Towards Full GAST-D Capability – Flight Testing DLR’s Experimental GBAS station

Abstract

Towards full GAST-D capability - Flight testing DLR’s experimental GBAS-station Ground Based Augmentation Systems (GBAS) are expected to be able to replace ILS in the near future. Ground stations meeting CAT-I (GBAS Approach Service Type C) requirements are already available have reached System Design Approval by the FAA in 2009. In Norway so-called special CAT-I systems (similar to GBAS but limited to one manufacturer for ground stations and one for avionics) have been in use since 2007. However, these implementations cannot provide sufficient integrity in the presence of ionospheric anomalies and therefore cannot support operations below the CAT-I minima. The German Aerospace Center (DLR) implemented an experimental GBAS test bed with three TOPCON Net-G3 ground reference receivers at Braunschweig-Wolfsburg Research Airport. They are connected to multipath limiting choke ring antennas with a spacing of 740-770m between the sites. It was originally installed in 2009 and then operated and flight tested as a GAST C station. In the recent months we implemented the necessary changes in both, the ground and airborne subsystems to comply with the new requirements for GAST D, which is the service type intended to support autoland operations in CAT-II/III conditions. At the end of August 2011 first real-time flight trials with the enhanced systems were carried out. In order to provide guidance all the way down to touch-down and roll-out enhanced monitoring and an advanced way of processing the navigation data is necessary. In the case of GBAS approach service type (GAST) C it is the ground station’s responsibility to provide integrity parameters which bound the positioning error of the onboard systems with an acceptably low integrity risk. Therefore several parameters have to be inflated to bound errors in case of unusual ionospheric activity. These assumptions are thus far too conservative for most of the time when the system is operated. In the currently proposed GAST D (CAT-II/III) requirements the ultimate responsibility for detection and mitigation of ionospheric anomalies as well as determination of the actual system performance is shifted to the airborne system. Therefore, no inflation of integrity parameters to bound rare anomalies is necessary. Depending on aircraft performance, a suitable geometry can now be selected by the airborne equipment to ensure safe operations. This allows assessing safety of the whole operation and does not only consider the navigational aspect. Additionally, further low-level requirements for mitigation of ionospheric threats in the ground as well as the airborne system have been introduced. The ground subsystem now provides pseudo range corrections from 30 seconds smoothed measurements in addition to the previously used 100 second corrections. Furthermore, Code-Carrier-Divergence (CCD) monitoring and B-value monitoring have been introduced and an absolute gradient monitor for detection of stationary iono fronts is necessary. On board the aircraft two position solutions are now calculated in parallel. One is based on 30 seconds and the second one on 100 seconds smoothed pseudoranges with the respective corrections from the ground station. DSIGMA compares both position solutions and thus is able to detect moving ionospheric fronts. Several further monitoring schemes like Code-Carrier-Divergence and other checks like Bias Approach Monitoring have to be performed. In preparation for the GAST D upgrade data from previous flight test have been post processed to determine necessary parameters, such as the Ground Accuracy Designator and the sigma_pr_gnd for the additional 30s filter constant. In the current flight trials we flew a total of 9 approaches on two days at the end of August 2011 with DLR’s VFW-614 research aircraft. The onboard receiver was as well a TOPCON Net-G3 which was operating at a rate of 5Hz. On the first day navigation data was only monitored and recorded during the flight while on the second day the GAST D navigation solution was used as input for aircraft guidance and coupled to the autopilot. Approaches were flown automatically down to 500ft above ground. Due to runway constraints no automatic landings were possible yet. To evaluate performance of the GBAS based navigation we postprocessed a dual frequency carrier phase trajectory in a combined forward and reverse solution as a truth reference. We will present the relevant parameters from the flight trials, putting special emphasis on the GAST D specific aspects. These mainly include the outputs of the CCD monitors, DSIGMA outputs and geometry screening aspects. In the effort to ensure safe guidance for automatic landings the total aircraft performance with respect to success criteria for a safe landing has to be evaluated. Part of this evaluation is determination of the aircraft’s flight technical error (FTE) when guided by GBAS navigation. When added to the navigation system error (NSE) the total system error can be determined.