  For our understanding of the origin of ultra high energy cosmic rays, the
energy region between 10^17 and 10^19 eV is of crucial importance. Previous
experiments have found indirect evidence that at these energies, the origin of
cosmic rays changes from predominantly Galactic to extragalactic. In addition,
weak evidence for an excess of cosmic rays from the direction of the Galactic
center in a narrow energy band around 10^18 eV has been claimed. However, so
far there is no direct evidence supporting this scenario. Neither Galactic nor
extragalactic sources have been unambiguously established. Given the importance
of this energy range, there is a strong case for a dedicated experiment to
study the EeV energy region with high precision. We present the conceptual
design of GRaNDScan, a mobile stereo air fluorescence detector optimized to
study the spectrum, composition, and arrival direction of cosmic rays in this
important energy range. If located at a site on the southern hemisphere with
good exposure to the Galactic center, this type of experiment will provide an
accurate map of the Galactic center region, long suspected to harbor one or
several sources of ultra high energy cosmic rays.
