The Local Area Augmentation System (LAAS) is a differential GPS navigation system being developed to support aircraft precision approach and landing with guaranteed accuracy, integrity, continuity and availability. While the system promises to support Category I operations, significant technical challenges are encountered in supporting Category II and III operations. The primary concern has been the need to guarantee contentment with stringent requirements for navigation availability. This paper describes how Position Domain Monitoring (PDM) may be used to improve system availability by reducing the inflation factor for standard deviations of pseudorange correction errors. The role of PDM in mitigating the continuity and integrity risks are also presented with recent test results.