Extended velocity range

Adjusting pulse separation may not always be a feasible option when configuring HR mode. In such cases, the Signature instruments as well as the Gen2 Vector offer a unique feature, the Extended Velocity Range (EVR), which is used to enhance the ambiguity velocity. The EVR is a technique used in pulse-coherent Doppler systems to extend the maximum measurable velocity while maintaining high precision. It works by introducing an additional pulse pair with two distinct time lags, allowing the system to resolve phase ambiguity that occurs when velocities exceed the unambiguous range.

How EVR improves velocity measurements:

  1. The standard lag (longer pulse separation) provides high precision velocity measurements but is limited by phase wrapping.
  2. The EVR lag (shorter pulse separation) provides a coarse velocity estimate with a velocity range approximately three to five times greater than the standard lag.
  3. By comparing the phase shift from both lags, the system determines which ambiguity branch the velocity measurement belongs to, effectively unwrapping the phase and extending the measurable velocity range.

 

Figure: EVR resolving phase wrapping in complex flow. The flow measurements (blue) wrap under conditions where the velocity exceeds the configured ±30 cm/s limit. EVR is enabled to unwrap and recover the full velocity signal (red), even in the presence of turbulence and extreme flow bursts. This technique enables accurate measurements in high-energy environments where standard configurations would fail.

 


The first lag is used to get a course estimate of the velocity, the second and larger lag aims to improve the estimate. The first lag defines the ambiguity range of the setting and the second is the one that is actually reported after we have used the first estimate to determine what ambiguity branch we are on. The actual lags change with the velocity setting. The velocity calculated using this technique is: 

 

 

\( V = \frac{c}{4}\pi f(\Delta_{\phi2} - \Delta_{\phi1)}(\Delta_{t2} - \Delta_{t1}) \)(1)
The ambiguity velocity then becomes:  
\(V_{amb} = \frac{c}{4} f (\Delta_{t2} - \Delta_{t1})\)(2)
    
 \(V\)-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 of the first and second pulse lag \( [rad]\)
 \(\Delta_{t}\)-time lag for first and second measurement \( [s]\)

 

This technique has the advantage of producing an extended velocity measurement for all cells in the profile (unlike the coarse / fine pulse-pairs technique). Signal noise effectively limits the minimum usable time difference. Note that EVR does not eliminate the possibility of ambiguity problems if velocities exceed the specified velocity range set by the user.

When enabling EVR, there are two important constraints to consider. First, the maximum internal ping rate is reduced to half of the non-EVR operation. The pulse pairs typically used for velocity profiling are now also utilized in the phase unwrapping process. Second, introducing an additional pulse in the water increases the risk of pulse interference from boundary echoes. Despite these limitations, lower sample rates and potential boundary interference, EVR is highly effective. In many cases, it allows the HR instruments to function in higher-energy environments, such as the surf zone.

Updated