Evaluation of LAAS availability with an enhanced GPS constellation

An aircraft landing system must not only meet certain accuracy requirements, it must also achieve the necessary levels of integrity, availability, and continuity of function. Integrity is a measure of the probability that the system will not provide the user with hazardously misleading information. Continuity of function is a type of availability which is a measure of the probability that the system will remain available throughout a precision approach, assuming the system was available at the start of the approach. The design of a landing system requires a compromise between integrity and availability. Since the vertical and horizontal position error statistics are a function of satellite position (due to geometry and the effects of elevation angle on noise and multipath), integrity, availability and continuity of function can only be accurately estimated using a 24-hour simulation. A local-area availability model has been developed based on the local-area augmentation system (LAAS) architecture being worked on within RTCA SC-159 Working Group 4. The model was used to determine availability of LAAS for CAT I, II, and III precision approach, as well as measuring the improvement in availability with such augmentations as pseudolites, geostationary satellites, and a larger (30 satellite) GPS constellation.