John W. Lavrakas, Advanced Research Corporation

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Abstract:

When the Global Positioning System was first deployed, long before it became operational, its operators realized the need for a means to notify users of satellite status, including identifying those satellites scheduled to be removed from service and those which had been removed from service due to an unscheduled action. As a result, they created the Notice: Advisory to Navstar Users or NANU. The NANU was modeled after the notice used by the aviation community, termed the Notice to Airmen, or NOTAM. The format they selected was quite robust, being human readable yet having fixed fields to support a consistent transfer of information. In November 2017, the U.S. Coast Guard Navigation Center began posting a fully machine-readable version of NANUs on their website navcen.uscg.gov. This file is called a satellite outage file and is a breakthrough in capability for those doing mission planning and post-mission assessment, for it provides a complete list of all satellite outages reported in NANUs, current and historical. For years the NANU format was used successfully to report satellite outages, although over time, users began to look for ways to automate their processing of the NANUs and minimize errors. In the early days, NANUs were generated by an airman entering the data at a keyboard, but this manual entry introduced errors into the NANUs, and the Air Force satellite operators looked for ways to streamline the process. They implemented GPS User Support Software (GUSS), which is a program run by the satellite operators to generate NANUs with standardized formats. This had an immediate effect in reducing errors in NANUs, including misspelled names and incorrect message types, but it did not resolve all problems. NANUs were still generated with incorrect dates or NANU types, which caused problems for many users. As a result of evolving user needs for higher fidelity and easier-to-process satellite outage information, in 2004 the GPS Operations Center at the Master Control Station developed a machine-readable file that would capture past, present, and future satellite outages, -- called the satellite outage file or SOF. The file is generated by a crew member in the 2nd Space Operations Squadron (2 SOPS) whenever NANUs are issued. The process of creating a SOF has been invaluable in catching errors in NANUs and ensuring users receive timely and correct information. In this paper we provide background on the satellite outage reporting process from its origins with NANUs to the present day SOF file. The paper describes the content and format of the SOF, when it is generated, what information it contains, and how it compares to the information that is in NANUs. It also describes ways the SOF is being used now to support applications as diverse as aviation operations, mission planning, and post-mission assessments, and reaffirms why the SOF is a critical piece of information for mission planning. The paper also describes how the SOF is used by various businesses and agencies in applications, using information gleaned from the US Coast Guard and various companies. It explains how the SOF can be used when assessing navigation performance and how easily it can be ingested, without introducing human error. The paper gives background on steps taken by the US Coast Guard and the US Space Force to make the SOF available to all GPS users, and explains how users can obtain the file.