Abstract: To solve the single-observer location using illuminators of opportunity, a constrained Weighted least squares (CWLS) location algo-rithm based on time differences of arrival is proposed. Firstly, introducing the range from the target to the observer as the additional variable, the TDOA measurement equations are linearized. Considering the relationship between the additional variable and the target location, the location problem is established as a CWLS model. To solve the CWLS model to obtain the estimation of the target posi-tion, the solution is derived based on Lagrange multiplier. Finally, the theoretical error and the Cramer Rao Lower Bound (CRLB) are also derived. Simulation results show that the proposed algorithm approaches the CRLB at high noise level. Moreover, from the Geometric Dilution of Precision (GDOP) figure, we can conclude that target Height and illuminator position are also important fac-tors affecting the localization accuracy.