The Need for a Robust Precise Time and Frequency Alternative to Global Navigation Satellite Systems

Positioning, Navigation, and Timing (PNT) services are key enablers of both essential safety and security applications and economically beneficial capacity and efficiency applications worldwide. Whether users are ground-based, sea-based or in the air, their primary/go-to source of PNT has become a Global Navigation Satellites System (GNSS), with the US Global Positioning System (GPS) being the most widely used. Starting in 2001, with the publishing of the landmark Volpe Transportation Systems Center’s GPS Vulnerability Report and leading up to the Department of Homeland Security sponsored GPS Interference Testing in 2012, the world has became much more aware of the vulnerability of GNSS-based services – especially in 2011, as the result of significant interest in using the spectrum directly adjacent to GPS for mobile communications services. This was an important wake up call to the world. But while users of GNSS positioning and navigation services are usually at least cognizant of the source of their services, many users of GPS precise time and frequency are oblivious to both the source of these services and their inherent vulnerability. In fact many time and frequency users are not even aware of how GNSS-provided time is crucial to their operations. The Federal Aviation Administration (FAA) has initiated an Alternate Position, Navigation, and Timing (APNT) program to research various alternative strategies. These strategies are necessary to ensure a safe, secure, and effective transition of the US National Airspace System (NAS) to the Next Generation Air Transportation System (NextGen). While discussing some of the position and navigation aspects of this program, this paper concentrates on the need for a robust time and frequency alternative to GNSS that will support aviation and have the potential to provide robust precise time and frequency services to other user communities. Alternatives strategies to be explored include use of existing NAS ground-based navigation aids, high power ground waves, antenna technologies, and alternative satellite constellations. INTRODUCTION To discuss the need for “a robust precise time and frequency alternative,” we must first define what is meant by “robust.” Although a number of alternative definitions can be found, the one preferred by the authors, as this is a systemic issue, is “the ability to overcome adverse conditions.” By extrapolating this basic concept, we provide the following basic axiom: Robust Time and Frequency Services denotes the provision of strong, sturdy precise time and frequency services that are able to withstand or overcome adverse conditions, and as we are dealing with radionavigation signals, the adverse conditions we must overcome can be categorized as interference. Radio frequency interference (RFI) comes is many “flavors.” It can be intentional or unintentional; predictable or unpredictable, manmade or environmental, crude or sophisticated (jamming or spoofing); and/or widespread or localized. When we speak of a harsh radionavigation environment, we envision one in which we must overcome some type of interference to arrive at the accuracy, availability, integrity, continuity, or coverage required by our specific applications. So why is “good” time so important? Because time is the means by which we precisely position and navigate, the means by which we can safely and efficiently separate airplanes in flight. Recently it has become increasingly apparent that it is through the denial or manipulation of good time that spoofers intend to adversely affect GNSS position and navigation users. The problem is clear – current GNSS time and frequency are not robust, many users are not aware of their dependence on GNSS time and frequency, and GNSS time and frequency services support the vast majority of critical infrastructure/key resource (CIKR) sectors. Figure 1 lists the 18 CIKR Sectors recognized by the US Department of Homeland Security (DHS) and shows that 15 of the 18 rely on GNSS provided time. As a part of the Transportation sector, the FAA clearly recognizes the need for precise time and has maintained a significant non-GNSS based infrastructure to ensure the safety and security of the NAS;