Enabling LPV for GLS equipped aircraft using an airborne SBAS to GBAS converter

Abstract

We built a prototype system intended to combine the advantages of both the ground based and the satellite based augmentation systems (GBAS, 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. 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 by even 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. 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 implemented the system in C++ on a 64 bit Linux PC and connected this PC to a Telerad VDB Transmitter. The necessary correction data was obtained from a Septentrio AsteRx3 GNSS receiver. We used the standard final approach segment (FAS) data blocks as published for Braunschweig-Wolfsburg Airport (ICAO Identifier EDVE) in the German AIP for runway 08 and runway 26. We ran ground bench tests, using customized software to calculate SBAS and GBAS Positions and Protections Levels using standard algorithms published in DO253D and DO229D. Data collected during one week starting on 24 October 2018 shows availability for the standard EGNOS approach service of 99.96% and a reduction to 92.4% availability using the converter system with the inflated protection levels. If it were possible to increase the VAL to the SBAS standard of 50m, the availability of the system would increase to 99.95The standards for data transmission in a GBAS system only allow a maximum value of 25.4m to be entered as a Final Approach Segment Vertical Alert Limit (FASVAL) as opposed to the SBAS LPV approach service FASVAL of 50 m. However, GBAS final approach segment alert limits are scaled with distance from the glide path intercept point (GPIP), which is typically about 1000ft upwind of the landing threshold. This may lead to an availability and continuity issue which has to be further evaluated. One possible solution is to find an acceptable height/distance combination, below which alert levels are more stringent than LPV and above which they are more relaxed. Setting the FASVAL to its maximum, this would occur at approximately 320m above aerodrome level without the penalty in availability being greater than discussed above. The GLASS system provides the LPV final approach segment to GLS-only equipped aircraft such as the 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. Moreover, the system could 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.