Passive geolocation of uncooperative radar emitters remains an important problem in radar electronic warfare. Several location estimation techniques have been investigated in the past. In this paper, we present a passive geolocation technique for radar emitters using Doppler frequency measurements. For uncooperative sources, neither the emitter location, nor its transmitted frequency is known a priori. The relationship between these unknowns and the measured Doppler frequencies is non-linear. In the special case where the moving receiver measures frequencies along a straight path at constant speed, the relationship becomes linear in the Cartesian location coordinates. A simple 1-D discrete search for the transmitted frequency is followed by a least squares (LS) estimator to provide a coarse estimate of the emitter coordinates. This is followed by Newton's algorithm to provide a maximum likelihood (ML) estimation. The simulation results demonstrate that the resulting ML estimator approximately meets the Cramer-Rao lower bound (CRLB).