Abstract: In this paper section 1 briefly describes how to get the 2-D SAR's range and azimuth high resolution. section 2 describes interferometric SAR. The phase difference by using the interferometric method, and other known values to get the height value of the target. Airborne SAR uses two antenna method therefore it needs two paths through the target area in order to get the 3-D picture. section 3 describes the error in height measurement. section 4 describes shadowing and layover. section 5 describes how to choose the baseline B and the orientation a in order to avoid phase ambiguity and if it is ambiguous how to solve it by using phase unwrapping method. setion 6 describes the airborne SAR suffered from the problems of motion errors such as aircraft roll etc. and the method to correct it. setion 7 describes differential interferometry.section 8 briefly describes the future prospects. section 9 points out the 3-D interferometric principle may be used on sonar as well.