Performance Evaluation of the Ionospheric Threat Mitigation Strategies in Dual-Frequency Multi-Constellation GBAS
Maria Caamano, Daniel Gerbeth, Hiroatsu Sato, German Aerospace Center (DLR); Mihaela-Simona Circiu, ESA/ESTEC; Michael Felux, Zurich University of Applied Sciences (ZHAW)
The Ground Based Augmentation System (GBAS) is a local-area, airport-based augmentation of Global Navigation Satellite Systems (GNSS) that provides precision approach guidance for aircraft. GBAS enhances GNSS performance in terms of integrity, continuity, accuracy, and availability by providing differential corrections and integrity information to aircraft users. Differential corrections, which are provided only for the L1 signals of GPS in current operational GBAS, enable the aircraft to remove most of the spatially correlated errors between the ground station and itself. Additionally, integrity parameters enable the airborne system to calculate bounds of the residual position errors and ensure the safety of the operation. However, abnormally large ionospheric gradients acting between the GBAS station and the aircraft on approach present a threat to users since the position errors caused by these gradients are not corrected through the application of the corrections. Thus, it is essential to monitor and exclude affected satellites to guarantee system integrity and safety. Several mitigation strategies are implemented in the current system, but they lead to a degradation of availability in areas with active ionosphere.
With the introduction of a second frequency (L5/E5a) usable for civil aviation and the development of multiple constellations (e.g. Galileo), new GBAS architectures and monitoring options arise and can be leveraged. One of the new GBAS architecture candidates, being developed primarily in the Single European Sky ATM Research (SESAR) program in Europe, proposes to use 100-second smoothed pseudoranges on the L1/E1 frequency to compute the position of the aircraft and the second frequency (L5/E5a) for monitoring purposes only. Forming an ionosphere-free (Ifree) combination of the dual-frequency measurements to compute the position, which removes the first order ionospheric error, is also one option. However, in this case the noise and multipath of both frequencies is combined, leading to a degraded nominal performance of the navigation solution. Therefore, a switch to the Ifree mode is triggered only when the performance of the single-frequency dual-constellation (SFDC) mode is not sufficient due to extremely high ionospheric activity. This first architecture is known as the GBAS Approach Service Type F (GAST F) [1,2]. Another option, known as GAST X and being developed primarily in the United States, proposes to send all relevant information to the aircraft and uses divergence-free positioning with a longer smoothing time constant . This solution also needs an ionospheric gradient monitor .
In previous work , we proposed a dual-frequency airborne ionospheric monitoring scheme to support the so-called GAST F architecture. The proposed monitor combines pseudorange corrections from two different frequencies transmitted from the ground station and compares these values with dual-frequency ionospheric delay estimations computed at the aircraft for each satellite. This allows individual satellites with large errors to be excluded without making conservative assumptions at the GBAS ground station. We designed a combined test statistic for multiple affected satellites simultaneously and proposed a threshold derived from operational requirements. This threshold, an adaptation of the one proposed in , is only dependent on the glide path angle transmitted by the ground station and has only been validated with very limited simulated data  and with real data in nominal conditions from static user receivers located a very close distance from the GBAS reference point.
In this paper, we revisit the threshold design and evaluate the performance of the monitor at different distances from the airport (i.e., from the touchdown point to the limits of the Precision Approach Region (PAR)). The performance of the monitor is compared using different thresholds: (i) a constant threshold derived from operational requirements, and (ii) a dynamic threshold that increases with distance to the airport in a similar way as the Alert Limits (ALs) increase. The objective of this study is to assess whether increasing the threshold when the aircraft is at distances further away than the Decision Height distance is more beneficial than maintaining a constant value. Increasing the threshold allows more ionospheric error within the position solution, but also enables the use of the SFDC mode without the need to switch to the Ifree solution for a longer period. A switch to the Ifree mode is irreversible for the remainder of the approach and, in general, worsens the performance because it combines the noise and multipath of two frequencies. Therefore, availability and continuity could be compromised in different cases (i.e., few satellites available, etc.) if these switches occur often and at close distances to the airport where the ALs are at their most stringent value. Furthermore, we compare the performance of these two modes (SFDC and Ifree) with the performance of the GAST X architecture, which uses the full ground pseudoranges and higher smoothing time constants.
First, we evaluate the performance of the monitor with the different thresholds and the performance of the different modes with simulated ionospheric gradients and a representative number of dual-constellation satellite geometries at different distances from the Tenerife Norte airport, Spain. Then, we perform these evaluations with real data collected by the Multipath Limiting Antennas (MLAs) installed at Tenerife Norte airport and a user receiver located at different distances from the airport.
Preliminary results show that with realistic noise and multipath conditions and simulated ionospheric gradients with vertical slopes below 300 mm/km the monitor proposed for GAST F is able to find a subset of satellites that provides sufficient performance at the required distances from the airport using a constant threshold derived from operational requirements. Results and conclusions for larger gradients will also be presented and discussed in the paper.
 ICAO Working Paper: NSP 5 WP 41 – DFMC GBAS Conceptual Framework – SESAR Joint Undertaking. 4.
 ICAO Working Paper: JWGs/8-WP/39rev1, “SESAR GAST F Architecture”.
 T. Murphy, M. Harris, G. McGraw, J. Wichgers, L. Lavik, M. Topland, M. Tuffaha, S. Saito, “Alternative Architecture for Dual Frequency Multi-Constellation GBAS”, Proceedings of the 34th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2021), St. Louis, Missouri, September 2021, pp. 1334-1374.
 T. Murphy, M. Harris, G. Balvedi, G. McGraw, J. Wichgers, L. Lavik, M. Topland, M. Tuffaha, S. Saito, "Ionospheric Gradient Monitoring for Dual Frequency Multi-Constellation GBAS," Proceedings of the 2022 International Technical Meeting of The Institute of Navigation, Long Beach, California, January 2022, pp. 1075-1097
 D. Gerbeth, M. Caamano, M.-S. Circiu, M. Felux, "Airborne Ionospheric Gradient Monitoring for Dual-Frequency GBAS," Proceedings of the 2022 International Technical Meeting of The Institute of Navigation, Long Beach, California, January 2022, pp. 1110-1122.
 M. Felux, M.-S. Circiu, J. Lee und F. Holzapfel, "Ionospheric gradient threat mitigation in future dual frequency GBAS,“ International Journal of Aerospace Engineering, 2017.