BIOGRAPHIES Daniel Gerbeth received a Bachelor and Master's degree in Electrical Engineering and Information Technology from Karlsruhe Institute of Technology in 2014. During Master studies he specialized in aerospace technology and navigation. After working in the field of sensor fusion and navigation aiding at Fraunhofer IOSB he joined German Aerospace Center (DLR) in May 2015 and is involved in the research on GBAS now. Michael Felux received a diploma in Technical Mathematics from the Technische Universität München in 2009. The same year he joined the German Aerospace Center (DLR) where he has was working on the development of the GBAS GAST C testbed at the research airport in Braunschweig and its upgrade to GAST D. He was involved in flight testing and ground validation of the station and the approach procedures. Since 2015 he is coordinating DLRs research on GBAS-based navigation.

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.

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.

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.

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BIOGRAPHIES Maria Caamano received a Master's degree in Telecommunications Engineering from the University of Oviedo, Spain, in March 2015. During Master studies, she specialized in the field of signal theory and communications. The same year she joined the German Aerospace Center where she is working on the impact of ionospheric irregularities in the single-frequency single-constellation GBAS. Daniel Gerbeth received a Bachelor and Master's degree in Electrical Engineering and Information Technology from Karlsruhe Institute of Technology, Germany in 2014. During Master studies he specialized in aerospace technology and navigation. After working in the field of sensor fusion and navigation aiding at Fraunhofer IOSB he joined German Aerospace Center (DLR) in May 2015 and is involved in the research on GBAS since then. Michael Felux received a diploma in Technical Mathematics from the Technische Universität München in 2009. The same year he joined the German Aerospace Center where he was working on the development of the GBAS GAST C testbed at the research airport in Braunschweig and its upgrade to GAST D. He was involved in flight testing and ground validation of the station and the approach procedures. Since 2015 he is coordinating DLR's research on GBAS-based navigation. Mihaela-Simona Circiu studied Computer Engineering at Technical University Gheorghe Asachi, Romania. She obtained a 2nd level Specialized Master in Navigation and Related Application from Politecnico di Torino, Italy in 2012. Since 2013 she has been working as a research associate at the Institute of Communications and Navigation at the German Aerospace Center. Her main focus is multi-frequency multi-constellation Ground Based Augmentation System.

, she is doing her Ph.D. as an external student of gAGE/UPC in the Doctoral Program in Aerospace Science and Technology (DOCTA). Her main research interests are the impact of ionospheric irregularities in the single-frequency single-constellation GBAS and the future development of multi-frequency multi-constellation 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. NAVIGATION. 2019;66:227-238. wileyonlinelibrary.com/journal/navi