A Notice Advisory to NAVSTAR Users (NANU) is a type of message issued jointly by the United States Coast Guard and the GPS Operations Centre in Colorado to inform users about changes or issues in the GPS satellite constellation. This includes updates such as satellite maintenance, outages, or decommissioning.

These notices are critical because they relate to the integrity of the system, ensuring users are aware of which satellites are available and reliable. Integrity in GNSS refers to the trustworthiness and accuracy of the information provided to users. If a satellite is not functioning correctly or is temporarily removed from service, users need to know to avoid relying on potentially faulty data.

In contrast, DGPS (Differential GPS) is focused on improving accuracy by correcting errors in GPS signals using ground-based reference stations, not on broadcasting system integrity notices.