Precise aeronautical ground based navigation using LDACS1

Kurzfassung

In order to cope with the expected growth of future air traffic, the entire navigation and communication structure employed in the sector of civil aviation is currently undergoing major changes. In the US and Europa this modernization of the air-traffic management (ATM) system is mainly conducted within the NextGen and SESAR projects. The modernization of the current communication infrastructure is especially important since most of the current, largely analog, systems have already reached the limits of their capacity. There is a common understanding within ICAO that a single data link technology is not capable of covering the communication needs for all phases of flight. As a result, the Future Communications Infrastructure (FCI) has been proposed [1], which comprises a set of data link technologies for aeronautical communications. Currently, a new digital aeronautical communication system in the L-band for air-ground communications, termed LDACS1, is being actively investigated. The LDACS1 system is largely based on the 4th generation cellular mobile communication technology and employs Orthogonal Frequency-Division Multiplexing (OFDM) transmission [2]. It is primarily designed to provide a reliable communication between the aircraft and the air traffic controllers. Its main advantages over the current analogue system is its vastly increased capacity, scalability, and efficiency. The modernized ATM system will also incorporate new navigational aids. Specifically, aircrafts will increasingly rely on the satellite-based navigation systems (GNSS) in all stages of the flight. GNSS-based operations, accompanied by the ground or satellite based augmentation system (G/SBAS), will eventually allow for a quality of navigation that satisfies the high standards of civil aviation. Nevertheless the deployment of GNSS also brings new challenges concerning the integrity, continuity and availability of the navigational information. Due to the low received power levels of the GNSS signals, this system is susceptible to either intentional or unintentional interferences. This necessitates the deployment of a parallel 'backup' navigational infrastructure, referred to as alternative positioning, navigation and timing (APNT),, that can be used when GNSS services are temporary unavailable. Currently different solutions as backup system for GNSS navigation are investigated. Most proposals are based upon extending the currently deployed distance measurement equipment (DME) infrastructure. This, however, exhibits several major drawbacks. First, a costly extension of the infrastructure is required. Also, the DME-interrogations might saturate in dense air-traffic environments, thus limiting the capacity of the system. Finally, such approach might have a severe impact on the operation of any future L-band communication system: frequency resources currently considered for communication might then not be available any more. In this case, a sustainable use of the L-band for both navigation and communication might not be achievable. An alternative, more flexible approach, discussed in this work, is to re-use the LDACS1 communication system infrastructure for navigation. Note, that the infrastructure of a communication network can be very efficiently used for navigation. In [3], A. Sayed outlines the general principles of positioning using a wireless network; in [4] the positioning with OFDM signals within the LTE standard is demonstrated. The deployment of LDACS1 for navigation has been first proposed in [5], where the theoretically achievable performance of LDACS1-based navigation has been investigated. In [6], the original synchronization algorithm of LDACS1 is used for computing the pseudorange between an aircraft and a ground station. Since the resulting performance of this approach still leaves a considerable gap towards the CRLB, we performed research for a more accurate ranging algorithm that exploits specifics of LDACS1 signals. In the final paper, two new approaches are proposed. Their performance is compared to the CRLB and the performance of the approch described in [6]. For a further improvement of the accuracy the information of several OFDM symbols and pilots is coherently combined increasing the integrity of the position estimate is drastically. [1] Framework for Spectrum Compatibility Analysis in L-Band for FCI technology Candidates, EUROCONTROL, Draft 1.0 [2] M. Sajatovic et al., LDACS1 System Definition Proposal: Deliverable D2, Eurocontrol Study Report, Edition 1.0, February 2009 [3] A.H. Sayed et al., Network-based wireless location: challenges faced in developing techniques for accurate wireless location information, Signal Processing Magazine, vol.22, no.4, 2005 [4] C. Mensing et al., Data-Aided Location Estimation in Cellular OFDM Communications Systems, IEEE GLOBECOM, USA, 2009 [5] M. Schnell et al., Using The Future L-Band Communication System For Navigation, Integrated Communications Navigation and Surveillance Conference, USA, 2011 [6] N. Schneckenburger et al., Ranging with LDACS1: First Results, GNSS Signals Conference, France, 2011