Introduction

Synthetic aperture radar is a category of imaging side looking radar (SLR) for which the resolution along the speed axis is determined by using the Doppler shift instead of the antenna beam footprint, which cannot be narrow enough in the spaceborne case because of the huge distance to ground. Indeed, standard SLR (non-SAR) is never used for imaging from space. Principles of SLR and SAR are defined in Part 5. This subsection focuses on the design specificity of a spaceborne SAR.

The progression of the original idea of SAR (Wiley 1951) from airborne to space was slow, since the adaptation of technology to the space environment was far from simple. Indeed, the introduction of satellite altitude and speed in the SAR equations as well as the operational requirements conventionally attached to space remote sensing greatly increase the SAR design constraints.

• As shown below, the maximum available swath over resolution ratio, called merit factor, is drastically reduced (by a factor of at least 25) because the ambiguity and timing constraints are tightened. The minimum required antenna area is boosted (by a factor of at least 1,250). It ranges from 5 m ² in the X band to 50 m ² in the L band and drives the overall vehicle dimension, which is far from the case in airborne. Moreover, in the spaceborne case the swath (or the resolution) depends strictly...