A method and apparatus for position determination is provided using measurements from both Global Positioning System (GPS) receivers and terrestrial-based Uplink Time Difference of Arpval (U-TDOA) receivers The method involves the transformation of downlink satellite measurements into equivalent U-TDOA measurements by computing comparable cross-correlation coefficients and time differences of arrival with respect to a U-TDOA reference station The method includes a weighting operation whereby the relative weights of the U-TDOA measurements and the relative weights of the GPS measurements are adjusted based on a theoretical scaling followed by empirical adjustments The method further involves the efficient computation and combining of metries that are used to minimize the weighted error between candidate location solutions and the U-TDOA and GPS measurements This is done efficiently in two ; dimensions for U-TDOA and in three dimensions for GPS measurements by increasing the complexity of searching operations as the optimal location solution is approached.