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

Impact of Satellite Biases on the Position in Differential MFMC Applications

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

Standardization of New Airborne Multipath Models

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

Antenna Group Delay Variation Bias Effect on Advanced RAIM

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 derivedDUFMAN 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

Antennas as Precise Sensors for GNSS Reference Stations and High-Performance PNT Applications on Earth and in Space

Satellite navigation is more and more important in a plethora of very different application fields, ranging from bank transactions to shipping, from autonomous driving to aerial applications, such as commercial avionics as well as Unmanned Aerial Vehicles (UAVs) . In very precise Positioning, Navigation and Timing (PNT) applications, such as in reference stations and precise timing stations, it is important to characterize all errors present in the system, in order to possibly account for them or calibrate them out. Antennas play an important role in this respect: they are indeed the “sensor” that captures the signal in space from Global Navigation Satellite Systems (GNSS) satellites and thereby strongly contribute to the overall achievable performance. This paper reviews the currently available antenna technologies, targeting specifically reference stations as well as precise GNSS antennas for space applications, and, after introducing performance indicators, summarizes the today achievable performance. Finally, open research issues are identified and possible approaches to solve them are discussed

Evaluation of GPS L5 and Galileo E1 and E5a Performance for Future Multi Frequency and Multi Constellation GBAS

In this paper, we show a performance analysis of different signals from the new Galileo satellites in the E1 and E5a frequency bands as well as GPS L5 signals in DLR’s experimental Ground Based Augmentation System (GBAS). We show results of noise and multipath evaluations of the available Galileo satellites and compare their performance to the currently used GPS L1 and the new GPS L5 signals which were presented in a recent paper. The results show that the raw noise and multipath level of Galileo signals is smaller than of GPS. Even after smoothing, Galileo signals perform somewhat better than GPS and are less sensitive to the smoothing time constant. Another issue to be considered in a future multi frequency system is inter-frequency bias. 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 and avoid errors introduced by different hardware in the airborne receiver and GBAS ground system. A residual uncertainty associated with the bias correction has to be taken into account. This can be modelled as part of σ_(pr\_gnd)

Concept for a Dual Frequency Dual Constellation GBAS

This paper proposes one possible concept for a dual frequency dual constellation GBAS architecture. It is based on a single frequency L5/E5a mode as primary processing scheme for best standard performance, a switch to an ionosphere free combination in case of ionospheric disturbances and supporting also classical GBAS approach service types (GAST) C and D for single frequency GPS-based CAT I and CAT II/III modes. The concept is supported by a proposal of how to transmit the required corrections in the existing capacity limited VDB broadcast and is backwards compatible to legacy GBAS. A discussion about the benefits and remaining issues of the proposed architecture concludes the paper

Initial results for dual constellation dual-frequency multipath models

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