ION GNSS 2012
Session B4: Preserving the Availability & Integrity of GNSS in Harsh Environments

Title: The Need for a Robust Precise Time and Frequency Alternative to GNSS
Author(s): M. Narins, P. Ahn, Federal Aviation Administration; P. Enge, S. Lo, and B. Peterson, Stanford University
Date/Time: Thursday, September 20, 2012, 2:35 p.m.
Room: 205 (NCC)

In the 21st century Positioning, Navigation, and Timing (PNT) services have become key enablers of both essential (safety and security) and economically beneficial (capacity, efficiency, etc.) 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. In 2011 the world became much more aware of the vulnerability of GNSS-based services as a result of the significant interest in using the spectrum directly adjacent to GPS for mobile communications services, and this was an important wake up call. However, while users of GNSS positioning and navigation are usually 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 vulnerability. Many 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 that will ensure that the PNT services necessary to safely, securely, and effectively to support the US National Airspace System´s (NAS) transition 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.

First, we will discuss and derive the timing capabilities desired by APNT to support critical operations. In particular, precise time synchronization is needed for technologies such as terrestrial passive ranging and multi-lateration. In addition, robust timing may be useful for communications in the NAS. Such communications and dependence on the information carried will only increase with NextGen.

The majority of the paper will address the different strategies and how their ability to meet the desired capabilities. Specific areas include network based timing, terrestrial transmissions and robust space based timing. For network time, we examine existing and upcoming protocols to support timing and synchronization such as network time protocol (NTP), precise time protocol (PTP) and J.211/DTI. The capabilities and appropriateness of the methods for APNT is assessed. Terrestrial technologies include use of low frequency transmissions such as eLoran or WWV as well as line of sight signals. Assessments of the capabilities of these systems are provided. For APNT, space based timing using one way measurements of satellite signals for timing as these currently present the gold standard in low cost, high accuracy time. Satellite signals must be used with care as they are typically weak. Hence, a critical part of the APNT assessment is to examine radio frequency interference (RFI) robust technologies such as directional antennas and controlled reception pattern antenna (CRPA). Such robustness could allow for use of space based augmentation system (SBAS) and global navigation satellite system (GNSS) signals, greatly limiting the effective area of RFI induced outages. The Iridium signal is another possibility which offers a 30 dB advantage over GNSS signals.

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