Accurate topographic data over the global land and ice masses have numerous applications in areas of geology, geophysics, hydrophysics and polar ice research. It is well established that interferometric synthetic aperture radar (InSAR) is a method which may provide a means of estimating global topography with high spatial resolution and height accuracy. ONe implementation approach, that of utilizing a single SAR system in a nearly repeating orbit, is attractive not only for cost and complexity reasons but also in that it permits inference of changes in the surface over the orbit repeat cycle echoes. This paper analyzes InSAR spatial geometry model and gives a phase error model. The paper also discusses the characteristics of InSAR echo signal. Finally, the general procedure of InSAR imaging is outlined.