Approach and Landing with the GBAS Landing System GLS

Presently, precision approaches of aircraft to airports with vertical and lateral guidance are only conducted as straight in approaches with a single glide path, mostly using VHF radio navigation technology. Here, the final approach track is usually several kilometers long (~10 nautical miles) and cannot be changed due to the linear propagation of radio signals. Using the technology of augmented Global Satellite Navigation Systems (GNSS) this final approach path can be made more flexible with several curved and straight segments with different vertical profiles in order to enable a safe descent with vertical guidance also in mountainous terrain, around obstacles or for noise abatement. Currently, a special type of laterally guided curved approaches is already implemented in very few places. This type of approach procedure is called Required Navigation Performance Authorization Required and is usually only available for specially approved operators. Here, the lateral guidance during the approach is usually based on a position solution provided by satellite navigation systems in conjunction with inertial navigation systems and the vertical guidance is still based on measurements of a barometric altimeter. This type of approach with vertical guidance is supposed to enhance the situational awareness for pilots compared to non-precision approach procedures (no vertical guidance) and reduce the risk for controlled flight into terrain. However, the accuracy of the barometric vertical guidance is inferior compared to precision approach procedures, temperature dependent and requires the correct pressure setting for each approach in the aircraft. Therefore, novel curved approach procedures should rely solely on satellite navigation augmented by either a ground based or satellite based system and only optionally by inertial measurement systems. When augmentation systems provide additional accuracy and integrity improvements, a vertical component of the position can be computed. Together with a predefined path, this highly accurate position solution can enable curved approaches with a vertical glide path guidance down to the required minimum obstacle clearance (~200ft above ground). Using a Ground Based Augmentation System (GBAS) at the airport, one or more Terminal Area Paths (TAP) could be broadcast to provide precision guidance to aircraft maneuvering in the terminal area. Such a use of the system would allow the provision of multiple curved approaches to multiple runway ends with only one ground installation and as such save cost to the airport and airline operator whilst providing the added benefit of noise abatement and enhanced obstacle clearance. The transmission of multiple curved or segmented steep approaches enables approaching aircraft to select the approach path most suitable for the given situation. An experimental GBAS station was installed at the research airport in Braunschweig. Data processing complied with the currently proposed requirements to support automatic landings. Corrections for GPS measurements and integrity parameters were sent to a research aircraft which was equipped with an experimental GPS receiver providing raw measurement data. The received data and measurements were then processed on board in real-time and provide approach guidance information to the experimental pilot in form of a flight director indication. To evaluate system performance the authors create a truth reference track from a post processed carrier phase solution. Finally, the GBAS outputs and the received ILS signals are compared to the truth reference. The system performed well within all specifications and showed full availability at all times during the flight. Compared to ILS, GBAS is significantly more precise and shows almost no noise