GNSS integrity, RAIM and augmentation

Can an accurate-looking position still be unusable?

Integrity asks whether misleading information can be detected in time.

Accuracy and integrity answer different questions

Accuracy concerns closeness to the true position. Integrity concerns confidence that the system can provide an appropriate timely warning when navigation information should not be used. A position can happen to be accurate while integrity monitoring is unavailable. A smooth display can also be wrong without an obvious jump.

Receiver autonomous integrity monitoring compares redundant measurements for consistency under its approved algorithms. In a basic unaided model, four satellites solve 3D position and clock, a fifth permits fault detection, and a sixth supports fault detection and exclusion. Barometric aiding and installed algorithms alter these basic counting examples; geometry and system requirements remain essential.

Availability is not the same as success

A RAIM prediction checks whether monitoring is expected to be available for a place and time under stated assumptions. It does not certify that no fault or interference will occur. If the receiver reports integrity unavailable or an alert, follow the approved procedure for the operation; do not continue by treating the last good position as indefinitely valid.

Redundancy can reveal inconsistency

With exactly enough ideal measurements to solve the unknowns, an erroneous range can shift the solution while still leaving a mathematical intersection. An additional independent measurement can fail to agree with that solution, providing evidence of inconsistency. Determining which measurement is faulty requires further information or redundancy; merely discovering that something is wrong is a different function from isolating and excluding the source.

In the laboratory, increase satellite count and compare the labelled basic capabilities, then deliberately degrade geometry. The count-based description remains conditional. A warning is valuable even if the current map happens to look correct, because the operation requires a timely basis for deciding when not to use it. Absence of a warning from an unavailable monitor is not evidence that the data have passed an integrity check.

What do SBAS and GBAS add?

Corrections and integrity information serve a defined service and approval.

Different delivery architectures

A satellite-based augmentation system uses a ground monitoring network and broadcasts corrections and integrity information through its service architecture. A ground-based augmentation system provides local corrections and integrity information for suitably equipped aircraft in its supported area. SBAS and GBAS are not interchangeable receiver modes.

Augmentation can improve usable performance but does not eliminate antenna masking, every interference threat or every system failure. The receiver and procedure must support the intended function. LPV and GLS describe different approach capabilities; the chart, installed approval, service status and operational requirements determine eligibility.

Compare a bound with the required limit

A protection level is a computed integrity bound under the system’s assumptions. An alert limit is the allowable bound for the operation. When the relevant protection level exceeds the applicable alert limit, that service cannot support the intended operation even if estimated position error looks small. The laboratory uses synthetic horizontal values to demonstrate this comparison and does not assign universal approach thresholds.

A performance gate can fail before the map moves

For a synthetic service, let the horizontal alert limit be 40 m. Initially the computed protection level is 25 m, so that comparison passes. If an integrity update increases the bound to 55 m while the displayed position remains visually stable, the comparison fails. The service is no longer supported for the assumed requirement even though the position estimate has not visibly jumped.

Restoring a 25 m bound would satisfy this comparison again but would not by itself establish that an approach is selected, authorised and supported by all required systems. An augmentation message is one component of a defined service. The approach type, installed receiver, operational approval where required, service availability and other applicable conditions still need to match. This is why the laboratory reports a conditional pass rather than an approach clearance.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.