Optimized Low Level Trajectories for Instrument Flight Rules in Alpine Areas

Roxane Pott, Sébastien Guillaume, Alain Geiger, Heinz Wipf

Abstract:	In the framework of establishing ICAO’s Performance Based Navigation (PBN) in Switzerland a number of special issues have been identified, especially for mountainous regions. The GNSS performance on aerial vehicles operating is of central importance for flight safety and operation efficiency, because the rugged topography might easily jeopardize the performance. Especially low-level operations in alpine areas need careful assessment of the protection levels and the GNSS performance. For the near future Switzerland’s lower air space will primarily be operated based on PBN. This paper focuses onto the implementation of Instrument Flight Rules (IFR) predefined tracks for low-level transfer flights of helicopters through mountainous valleys. One of the main stakeholders of this ongoing research are Helicopter Emergency Medical Services (HEMS), which have to transfer patients in medical emergency from secondary hospitals in the Alps to primary ones in urban centers. Another domain of application is the military helicopter air transportation e.g. for disaster relief. Theoretical studies were conducted to assess the feasibility from the point of view of the GNSS performance. We present a mathematical model, which allows assessing GNSS performance avoiding tedious simulations or expensive in-flight measurements. The use of continuous satellite distributions rather than discrete satellite positions leads to closed formulae describing the impact of erroneous or obstructed measurements. It becomes possible to quantify in a generalized manner the decrease of position’s accuracy caused by descending into a valley. Additionally an optimization algorithm was developed to automatically create the IFR low-level trajectory through a given valley. Different optimization parameters were taken into account such as height, which has to be low enough to avoid icing on the rotor blades. At the same time, the trajectory shall be constrained by an upper bound of collision probability with terrain and obstacles. These constraints of utmost importance asked for a rigorous probabilistic calculation. Not only the position's uncertainty is taken into consideration, but also the anticipated ground/obstacle collision probabilities, given the aircraft speed. Traditionally, instrument flight procedures are developed by applying instructions from ICAO’s Required Navigation Performance (RNP) documentations. This leads to geometrically rigid primary and secondary protection surfaces, a most conservative design. In consequence, for a flight path following a valley, the established RNP 0.3 designing procedure results in a non-optimal trajectory above the surrounding mountain peaks, prone for waste of time and fuel while running the chance of collecting ice. The paper, presents how probabilistic considerations enable the design of procedures without rigid protection areas. Currently, no method exists, which uses terrain and obstacle collision probabilities together with aircraft speed to devise an optimized flight path. The computation of a field of collision probabilities is the salient point of this paper. It is the basis of the trajectory optimization process, which takes into account the maximum tolerable terrain and obstacle collision probabilities, and creates the lowest and fastest flight path possible. The method is flexible when it comes to optimization parameters like smoothness (of importance for HEMS flights) and the terrain collision probabilities. A path, resulting from this optimization process, was entered into a full flight helicopter simulator. First simulator test flights proved the practicability of the results. The presented theoretical method gives a generic insight into error propagation within a satellite navigation system. Further, the development of optimization methods enables an automated process, and eliminates the need to delineate protection surfaces and allows for optimally adapting trajectories. The presentation will cover the description of the methods and will show first results in the Swiss Alps.
Published in:	Proceedings of the 32nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2019) September 16 - 20, 2019 Hyatt Regency Miami Miami, Florida
Pages:	1464 - 1487
Cite this article:	Pott, Roxane, Guillaume, Sébastien, Geiger, Alain, Wipf, Heinz, "Optimized Low Level Trajectories for Instrument Flight Rules in Alpine Areas," Proceedings of the 32nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2019), Miami, Florida, September 2019, pp. 1464-1487. https://doi.org/10.33012/2019.16909
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