Flight Testing GBAS for UAV Operations
Michael Felux, Sophie Jochems, Philipp Schnüriger, Valentin Fischer, Patrik Steiner, Zurich University of Applied Sciences, Center for Aviation; Michael Jäger, Luciano Sarperi, Zurich University of Applied Sciences, Institute of Signal Processing and Wireless Communications; Natali Caccioppoli, Eurocontrol
The Ground Based Augmentation System (GBAS) is a precision approach and landing system which provides corrections for GPS navigation signals to airborne users. The correction and integrity parameters are transmitted via a VHF data broadcast. The broadcast is freely accessible and can therefore be received and decoded by virtually everyone. A GBAS ground station is located within the security perimeter of an airport, uses high-quality equipment that allows to keep errors to a minimum and furthermore, provides guaranteed levels of performance by specification and continuously self-monitors the integrity of the information provided.
This augmentation system, by design only serves a very specific purpose: in civil aviation, aircraft equipped with the appropriate avionics can use the provided information to correct GPS signals, ensure that all integrity requirements are met, and use the information regarding the approach geometry in the GBAS message to fly precision approaches. However, GBAS to date is not very widely used because i) not many airports are equipped with a GBAS ground station and ii) depending on the airport, not much of the fleet operating in and out is equipped with the GBAS landing system GLS. The reason for this is a classic chicken/egg problem: On the one hand, the Air Navigation Service Provider (ANSP), or in some cases also the airport operator, is responsible for the installation, operation and maintenance of a GBAS ground station. As this is rather expensive, not many ANSPs and/or airports are willing to install such a ground station without a solid business case. Especially because often only a few aircraft operating at this airport are actually suitably equipped to fly a GLS approach, it is currently still very difficult to generate operational benefits from a GBAS. On the other hand, there are the aircraft operators. For them, it is also financially not very rewarding to equip their aircraft with the required receiver as there may be only a few destinations within their network where flying a GLS approach is actually possible and might bring operational benefits.
In previous work, we analyzed the performance of the envisioned Differentially Corrected Positioning Service (DCPS) for GBAS, a service type that enables other use-cases than precision approach guidance of large aircraft. For that purpose, GBAS messages transmitted by the GBAS station at Zurich airport (ZRH) were received and decoded. The protection levels for the GBAS DCPS were then calculated and compared to the SBAS protection levels. The results have shown that the use of GBAS could provide operational benefits when staying within a radius of around 57 km from the airport. Above a distance of 57 km away from the airport, the GBAS protection levels are larger than the SBAS protection levels and therefore no longer provide any advantage over the use of SBAS.
In this paper we continue our previous work and focus on the use of GBAS corrections in the emerging field of UAV navigation. One main challenge is the limited possibility to receive the VHF-based transmission of the corrections and parameters from a GBAS at the low flight altitudes typical for this kind of operations. The concept explored in this work is therefore the reception and decoding of the GBAS messages at a location close to the airport. This is accomplished by a software-defined radio receiver set up near the airport of Zurich with its operational GBAS station where good reception is ensured. The corrections are then forwarded via the cell phone network to the UAV on which a GNSS receiver capable of receiving and applying the received corrections is mounted. The flights are taking place in Winterthur, approximately 15 km away from Zurich airport. The paper then evaluates the accuracy and protection levels achievable by applying the GBAS corrections and integrity parameters in the envisaged GBAS positioning service and compares them to the SBAS protection levels at the receiver.
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