Design and testing of an impulse ground-penetrating radar (GPR) for possible rover deployment on Mars in 2009 is well underway. The GPR has mass, power, volume, and data-rate targets of 0.5 kg, 3 W (peak), 3400 cm3, and 0.3 Mb/d and will possess easily modified bistatic high-frequency (e.g., 500--600 MHz) and monostatic rat-tail (e.g., 100 MHz) antennas. The GPR should be capable of measuring in situ radar properties on Mars to depths of up to 10--20 m, thereby helping to constrain near-surface geology and structure. Initial GPR tests at the site of the 2001 FIDO rover field trials near Cronese Lake, California, confirm the value of the instrument in defining local geologic setting and in providing context for data collected using other rover science payload elements. A rover-deployed GPR may enable three-dimensional mapping of local stratigraphy, which in turn can assist in evaluating the history of aqueous activity and the accessibility of near-surface water.