What is an Alert Limit?
The Alert Limit (AL) is a critical parameter in the positioning system integrity concept. It defines the maximum allowable position error for safe operation in a given application. If the system’s calculated Protection Level (a statistical bound on the possible position error) exceeds the alert limit, an alert is triggered to indicate that the positioning information should not be trusted for safety-critical functions. The alert limit is set based on the requirements of the specific use case; for example, automotive applications may require an alert limit of 1–3 meters to ensure lane-level safety. The Integrity Risk is the probability that the actual position error exceeds the alert limit without timely warning.
The alert limit concept is foundational in both aviation and automotive safety standards, ensuring that users are notified whenever the system cannot guarantee the required level of accuracy and safety. In practice, if the protection level calculated by the system exceeds the alert limit, the system will disable or issue a warning to prevent unsafe operation.
