## 1 Introduction  
Bombardment by meteorites was a major geological process during the early history of Earth [1]. Unlike the Earth, the lack of the geological and atmospheric process on the Moon and other planets have prevented erosion of their impact craters. However, scars of these geologic forces and the past cratered Earth's surfaces have been deeply erased by erosion and plate tectonic processes [2]. The discovery of new impact structures is not only of high importance to gain an understanding of the Earth's early history, but also has economic significance as possible hosts of valuable ore deposits (e.g., gold and uranium) and hydrocarbon resources (e.g., oil and natural gas) [3, 4]. For example, in North America alone, approximately nine out of 19 confirmed impact craters were exploited for oil and gas with a daily production of 2-30 million barrels of oil and more than 42 million m\({}^{3}\) of gas of gas [5]. Consequently, it is worthwhile to search for new unknown terrestrial impact structures. Terrestrial craters are composed largely of two basic types: small, simple craters (<4 km in diameter) and large, complex craters (>4 km in diameter) [6]. A simple crater is a small bowl-shaped depression, with a structurally uplifted rim, which includes an overturned flap and ejecta. A complex crater, on the other hand, exhibits a structurally complex outer rim with a terraced wall, a topographically lower annular trough, and a central topographic peak [7]. Today, there are about 190 confirmed terrestrial impact craters (simple and complex) listed in the Earth Impact Database ([[http://www.unb.ca/passc/ImpactDatabase](http://www.unb.ca/passc/ImpactDatabase)]([http://www.unb.ca/passc/ImpactDatabase](http://www.unb.ca/passc/ImpactDatabase))). The distribution of these impact structures, as mapped by [4], illustrates a significant crater clustering in North America, Eurasia, and Australia, whereas the remaining continents, particularly Africa, lack their share of impact craters recorded at the same level. To date, only 20 structures were confirmed to be of an impact origin in the entire African continent. In addition, 49 structures are currently cited as being of potential impact origin and still awaiting field verification. Approximately, 55% of the confirmed structures in Africa are found in the Great Sahara alone [4] (Figure 1). In fact, the Sahara has long been known to host a large number of these structures which were found either during oil field explorations (e.g., the B.P. confirmed structure by [8]) or during the mapping of ancient river courses and lake basins for groundwater exploration (e.g., the Ibn-Battuah structure by [9]). There are seven structures that still remain to be discovered, buried beneath the dry sand of the desert. Presently, the harsh conditions and difficult accessibility are impeding the exploration of the vast area of the Great Sahara by ground-based techniques, making satellite data the only feasible means to hunt for suspected impact structures (e.g., [9; 10]). Figure 1: Confirmed and potential impact structures in the Great Sahara of North Africa. (**a**) Distribution of the structures plotted on an EarthSat mosaic. Confirmed structures are mapped in green, whereas proposed structures (including Rimaal), which are awaiting field confirmation, are plotted in red. (**b**) Confirmed and proposed craters in the Great Sahara with their reported diameters. Over the last four decades, the growing number and quality of space imagers has strongly contributed to the identification of possible impact structures (e.g., [9; 11]) which usually appear as distinct ring anomalies marking the original Earth surface. Unlike optical sensors, which can only image the surface of the desert, long-wave microwave sensors can unveil sand-hidden subsurface features [12]. In particular, radar images at C and L bands have successfully revealed sand-buried paleoriver courses and lake basins in several parts of the Great Sahara [13; 14; 15; 16; 17].