Airport Surface Movement – Performance Requirements & Navigation Algorithms

This paper derives the navigation system performance requirements for the guidance function under zero visibility conditions of an Advanced Surface Movement Guidance and Control System (ASMGCS), taking as basis the airport design requirements. The stringency of these requirements suggests that code-based Global Navigation Satellite Systems (GNSS) in stand-alone mode or augmented by Spaceor Ground-Based Augmentation Systems (SBAS, GBAS) may not be suitable for airport surface navigation, and that multi-frequency carrier-phase Real-Time-Kinematic (RTK) techniques are required. The second part of this paper gives an overview of, and characterises, the performance of novel code-based and carrier-phase based algorithms developed at Imperial College London, exploiting the redundancy and improvement in geometry as well as the multiple frequencies of a combined GPS and GALILEO constellation, to improve the integrity, reliability and availability of airport surface movement. INTRODUCTION As a result of the rapid increase in air travel, satellite navigation has become increasingly important to assure efficiency and safety within the aviation industry. Phase-smoothed code-based airborne GNSS receivers are already being used to support aircraft navigation for en-route phases of flight, and GBAS-based navigation system architectures for the more stringent phases of flight (CAT III landings) are currently under development. In order to realise the gate-to-gate concept with GNSS, it would be of interest if such a GBAS architecture would also be able to support surface movement and be integrated into any future ASMGCS architecture, to support the following four primary functions [3]: Surveillance: Captures the information on aircraft, vehicles and objects within the coverage area and under specified operational conditions, and updates data needed for guidance and control. Routing: Provides assignment of a route to individual aircraft and vehicles, which provide safe and efficient movement for its current position to its intended final position. Guidance: Provides guidance necessary for movements through clear and continuous indications allowing pilots and vehicle drivers to maintain their positions on intended routes and for situational awareness. Control: Provides a safe and efficient means of managing movements and planning for required movements, detects conflicts/incursions and provides solutions. In order to support all of these functions, the navigation system must be able to support the most stringent navigation performance requirements in terms of accuracy, integrity and continuity. Based upon airport design requirements, mandated by ICAO, and operational considerations, and relating these to the allowable total system error (TSE), the first part of this paper derives the navigation system performance requirements for the guidance function for each of the commercial airport categories. Making general assumptions about the path definition error and the path steering error of the aircraft, the navigation system error (accuracy) requirements are derived. The paper then derives the integrity and continuity requirements based upon target levels of safety (TLS) established by the ICAO. The results are compared to existing requirements developed by the RTCA and ICAO. Based upon these findings, both code-based and carrier-phase based multi-frequency positioning and integrity algorithms are developed to test their suitability for surface movement. Carrier-phase based methods under static conditions have already shown the capability to determine positions with centimetre or even millimetre accuracy. Potential has also been demonstrated for kinematic applications, although the reliability and availability of ambiguity resolution are currently not sufficient for safety critical applications. Amongst the challenges for airport surface movement is the proximity of the aircraft to buildings and other aircraft, resulting in potentially significant variations in multipath and sudden obstructions of the satellites in view, hence affecting the on-the-fly integer ambiguity resolution and validation. A further challenge is the accurate modelling of airframe-induced multipath. The second part of this paper gives an overview of a software platform with novel positioning and integrity monitoring algorithms, which exploit the redundancy and improvement in geometry as well as the multiple frequencies of a combined GPS and GALILEO constellation to improve the integrity, reliability and availability of both code-based and RTK algorithms for airport surface movement. This is followed by a limited characterisation of the initial performance of these algorithms. AIRCRAFT AND AIRPORT CATEGORIES Performance requirements are dependent upon the airport and aircraft categories. In Table 1 categories of airport used by commercial aircraft are shown. Whilst different conventions are used by the RTCA and ICAO to designate each of these categories, for the sake of clarity, the ICAO coding is used in this paper. Each category of airport can accommodate aircraft up to the specified under-carriage widths and wing spans. This means for example that an airport of CAT-E can accommodate aircraft of CATC, D and E (but not F). The most stringent surface movement navigation performance requirements are discussed for each airport category and are derived based upon the airport design requirements, discussed in the next section. Aircraft/Airport Category Examples Under-carriage width (UCW) Wing Span (WS) Commercial Aircraft C A320 B737 6 9 m 24 36 m D A300, A350 B757, B767-200 9 – 14 m 36 52 m E A330, A340 B747, B767-400, B777-200 9 – 14 m 52 65 m F A380, B777-300 14 – 16 m 65 80 m Table 1: Airport/Aircraft Categories [1] AIRPORT DESIGN REQUIREMENTS Surface movement consists of a number of phases, including rapid exit from the runway, high speed taxiing, slow taxiing and movement in the gate area. Each of these phases is associated with a given travel surface, and the assumption is made that the geometric design requirements of each of these surfaces is compliant with specifications in [2]. Travel surfaces are divided into the following categories: Taxiways – high-speed taxiing: exit (80 kts), normal (50 kts) and curved (20 kts) Taxilanes – slow taxiing (10 kts) and Gate Areas – stands (10 kts). In order to be able to derive the navigation system performance requirements, a prerequisite is the accurate knowledge of the airport design requirements for each of the travel surfaces. These requirements are expressed in terms of factors such as the dimensions of the travel surface in relation to the dimensions of the aircraft, the distance to the nearest obstacles, as well as the speed of taxiing. In line with previous studies [3, 4], these requirements are analysed for each of the three main travel surfaces. TAXIWAYS: In order to guarantee the safety of surface movement on the taxiways, a minimum travel surface clearance – TSC (see Figure 1 ) is mandated [1, 3]. Figure 1: Travel surface parameters This TSC together with the maximum undercarriage width (UW) determine the minimum travel surface width TSW (note that this width does not include the wingspan of the aircraft):