It reflects the inherent characters in the height measuring of InSAR, because it is not only the source of coherence but also the root of baseline decorrelation, and it defines the limitation of InSAR by a critical baseline.

A conclusion can be gotten after researching,that the main errors are arisen by means of thermal noise such as design error,space synchronization,phase synchronization and data processing decorrelation,and the effect and demand also have been analyzed.