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The soft-touchdown, ldquosky-cranerdquo concept employed by the 2009 NASA Mars Science Laboratory mission requires an order-of-magnitude improvement from previous missions in the sensing of vehicle velocity and altitude. This paper describes the development of a new radar ldquoTerminal Descent Sensorrdquo that provides decimeter-per-second velocity accuracy while also providing better than 2% range accuracy on six unique beams. This sensor design uses a millimeter-wave center frequency (Ka-band) and pencil beam antennas to achieve the required velocity precision and to overcome the problems that angle-of-arrival errors can cause in velocity reconstruction. Included are discussions of the design concept, driving requirements, hardware architecture, and results from a high fidelity performance simulation.