Next Generation Low Frequency Solutions for Alternative Positioning, Navigation, Timing, and Data (PNT&D) Services and Associated Receiver Technology

A. Helwig, G. Offermans, C. Stout, C. Schue

Abstract:	The use of the Global Positioning System (GPS) in everyday life is pervasive and its use increasing, especially as additional multi-constellation Global Navigation Satellite Systems (GNSS), i.e. Galileo, Compass, and GLONASS, and Regional Navigation Satellite Systems (RNSS) become fully operational and “fill the world’s skies.” In many applications, its use is essential even when the user doesn’t know that it is being used (e.g., cell phone service). There are many critical infrastructure applications involving safety, national security, and our economy that require GPS as the primary means for providing Position, Navigation, and Timing (PNT). Examples include: wired & wireless telecommunications, banking & financial transactions, utilities & power delivery/control systems, transportation & shipping, surveillance systems, computer networks, first responder operations, Next Generation Air Transportation system (NextGen), Intelligent Transportation Systems (ITS), and the US Railway Safety Improvement Acts’ Positive Train Control system (PTC). The Federal Communications Commission (FCC) is currently working out its plan to require telecommunications carriers to meet more stringent location accuracy standards that currently apply to handset solutions. GPS has become the sole means in many instances for obtaining crucial precise timing signals. The US Department of Homeland Security (DHS) has identified fifteen (15) Critical Infrastructure and Key Resource (CIKR) sectors that use GPS for timing. For eleven (11) of the sectors, GPS timing is deemed essential for successful operation. GPS/GNSS is inherently vulnerable to interference, disruption, jamming, and spoofing, whether intentional or otherwise, and in many cases operates without an additional system to provide independent PNT information for validation and backup. GPS is also subject to system failures and anomalies leading to degradation in service or complete outages. Many government and civilian organizations around the world are studying the problem of what to do when GNSS-based services are unavailable to provide PNT and Data (PNT&D) information to the extensive public and private sector user communities. The DHS and the Federal Aviation Administration (FAA) are undertaking efforts to study this problem and identify potential solutions. The International Maritime Organization (IMO) in developing its e-navigation concept has recognized that “although additional GNSS services (e.g. Galileo) will become available and robustness will increase, such space-based systems will be vulnerable to jamming and unintentional interference” and that “enhanced navigation system resilience, leading to improved reliability and integrity” is needed. In its World Wide Radio Navigation Plan (WWRNP), the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) acknowledges that “…the vulnerability of GNSS to accidental or deliberate interference is well known and the need for more than one position input to e-Navigation is recognized.” A robust alternative to satellite-based PNT&D services is terrestrial-based Low Frequency (LF) technology. Because of the very different system characteristics of LF ground wave systems compared to GNSS technology, an LF PNT&D service can offer superior availability and resilience to both intentional and unintentional interference compared to satellite technology. The concept of LF PNT services is not new. Its best known implementation to date, the Loran system, is and has been in use for decades around the world. Exhaustive study, analysis, and field trials led by several international authorities have shown that the eLoran system can meet the accuracy, availability, integrity, and continuity requirements for Maritime Harbor Entrance and Approach (HEA) and aviation Required Navigation Performance (RNP 0.3) described as minimum system requirements. eLoran has inherent security and integrity, and system infrastructures exist in many countries around the world. We will present the findings from our investigation, studies, and trials over the past several years on system concepts for an LF/MF APNT system, which includes transmission and reception topology, that could meet the following minimum system requirements and system considerations: be independent from GPS; be able to coexist with GPS; have a data channel capable of at least 1,200 bps; co-exist with and allow Loran and eLoran to remain as international modes" of operation; use existing "protected" spectrum; incorporate UTC timing to an accuracy of at least 50ns; have integrity and security; be a Safety-of-Life System; consider navigation accuracy, availability, integrity, and continuity as paramount; and ensure that the provision of data does not compromise the reliable delivery of navigation information. We examined new methods which may improve the navigation capabilities of future LF-pulsed positioning systems and considered several enhancements to the LF-pulsed positioning technology used in eLoran. We will present LF system options which can co-exist within the international LF ecosystem, bridge GNSS capability gaps, provide user services that are interchangeable with GNSS, and contribute to international Interference Detection and Mitigation (IDM) programs. We will also discuss the development and production of our first generation LF timing receiver which has been undergoing testing and evaluation using on-air signals since March 2011 by an independent research and development company. The results demonstrate that our receiver performs to Stratum-1 levels and meets the International Telecommunication Union (ITU) requirements for Primary Reference Clocks (PRCs) in telecommunication networks. We will also present the development of our new, multi-channel, multi-frequency OEM LF module that can take full advantage of LF APNT&D services, while having the ability to simultaneously process other LF or MF signals of opportunity, as well as integrating GPS information. Because the United States has abandoned the spectrum between 90-110 kHz, a “Blue Ocean” opportunity exists to demonstrate that our LF technology can provide a precise timing and frequency source, either standalone from, in conjunction with, or as a backup to GPS. During the week of February 27, 2012, advanced LF signals were transmitted using modern technology from the former Loran Support Unit site in Wildwood, NJ. As a result of a Cooperative Research and Development Agreement (CRADA) between the US Coast Guard and UrsaNav, live testing of a wide-area precise timing solution has begun. These initial tests included a comprehensive pallet of signals that are being evaluated for their ability to provide a robust, wide-area, wireless precise timing alternative that can operate cooperatively with GPS, or during periods of GPS unavailability. Testing included eLoran and advanced LF timing signals. We installed various UrsaNav LF Timing receivers at five sites at distances between 140-500 miles from the broadcast site. We collected timing and data channel information and we present preliminary results in this paper. Additional on-air tests are planned at various sites throughout the United States. Broadcasts will test several different frequencies, waveforms, and modulation techniques using evolutionary, state-of-the-art technology."
Published in:	Proceedings of IEEE/ION PLANS 2012 April 24 - 26, 2012 Myrtle Beach Marriott Resort & Spa Myrtle Beach, South Carolina
Pages:	1221 - 1232
Cite this article:	Helwig, A., Offermans, G., Stout, C., Schue, C., "Next Generation Low Frequency Solutions for Alternative Positioning, Navigation, Timing, and Data (PNT&D) Services and Associated Receiver Technology," Proceedings of IEEE/ION PLANS 2012, Myrtle Beach, South Carolina , April 2012, pp. 1221-1232. https://doi.org/10.1109/PLANS.2012.6236978
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