A Notice Advisory to Navstar Users (NANU) is a message issued jointly by the United States Coast Guard and the GPS Operations Center at Schriever Space Force Base in Colorado.^[1] Such notices (NANUs) provide updates on the general health of individual satellites in the GPS constellation. NANUs are typically issued approximately three days prior to a change in the operation of a GPS satellite, such as a change in orbit or scheduled on-board equipment maintenance.

NANU types

Forecast outages

• FCSTDV, or Forecast Delta-V, gives scheduled outage times for Delta-V maneuvers. The satellite is moved during this maintenance and the user may be required to download a new almanac.
• FCSTMX, or Forecast Maintenance, gives scheduled outage times for Ion Pump Operations or software tests.
• FCSTEXTD, or Forecast Extension, extends the scheduled outage time "Until Further Notice"; references the original NANU.
• FCSTSUMM, or Forecast Summary, gives the exact outage times for the scheduled outage, including the FCSTEXTD; sent after the maintenance is complete and the satellite is set healthy to users; references the original NANU.
• FCSTCANC, or Forecast Cancellation, cancels a scheduled outage; new maintenance time not yet determined; references the original NANU.
• FCSTRESCD, or Forecast Rescheduled, reschedules a scheduled outage; references the original NANU.
• FCSTUUFN, or Forecast Unusable Until Further Notice, scheduled outage of indefinite duration not necessarily related to Delta-V or maintenance activities.

Unscheduled outages

• UNUSABLE, or UNUSABLE with a reference NANU, closes out an UNUSUFN NANU and gives the exact outage times for the outage; references the UNUSUFN NANU.
• UNUSUFN, or Unusable Until Further Notice, notifies users that a satellite will be Unusable to all users until further notice.
• UNUNOREF, or UNUSABLE with no reference NANU, gives times for outages that were resolved before a UNUSUFN NANU could be sent.

Other

• LEAPSEC, or Leap Second, is used to notify users of an impending Leap Second and all data after the subject line is free flowing text.
• LAUNCH, is used to notify users of a recent GPS launch.
• DECOM, is used to notify users that an SV has been removed from the current constellation identified within the broadcast almanac, but does not necessarily signify permanent disposal.
• USABINIT, or Initially Usable, notifies users that a satellite is set healthy for the first time.
• GENERAL, informs the user of general GPS information and is written using free flowing text.