An introduction to Wide Area Augmentation System and its predicted performance

The Federal Aviation Administration is developing a Wide Area Augmentation System (WAAS) to GPS that will broadcast clock, ephemeris, and ionospheric corrections. Aviation user equipment will apply the corrections to GPS meas urements and also convert error bounds into the position domain. Flight ope rations can be conducted using WAAS guidance only when the horizontal, and in some cases also the vertical, position error bound is less than a thresh old that depends on the phase of flight. The expected fraction of time that a given flight operation can be conducted is termed its availability. Geom agnetic storms sometimes are accompanied by large spatial and temporal grad ients in ionospheric delay that result in an increase in ionospheric and po sition error bounds, which reduces availability. The paper estimates availa bility of precision approach operations under various scenarios.