Impact of the DME interference on the LDACS1 ranging performance

Kurzfassung

The communications, navigation and surveillance (CNS) technologies in air traffic management for civil aviation are currently undergoing a major modernization process. The legacy systems, which were established over several decades ago, such as analog voice communication systems, distance measuring equipment (DME) and VHF omnidirectional range (VOR) may not meet the continuously growing demands in the near future. In communications, L-band digital aeronautical communication system – type 1 (LDACS1) is one of the most promising candidates for the future air traffic management data link. Using digital communication systems, more complex information can be exchanged between pilots and controllers as well as high-capacity data transfer can be achieved. In navigation, the Global Navigation Satellite System (GNSS) is foreseen to be the primary means for high precision navigation and complex flight trajectories. However, there are still concerns about possible GNSS outages due to intentional or unintentional interference. As a result, a parallel backup navigational infrastructure commonly-referred to as alternative positioning navigation and timing (APNT) is needed. The proposed APNT solutions mainly focus on utilizing ground-based navigational systems because their source of errors would be different from that of GNSS. Several works have been done on the extension of LDACS1 functionality toward navigation and surveillance, and the LDACS1 system has been proposed as one of the APNT solutions. The LDACS1 system was not primarily designed for ranging application but only a few minor modifications are needed to exploit its navigation functionality. LDACS1 is a ground-based cellular system. The networks of LDACS1 ground stations can be used for an aircraft to estimate its position using trilateration. At the moment, enhancing and intensifying the use of the DME system is the most favoured solution for APNT. The major disadvantages for this option are the cost of installing additional DME infrastructure and its inefficient use of L-band spectrum. One of the advantages of using LDACS1 as APNT is that the deployment of LDACS1 ground stations is planned for the future aeronautical communication systems, and navigational functionality can be added without installing any additional infrastructure. Another key advantage of LDACS1 is its spectral deployment, the LDACS1 signal has 500 kHz effective bandwidth and is planned to be allocated in the same frequency band as DME, i.e. the lower part of L-band (960 - 1164 MHz), using inlay approach. The 500 kHz LDACS1 channels will be placed in the 1 MHz spectral gaps between two adjacent DME channels. The LDACS1 system is planned to be able to work in parallel with DME, and therefore both systems can work together as APNT solutions by combining the range calculation in the situations where either of the systems does not have a sufficient number of ground stations available. It is important to ensure the good performance for both systems and several studies on this topic are ongoing. The German Aerospace Center (DLR) has conducted two measurement campaigns in 2012 and 2013 to assess the usability of the LDACS1 system as APNT solution. Several flight scenarios were performed to evaluate the LDACS1 ranging performance and the analysis of the measurement data gave the conclusion that LDACS1 signals offer an excellent ranging source [1], [2]. One of the measurement scenarios recorded during the 2013 measurement campaign focused on the impact of DME interference on the LDACS signals. During this scenario, the measurements aircraft (LDACS1 receiver) flew at FL250 directly over a testing DME station. To test the feasibility of LDACS1 inlay deployment, similar flight route was flown twice and the DME channel is tuned to 500 kHz and then 1.5 MHz away from the tested LDACS1 carrier frequency. Different signal-to-interference ratio (SIR) can be observed along the flight paths. Additionally, the co-site DME interference, i.e. the interrogating DME signal transmitted from the aircraft, were visible at the receiver in most of the flight scenarios. This event occurred with a very low duty cycle but the damage to LDACS1 received signal can be very severe as the on-board DME transmitted power is much higher than the LDACS1 received power from the ground. The worst case scenario occurs when the LDACS1 received signal is heavily faded due to banking as the view of the LDACS1 receiving antenna was blocked. The LDACS1 ranging performance with the presence of DME interference is analyzed based on the measurement data and mitigation techniques are investigated. The in-band DME interference can be suppressed using methods based on pulse blanking. These methods have been studied and proved to be effective for the interference mitigation in the case of LDACS1 communication systems [3]. In this work similar approaches are adopted for the LDACS1 ranging applications. Since the LDACS1 ranging errors caused by DME interference occur in bursts with low duty cycle, the ranging algorithms that accommodate prior information in the range estimation are suitable methods to suppress this type of error bias. In this work, we propose the use of particle filtering and Doppler smoothing filter for the DME interference mitigation. Particle filtering is a variant of sequential Bayesian filter where the dynamic of the estimated parameters is taken into account in the estimation. Doppler smoothing filter is one interpretation of a well-known Hatch filter. This method uses the Doppler information to smooth the raw LDACS1 range estimates from maximum likelihood algorithm or particle filtering and, as a result, sudden error bursts can be suppressed. [1] D. Shutin et al., “LDACS1 Ranging Performance - An Analysis of Flight Measurement Results,” Digital Avionics Systems Conference (DASC 2013), USA, 2013. [2] T. Thiasiriphet et al., ““Application of Bayesian Filtering for Multipath Mitigation in LDACS1-based APNT Applications,” in ION GNSS+ , USA, 2014.” [3] U. Epple, M. Schnell, “Overview of Interference Situation and Mitigation Techniques for LDACS1”, 2011 Digital Avionics Systems Conference (DASC 2011), USA, 2011.