Ranging with LDACS : Results from Measurement Campaign

Abstract

The current air traffic management (ATM) system in civil aviation is globally standardized, well-established and highly reliable. However, it still relies on techniques which have been developed several decades ago. With the continuously and rapidly growing demand for air transportation, it is predicted that the ATM system will reach its capacity limit in the near future. A major ATM modernization process including communications, navigation and surveillance (CNS) technologies is currently ongoing under SESAR in Europe and NextGen in the US. For the future aeronautical communications, analog voice shall be replaced by digital data to support more complicated information exchange as well as higher capacity. L-band Digital Aeronautical Communication System (LDACS) is the proposal for the future ATM data link. In this system, multicarrier OFDM signals with 500 kHz effective bandwidth per channel are used for transmission. With respect to aeronautical navigation, the performance and capacity of the legacy systems, e.g. distance measurement equipment (DME), are limited and will not be able to catch up with the growing demand for higher precision and more efficient flight-route management. The future navigation system will heavily rely on global navigation satellite systems (GNSS). Despite the far superior performance, the main concerns for the satellite-based approach are availability, continuity and integrity. The fundamental problem for satellite-based systems is the weak receiving power because of the large distance between the satellites and the aircraft. As a result, the satellite-based navigation system can be easily interfered, either intentionally or unintentionally. There is a need for a parallel backup navigational infrastructure to ensure availability, continuity and integrity. This backup is commonly referred to as alternative positioning, navigation and timing (APNT). The solutions for APNT are mostly ground-based systems as the sources of error would be different from that of satellite-based systems. Intensifying and enhancing the already existing DME system is the most commonly discussed and most-ready solution. The major drawback of this solution is that DME is using the precious aeronautical spectrum very inefficiently. Intensifying DME usage would even increase this inefficiency and would finally block the L-band spectrum for any other future CNS system – a consequence which shall be considered thoroughly. Aiming at efficient spectrum usage, the German Aerospace Center (DLR) has proposed to consider LDACS as APNT solution. Although the LDACS system is primarily designed for communications, the navigational function can be added without any major modifications. The network of LDACS ground stations can be used as pseudolites for estimating the aircraft positions. Additionally, LDACS has capability to be operated in parallel with DME, and therefore it is possible to combine the range calculation from both systems. Two LDACS flight trials to assess the usability of the system as APNT solution were conducted by DLR in November 2012 and November 2013. The goal of the first campaign was to implement a core structure of the LDACS system for navigation and test its performance in a realistic scenario with an airborne receiver. The results confirmed that the LDACS signal is a good ranging source for navigation. Multipath propagation and co-site DME interference were identified as significant factor on the performance. More understanding of their impact in different scenarios and developing techniques for mitigating these effects can significantly improve the ranging performance. As a result, the second measurement campaign was performed in November 2013. This contribution will focus on the measurement setup and the LDACS ranging results from this campaign. In this campaign, only one ground station (LDACS transmitter) was considered, therefore the results are assessed by ranging performance. The main goal was to verify the LDACS ranging performance in diverse flight scenarios. Several flight scenarios such as flying in different altitudes from FL100 to FL380, circling around the ground station in low altitude and flying in a strong in-band DME interference environment were performed with the measurements aircraft (LDACS receiver). The LDACS ranging performance is analyzed based on the measurement data and several mitigation techniques are investigated. We propose particle filtering and the Doppler smoothing filter to improve the ranging performance as well as mitigating the interference from the multipath propagation and DME. The results show that the proposed algorithms can greatly improve the ranging performance with reasonable computational complexity.