The navigation and fuel plans of the earlier modules were built from published sources: charts, the AIP, the manual. All of it describes the world as it was at press time. The NOTAM briefing is where the plan meets the world as it is this morning, and the air law course's machinery of aeronautical information (NOTAM, supplements, the AIRAC cycle) now becomes a working tool: the pre-flight information bulletin (PIB) collects every current NOTAM relevant to the route, and the planner's job is to read it against the flight.
Facilities and services
The first check is the broadest: every ground- and satellite-based facility and service the flight depends on must be available and adequate. Available means working and open; adequate means suitable for the use planned: a VOR that is on the air but NOTAMed unusable in the sector the route crosses fails the check as surely as one switched off, and a navaid the approach depends on matters more than one the route merely passes.
Aerodromes: departure, destination, alternates
For the departure, destination and every alternate, the briefing asks for the latest state of the ground the flight intends to touch:
Opening hours — of the aerodrome and its services; an alternate that closes an hour before the diversion could arrive is no alternate.
Work in progress (WIP) — shortened or closed runways and taxiways, displaced thresholds, cranes.
Special procedures due to WIP — the temporary routings, holding points and restrictions the work imposes.
Obstructions — new or temporarily lit obstacles affecting departure, approach or circling.
Frequency changes — communications, navigation aids and facilities whose published frequencies no longer hold.
The en-route check
Between the aerodromes, the same question is asked of the route itself:
Airways and routes — segment closures, level restrictions, and the day's conditional-route availability from the module-2 machinery.
Restricted, danger and prohibited areas — activations and new establishments; a route legal on the chart can be blocked by this morning's activity.
Frequency changes — en-route communications and navaids, so the flight log's boxes match what will answer.
Satellites in the briefing
Satellite facilities get the same treatment with one refinement: availability must hold during the expected time of use, not merely today. A constellation healthy at briefing time can have a satellite scheduled out of service exactly when the flight needs it over the destination, so satellite checks are made against the arrival window. The same clock-bound question is asked of the augmentations: is the GBAS at the destination, or the SBAS service (EGNOS in Europe), notified available for the period the approach will fly it?
GNSS achievability
Ground aids fail one at a time and in place; GNSS capability moves. Whether an RNP approach will actually be flyable depends on the satellite geometry and integrity checking at the place and time of use, and both change hour by hour as satellites orbit, fail and are withdrawn for maintenance. That is why GNSS achievability is checked before flight: the capability the aircraft carries is only a licence to use what the constellation happens to offer at the ETA. Three message types feed the check:
RAIM — receiver autonomous integrity monitoring, the radio-navigation course's consistency check among range measurements: five satellites in view to detect a faulty signal, a sixth to isolate and exclude it.
GNSS NOTAMs — the aeronautical information channel for satellite outages and augmentation service interruptions.
NANU — notice advisory to Navstar users: the GPS operators' own satellite-health messages, announcing planned maintenance about 72 hours ahead and reporting unscheduled outages.
Augmented, non-augmented, planned, unplanned
How the check is run depends on the avionics. A non-augmented receiver relies on RAIM for integrity, so the planner runs a RAIM prediction: receiver-specific software fed with the latest constellation status computes whether enough satellites will be usable at the destination around the ETA, and a predicted outage bracketing the arrival means delaying, replanning the approach or choosing another aid. An SBAS-equipped aircraft inside confirmed coverage takes its integrity from the augmentation instead, so no RAIM prediction is needed; the homework shifts to the SBAS NOTAMs, because an EGNOS interruption takes the LPV minima with it.
The outage's character matters as much as its existence. A planned outage is announced in advance through NANUs and NOTAMs and therefore appears in the prediction: the planner sees it coming and plans around it. An unplanned outage is a failure without warning, invisible to any pre-flight prediction, and is caught only in flight by the receiver's own monitoring — which is precisely why the airborne integrity function exists and why the flight plan carries a conventional fallback.