Availability Enhancements for CAT IIIB LAAS

Abstract

: Enhancements are under consideration for the Federal Aviation Administration's (FAA) Category I (CAT I) Local Area Augmentation System (LAAS) to meet the more stringent performance requirements of CAT IIIb operations. This paper analyzes the relative improvement predicted for various alternatives designed to enhance LAAS CAT IIIb availability. These enhancements include increasing the number of ranging sources, using less-conservative error models, taking credit for redundant GPS navigation sensors in the aircraft, using a second frequency to overcome the effects of ionospheric delay, or using differential carrier-phase measurements in the position solution. Availability under each scenario is computed for a vertical alert limit (VAL) ranging from 5.3 m (current requirement) to 10.0 m. Results show that unless VAL is increased beyond about 5.3 m, adequate (0.999) availability cannot be achieved merely by extending the CAT I architecture or by using any one of the above methods alone, with the exception of using dual frequency in both the ground facility and the aircraft. On the other hand, if VAL can be increased to approximately 7.5 m, some other alternatives alone and most pairwise combinations of these alternatives will provide adequate availability.