A NANU is an advisory message to inform users of a change in the GPS constellation. These messages are released 72 hours in advance for planned maintenance. These messages are also used to notify users of unscheduled outages. General NANUs can be used to disseminate general GPS information.

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

Unscheduled Outages

UNUSUFN, or Unusable Until Further Notice - notifies users that a satellite will be Unusable to all users until further notice.

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.

UNUNOREF, or UNUSABLE with no reference NANU - gives times for outages that were resolved before a UNUSUFN NANU could be sent.

Other

USABINIT, or Initially Usable, notifies users that a satellite is set healthy for the first time.

LEAPSEC, or Leap Second, is used to notify users of an impending Leap Second.

GENERAL, informs the user of general GPS information.