An Aeronautical Air-Ground Channel Investigation for APNT Applications

Kurzfassung

Over the course of the last years, civil aeronautics have been undergoing a perpetual innovation: the way we communicate, navigate, and survey the airspace is currently being redefined. In the past, pilots mainly relied on DME (distance measuring equipment) and VOR (VHF omnidirectional radio range) for navigation. Compared to state-of-the-art navigation aids these systems offer only a limited performance while being spectrally inefficient. Therefore, it is planned in the future to rely on global navigation satellite systems (GNSS) for navigation. GNSS offers a highly superior navigation performance as compared to that obtained with legacy DME/VOR infrastructure. To guarantee the required degree of integrity, 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 in-orbit satellites, GNSS signals are susceptible interferences, both intentional and unintentional. Hence a ground based navigational backup system, referred to as alternative positioning, navigation and timing (APNT), needs to be employed. These systems should be used when GNSS services become temporarily unavailable. Different proposals for APNT are currently being discussed. One approach is based on the legacy system DME. The major advantage of DME is that it allows reusing the existing infrastructure; in fact, DMEs can be currently used as an APNT system. However, a large drawback of this solution is the continued use of old, spectrally-inefficient technology. But, what is more important, is that once the transition to GNSS-based air traffic management has been realized, a DME-based APNT solution will not be able to support the same level of navigational information in terms of accuracy or precision. Thus, alternative APNT approaches that provide accurate positioning, while at the same time allowing for a “gentle” phasing out of DMEs, are needed. One such example is the future communication system LDACS1, which signals can be also used for ranging and positioning. Hereby, communication infrastructure can also be employed for navigation purposes. Independently of the underlying technology, the majority of the APNT proposals are ground-based systems with planned allocation in the L-band. If, however, the future APNT systems are to deliver highly accurate positioning information, it is crucial to understand the characteristics of the ground-to-air (G2A) L-band wireless propagation channel in more detail. Yet this remains a challenging task as channel models for L-band G2A useable for testing and validation of range estimators are sparse. The existing models are extensions of classical statistical channel models developed for wireless communication applications. Thus, dependable simulations of the navigation performance of the proposed systems are hard to realize. To address this challenge, DLR has conducted in 2013 an extensive channel measurement campaign in the L-band. To this end, the setup consisted of a ground-based transmitter located on top of a building in an airport, and a receiver in a research aircraft Dassault Falcon 20E . For both the transmitter and receiver commercial L-band antennas were employed. The measurement bandwidth was set to 10MHz with a center frequency at the lower end of the L-band at 965 MHz; the transmit power was set to 39 dBm. Different flight patterns were tested in order to allow an extensive investigation of the channel characteristics. These include flights at different altitudes and transmitter-receiver separations, approach on the airport, take-off and landing. First preliminary analysis of the recorded data identifies an interesting characteristic of the L-band G2A channel relevant for navigation applications: for all altitudes multiple propagation paths can be observed. This effect is especially profound at low altitudes. The multipath is prone to occur in close vicinity to the transmitter, but excess delays sometimes reaching up to a few kilometers. The additional propagation paths are also strongly correlated in Doppler domain. As the result, for ranging and positioning applications such multipaths cause a strong interference to the direct path, significantly biasing the output of the correlator tuned to the line of sight (LOS) path. Furthermore, the spatial proximity of the different propagation paths complicates their separation and detection. This makes the compensation of the multipath effects a very challenging task. Especially for systems with low bandwidths, such as DME or LDACS1 that are considered for APNT services, multiple propagation paths represent a significant error source. Data collected in previous flight trials show that errors in the region of hundreds of meters are possible when ranging with a low bandwidth system in strong multipath environment. In the final paper we will discuss the above mentioned problems in more details. We will begin by outlining the goals of the channel measurement campaigns and explain the measurement setup used in our measurements. The latter includes a short description of both the hardware and software employed during the campaign as well as the applied algorithms. Then, we will concentrate on the analysis of the measurement data. Hereby a special focus is placed on the multipath characterization of the G2A channel using a novel superresolution multipath estimation algorithm. The paper is concluded with a discussion of the obtained results and outline of the future investigation directions.