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 measurements and also convert error bounds into the position domain. Flight operations can be conducted using WAAS guidance only when the horizontal, and in some cases also the vertical, position error bound is less than a threshold that depends on the phase of flight. The expected fraction of time that a given flight operation can be conducted is termed its availability. Geomagnetic storms sometimes are accompanied by large spatial and temporal gradients in ionospheric delay that result in an increase in ionospheric and position error bounds, which reduces availability. The paper estimates availability of precision approach operations under various scenarios. ¿ 2001 American Geophysical Union