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Reference stations are a critical element of the GNSS infrastructure and provide valuable measurements for constellation monitoring [1]. It is however important to obtain high-quality measurements from these stations and therefore any kind of local error source shall be minimized or properly accounted for. When choosing the site for a new reference station or when analyzing the performance of a current one, particular attention shall be devoted to multipath. The multipath (and a poor suppression of it) is one of the major error sources and can lead, in an extreme case, to non-usability of measurements of the reference stations’ network and/or to the need of expensive countermeasures to be put in place after installation [2]. Currently, however, there is, to the authors’ best knowledge, no scientific way to assess the multipath conditions of a given site before installation of a new antenna. It was demonstrated in previous work from the authors [3] that the amount of multipath available at the receiver in a given installation site strongly depends on the so-called antenna “multipath suppression capability indicators” (MPSI). Antennas with similar MPSIs placed in the same position are expected to deliver similar amount of multipath error to the receiver. It is therefore feasible to perform a generic characterization of the site by using a reconfigurable “probe” antenna, capable of assuming different levels of MPSI. The results obtained by this set of measurements will then form a sort of “database of multipath” for the site for each MPSI level. Such technique will then allow to foresee beforehand the performance obtainable with any other antenna, just by comparing its MPSI values with the multipath obtained with a corresponding MPSI of the “probe antenna”. This paper will show the design of the reconfigurable antenna. The results of the electromagnetic measurement of the antenna will also be shown together with the different values of MPSI achievable by reconfiguring the antenna. Moreover, MPSI values from different commercial antennas will be derived for comparison. An experimental validation of the method will be reported, in which the multipath probe antenna will be used to estimate the multipath environment in a given site.

This paper investigates the impact of the range error caused by the antenna group delay variations (noted as AGDV) on the advanced receiver autonomous integrity monitoring (ARAIM). The new multipath and AGDV error models for aviation use of new GPS and Galileo signals developed within the Dual Frequency Multipath Model for Aviation (DUFMAN) project and relevant AGDV measurements analyzed in DUFMAN [1], [2] are applied for the assessment of the impact. In this work, several approaches are taken to address the contribution of the AGDV error in the current ARAIM airborne algorithm: consideration of the user antenna bias error as a measurement bias term or as a random process sigma term. We performed ARAIM service volume simulations for Localizer Performance with Vertical guidance (LPV)-200 by applying the proposed error modeling methodologies and integrity support message (ISM) parameters in line with the current ARAIM framework. We compared availability performances as well as protection levels between the methods. It was found that 99.5% LPV-200 availability increased by approximately 5% when the newly derived DUFMAN multipath and AGDV error models were applied. On the other hand, despite the maximum improvement of roughly one meter in the vertical protection level, the AGDV effect considered as the bias term in the worst-case sense appears to be marginal to the ARAIM availability performance.

This paper proposes DFMC airborne multipath models and antenna error models derived from measurement and supported by simulations. Based on the data evaluated, new multipath models (including the contribution from the antenna) for Galileo E1 and GPS L1 and Galileo E5a and GPS L5 are discussed. Furthermore, a model for the Ionosphere-Free combination of the signals is proposed.

Global navigation satellite systems (GNSS) are used in many applications and have been part of our daily life since years. In this work error contributions based on the satellite hardware have been investigated and their impact on safety critical applications has been assessed. The paper starts with an introduction of the measurement facility and analysis of the satellite payload imperfections as well as the impact of signal deformations on the pseudo-range for different GPS (Block IIF) and Galileo satellites. The analysis is based on in-phase (I) and quadrature-phase (Q) data captured with a high gain antenna, which offers almost interference and multipath-free signal reception. Using these measurements, the biases for different receiver parameters in terms of correlator spacing and bandwidth are derived. The differential code biases are estimated using a fixed configuration for the ground station, e.g. 0.1 chips for L1 and 1 chip for L5 as specified in the current DFMC MOPS (EUROCAE WG62) and 24 MHz (double-sided) bandwidth. For the user receiver, we extend the receiver parameters to the following design space: 0.01 - 1 chips correlator spacing and 2 – 50 MHz bandwidth for L1, 0.01 – 0.5 chips correlator spacing and 4 – 50 MHz bandwidth for E1, and 0.01 - 1 chips correlator spacing and 10 – 50 MHz bandwidth for and L5/E5a signals. In addition, an analysis of the magnitude of the differential satellite code biases and position errors within the design space defined for GPS L1 (RTCA DO-229E), and within the DFMC design space proposed in the DFMC SBAS MOPS (EUROCAE WG62) is shown. Based on the derived satellite differential code biases, the impact on the position solution for one location and different geometries is presented.

A current baseline of ARAIM from the WG-C Advanced RAIM Technical Subgroup (ATSG), described in the recently updated (v3.1) Reference Airborne Algorithm Description Document [1] makes use of the Airborne Accuracy Designator Model B (AAD-B) [2] in order to characterise the multipath contribution and receiver noise term for GPS L1/L5 and a lookup table of multipath and noise error for Galileo E1b and E5a signals, dependent on satellite elevation. The AAD-B model is derived from GPS L1 measurements only and does not characterise the effects of noise or multipath on the modernised signals, those being Galileo E1b, E5a and GPS L5. Therefore the use of the AAD-B model requires updating to ensure it is representative of the nominal errors experienced on these other measurements. Initial dual-frequency, dual-constellation (DFDC) multipath models have been presented in [4] and [5] using measurements from simulations and GNSS receivers including DFDC avionics hardware. Work is currently being conducted within the European Commission funded Dual Frequency Multipath Model for Aviation (DUFMAN) project to produce new multipath models that characterise the performance of each of the aforementioned signals, based on the collection and processing of data from aviation environments using aviation GNSS receivers. The purpose of these new models is to provide GNSS aided aviation navigation algorithms such as ARAIM and DFDC Satellite-Based Augmentation System (SBAS), with a complete DFDC multipath model to safely overbound integrity risks emanating from multipath for GPS L1c/a, L5, Galileo E1b and E5a signals. This paper shall present the findings of an investigation of the impact of these preliminary DFDC multipath models from the DUFMAN project on the performance of ARAIM by simulating global availability against the most stringent RNAV approach requirements; Localiser Performance with Vertical guidance (LPV)-200. In addition, we will present a series of comparisons to a reference control simulation when using not only the new multipath models but also alternative methods of modelling user antenna bias terms (a component of the analysis performed within the frame of DUFMAN) and finally arriving at a consideration of user antenna bias as deterministic and predictable to a degree of certainty.

This paper proposes initial versions of DFMC airborne multipath models derived from measurement and supported by simulations. Based on the data evaluated, new multipath models for Galileo E1 and GPS L1 are discussed. Furthermore, two new multipath models one for Galileo E5a and GPS L5 and another one for the Ionosphere-Free combination of the signals are discussed and suggested.

In aeronautical navigation the use of Global Navigation Satellite Systems (GNSS) is becoming ever more important. GNSS are one of the cornerstones of the performance based navigation (PBN) concept. They are currently used for navigation en-route, as well as during arrival procedures and for lateral approach guidance. Together with satellite-based or ground-based augmentation systems (SBAS, GBAS) satellite navigation can provide precision approach guidance down to CAT-I minima. In order to ensure sufficient global availability of these services and enable new services, such as Advanced Receiver Autonomous Integrity Monitoring (ARAIM) for providing services with higher performance levels, including in regions with active ionospheric conditions, existing integrity concepts and augmentation systems are upgraded to incorporate not only GPS but multiple GNSS constellations and also navigation signals on a second frequency. On the side of GNSS, all GPS satellites since the Block IIF generation with currently 12 operational satellites provide signals in the L5 band (in addition to the most commonly used signals in the L1 band), a second frequency band usable for aeronautical applications. The Galileo constellation has currently 22 operational satellites in orbit that all provide signals on the E1 and E5a frequency bands. Other constellations, such as Glonass and BeiDou are also launching further satellites so that a large number of navigation satellites are available to users. The use of dual-frequency and multi constellation techniques will mitigate the impact of most ionosphere-related disturbances, significantly increasing service availability. All GNSS-based navigation methods have in common that they need appropriate integrity concepts safely bounding any residual errors that may prevail in the position solution. With the ionospheric errors largely addressed by dual-frequency and multi-constellation methods, the residual noise and multipath becomes the most significant contributor to the residual errors. In order to bound these errors, standardized error models are used. The existing multipath model was developed based on extensive data analysis, however, using only the legacy GPS signal in the L1 band. Galileo is using a different modulation for the E1 signals which is less susceptible to multipath. The GPS and Galileo signals in the L5/E5a band are using a 10-times higher chipping rate than the L1/E1 signal. Therefore, also for these signals, the multipath envelope is significantly smaller, potentially allowing to have smaller error models for these signals. When using dual-frequency methods to remove the ionospheric delay, the receiver tracking noise and multipath error from the signals on both frequencies are combined. For all these cases the existing model is not well suited for error modelling. Within the frame of the DUFMAN project funded by the European Commission new multipath models for the new signals are developed in order to be able to exploit the potential benefits for aviation users. Previous papers on the project were addressing the methodology, described the results of the studies and the influence of the antenna. This paper explains the standardization activities and discusses choices that were made in setting up the data collection campaign and the subsequent steps to standardized models. Regarding standardization, the International Civil Aviation Organization (ICAO) is producing Standards and Recommended Practices (SARPS) for DFMC SBAS which will make use of the DFMC multipath models. Further requirements on the hardware exist e.g. in form of Minimum Operational Performance Standards (MOPS) that specify performance of certain components, such as the airborne antenna. A variety of antennas differing significantly in performance is available on the market. Furthermore, the airborne receiver hardware may use different correlator spacing and receiver bandwidth settings which may also have an impact on the results. In the effort to characterize the multipath errors, hardware and processing choices had to be made taking into account all those requirements and the impact on the final models. The paper discusses the interdependency between different standards and explains the choices that were made in the project, as well as results in terms of standardization.

Current satellite navigation systems are providing more and more dual frequency capabilities, enabling an improved navigation accuracy and reduction of residual errors (e.g. from ionosphere)[1,2]. Recently, the aviation community has started an effort to achieve new standardized minimum operational performance specifications (MOPS) for GNSS equipment, in order to allow for use of dual frequency multi constellation (DFMC) systems in the future, with clear benefits in terms of obtainable navigation performance. In such conditions, residual errors due to the GNSS antenna become more and more relevant and actually need to be properly identified and bounded, both in the antenna specifications and, for instance, in the models for aircraft multipath. Previous work from the authors [3] has shown the methodology developed in order to accurately simulate the installed performance (both in terms of antenna pattern distortion and multipath) of commercial antennas mounted on commercial aircraft. The antennas are measured in an anechoic chamber and then the data are imported into electromagnetic simulation software. Combining these measurements with a proper CAD model of an airplane, they are then used to simulate the real-life installation on the airplane. In this work, an analysis of the multipath performance obtainable by different antennas mounted on different aircraft (single aisle and wide body) will be shown: such results will provide a good indication for the performance to be expected in real-life scenarios and will also be used to further validate (and ensure the general applicability of) the new proposed model for multipath, obtained from flight data and shown in a concurrent paper at the conference. Moreover, the multipath suppression indicator (MPSI) will be introduced which is able to quantify the multipath suppression capability of the antennas: its applicability in the avionic context will be validated against the data obtained by the installed performance analysis and a proposal for its use in future antenna MOPS will be given.

Navigation : journal of the Institute of Navigation 67(1), 61-81 (2020). doi:10.1002/navi.344, Published by Wiley-Blackwell, Hoboken, NJ ˜[u.a.]

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.

The current increase in the number of available satellites from different constellations as well as in the number of satellites emitting signals at multiple frequency bands is promising to strongly increase the positioning performance of satellite navigation receivers. Such improvement is for instance due to the possibility to use dual frequency corrections for the ionospheric effects. Dual frequency multi constellation (DFMC) systems are hence currently being deployed worldwide for different applications. In the aeronautical field, a standardization effort for DFMC navigation systems is currently being coordinated by RTCA, with the aim to define the minimum operational performance specifications (MOPS) of future avionic systems. In such framework, antennas assume a central role: on the one hand, the impact of antenna imperfections on the overall pseudorange error will become more relevant with DFMC systems. This is due to the minimization of other error terms, such as the ionospheric one, thanks to multifrequency processing. Such contribution needs therefore to be properly characterized. On the other hand, antenna characteristics impact the multipath error at the receiver, which is due to the multipath susceptibility of the antenna itself: such contribution also needs to be further investigated and when possible, modelled. This paper aims at addressing both these antenna-related error terms, focusing on airborne applications. A method for a precise antenna characterization will be shown, also considering, through electromagnetic measurements and simulations, the (multipath-generating) installation on the aircraft. An initial comparison of the results obtained through this method with data collected from flight tests will be shown, validating the correctness of the method. Moreover, a further insight will be given regarding the different performance levels achievable by different commercial antennas.

This paper presents an update of the ongoing work to develop dual frequency dual constellation airborne multipath models for Galileo E1, E5a and GPS L1 and GPS L5 in the frame of the project DUFMAN (Dual Frequency Multipath Models for Aviation) funded by the European Commission. The goal of this activity is to support the development and implementation of airborne GNSS-based navigation solutions, such as Advanced Receiver Autonomous Integrity Monitoring (ARAIM), dual-frequency multiconstellation Satellite Based Augmentation System (SBAS) and dual-frequency multi-constellation Ground based Augmentation System (GBAS). Previous work described the methodology proposed to derive the airborne multipath models and presented preliminary multipath models obtained from an experimental installation. In this paper we present the initial results obtained from flight campaigns conducted within DUFMAN on Airbus commercial aircraft. The measurements are collected from prototypes of dual-frequency multi-constellation avionics receiver and the antenna installed on the aircraft has been selected to meet at best the current dual-frequency dual-constellation antenna requirements. In addition to the initial results obtained from avionics hardware, the impact of the different receiver correlator spacing and bandwidth is investigated and discussed.

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.

Current standardization activities in the aeronautical community are paving the way to using multifrequency multiconstellation GNSS as a primary means of aircraft navigation. The increase in the number of satellites from different constellations as well as the increasing use of multiple frequency GNSS receivers promise improvements in the achievable accuracy in GNSS positioning. The effects of receiver antennas shall be investigated because of the stronger impact of possible imperfections on the overall error budget for multifrequency combinations. The scope of this paper is to show the possibility of properly modeling and indeed characterizing the antenna-induced GNSS pseudorange errors, through simulations and electromagnetic measurement. An insight into antenna characteristics giving rise to such errors will be given, by analyzing the impact of the feeding technique on the achievable pattern uniformity and hence on antenna-related pseudorange errors. The technique is then validated through GNSS field measurement.

This paper presents the initial results of an effort to develop and establish airborne multipath models for the GPS and Galileo L1/E1 and L5/E5a signals. These models will be used in dual frequency and multi constellation aeronautical navigation techniques, such as ARAIM, SBAS and GBAS. Development of the models is based on electromagnetic simulations and a large-scale data collection campaign on a variety of different airframes. In this work, results of the evaluation of the first batch of about 200 hours of flight test data with experimental equipment are shown. At a later stage, more data with avionics hardware will be collected and evaluated. This paper mainly aims at establishing the methodology and obtaining initial results as a first indication. For this purpose flight data is analyzed for periods where satellites are tracked continuously. Then, the carrier phase measurements are subtracted from the code measurements, the ionospheric divergence is removed and the bias introduced by the carrier phase ambiguities is removed. Finally, all the data is sorted, overbounded and inflated to account for the limited number of samples. The methodology also includes the removal of code errors induced by the antenna group delay, as these are suggested to be treated separately from the multipath errors. The absolute values of the bounds for GPS L1 and Galileo E1 are larger than the existing GPS L1 model. This could be expected when being conservative at each step of the evaluation process and when using experimental hardware that is not entirely compliant with current minimum performance standards for airborne equipment. Without the conservatisms applied, the observed errors are below the Airborne Accuracy Designator (AAD A) curve. The level of residual multipath on L5/E5a is significantly smaller than that on L1/E1 and the model for the ionosphere free combination is significantly larger than the bound on L1/E1. Both these properties found in our experimental results match with theoretical expectation.

With the increasing number of satellites broadcasting signals on a second ARNS frequency (L5/E5a) available the dual frequency processing becomes a promising option for the next generation aviation users. In this context, characterizing the error sources of the navigation solution is crucial. This evaluation is particularly important because the next generation aviation users of GNSS will be able to utilize dual frequency measurements and eliminate the previously dominant ionospheric error term from the user error model. Accordingly, the multipath and code tracking errors will then constitute a much larger relative contribution to the total user ranging error than for a GPS L1 user. It is therefore necessary to carefully analyze the code noise and multipath error distribution to derive appropriate integrity figures for use in SBAS, ARAIM, and GBAS protection level equations. This paper presents the methodology to build the multipath models and proposes several improvements. The estimation of the multipath makes use of the carrier phase measurements, thus it is affected by the integer ambiguities. A new method for removing the ambiguities from the multipath estimation is presented. The method suitable for measurements from flight data and is able to exclude measurements highly affected by multipath by using a derived ???? based on the receiver thermal noise only. Next, an overview on the error sources relevant for the multipath estimation is given and a first concept to separate the receiver antenna errors from the multipath errors is presented.

Multipath is recognized as a limiting factor for precise GNSS positioning. Therefore, a strong interest in identifying and mitigating it both at antenna and receiver side has led to much research in this topic and solutions being developed and commercialized. However, from the antenna point of view, a clear analysis of the actual antenna multipath susceptibility (or in an opposite way, its rejection capability) for multiple and more complicated cases than single bounce from ground has, to the best knowledge of the authors, not been extensively performed. Scope of this work is therefore to analyze the multipath rejection capability of different GNSS antennas (both at L1/E1 and L5/E5a band) in different situations, such as in tripod-like scenarios (hence being affected by multipath from horizontal surfaces) or in scenarios with multipath coming from slanted obstacles or in the vicinity of buildings (hence in cases with multipath coming from vertical surfaces). Such results will then be correlated with the antenna characteristics, in terms for instance of cross polarization discrimination and down-to-up ratio (DUR) and an optimal performance indicator for the different cases will be sought for. It will be shown that the antenna electromagnetic characteristics deterministically determine the rejection capabilities, but also that different parameters shall be taken into account for each category of multipath.

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.

The Ground Based Augmentation System (GBAS) is a landing system for aircraft based on differential corrections for the signals of Global Navigation Satellite Systems (GNSS), such as GPS or Galileo. The main impact on the availability of current single frequency systems results from the necessary protection against ionospheric gradients. With the introduction of Galileo and the latest generation of GPS satellites, a second frequency is available for aeronautical navigation. Dual frequency methods allow forming of ionospheric free combinations of the signals, eliminating a large part of the ionospheric threats to GBAS. However, the combination of several signals increases the noise in the position solution and in the calculation of error bounds. We, therefore, developed a method to base positioning algorithms on single frequency measurements and use the second frequency only for monitoring purposes. In this paper, we describe a detailed derivation of the monitoring scheme and discuss its implications for the use in an aviation context.

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.

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.

This work shows the design and test of a dielectric resonator antenna (DRA) able to cover all GNSS bands with good polarization purity and minimal group delay variations, thus enabling minimal antenna-related contributions to the GNSS pseudorange error. Both electromagnetic simulations and measurements in DLR’s near field chamber are shown, proving the excellent performance of the antenna. Moreover, the antenna related GNSS error is shown by means of a GNSS software receiver.