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The growth of the civil air traffic in recent years has made a more efficient management of the air space necessary. This requires both a higher degree of positioning performance and a higher communication capacity. Therefore, the communication, navigation, and surveillance (CNS) infrastructure for civil aviation is currently undergoing a major modernization process. In communications, the current analog voice radio system will have to be substituted by more efficient digital systems for two reasons. First, to keep pace with the steady growth of civil air traffic. Second, to enable modern air traffic management (ATM) concepts which require data rather than voice exchange. One of the most promising candidates for the future ATM data link is the L-band digital aeronautical communication system – type 1 (LDACS1) [1]. LDACS1 is largely based on 4th generation telecommunication technology and employs orthogonal frequency-division multiplex (OFDM) as modulation. Compared to the current analog systems, it offers a vastly increased capacity, scalability, and efficiency. As for communications also for navigation, a paradigm shift is taking place currently. In the past, pilots had to rely solely on DME (distance measuring equipment) and VOR (VHF omnidirectional radio range) for means of navigation. Compared to state-of-the-art navigation aids, these systems offer only a limited performance while being spectrally inefficient. Therefore, in the future aircraft will increasingly rely on GNSS (global navigation satellite systems) offering a highly superior navigation performance compared to DME/VOR. To guarantee the required degree of integrity, the GNSS systems will be accompanied with ground or satellite based augmentation systems (G/SBAS). However, an increased use of GNSS for aviation brings new challenges. Especially, integrity, continuity, and availability of navigational information are of exceptional importance in a safety-of-life environment. Due to the low power levels received from distant satellites, navigation is susceptible to intentional or unintentional interference. Hence a parallel backup navigational infrastructure referred to as alternative positioning, navigation and timing (APNT), needs to be employed. This system are used when GNSS services are temporary unavailable. Currently, different approaches to design a backup system for GNSS exist. Most proposals rely on an increased use of the DME technology. However, this exhibits two main drawbacks: First, the required infrastructure extension is costly. Second, precious spectrum is assign to an old and spectrally inefficient technology. This assignment would last for decades and avoid sustainable use of the L-band for modern CNS approaches which are imperatively needed for future aviation. To support sustainable spectrum usage, we propose to consider the future communication system LDACS1 as APNT solution. To verify the practical ability of LDACS1 to act as APNT system for GNSS backup, DLR has carried out a measurement campaign in November 2012. The goal of the campaign is to examine whether LDACS1 is capable to deliver ranging/positioning results with sufficient accuracy as required for APNT. For the measurements, four LDACS1 ground stations were set up in a quadrangular shape about 40 kilometers apart from each other and used as transmitters. A Dassault Falcon 20 aircraft with respective onboard equipment served as receiver. Several patterns were flown within the test area at several altitudes. This allows an evaluation of the systems performance for various real world geometries. According to the Cramer-Rao lower bound the nominal LDACS1 transmit power 43 dBm should deliver reliable and precise navigation for the aircraft. Nevertheless in any real measurement different error sources exist. Therefore, the main question of the campaign is, how much the performance is degraded due to those various error sources. Firstly, the requirement of synchronization accuracy in the range of nanoseconds in order to avoid concealment of measurement errors put hard constraints on the synchronization setup of the four ground stations. Thus, the station setup included oven-controlled Rubidium clocks at the stations as well as dual frequency GNSS timing receivers, the latter serving to monitor those clocks. The setup of each station was calibrated individually to mitigate the effect of hardware biases affecting synchronization performance. Application of a common-view time transfer technique independent of the GNSS receiver's position, velocity, and time (PVT) solution allows an analysis of the clock biases and drifts up to nanosecond level. Secondly, the typical challenges known from GNSS systems exist, e.g. multipath resolution or interference from other systems like the onboard DME. In particular, the LDACS1 bandwidth of 500 kHz makes the resolution of close multi-paths challenging. The analysis so far gives very promising results. With the four stations visible from the aircraft the LDACS1 system offers a performance considerably better than DME systems in almost all cases. However, under certain conditions, and without applying appropriate countermeasures, the performance may be degraded, e.g. in cases of strong interference or strong multipath situations. The final paper starts with a brief review of the LDACS1 system. This is followed by a description of the measurement campaign setup, including the specific challenges which had to be solved to conduct the campaign. Hereby, a detailed analysis of the different error sources and their influence on the performance of the measurement system is given. This includes errors due to clock synchronization and unknown hardware delays. In addition, the error sources which would impair the ranging performance also in an operational system are assessed, like Doppler effects, multipath propagation, and interference from onboard systems. Taking those error sources into consideration information on both positioning precision as well as integrity is presented. This includes a discussion of the employed algorithms and methods. The paper concludes with an outlook on future work to be conducted in the field of LDACS1 navigation. |
