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SAM PULLEN, Stanford University A: Since at least 1997, the Global Positioning System (GPS) Control Segment (CS) has issued what are known as “Notice Advisor(ies) to NAVSTAR Users” or “NANU(s).” As originally reported in Section 3.5 of the 2001 version of the GPS SPS Performance Standard (GPS SPS PS), these provide “notification of changes in constellation operational status that affect the service being provided to GPS users, or if the U.S. Government anticipates a problem in supporting performance standards established in this document.” The most recent 5th Edition of the GPS SPS PS [2] explains that a key purpose of NANUs is to alert GPS users of foreseeable future GPS satellite outages so that users are not surprised by them and potentially lose continuity. Specifically, Section 2.3.5 of [2] states the following:

United statesEl segundoBrent renfroDepartment of defenseAnalysis centerRussian defense ministryRussian ministry of defenseService definition documentCentral research institute of machine buildingChina satellite navigation officeUs coast guard navigation centerGalileo open serviceService performance standardUser support community interfacesBeidou open service performance standardRoscosmos state corp