Time Source Options for Alternate Positioning Navigation and Timing (APNT)

Precise time synchronization is an essential element for APNT technologies, such as wide area multilateration (WAM) and passive pseudo-ranging. The goal is to provide a time service/source that allows APNT technologies to support a required navigation performance 0.3 (RNP 0.3) and provides the navigation integrity and accuracy for surveillance systems that is required for three-mile aircraft separation. The Global Positioning System (GPS) is the key enabler of these capabilities; however, because of its vulnerability to interference, it is critical that an APNT solution be robust (i.e., able to overcome adverse conditions) in the event of GPS outages and interference. The availability of precise and trusted time synchronization is integral to achieving the navigation accuracy performance. This paper outlines the ongoing efforts by the APNT Team investigating technologies capable of providing the precision and widespread delivery of time needed for the APNT goals. OVERVIEW OF APNT TIMING Precise time synchronization is an essential element for APNT technologies, such as wide area multilateration (WAM) and passive pseudo-ranging. Precise time synchronization is key to achieving the required navigation accuracy and surveillance performance needed to support required navigation performance 0.3 (RNP 0.3) and allow three-mile aircraft separation. It is imperative that the alternative timing system not fail in a manner that reduces the performance or capacity of its supported APNT even in the presence of deliberate jamming and interference of GPS. Hence, the goal of APNT timing is to provide a time service that allows APNT technologies to support targeted goals while being robust to GPS outages and interference. A precise, robust time system could also provide utility to other aviation assets, such as networking and communications. Precise timing is also needed to support surveillance operations using multilateration to provide verification and redundancy for the automatic dependent surveillance broadcast (ADS-B) system. Networks coordinate communications by precise synchronization to a common time standard such as Coordinated Universal Time (UTC), often using GPS. An APNT timing capability, synchronized to UTC, would provide a high quality, robust source of time for these functions.