The spectrum of tropospheric phase fluctuations expected for a radio antenna at timescales <100 s on a space to ground link has been calculated. A new feature included in these calculations is the effect of aperture averaging, which causes a reduction in delay fluctuations on timescales less than the antenna wind speed crossing time, ~D/(8 m/s) (D is the antenna diameter). On timescales less than a few seconds, the Allan deviation &sgr;y(Δt)∝(Δt)+1, rather than &sgr;y(Δt)∝(Δt)-1/6 without aperture averaging. Calibration of tropospheric phase fluctuations with water vapor radiometers will not be possible on timescales less than ~10 s, due to thermal radiometer noise. However, the tropospheric fluctuation level will be small enough that phase measurements on timescales less than a few seconds will be limited by the stability of frequency standards and/or other nontropospheric effects. ¿ 1998 American Geophysical Union