Impact of Ground Multipath on Terrestrial Radio Navigation Performance

Terrestrial radio navigation systems such as VHF Omni-directional Range (VOR) and Distance Measuring Equipment (DME) have been used for decades to provide navigation information, for example bearing angle (VOR) and slant range (DME), to aircraft worldwide. Increased performance requirements necessitate a closer look at the propagation-induced errors of terrestrial radionavigation. An ongoing global effort (ICAO, FAA, Eurocontrol) is aiming to increase the efficiency and safety of aviation while reducing its cost and environmental impact. To achieve this goal, a shift from existing sensor-based navigation to Performance-Based Navigation (PBN) is instrumental. The PBN concept specifies that the aircraft RNAV (Area Navigation) system performance requirements be defined in terms of the accuracy, integrity, availability, continuity, and functionality, which are needed for the proposed operations in the context of a particular airspace concept [1]. To achieve the performance improvements defined in PBN, the use of Global Navigation Satellite Systems (GNSS) is heavily relied on. However, within PBN, robust and assured navigation performance is also essential during a (prolonged) outage of all satellite-based navigation systems due to, for example, RF interference, jamming, or ionosphere events. Thus, a backup or alternate system is required to mitigate the impact of a GNSS outage. Several terrestrial radio navigation systems are considered for this function. This paper focuses on the impact of ground multipath on these systems. In radio navigation, multipath refers to one or more propagation paths that reach the antenna in an indirect path due to, for example reflection and diffraction. Generally, multipath causes the distortion of the direct signal, therefore influences the ranging performance of radio navigation systems. Ground multipath, in particular, refers to the multipath reflecting off the ground. In terrestrial radio navigation system, due to the limitation on the antenna physical height, the ground multipath typically has a small grazing angle for most of the coverage. This grazing incidence causes very high reflectivity of the electromagnetic wave regardless of the surface, resulting in ground multipath that is close to the direct signal in amplitude. This near-perfect reflecting ground multipath can give rise to almost full cancellation of the direct signal when it is completely out-of-phase, resulting in significant range measurement errors. Ohio University has developed a high-accuracy DME measurement setup in [2-4]. A legacy interrogator is installed in a flight-test aircraft to interrogate a legacy transponder under normal conditions. Two precisely time-synchronized Radio Frequency (RF) data collection systems are installed in the airplane and on the ground and collect all incoming and outgoing DME RF signals. In post-processing, this setup allows for high fidelity measurement of DME signal level and ranging performance. In this paper, this DME measurement setup is used to assess the impact of ground multipath on terrestrial radio navigation systems. In previous flight tests [4], it has been observed that ground multipath causes some significant dips in signal level in a typical DME setup. This paper presents multipath simulations in conjunction with flight test data. The measured multipath fading is matched with the movement of the reflection point on the actual terrain, derived from a high-precision terrain database. Next, the actual transponder and interrogator pulse waveforms are used to simulate and match the impact of multipath on Time-Of-Arrival (TOA) measurement errors and received signal strength. Finally, the findings are extrapolated to terrestrial radionavigation using different forms of modulation under a variety of representative conditions. [1] International Civil Aviation Organization (ICAO), “Performance-based Navigation (PBN) Manual,” ICAO Doc 9613, 3rd edition, 2008, ISBN 978-92-9231-198-8 [2] K. Li and W. Pelgrum, “Optimal Time-of-Arrival estimation for Enhanced DME”, Proceedings of the 24th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS 2011), Portland, OR, September 2011, pp. 3493-3502. [3] K. Li and W. Pelgrum, “Flight Test Performance Assessment of DME (eDME)”, Proceedings of the 2012 International Technical Meeting of The Institute of Navigation, Newport Beach, CA, January 2012, pp. 131-141. [4] K. Li and W. Pelgrum, “Robust DME Carrier Phase Tracking Under Flight Dynamics”, Proceedings of the 2013 International Technical Meeting of The Institute of Navigation, San Diego, CA, January 2013.