A method and system for knowing positioning system standard time at a mobile unit with respect to a system, such as the Global Positioning System, when normal, direct measurement may be impracticable owing to low signal-to noise ratio, by calibrating the timing signal of an available communication network, such as a cellular telephone transmission network (610, 615). A time reference is set for the communication network with respect to the positioning system at a time when an adequate signal-to-noise ratio prevails and the offset of a timing event in the communication network control signal measured by means of the mobile unit's internal clock may be determined (620, 625). Subsequent times are measured with respect to this time reference by using the internal clock to measure time intervals therefrom.