We have examined the precision of interferometric synthetic aperture radar (SAR) measurements of surface deformation of the Earth using 24-cm wavelength data acquired by the Japanese Earth Resources Satellite 1 (JERS 1) spacecraft, over the Izu Peninsula, Japan. Radar images acquired from 1993 to 1994 were made into 11 independent interferograms. Apparent water vapor signatures with equivalent path delay up to 16 cm preclude reliable estimates of small deformation from only one interferogram. Averaging of interferograms improves the measurement, such that residuals between precise leveling data and corresponding radar observations are less than 1 cm RMS. There is a clear correlation between cloud cover extent on a given day and the spectral power of the water vapor signature in an interferogram formed from the corresponding day's SAR image. Also, several water vapor features associated with local topography are consistent with accumulation of moisture on the windward side of an on-shore flow. Analysis shows that decorrelation in mixed conifer and deciduous forest is a weak function of the time between observations, imposing a roughly constant level of additional decorrelation relative to scatter-stable urban areas. Furthermore, in high-relief areas, correlation is high when the baseline is tens of meters but drops rapidly with increasing baseline length. With strict control of the interferometric baseline length, adequate correlation in interferograms over very long time periods is possible even in forested, high relief regions. Observed inflation over Ito city is consistent with a localized magma source at depth, suggesting a different mechanism from the previous episode in 1989.