Flight Testing GLS Approaches using SBAS with the DLR A320 Advanced Technology Research Aircraft

Abstract

We designed and built a system intended to combine the advantages of both the ground based and the satellite based augmentation systems (GBAS, SBAS) by using a converter between them. We installed a prototype system at Salzburg Airport and flight tested it on 12th of February 2020 using DLR's A320 test aircraft equipped with flight test instrumentation. Using our system, 3D GLS type approaches are possible at any airport within the coverage of the SBAS. The system includes an SBAS-capable global navigation satellite systems receiver with a database and a GBAS-compatible data link. The correction and integrity data received from the SBAS satellite are automatically translated into GBAS compatible structures and sent to the airborne GBAS receiver using the final approach segment data block Without SBAS the system can revert to differential GPS. In both GBAS and SBAS, instant integrity information is provided by estimating protection levels, a high probability bound for the computed position. This is then compared to the alert limit of the respective system. Since both systems are quite similar, and the SBAS signal can nowadays be decoded even by low cost receivers, one can receive the augmentation data from the SBAS, slightly modify it to fit into the GBAS data structure and broadcast this data to a GBAS equipped aircraft. Said aircraft could execute a RNP approach with the Localizer Performance and Vertical guidance (LPV) final approach segment which would otherwise not be available. This may come especially handy in places where no non-precision minima are published, such as the RNP-E approach into Innsbruck and Salzburg. Since there are slight differences between the two systems, we made sure that integrity for the safety-of-life approach service is ensured. We named the system GLASS (GLS Approaches using SbaS), built a prototype and tested it with real GBAS avionics hardware. We performed 4 approaches to Salzburg Airport in Austria (LOWS). Salzburg is now equipped with a RNP approach using LPV only to runway 15 with significantly a lower minimum than the RNP approach with LNAV minimum called the RNP E 15. This is due to the location of the new missed approach point. Using the GLASS system, all GLS equipped aircraft would be able to take advantage of this new minimum line. We followed the approach track using FMS guidance and recorded the ARINC 429 output from the Collins GLU925 Multimode Receiver (MMR). The GLASS guidance was provided to the pilot on the electronic flight bag display for reference. We show a complete analysis of integrity data, MMR status information and MMR output guidance. We compare the GLS data from the MMR with standard SBAS data from an onboard Septentrio PolaRx3 receiver. The GLASS system provides the LPV final approach segment to GLS-only equipped aircraft such as the Boeing 737-800. This can enable increased access to airports that are currently not equipped with an xLS type approach such as Innsbruck (LOWI). Especially approaches in France could be of interest, since the government has officially declared to decommission all category I ILS installations in favor of RNP approaches with LPV. The system could also be carried on the airborne side rather than be a fixed installation on the ground. With a pilot selectable FAS block, it could enable LPV approaches without modifications to existing airborne hardware. Thus, any GLS capable aircraft could fly LPV approaches without requiring ground infrastructure modifications. In this case, the protection level scaling from GBAS is not an issue, since it can be compensated for by the GLASS system.