Gate-to-Gate with Modernized GPS, GALILEO and GBAS – Harmonization of Precision Approach Performance Requirements

Satellite Navigation has become increasingly important in the optimization of the efficiency and safety within the aviation industry. ANASTASIA (Airborne New and Advanced Satellite techniques and Technologies in A System Integrated Approach) is a European Commission project within the Sixth Framework Program, with the basic objectives to define and implement future (beyond 2010) communication and navigation avionics based on satellite services, exploiting the multi-constellation and multi-frequency architectures in combination with multiple onboard sensors, to provide a worldwide gate-togate service. Included in the objectives are the preliminary development of advanced airborne systems for flight trial evaluation and the dissemination of results for standardisation activities. Studies have shown that stand-alone Global Navigation Satellite Systems (GNSS GPS and GALILEO) or stand-alone GNSS augmented by Space Based Augmentation Systems (SBAS) cannot satisfy the demanding performance requirements of Category-II/III precision approaches or of surface movement. To satisfy these requirements, Ground Based Augmentation Systems (GBAS) are needed. To date, performance requirements have only been firmly established for the various phases of flight up to Category-I precision approach. Two methods have been used to derive the performance requirements for Category-II and III precision approaches: the "ILS (Instrument Landing System) Look-Alike Method" and the "Autoland Method". The "ILS Look-Alike Method" is based upon the concept of matching system performance at the Navigation System Error (NSE) level through linearization of the ILS performance specifications at a given height. The "Autoland Method" is based on the idea of evaluating the required performance to protect the safety of the landing operation, rather than by extrapolating the equivalent NSE performance from existing ILS specifications. Both methods lead to significant discrepancies in the performance requirements. This paper analyses each method, and identifies key differences. Potential solutions to harmonize the performance requirements obtained from these two methods are proposed. INTRODUCTION Defining the performance requirements for a particular phase of flight is a key element of operational safety. It is a pre-requisite to determining whether a given navigation system is suitable for this particular phase of flight. Defining these requirements for precision approach phases is the foundation of research in ANASTASIA (www.anastasia-fp6.org). Originally, navigation capability was associated with the mandatory carriage and use of specific navigation equipment. More recently, the International Civil Aviation Organization (ICAO) developed the Performance Based Navigation (PBN) concept. PBN specifies system performance in terms of accuracy, integrity, continuity, availability (the parameters used depend on whether RNP or RNAV is specified) and functionality required for the proposed operation in the context of a particular airspace concept. Table 1 shows the latest values of the required navigation system performance for precision approaches. While the Signal-In-Space (SIS) performance requirements for Category-I approaches are well established, those for Category-II and III approaches have been under debate by the two main regulatory agencies – the EURopean Organization for Civil Aviation Equipment (EUROCAE – EU) and the Radio Technical Commission for Aeronautics (RTCA – USA) for several years. Two separate methods were used in the derivation of these requirements, with significantly different results, as shown in Table 1. (Note, as an example for Cat-IIIb accuracy requirements, 6.2 m was derived by the RTCA and 3.6 m by EUROCAE). Early attempts by the Federal Aviation Administration (FAA) in the USA and the International Civil Aviation Organization (ICAO) All Weather Operations Panel (AWOP) to develop requirements for GNSS to support Category-II and III operations were based on the concept of matching system operational performance at the Navigation System Error (NSE) level through linearization of the ILS performance specifications (errors) at a given height. This resulted in the so-called ILS look-alike approach that was originally used to define the performance requirements for Category-I approaches and was adopted by EUROCAE to derive the allowed Navigation System Error (NSE) and the Alert Limits (AL) for Category-II and III approaches. The synthetic model in the ILS Collision Risk Model (CRM) was used to validate these results [2]. The RTCA adopted the performance requirements derived from the touchdownrequirements laid out in [3, 4], defining the maximum probabilities with which an aircraft is allowed to land outside the touchdown box. This so-called Autoland method aims at deriving performance requirements for GBAS equivalent to ILS in terms of operational safety. This paper attempts to reconcile the two approaches and to explain any differences between them, with emphasis on the vertical performance requirements as these constitute the requirements that will ultimately determine the GBAS architecture that will be needed to satisfy Category-III approaches and landings. ILS LOOK-ALIKE METHOD The Instrument Landing System (ILS) uses two carriers (one for the localizer and one for the glide-slope) each of which is amplitude modulated by two tones. The depth of modulation (DM) of these two tones is a function of the angle of displacement from the centreline. The ILS aircraft receiver measures the difference in DM (DDM) between the two tones to compute the angular position. Any change in the transmission of the ILS signal that causes a change in the DDM at a given angle with respect to the nominal (expected) DDM at that angle (either due to transmitter or receiver failures) contributes towards the navigation system error (NSE). The various sources of error that can be identified from [5, 6] are: • course alignment (variation in the mean ILS course line from the intended geometric approach centreline) • beam bends (causing the ILS course line to fluctuate around the mean ILS course line) • angular displacement sensitivity, corresponding to variations in the rate of change of DDM (as picked up by the aircraft receiver) • the receiver centring error and • various other sources of error such as the polarization of the carrier, receiver displacement sensitivity and receiver displacement linearity as well as noise as a result of RF interference and power supply interference. The rationale for using the ILS performance specifications in the derivation of the GBAS performance requirements is the validation of ILS through many years of operational service, GBAS being intended to meet the same operational requirements as, with equivalent performance to, ILS. Accuracy – Navigation System Error (NSE) The errors in [5, 6] are expressed in terms of angles and were conservatively assumed to be given as 3-sigma values in [2]. Various assumptions about the geometry of the runway and location of the localizer and glide-slope transmitters are made to convert the angular errors into Accuracy Integrity Continuity Phase of Flight SIS (2σ) Alert Limits Integrity Risk TTA Continuity Risk Availability Cat-I 16 m (L) 4 m (V) 40 m (L) 10 m (V) 2E-7/150 s 6 s 8E-6/15 s 0.99 – 0.99999 Cat-II 6.9/6.1 m (L) 2.0/1.4 m (V) 17.3/17.9 m (L) 5.3/4.4 m (V) 1E-9/15 s 2 s 4E-6/15 s 0.99 – 0.99999 Cat-IIIa 6.2/3.6 m (L) 2.0/1.0 m (V) 15.5/10.4 m (L) 10.0/2.6 m (V) 1E-9/15 s 2 s 4E-6/15 s 0.99 – 0.99999 Cat-IIIb 6.2/3.6 m (L) 2.0/1.0 m (V) 15.5/10.4 m (L) 10.0/2.6 m (V) 1E-9/30 s (L) 1E-9/15 s (V) 2 s 2E-6/30 s (L) 2E-6/15 s (V) 0.99 – 0.99999 Table 1: Signal-in-Space Performance Requirements for the various phases of aircraft operation [1] linear errors at a given distance from the landing threshold (and therefore at a given height above the threshold – assuming a nominal glide-path angle of 3 degrees). Figure 1 illustrates these considerations.