SBAS Knowledge Base

Alert Limits

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Alert Limits

Scope and reader profile

This page explains alert limits in the SBAS integrity learning path. It is intended for readers who need to understand how operation-specific thresholds relate to protection levels, alerts, and use-or-non-use decisions.

Exact alert-limit values, dimensions, procedure-specific requirements, and alerting behavior must be verified against authoritative standards, service definitions, equipment documentation, and approved operational material.

Executive summary

An alert limit is an operation-specific threshold used to decide whether the navigation solution is acceptable for continued use in that operation.

In simplified form:

If the relevant protection level is within the relevant alert limit,
the navigation solution may be usable for the operation, provided all other required conditions are also satisfied.

The alert limit is not a generic warning preference. It is tied to the operation and must be interpreted through the applicable aviation approval context.

Relationship to protection levels

Alert limits and protection levels are paired concepts. Use SBAS Core Claim Routing for source-family routing and SBAS Operational Validation Dashboard before treating any alert-limit discussion as operational approval evidence.

ConceptRole
Protection LevelsExpress a conservative bound on navigation error
Alert limitsDefine the maximum acceptable bound for the intended operation
SBAS IntegrityProvides the broader monitoring and alerting function

A simplified relationship is:

Protection level ≤ alert limit → operation may be supported, if all other conditions are satisfied.
Protection level > alert limit → operation is not supported under that basis.

This is conceptual. The exact logic, timing, annunciation, and operational response must come from the applicable standards and approved operating procedures.

Why alert limits are operation-specific

Different operations tolerate different navigation-error bounds. En-route, terminal, non-precision, and vertically guided approach contexts do not have identical safety requirements.

Alert limits therefore depend on:

  • phase of flight;
  • procedure type;
  • lateral versus vertical dimension;
  • aircraft equipment and receiver mode;
  • service definition and regional approval;
  • procedure publication and operational authorization.

This is why this knowledge base avoids publishing free-floating alert-limit numbers unless they are tied to a verified source and context.

What alert limits are not

Alert limits are often misunderstood. They are not:

  • the actual navigation error;
  • a generic accuracy target;
  • an arbitrary cockpit warning threshold;
  • evidence by themselves that a procedure is authorized;
  • interchangeable across regions, systems, service definitions, or operation types.

They are part of a structured integrity decision chain.

Operational interpretation

For aviation use, alert limits help determine whether the current navigation mode remains acceptable for the intended operation.

A useful institutional phrasing is:

The alert limit defines the operation's tolerance boundary; the protection level indicates whether the current bounded solution fits inside that boundary.

If it does not, the operation must transition according to the approved procedure and aircraft/operator guidance. This page does not define that response.

Source anchors and current maturity

Relevant current source scaffolds include:

Institutional grounding note (2026-05-03): The ICAO APAC ITF/7 Preliminary Draft SBAS Implementation Guidance Document (A3-WP05, rev3, 14 May 2025) confirms LP, LPV, LNAV/VNAV, LNAV as recognized SBAS operational types and Class 3/4 as SBAS avionics classifications as of May 2025. Alert-limit values are operation-specific and must be sourced from the applicable service definition, DO-229F MOPS, and regulator material before operational use.

This page remains source-scaffold-linked. It provides a careful conceptual framework, but not yet a verified table of alert-limit values. Any future values must be tied to exact source family, operation type, revision/date, and applicability conditions; DO-229 alone is not enough to publish operational minima or procedure-specific thresholds.

Use in the knowledge base

Alert-limit language appears in:

This page should be the preferred concept anchor for future cleanup of alerting and threshold language.

Implementation relevance

For implementation planning, alert limits affect:

  • service-performance assessment;
  • procedure eligibility and publication;
  • cockpit mode availability and annunciation;
  • safety case and hazard analysis;
  • contingency procedures and downgrade logic;
  • training and operational documentation.

For regional SBAS planning, they highlight why a signal or coverage footprint is not enough. The service must support the operation’s integrity requirements under the relevant regional conditions.

Open verification tasks

  1. Verify authoritative source language for alert-limit definitions and dimensions.
  2. Audit procedure pages for unsourced alert-limit values and remove or qualify them.
  3. Separate alert-limit concepts from generic weather minima, decision altitude/height, and procedure chart minima.
  4. Add source-backed values only when the source, operation type, dimension, and revision are explicit.
  5. Connect verified alert-limit material to LPV-Approach-Procedure and SBAS in Civil Aviation MOC.

See also

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