Description of a GNSS availability model and its use in developing requirements

This paper discusses the use of the Global Navigation Satellite System (GNSS) Air Traffic Operations Model (GATOM) to assess GNSS availability. Many models estimate average availability. In addition to the average value, the GATOM output also includes the little-studied temporal and spatial characteristics of GNSS availability. These characteristics must be understood so that operationally meaningful availability requirements can be defined, and augmentations to GNSS developed to allow the overall system to meet these requirements. GATOM is a comprehensive, Markov process model that calculates a variety of statistical measures of the required services at locations specified by the user; the primary GNSS services modeled at present are navigation and the receiver autonomous integrity monitoring (RAIM) detection and isolation functions. The current version of the model is briefly described, and then the results are presented of an analysis conducted with the model regarding availability during Category I and nonprecision approaches. The analysis addresses Global Positioning System (GPS) constellations of 24 to 32 satellites, and augmentation with baro-altimeter and geostationary satellites.<>