Mihaela-Simona Circiu

Nominal Performance of Future Dual Frequency Dual Constellation GBAS

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 Gradient Threat Mitigation in Future Dual Frequency GBAS

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.

Multi-constellation GB AS: How to benefit from a second constellation

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.

Ionospheric monitoring in a dual frequency GBAS

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.