Target tracking with multiple synchronous passive sensors is well studied in the literature. In such a system, each sensor provides a line-of-sight (LOS) measurement to the target and by fusing all the measurements from a given time, a composite measurement is formed to obtain the target position. If the LOS measurements are synchronized, two LOS measurements can be used to form a composite measurement. However, if the LOS measurements are not synchronized, the formation of composite measurements is not possible from two LOS. In this paper, a method is derived for forming composite measurements when the LOS measurements from multiple sensors are asynchronous. The required minimum number of asynchronous LOS measurements is addressed as a function of the number of passive sensors and the offset between successive LOS measurements. The procedure is used assuming the motion is constant velocity (CV), i.e., with process noise.