EXECUTIVE SUMMARY
ES.1 PERFORMANCE
An independent risk assessment was conducted by the Johns Hopkins University Applied Physics Laboratory (JHU/APL) to determine if the Global Positioning System (GPS) and augmented GPS can satisfy the performance requirements to be the only navigation system installed in an aircraft and the only service provided by the Federal Aviation Administration(FAA) for operations anywhere in the National Airspace System (NAS). This report quantifies the ability of GPS, GPS with the Local Area Augmentation System (LAAS), and GPS with the Wide-Area Augmentation System (WAAS) to satisfy navigation performance requirements as expressed by accuracy, integrity, continuity, and availability requirements. Oceanic through Category III Precision Approach operations were evaluated with risks that present both normal and abnormal degrees of performance degradations. The primary conclusion is that GPS must be augmented to meet these requirements and that WAAS/LAAS can provide the required navigation performance. The study considered all known risks and its primary conclusion assumes the identified mitigation actions are instituted, and specific WAAS/LAAS configurations are implemented. The main conclusions of the study are as follows：
a. GPS with appropriate WAAS/LAAS configurations can satisfy the required navigation performance as the only navigation system installed in the aircraft and the only navigation service provided by the FAA.
b. Risks to GPS signal reception can be managed, but steps must be taken to minimize the effects of intentional interference.
c. A definitive national GPS plan and management commitment is needed to establish system improvements with civil aviation users and to provide greater informational access to the civil aviation community.
In particular, the final conclusion points to the need to develop a combined GPS and augmentations system design based on cost and performance trades among GPS system improvements, GPS operational policies, and WAAS/LAAS capabilities. Study findings with regard to the three system configurations considered are summarized in the following subsections.
ES.1.1 SATELLITE CONSTELLATIONS
Currently, 27 GPS satellites are operating. They provide the minimum basic configuration of 24 satellites (6 orbit planes of 4 satellites each) and 3 active on-orbit spares. The number of operating satellites could slip to 24 before additional replacements are added. In this study, the current constellation is assumed to be the nominal basic 24-satellite constellation (i.e., 6 by 4). The next logical extension of this geometry would be a 30-satellite constellation (i.e., 6 by 5), and that geometry was considered to represent an expanded GPS constellation that might practically be implemented.
The current GPS/WAAS test configuration is based on the current GPS constellation supported by two geostationary satellites (GEOS). Therefore, the base constellation for GPS/WAAS analysis was 24 GPS satellites and the current 2 GEOS. Improvements considered expansions up to five GEOS. GPS/LAAS analyses were based on the minimum acceptable GPS/WAAS configuration-a 24-satellite and a 30-satellite GPS constellation. Airport pseudolites (APLs) were also included to improve local geometry.
