Let i_K denote the index from the previous capture (the procedure to get this during acquisition was described in Section 2.2.1.1) for previous burst j–1 and let j denote the current capture. Now, the algorithm has the following steps:

1. The algorithm checks if , if this is not true then it follows steps (2) and (3). If it is true, then the algorithm goes to step (4).
2. If however then i_k is set equal to i_k–1 and if then i_k is set equal to i_k+1.
3. The check in step (1) is now repeated on this new index i_K. In the water flow meter USS Software Library, a maximum of one such update in step (2) is allowed. If the algorithm still does not satisfy the inequality in step (1) then a full acquisition as in Section 2.2.1.1 is triggered.
4. The offset delay of the maximum from index i_K is then calculated from the parabolic interpolation equation. denotes the effective time.

The upstream absolute time of flight is thus given by Equation 13.

In this equation, T_proop is the propagation time that is specified using #define USS_ACOUSTIC_LENGTH in USS_userConfig.h and is maintained in USS_Meter_Configuration.acousticLength as part of the application meter configuration. It corresponds to the approximate propagation time for ultrasound in the given meter. Typically T_proop is 35 to 40 µs. T_offset corresponds to the number of cycles to back track from the lobe corresponding to the index i_K and is a function of the threshold η. In the USS Software Library, T_offset is set to 5 µs and can be modified using the user configuration #define USS_ALG_ADC_ADDITIONAL_CAP_DLY. It is maintained in the variable USS_Algorithms_User_Configuration.ADCAdditionalCaptureDelay.

The calculations of acquisition and tracking for the upstream direction are repeated for the downstream direction. The software keeps track of another index i_K for the downstream data for each burst j.