When a flight is planned to use GNSS services, the pilot is of course required to check during his pre-flight preparation that the planned satellite services will actually be available at the desired time of use. Planned outages are announced in advance by NOTAM messages. In addition to NOTAMs, another type of message informs GNSS users about satellite health and planned outages in the GPS satellite constellation, the Notice Advisory to Navstar Users (NANU). It is usually issued three days in advance and covers scheduled maintenante, changes in satellite orbit which requires the user to download a new almanac, etc.