These avionics systems will use ionospheric corrections broadcast by WAAS, or self-generated onboard dual frequency corrections, depending on which one is more accurate.
These corrections are considered user position-independent, which means they can be applied instantly by any receiver inside the WAAS broadcast footprint.
The WAAS specification mandates availability as 99.999% (five nines) throughout the service area, equivalent to a downtime of just over 5 minutes per year.