Loran Fault Trees for Required Navigation Performance 0.3

Long Range Navigation or Loran is an attractive candidate for providing redundant services for GPS because of its complementary RNAV, stratum 1 timing, and data channel capabilities. However, for Loran to be accepted as a redundant navigation system for aviation, it must meet the accuracy, availability, integrity, and continuity standards for Required Navigation Performance 0.3 (RNP 0.3). The Loran Integrity Performance Panel (LORIPP), a core team of experts, has been chartered to assess Loran’s potential to meet the RNP 0.3 performance. Proof that the system meets RNP 0.3 requirements involves demonstrating that all system threats are accounted. The LORIPP is developing a comprehensive hazard list and fault trees for requirements such as integrity and continuity for Loran. The hazard list enumerates the noteworthy faults that can cause integrity, continuity, availability or accuracy concerns. The fault tree allocates the acceptable error probabilities for each fault with regard to the requirement. For example, the integrity fault tree list shows all faults that can cause an integrity failure or Hazardously Misleading Information (HMI) and the probability that the fault will cause the failure. The probability allocations are selected based on what is known, what can be proven or what is required to meet overall the system requirement. In the case of integrity, the requirement is that the probability of HMI be 10 per hour or less. The paper details the Loran integrity and continuity fault tree that is being designed and developed by the LORIPP. The key elements of the design such as the division of faults and the fault allocations are explored. The paper details the decisions made in the design of the fault trees and how the design aids requirements analysis and book keeping. The anticipated result of the Loran fault tree analysis is to prove that Loran can meet RNP 0.3 integrity and continuity requirements. This enables Loran to provide cost effective redundant aircraft navigation services for GPS.