It is a difficult task for an unmanned ground vehicle using regular sensor to sense obstacles in unstructured environments or out fields, especially on negative obstacle sensing. In this paper, we present an approach for negative obstacle sensing, based on ultra-wideband synthetic aperture radar (SAR) image characteristic of negative obstacle sensing. And real data on negative obstacle sensing is collected by ultra-wideband synthetic aperture radar (SAR). In all, we carried out three experiments by using three different frequency band antennas, and experimental results show that ultra-wideband synthetic aperture radar (SAR) can display edge information of negative obstacle in synthetic aperture radar (SAR) images.