The pulse-coherent Doppler technique is a high-precision method used in several Nortek instruments, available as a HR mode for specific profiler instruments or as the standard measuring technique for the velocimeter, which measure water velocity with exceptional accuracy.
Pulse-coherent processing determines velocity by analyzing the phase shift between two closely spaced acoustic pulses.
The key steps are:
- The instrument transmits two short identical acoustic pulses separated by a known time lag ( \(t\)).
- These pulses reflect off suspended particles (scatterers) moving with the water.
- The returning echoes are received, and the phase shift between the returning pulses is measured.
- This phase shift is used to calculate the along-beam velocity (V) of the water.
The velocity along the beam is given by:
| \( V = \frac{\Delta \phi c}{4 \pi f \Delta t} \) | (1) | |||
| \(V\) | - | measured velocity along the beam axis \( [\frac{m}{s}]\) | ||
| \(c\) | - | speed of sound in water \( [\frac{m}{s}]\) | ||
| \(f\) | - | acoustic transmit frequency \( [\frac{1}{s}]\) | ||
| \(\Delta_{\phi}\) | - | phase shift between two consecutive pulses (lag) \( [rad]\) | ||
| \(\Delta_{t}\) | - | time difference between two consecutive pulses \( [s]\) | ||
The figure below demonstrates the principle of pulse-coherent Doppler processing for a Vector by comparing the timing of echoes received after a transmitted acoustic pulse. In both plots, the same transmitted signal (black) is reflected by particles in the water and received by the three transducers. In the upper plot, the echoes from the receivers are only slightly delayed, indicating a stationary or slowly moving particle. In contrast, the lower plot shows greater delays between the transmitted signal and the received echoes, consistent with a particle moving away from the instrument. This delay appears as a phase shift, which is used by the system to calculate particle velocity, which is assumed to be the same velocity as the current.
This phase-based approach allows for highly accurate velocity measurements over short ranges. However, it introduces limitations such as velocity ambiguity, where phase shifts exceeding ± radians become indistinguishable, leading to potential measurement errors. To mitigate this, the system's maximum measurable velocity (\(V_{max}\)) is constrained by the chosen time lag, calculated as:
| \( V_{max} = \frac{c}{4 f \Delta t} \) | (1) | |||
| \(V\) | - | measured velocity along the beam axis \( [\frac{m}{s}]\) | ||
| \(c\) | - | speed of sound in water \( [\frac{m}{s}]\) | ||
| \(f\) | - | acoustic transmit frequency \( [\frac{1}{s}]\) | ||
| \(\Delta_{t}\) | - | time difference between two consecutive pulses \( [s]\) | ||
Selecting an appropriate time lag is critical in pulse-coherent Doppler processing, as it balances the trade-off between measurable velocity range and precision. This method offers exceptional temporal resolution, low noise, and high precision, making it ideal for capturing fine-scale turbulence, rapidly varying flows, and near-boundary measurements, particularly in low-energy environments. However, its effectiveness is constrained by a short profiling range due to pulse separation and phase coherence limits. In high-velocity or highly turbulent conditions, the risk of velocity ambiguity increases, and phase unwrapping techniques may be required to maintain measurement accuracy.
Updated