We report here observations of tropospheric phase fluctuations at 5 GHz on the near-vertical paths relevant to a number of astronomical and geophysical measurements. Using the Very Large Array near Socorro, New Mexico, the spatial structure function of phase was measured in the scale size range 1–35 km. The 23 data sets reported here were taken over a 2.5-year interval. During 15 observing sessions there is a clear detection of tropospheric phase fluctuation, while only upper limits can be constructed on the remaining 8 days. The measured structure functions vary as (baseline)x, with the average value of x being 1.4. Where comparisons with other microwave observations (e.g., water vapor radiometry) are possible, there is reasonably good agreement in both the level and shape of the power spectrum of these tropospheric phase fluctuations. Some implications of these observations for precision Doppler tracking of spacecraft and geodesy/radio interferometry observations are discussed. Although the level of these tropospheric scintillations is highly time variable, very low frequency gavitational radiation searches using Doppler tracking of distant spacecraft will have, if uncalibrated, tropospheric propagation noise at a typical level Δƒ/ƒ∼ 5 × 10−15 for 1000-s integration times.