Planes report pressure altitude via their transponders. 20-30 feet up and down is very normal for an autopilot. GPS altitude is used for vertical guidance for

Planes report pressure altitude via their transponders. 20-30 feet up and down is very normal for an autopilot. GPS altitude is used for vertical guidance for certain types of GPS approaches (i.e. "LPV" approaches[1]) and requires the airplane's avionics to be equipped with a WAAS[2] receiver that provides accurate altitude information. [1] https://en.wikipedia.org/wiki/Localizer_performance_with_vertical_guidance https://en.wikipedia.org/wiki/Localizer_performance_with_ver... [2] https://en.wikipedia.org/wiki/Wide_Area_Augmentation_System https://en.wikipedia.org/wiki/Wide_Area_Augmentation_System