Ionospheric Gradient Threat Mitigation in Future Dual Frequency GBAS

Abstract

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.

Ground-Based Augmentation Systems Operation in Low Latitudes - Part 1: Challenges, Mitigations, and Future Prospects

“…As a conclusion for ionosphere-free observables in position solutions, Felux et al (2017a) suggest using the ionosphere-free mode solely for monitoring gradient threats while calculating position solutions using the single-frequency approach with two GNSS constellations, if available. Extending this concept, Felux et al (2017b) present a method in which gradients are monitored by having each aircraft comparing ionospheric delay estimates from the ground station with those that it estimates. The authors argue that this approach to sharing the responsibility of monitoring threatening gradients between the ground facility and the airborne subsystem is less conservative and more realistic.…”

“…Currently, there is no space in the VDB messages to transmit ionospheric delays estimated by the ground station. For this reason, in the method proposed by Felux et al (2017b), which is improved and tested by Felux et al (2019), the test statistic for ionospheric gradient monitoring uses the pseudorange corrections broadcast for each frequency. In this way, ionospheric delays affecting both the ground and the aircraft are estimated by the airborne subsystem using a quantity called pseudo ionospheric delay.…”

Airborne Ionospheric Gradient Monitoring for Dual-Frequency GBAS

“…In previous work Felux et al developed in [5] and validated in [6] a method to mitigate the threat of ionospheric gradients for singlefrequency positioning by using the second available frequency for monitoring purposes. One of the main design drivers for new GBAS service types was to maintain backwards compatibility and use the existing VHF Data Broadcast (VDB) with its limited capacity [7].…”

Markov Chain‐Based Stochastic Modeling of Deep Signal Fading: Availability Assessment of Dual‐Frequency GNSS‐Based Aviation Under Ionospheric Scintillation

“…Both of these frequencies are within aeronautical radio navigation service (ARNS) frequency bands, which is intended for civil aviation operations (FAA, 2010). Following existing single‐frequency GPS L1‐based aviation systems, the development of dual‐frequency GPS L1/L5‐based systems for aviation operations is ongoing (Felux et al., 2017; Gerbeth et al., 2016; Walter et al., 2012). Furthermore, the worldwide development of GNSS constellations, including GLONASS, Galileo, and BeiDou is in progress to provide a sufficient number of satellites that transmit dual‐frequency signals in both L1/E1 and L5/E5 frequency bands for civil aviation users (ICAO, 2018).…”