A method of operating an ultrasonic flow meter to measure a flow rate of a fluid, the ultrasonic flow meter comprises: - at least two ultrasonic transducers (5A, 5B) arranged for transmission and receipt of ultrasonic signals in opposite directions across a measuring distance; - a generator circuit for controlled generation of an acoustic wave packet to be transmitted from the at least two ultrasonic transducers; - a receiver circuit for receiving the transmitted signals from the at least two ultrasonic transducers; - a processing means for digital signal processing; wherein the method comprises: for at least two signal paths along generally opposite directions: - generating the acoustic wave packet to be output into the measuring distance at a first of the at least two ultrasonic transducers, the wave packet being generated at a signal frequency; - receiving the acoustic wave packet at a second of the at least two ultrasonic transducers as a received signal; - digitizing the received signal at a sampling frequency below the Nyquist-limit to generate a digitized received under-sampled signal for the given signal path; characterized by - from at least two digitized received non-reconstructed under-sampled signals along generally opposite directions, determine the difference in propagation time along the measuring distance: either by performing a correlation between the at least two digitized received under-sampled signals along generally opposite directions, and determine the maximum correlation between the at least two signals, or by determining the phase-shifts between the digitized received under-sampled signals along the generally opposite directions; - and averaging the determined time difference obtained over a number of measurements. - at least two ultrasonic transducers (5A, 5B) arranged for transmission and receipt of ultrasonic signals in opposite directions across a measuring distance; - a generator circuit for controlled generation of an acoustic wave packet to be transmitted from the at least two ultrasonic transducers; - a receiver circuit for receiving the transmitted signals from the at least two ultrasonic transducers; - a processing means for digital signal processing; - generating the acoustic wave packet to be output into the measuring distance at a first of the at least two ultrasonic transducers, the wave packet being generated at a signal frequency; - receiving the acoustic wave packet at a second of the at least two ultrasonic transducers as a received signal; - digitizing the received signal at a sampling frequency below the Nyquist-limit to generate a digitized received under-sampled signal for the given signal path; - from at least two digitized received non-reconstructed under-sampled signals along generally opposite directions, determine the difference in propagation time along the measuring distance: either by performing a correlation between the at least two digitized received under-sampled signals along generally opposite directions, and determine the maximum correlation between the at least two signals, or by determining the phase-shifts between the digitized received under-sampled signals along the generally opposite directions; either by performing a correlation between the at least two digitized received under-sampled signals along generally opposite directions, and determine the maximum correlation between the at least two signals, or by determining the phase-shifts between the digitized received under-sampled signals along the generally opposite directions; - and averaging the determined time difference obtained over a number of measurements.