The GLASS project aims to achieve an SBAS-based extension of GLS approaches.The objective is a cost-effective system to enable satellite-based approaches with vertical guidance and automated landings in freely definable locations.
At present, automated landings can only be carried out with precision guidance systems such as the Instrument Landing System ILS, Microwave Landing System MLS, or the GBAS (Ground-Based Augmentation System) landing system GLS. What all of these systems have in common is that the guidance signals are routed to the autopilot directly from the receiving device. The autopilot then takes over control of the aircraft during landing. The receivers for these three systems are often combined in a multimode receiver or MMR.
Navigation using satellite signals is based on signal propagation time measurements from the satellite to the receiver, knowledge of the satellite position, and subsequent triangulation. However, due to atmospheric interference and noise in the horizontal direction this is only possible with an accuracy of several metres. Position resolution in the vertical direction is even more imprecise due to the absence of signals originating below the receiver.
For ground-based GLS, corrections for the signals from the individual satellites are transmitted from a ground station via VHF. These corrections can be used within a radius of around 50 km from the ground station. On board the aircraft they are applied to the propagation time measurements received. A highly accurate position is then calculated based on the corrected measurements, the accuracy of which is also adequate in the vertical direction, in order to enable aircraft guidance in three dimensions. The ground station further transmits integrity information which guarantees the reliability of the correction signals. Finally, the GBAS station also transmits approach information such as runway threshold coordinates, landing direction and approach angle in what is known as the final approach segment (FAS) data block for each approach.
From the FAS data block the MMR is able to calculate angular deviations from the target path and transmits these directly to the autopilot. With a satellite-based augmentation system (SBAS) such as EGNOS, correction signals and integrity information are sent to the user in a similar way. Unlike with GLS, the data are transmitted via the datalink of a geo-stationary satellite and are valid for a larger area.
Set-up and function of the GBAS (Ground Based Augmentation Systems) landing system GLS
Set-up and function of the GLASS Systems (GLS approaches based on SBAS)
The FAS data blocks however are stored in the database of the flight management system (FMS) on board the aircraft. The angular deviations here are calculated by the FMS and forwarded to the autopilot via the FMS. Correction information and FAS data are nevertheless largely identical in both systems (GLS and SBAS). Automated landings can only be carried out by aircraft with the appropriate equipment and with the assistance of a precision landing system such as ILS, MLS or GLS. However, these current systems do not allow automated landing with guidance by the FMS as detailed database integrity values would have to be guaranteed and observed.
A complementary examination would also need to be made for the Flight Management Computer (FMC). This would mean that the autolanding system would have to be recertified from beginning to end. This is a costly process.
Combining augmentation systems
The GLASS system is intended to bring together the advantages of both augmentation systems. It combines an SBAS-capable GNSS receiver with a database and a GLS-compatible data link. The correction and integrity data received from the SBAS satellite are automatically translated into GLS-compatible structures and sent to the multi-mode receiver using the FAS data block.
This receiver can now send deviations directly to the autopilot making automated landings possible. The device can be installed on the ground as well as in the aircraft. An extended version generates ad-hoc FAS data blocks via a user interface by inputting basic data such as approach trajectory and glide angle, enabling faster set-up and commissioning. This would make mobile use possible for setting up a landing system. A cost-effective system could thus be developed with which smaller airfields would also be in a position to offer landing systems for automated landings.