Preliminary Analysis of Requirements for CAT IIIB LAAS*

The FAA is in the early stages of performing analyses leading to the specification for a Performance Type 3 (PT 3) Local Area Augmentation System (LAAS) Ground Facility (LGF) capable of supporting Category IIIb (CAT IIIb) precision approach and landing operations. This paper presents a preliminary examination of integrity, continuity and availability requirements for a PT 3 LGF. The requirements for PT 3 are more stringent than for PT 1 (supports CAT I), particularly with regard to integrity performance. Consequently, the probabilities of missed detection for PT 3 ranging source fault monitors are found to be somewhat smaller than for PT 1. However, the exposure time for the more stringent integrity requirements is shorter than for PT 1. Therefore, the probability of failure of a ground reference receiver during the exposure time for PT 3 is actually less than was previously assumed. As a result, new smaller values of parameters used in aircraft protection level computations are proposed. An initial allocation of continuity requirements is made based on rationale similar to that for PT 1. However, the multiple reference consistency check (MRCC) used in the LGF to compare corrections among reference receivers must meet requirements for both continuity and integrity. The analysis indicates that the PT 3 MRCC threshold needs to be set higher than for PT 1, in order to meet the more stringent integrity requirement for wrong exclusion of a reference receiver. The higher MRCC threshold leads to the achievement of lower continuity risk, allowing a reallocation of continuity requirements to other risk areas. Predicted availability of GPS satellite geometry to meet PT 3 requirements is found to be 0.994 for the originally proposed Vertical Alert Limit (VAL) of 5.3 m. However, if PT 3 VAL can be kept at the 10-m level used for PT 1, availability is predicted to be 0.999. * The contents of this material reflect the views of the author. Neither the Federal Aviation Administration nor the Department of Transportation makes any warranty or guarantee, or promise, expressed or implied concerning the content or accuracy of the views expressed herein.