AVAILABILITY CHARACTERISTICS OF GPS AND AUGMENTATION ALTERNATIVES.

Acceptance of GPS as a sole-means navigation aid in the National Airspace System (NAS) will necessitate meeting stringent availability and continuity-of-service requirements that may be unachievable for GPS without augmentation. This paper compares availability characteristics for GPS and GPS augmented by barometric altimeter (baro)-aiding and/or geostationary satellites (GSs) with a GPS-like ranging capability. Availability is presented as the probability of achieving a specified performance level in terms of either horizontal/vertical dilution of precision (HDOP/VDOP) or user navigation error (UNE). Availability distributions are determined for users in the contiguous United States (CONUS), based on the planned operational GPS constellation and various GS deployments, with full consideration of operational probabilities for both GSs and GPS satellites. The impact of baro-aiding is included by parameterizing baro-altitude accuracy relative to GPS/GS pseudorange accuracy. The results are applied to assessment of GPS augmentation alternatives by comparing UNE performance with FAA accuracy requirements and integrity protection limits.