A method for the determination of the time of flight of a signal transmitted between a transmitter (42, 44) and a receiver (44, 42). In one form, the method involves transmitting a first signal and a second signal having a waveform modification introduced at a predetermined point in time of the duration of the second signal; receiving said first and second transmitted signals; determining a point of diversion between the first and second received signals to determine an arrival time of the introduced waveform feature modification at the receiver. In addition, the invention provides an accurate time of flight determination of ultrasonic signals in a flow sensor (24) adapted for a smoke detector system (10).