KIGAM and KNU are developing a ground-based Arcscanning SAR system (ArcSAR) mounted on a truck. The system achieves the coherent integration of radar returns from ground targets by the circular motion of the antennae attached to the end of an extendable arm. Precise control of antenna position and the extended coherent-integrationlength enable the formation of high-resolution, highprecision and phase-preserving SAR images. Based on the Polar Format Algorithm, two SAR-focusing algorithms were developed for the data acquired from two different scanning modes of this unique system: the scan mode and the spot mode.