Estimation of the time-of-flight is the most recently used procedure in material thickness measurement, ultrasound velocity estimation, mechanical load and stiffness evaluation, curing or melting monitoring, temperature measurement etc. A review of the pulse time of flight estimation techniques is given. Simple techniques like thresholding, zero crossing or phase shift are discussed. For high accuracy measurements more advanced techniques are needed to exploit whole information contained in signal. Advanced techniques use maximum likelihood criteria for time position estimation. Likeness between the reference (transmitted) signal and received signal is analyzed here. A comparison of the correlation maximum, difference L1 norm minimum and difference L2 norm minimum techniques is given. Processing in digital domain introduces additional errors. Augmentation of the noise component of the Cramer-Rao equation accounting the analog-to-digital conversion influence (quantization and clock jitter) is presented. Estimate of the time of flight in digital domain is discrete. Subsample interpolated value can be obtained using sinc function, but usually truncated version is used. Bias errors introduced by parabolic, cosine and Gaussian interpolation are analyzed and compared to frequency domain interpolator. Application of the findings discussed above for ultrasonic system engineering is given.
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