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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.

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 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.

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.

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 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.

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 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.