GBAS Approach Guidance Performance – A comparison to ILS

Abstract

In this paper we show the results of flight testing the proposed GBAS Approach Service Type D navigation - the service type intended to support autoland operations - and compare its guidance capabilities to the performance of an ILS. The evaluations shown here are based on a flight test campaign carried out in 2011 and 2012. An experimental GBAS ground station consisting of three reference receivers was installed by DLR at the research airport in Braunschweig-Wolfsburg in northern Germany. Data processing in this station complies with the requirements to support the most stringent requirements for approaches and landings under CAT-II/III weather conditions. GPS pseudorange corrections and integrity parameters were calculated and sent to our research aircraft. We then processed the received navigation data from the experimental onboard GPS and applied the corrections and integrity parameters from the ground station. The obtained position was used to calculate ILS-like deviations which were displayed to the pilot. The base aircraft's measurement data (including actual ILS deviations) are available in our test aircraft. To evaluate system performance we created a truth reference trajectory from a post processed dual frequency carrier phase solution and compared the GBAS outputs and the received ILS signals to this reference. This setup allowed us to evaluate quality of the signals provided and compare performance of two systems under nominal conditions and with a representative aircraft. Results show that GBAS is well suited to support precision approach operations under such conditions. The experimental system was performing well within all limits and specifications and showed full availability at all times during the flight. While ILS errors reached up to about 8 meters for the glide slope and about 12 meters for the localizer signals, GBAS errors remained below one meter laterally and vertically at all times and provided a significantly less noisy guidance signal.