Target Integrity Risk (TIR) is the specified maximum acceptable probability that a positioning system will provide hazardously misleading information without timely warning, setting the quantitative safety threshold that integrity monitoring systems must achieve. TIR values reflect application-specific safety requirements and drive the design of protection level computation, fault detection algorithms, and overall system architecture.
The TIR specification establishes the probability bound that actual position error may exceed the protection level or alert limit without the system issuing an alert. For example, a TIR of 10⁻⁷ per hour means the probability of undetected hazardous position error must remain below one in ten million per hour of operation. This demanding requirement reflects the serious consequences of misleading information in safety-critical applications, an autonomous vehicle or aircraft acting on incorrect position data could cause casualties.
Different applications specify different TIR values based on hazard analysis and acceptable risk levels. Aviation precision approaches may require TIR values of 10⁻⁷ per approach or lower. Automotive lane-level positioning may specify TIR between 10⁻⁷ and 10⁻⁸ per hour depending on vehicle speed and operational context. The TIR must be allocated across subsystems, GNSS corrections, positioning engine, sensor fusion, integrity monitoring, with each component contributing only its share of the total risk budget.
Demonstrating TIR compliance presents significant verification challenges. Direct testing to validate one-in-ten-million failure rates would require billions of operating hours. Instead, compliance arguments combine analysis (identifying and bounding all potential failure modes), simulation (modeling system behavior under nominal and fault conditions), and testing (validating critical parameters and algorithms). Regulatory frameworks for aviation provide established methodologies; automotive integrity frameworks are still maturing. Achieving and documenting TIR compliance is essential for any GNSS system intended for safety-critical applications.
