GNSS, augmentation and performance-based navigation
Satellite navigation combines signal measurements, a position solution and information about whether that solution meets the required use. Seeing plausible coordinates does not establish accuracy, integrity or approval for an instrument procedure. Separate the receiver’s capability from its current performance, installation eligibility, database and operational requirements. These are related checks, not interchangeable labels.
Position and integrity
A basic pseudorange measurement includes a clock-error component as well as geometric distance and other errors. Satellite geometry affects how measurement errors influence a computed position; dilution-of-precision measures describe that geometry. A good-looking geometry value does not eliminate every other source of error or guarantee that a signal is genuine. Multipath involves reflected arrivals, while interference and misleading signals can produce other problems.
RAIM is receiver-autonomous integrity monitoring. The available monitoring function depends on the observations, geometry, algorithm and any approved aiding. Fault detection is different from detecting and excluding a faulty input while continuing a valid solution. Do not give one satellite-count rule without checking whether the question includes barometric aiding or another design assumption. A displayed position and an available integrity-monitoring function are distinct information states.
Augmentation and sensor integration
SBAS provides wide-area corrections and integrity information; GBAS uses local reference information and a ground broadcast for its intended service. Neither acronym means the same thing as a conventional ILS transmitter. An FMS may integrate several sensors, but the actual weighting, failure response and eligible procedure depend on its implementation and documentation. A second display using the same input is not an independent cross-check merely because it appears on another screen.
PBN and the specified operation
RNAV permits navigation along desired paths within the applicable coverage or limits of the navigation system. RNP specifications add onboard performance monitoring and alerting requirements. Accuracy is only one part of a navigation specification; integrity, continuity, availability and functionality also matter. An accuracy statement expressed for a percentage of flight time is not an absolute guarantee that no deviation can ever occur.
A fly-by waypoint allows turn anticipation, while a fly-over waypoint requires the relevant point to be overflown before the next turn under the procedure definition. Radius-to-fix legs involve a defined curved path and the appropriate capability. A current published procedure, valid database and actual aircraft eligibility cannot be replaced by drawing a similar route manually. This bank contains no approval to continue an approach after a downgrade or to substitute one minima line for another at an unspecified point.