Super-resolution Analysis of the L-band Air-to-ground Radio Channel

Nicolas Schneckenburger, Dmitriy Shutin, Thomas Jost, Michael Schnell, Uwe Fiebig

Abstract:	Since the early days of aviation, reliable navigation of aircraft has always been a fundamental challenge. In the past decades, pilots mainly relied on DME (distance measuring equipment) and VOR (VHF omnidirectional radio range) for navigation. However, currently a transition to new ways of navigation is taking place: In the future, navigation in civil aviation will increasingly rely on GNSS, e.g. GPS, accompanied by ground or satellite based augmentation systems (G/SBAS). Although offering a high performance, an intensified use of GNSS for aviation raises new challenges. Due to low power levels received from in-orbit satellites, GNSS signals are susceptible to interference, both intentional, such as spoofing [1], and unintentional. Hence, a navigational backup system commonly referred to as alternative positioning, navigation, and timing (APNT), needs to be employed to be able to cope with possible GNSS outages. Different proposals for the APNT implementation exist, mainly in the protected L-Band frequency range [2, 3, 4]. These proposals employ a ground-based infrastructure of signal transmitters or transponders. The aircraft can estimate their position by measuring ranges or pseudoranges to these ground stations. Thus, if the future APNT systems are to deliver highly accurate positioning information, it is necessary to understand the ranging characteristics of the air-to-ground (A2G) L-band wireless propagation channel. Yet this remains a challenging task: Currently, there are no models of the L-band A2G channel applicable in the context of ranging. In [5], an extensive flight measurement campaign dedicated to measure the L-band A2G channel was presented. A channel sounding signal with a bandwidth of 10 MHz is emitted by a ground based transmitter and recorded by an airborne receiver. The high bandwidth allows a detailed channel characterization. Different flight patterns are tested in order to allow determination of the channel characteristics under different geometrical configurations. These patterns include different phases of the flight, i.e. approach on the airport, takeoff and landing, as well as an enroute scenario. The first results presented in the paper indicate a strong presence of multiple propagation paths. Multipaths with a power exceeding -10dB compared to the direct line of sight (LOS) path power can be observed. The propagation paths almost exclusively originate from scatterers in the direct surrounding of the ground transmitter. The statistics of the individual multipaths depend on the distance and geometry between aircraft and ground station. For a large separation between receiver and transmitter, more specular multipaths with a long lifetime are experienced. In case of a smaller distance between aircraft and ground antenna they become more diffuse. A drawback of the previous analysis of the measurement data is that for their generation no super-resolution techniques are applied. Therefore, the resolution in both delay and Doppler frequency domain is limited by the physical signal parameters of the channel sounding sequence: Thus, the ability to analyze multipaths with a very similar delay and Doppler frequency compared to the LOS is limited. Unfortunately, these constellations are of great interest as close multipaths can significantly degrade a standard range estimator such as a delay-locked-loop (DLL). To allow a better characterization of close multipaths, we apply a novel super-resolution algorithm to the data collected in 2013. The algorithm combines Bayesian parameter estimation with Kalman filtering similar to [6]. The parameter estimation allows the resolution of individual multipath components even if they are very similar in terms of delay and Doppler frequency. The Kalman filter makes a continuous tracking of the different propagation paths over time possible. Therefore, we are able to characterize a multipath not only by its amplitude, delay, and Doppler frequency, but also its lifetime. Additionally, due to the smoothing property of the filtering and the possibility to use the Kalman Filter’s prediction as initialization for the Bayesian estimation, the parameter estimation is improved. The reliable characterization of the individual multipaths also allows an assessment of errors to be expected with different APNT candidate systems such as LDACS1 or DME. In the final paper we will discuss the above mentioned problems in detail. We will begin with a very brief summary of the setup of the 2013 flight trials. Next is a description of the employed super-resolution algorithm. In the following section, we apply the algorithm to the collected measurement data for detailed investigation of the L-band A2G radio channel. Hereby, a special focus is laid on the consequences of multipaths on the process of range estimation. The paper is concluded with a discussion of the obtained results and an outline of the future investigation directions. [1] Joe C. Grabowski, “Field Observations of Personal Privacy Devices,” in Proceedings of the 2012 International Technical Meeting of The Institute of Navigation, Newport Beach, CA, 2012, pp. 689 – 741. [2] Michael Schnell, Ulrich Epple, Dmitriy Shutin, and Nicolas Schneckenburger, “LDACS: Future Aeronautical Communications for Air-Traffic Management,” Communications Magazine, IEEE, vol. 52, no. 5, pp. 104–110, 2014. [3] Sherman Lo, Yu Hsuan Chen, Per Enge, Benjamin Peterson, and Robert Erikson, “Distance Measuring Equipment Accuracy Performance Today and for Future Alternative Position Navigation and Timing (APNT ),” in Proceedings of the 26th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2013), Nashville, TN, 2013, pp. 711 – 721. [4] Wouter Pelgrum, Kuangmin Li, Matt Smearcheck, and Frank van Graas, “eDME architecture development and flight-test evaluation,” in 25th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2012), Nashville, TN, USA, 2012. [5] Nicolas Schneckenburger, Dmitriy Shutin, Thomas Jost, Michael Walter, Thanawat Thiasiriphet, Alexandra Filip, and Michael Schnell, “From L-Band Measurements to a Preliminary Channel Model for APNT,” in 27th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2014), Tampa, FL, USA, 2014 [6] Thomas Jost, Wei Wang, Uwe-Carsten Fiebig, and Fernando Perez-Fontan, “Detection and tracking of mobile propagation channel paths,” IEEE Trans. Antennas Propagation, vol. 60, no. 10, pp. 4875–4883, 2012.
Published in:	Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015) September 14 - 18, 2015 Tampa Convention Center Tampa, Florida
Pages:	1381 - 1402
Cite this article:	Schneckenburger, Nicolas, Shutin, Dmitriy, Jost, Thomas, Schnell, Michael, Fiebig, Uwe, "Super-resolution Analysis of the L-band Air-to-ground Radio Channel," Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015), Tampa, Florida, September 2015, pp. 1381-1402.
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