SIR-A and SIR-B images or three sand dunes and two bedrock regions in Alashan Plateau of north central China have been studied and compared to the digital processing Landsat imagery and field investigation data. The results of the study show that radar illuminating direction is a key factor

which results in the echo strength change for the same type and size of sand dunes. The sand dunes in which the slipfaces are oriented to radar beam have a bright-point response and the dunes in which the gentle slopes face the radar beam have a dark signature on the radar images. Two Precambrian metamorphic rock bodies buried beneath the thin layer of alluvial material or aeolian sand have been detected by using the spaceborne radar imagery. A preliminary scattering model was given to explain the imaging mechanism.