Observations of amplitude and phase scintillation were made at the Millstone Hill radar facility by using phase coherent sources at 150 and 400 MHz on the U.S. Navy navigation system satellites. These observations have been processed to yield power spectra of the fluctuations in the logarithm of the received power (log power) at each frequency and the fluctuations in phase path length difference (differential phase) between the two frequencies. A theoretical analysis was performed to predict the log power and differential phase power spectra. The theoretical model was based upon weak scatter theory for a thick screen with a three-dimensional power law power spectrum representation for the electron density fluctuations. A -4 exponent for the power law best fit the observed spectra. For weak scintillation (rms fluctuations in log power less than 5 dB or S4<0.9) the theoretical calculations were in excellent agreement with the observations. For strong scintillation, saturation was observed, and the measured spectra were broader than those predicted on the basis of weak scatter theory. An increase in electron density fluctuations relative to the value predicted by the power law model was observed at scale sizes smaller than 0.8 km. This increase may be responsible for the anomalous scintillation observations at gigahertz frequencies.