This chapter introduced the basic concepts of SAR and the principle of 2D target resolution. Obtaining high range resolution with wideband signal and high azimuth resolution with long synthetic array is the basis of the 2D high-resolution imaging. RDA is widely used in SAR imaging processing. Azimuth-translation invariance is the key of RDA. This chapter described the major processing step of RDA, including range compression, range migration correction and azimuth compression, and derives the signal expression of the key step. With the development of SAR imaging requirements, SAR has developed a variety of working modes. This chapter also introduced the working geometry of Stripmap mode, Spotlight mode, Sliding Spotlight mode, TOPS mode and Scan mode, established the unified signal model for multi-modes and analysed the signal property. In imaging processing for multi modes, a unified focusing algorithm based on FrFT is shown, which can simultaneously process Stripmap, Spotlight, Sliding Spotlight and TOPS SAR mode data. The focusing algorithm is parameterised into a rotation angle or a rotation factor, which is determined by the rotation centre distance. Real data of different modes can be well focused by using different and appropriate rotation angles.