An airborne radar altimeter operating at 36 GHz is uniquely capable of measuring the topography of water, land, and ice surfaces. The Multimode Airborne Radar Altimeter (MARA) was designed to combine a narrow transmitted pulsewidth, a high transmitted power level, and a narrow antenna beam to produce a high-precision ranging capability at the nadir of the aircraft platform and at four fixed off-nadir angles out to 12/spl deg/ in the multibeam mode described in this paper, or a single beam scanning between /spl plusmn/22/spl deg/ in the scanning radar altimeter mode. Data collected over water and land surfaces are presented to demonstrate the potential of MARA for topographic mapping with accuracies and precisions of value in dynamic oceanography, geodesy, geology, hydrology, biogeochemistry, biogeography, and glaciology. >