New potential of low-frequency radionavigation in the 21st century

Abstract

New potential of Low Frequency radionavigation in the 21st century

PhD dissertation of Wouter Pelgrum, Delft University of Technology


Summary


GPS (Global Positioning System) has enabled accurate, affordable and almost ubiquitous positioning and timing. This has resulted in its wide spread usage, increased popularity, numerous new applications but also into an increased dependency of the Global Navigation Satellite Systems. The ever increasing performance of GPS has long fueled the thought that GPS ¿ and GPS alone ¿ was to be the designated future of radio positioning and timing. The 2001 Volpe study and later the 2004 proposed ERNP (European Radio Navigation Plan) stated otherwise: although very accurate, GPS and other satellite navigation systems are considered not reliable enough to be used as sole-means for safety, environmental and/or economically critical applications. Those critical applications need a backup system with dissimilar failure modes. The solution suggested by the Volpe-report and by the ERNP-proposal is - perhaps rather surprisingly - an old and almost forgotten radionavigation system: Loran-C. This system with its high-energy low-frequency pulses is largely dissimilar to GPS. The combination of Loran-C and GPS therefore has the potential to be far more robust than either system on its own. However, the `official¿ performance of the 1958 Loran-C system is not a match for the stringent requirements of most modern applications. Fortunately, this Loran performance is related to the capabilities of outdated technology rather than on the fundaments of low frequency radionavigation. The question now arises:

What are the fundamental limits of Low Frequency radionavigation and how do they affect potential applications?

Loran-C is currently the only operational and publicly available low frequency radionavigation system with regional coverage. This dissertation therefore mainly focuses on Loran-C although most results will also be applicable on other low frequency radionavigation systems. Chapter 2 introduces the system details of Loran-C.

The search for the fundamental limits is started by first identifying the potential error sources. Chapter 3 contains a thorough system analysis, starting at transmitter, via propagation, antenna, receiver algorithms to finally a calculated position and time.

Low frequency ground waves experience delays as a function of ground conductivity, topography, seasons and weather. These propagation delays can cause significant position errors if left uncompensated. Chapter 4 discusses the usage of a differential reference station to compensate for the temporal fluctuations in propagation. A spatial correction map further reduces the propagation related positioning errors. The resulting positing accuracy is potentially sufficient for e.g. the stringent 20 m 95% accuracy requirement of the maritime Harbor Entrance and Approach procedure.

Chapter 5 pays special attention to the H-field antenna: error sources such as noise, E-field susceptibility, tuning and cross-talk are discussed thoroughly as also novel mitigation techniques and their successful implementation.

The presented theory is brought into practice in chapter 6 which discusses various measurement campaigns. A highly accurate measurement system is developed throughout the PhD research. The Reeuwijk measurements show the first step with precise dual-difference measurements; local propagation effects are revealed in both the temporal and spatial domain. The measurement setup is further expanded for the land-mobile measurement campaign in Boston. Here both E-field and H-field are measured simultaneously allowing unprecedented analysis of re-radiation and an assessment of the applicability of low frequency radionavigation in a land-mobile environment. Differential corrections and H-field antenna calibration are introduced for the Tampa Bay campaign resulting in an unprecedented measurement performance. The effect of bridges on positioning performance is shown quantitatively as also the successful results of a novel re-radiation detection algorithm. This algorithm enables detection of local disturbances allowing a timely warning of potential erroneous position information. Finally, during a realistic Harbor Entrance and Approach scenario a positioning performance of better than 10 meter 95% is achieved.

This dissertation is concluded with an assessment of the potential of LF radionavigation, based on the results of the PhD research combined with personal views of the author.