Conventional nadir radar altimetry is already in its third decade and has proven invaluable in understanding ocean sea surface height (SSH) from mesoscales to basin scales (see, e.g., Fu and Cazenave 2001, and other contributions in this volume). There is also a growing community of hydrologists using nadir altimetry data to monitor large lakes and rivers (see, e.g., Alsdorf et al. 2007). Significant advances have been made using small constellations of nadir altimeters to improve the coverage and sampling possible with a single altimeter instrument. However, limitations in nadir altimeter spatial resolution, or space–time sampling of the constellation, have not yet allowed the systematic mapping of small mesoscale and sub-mesoscale ocean features or smaller surface water bodies.