private campaign in Italy. The dataset, related to the Pinerolo-Torino line, was used to test and debug the toolset, and the results provided in this document refer to that line and campaign. The Consortium thanks RFI, ASTS and TRI for the investment done to this aim and the availability of needed dataset in order to respect the scheduling of the project. The toolset will be definitively designed after the test campaigns on field, which will confirm the assumptions and in particular the decision logic approach. In fact, once the test campaigns foreseen in the ERSAT GGC will be performed - as planned in the WP4.3 task - the final implementation of the toolset prototype will be released based on those data. A conclusive issue of this deliverable will be then provided with the results of the test campaigns.

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.

When incorporating multiple constellations into future Ground Based Augmentation Systems (GBAS) a problem with limited VDB (VHF Data Broadcast) capacity might arise. Apart from that, the number of airborne receiver tracking channels could be insufficient to use all satellites in view as well. One way to overcome these issues is to perform any kind of satellite selection to limit the number of used satellites with least possible influence on performance. While the general process of how to select most useful satellites efficiently has been largely covered lately, this paper focuses on the incorporation of such algorithms into GBAS. It investigates different factors that constrain the approach of simply selecting "the best set in every epoch independently" 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 of use. Various protection level simulations are performed to show the influence of the named factors on the nominal performance. Additionally, this paper describes methods how airborne users can perform stand-alone selection in a most efficient way and later transition into GBAS, which requires using the satellites actually provided with corrections from the ground. Taking into account all these contextual influences on top of the selection process, results show satellite selection is still applicable in GBAS ground stations.

In this paper we describe a method to monitor for a difference in the ionospheric delay observed by a ground station of a Ground Based Augmentation System (GBAS) and an airborne user. In case of detection the affected satellite can be excluded or a switch to an ionospheric free processing mode can be triggered. As it is not possible to estimate the absolute ionospheric delay at the ground station from the transmitted corrections directly, we compare a pseudo-ionospheric delay estimate from the corrections with an ionospheric estimate after removing biases. We then show the performance of the proposed monitoring architecture in flight trials from our GBAS test environment for different scenarios. In order to obtain results for a full constellation we considered an L1/L2 dual frequency combination where the expected noise and multipath is larger than in the L1/L5 case which will be used in an operational GBAS. Results show that even with the larger test statistic and in the single constellation case the monitor is feasible and provides good results. Furthermore, we also show results using the available L5/E5a signals collected during flight tests. Also in this case the performance is good and the threshold was not exceeded despite the low threshold of the monitor due to the marginal geometry. In order to test the reaction of the monitor to ionospheric gradients we injected a simulated error into the airborne measurements. As the monitoring threshold depends on the satellite geometry we show some exemplary results of the impact of different gradient slopes on the test statistic of the affected satellites.

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.

In this paper we present the expected performance of different processing modes for a future dual-constellation dual-frequency Ground Based Augmentation System (GBAS). The evaluations are based on measurements collected during flight trials. We discuss the changes to the residual differential error for each processing mode. In our previous work we derived preliminary airborne multipath (σair) and ground multipath (σgnd) curves for Galileo E1 and E5a signals and GPS L5 signals. We use these models in our evaluations. After discussing the benefits of adding a second constellation in the GBAS implementation we will focus on the modes enabled by the addition of a second frequency, mainly the single frequency L5/E5a mode and the Ifree mode. We compare the errors and the nominal protection levels of the new processing modes with the current GAST-C and GAST-D. This work contributes to the analysis and trade-off considerations in the development of a future DFDC GBAS standard.

In this paper we analyze and discuss the impact of ionospheric scintillations and multipath on the availability of the current single-frequency single-constellation Ground Based Augmentation System (GBAS). Scintillation effects, which usually occur around plasma bubbles, cause the receiver to lose lock of one or more satellites, leading to potentially unfavorable satellite geometries for an airborne user. We simulate different bubble dimensions and distinct locations of the bubble in the sky in order to illustrate that the use of a second constellation improves the performance of the system in terms of availability for the situations where the single constellation system is unavailable. Furthermore, we investigate the impact of multipath in the availability of the system. During the touch-down and roll-out of the aircraft on the runway, multipath coming from ground reflections becomes important, especially for low elevation satellites producing large position errors. The results show that the use of a second constellation allows the removal of low elevation satellites from the position solution, which are typically strongly affected by multipath. Elevation masks of 10° or even 15° do not degrade the availability of the dual-constellation system in any of the scenarios considered.

This paper proposes a method for monitoring the difference in the ionosphere between a GBAS ground station and an airborne user from single frequency L1/E1 corrections and single frequency L5/E5a corrections. The absolute ionospheric delay at the ground station cannot be estimated from the corrections only. Therefore, the monitored test statistic is the difference of an ionospheric delay estimate at the airborne system and a similar pseudo-ionospheric delay estimation from the single frequency corrections from the ground station. In order to make the two estimates comparable the common part of the airborne ionospheric delay has to be removed. This test is affected by noise and multipath from the airborne measurements and from the ground corrections. Based on airworthiness performance requirements a monitoring threshold per satellite is derived. The decision then enables the airborne equipment to either exclude individual satellites from the single frequency processing or trigger a switch to an ionospheric free dual frequency processing.

In the next generation of Ground Based Augmentation System (GBAS) corrections and integrity parameters for two frequencies and multiple constellations will be provided. However, the capacity of the VDB (VHF Data Broadcast) link providing those parameters to users is strongly limited. Thus providing all those new corrections and parameters would either require a longer update rate of corrections or a limitation on the number of satellites to which corrections are provided. In this paper we propose a new satellite selection method which allows fast selection of a variable sized, quasi-optimal subset of all visible GNSS satellites. The proposed selection heuristic bases on a strong correlation between the third (i.e. vertical) component inside the S-matrix for each satellite and the probability of the related satellite being part of a subset which provides a favorable vertical protection level. Utilizing this correlation we design an algorithm which converges fast and leads to optimal results for a majority of constellations. As the selection can be performed for an arbitrary number of satellites and the complexity is not exponentially scaling with the number of available satellites, as an exhaustive search (brute-force) does, the heuristic is flexible and suitable for different applications even beyond GBAS’s. In the context of GBAS these subsets still provide reliable protection levels as the contribution to the accuracy is dropping with every additional satellite. We show the feasibility of using for instance only 14 satellites in global protection level simulations. In a multi-frequency multi-constellation GBAS architecture the approach of selecting only an optimal subset allows to keep the current 2 Hz update rate. This could mitigate problems with VDB capacity or remove the necessity of additional, more frequent integrity messages.

In this work an overview of numerous possible processing modes in future dual frequency, dual constellation GBAS is given and compared to the current GAST D standard. We discuss the individual error contributions to GBAS protection levels and give an overview of the general processing. Based on this the consequences when adding a second constellation as well as frequency are investigated. Geometrical implications and changes to the residual differential error bounds are studied separately first. In terms of geometry a comparison between the single and dual constellation case is presented using dilution of precision as metric. The influence on the different sigma contributions when using new satellites (Galileo) and signals (E1, L5, and E5a) is individually discussed based on recent measurements. Final simulations for different varying parameters are carried out to compare relevant processing modes in terms of achieved nominal protection levels. A concluding discussion compares the outcomes and analyzes the implications of choosing one or the other mode.

Ionospheric anomalies, like ionospheric gradients, might produce a difference between the ionospheric error experienced by the Ground Based Augmentation System (GBAS) reference station and the aircraft on approach. This ionospheric delay difference could lead to position errors if undetected. For that reason, the GBAS Approach Service Types (GAST) C and D provide solutions against this threat, but the methods employed still face challenges. In the case of GAST C, the GBAS integrity parameters are inflated in order to exclude potentially usable satellite geometries that could produce unacceptably large position errors if affected by a worst case gradient. The GAST D concept contains a significant amount of monitors to cover different regions of the ionospheric threat space, which includes a combination of gradient sizes, directions and speeds. Both methodologies have a negative impact on the availability of the system, especially when trying to implement GAST C and D in regions with severe ionospheric behavior, i.e. equatorial regions, because of the exclusion of too many satellite geometries in the case of GAST C and a high rate of false alarms in GAST D. This paper proposes a real-time ionospheric monitoring approach that supports the already existing GAST C and D solutions without changing the overall integrity concept. The proposed concept is based on a multi-frequency multi-constellation receiver network external to the GBAS installation that provides real-time ionospheric information to the GBAS stations. A GAST C reference station could use this information to reduce the maximum gradient to consider in the inflation of the integrity parameters while a GAST D station could benefit from prior probabilities of appearance relaxing the monitoring thresholds and thus provoking less false alarms. The monitoring approach is first explained and then evaluated with simulated data. This data is constructed based on real measurements of noise and multipath taken with both Choke-ring and Multipath Limiting Antennas (MLAs), and models for the nominal ionospheric decorrelation. Simulation results show that a considerable reduction of the threat space that the remaining monitors in GAST C and D have to consider seems feasible. Furthermore, the main effects of the presence of high levels of noise and multipath in the measurements used are discussed.