High resolution (HR) mode

This article explores how the Vector 2 applies the high-resolution pulse-coherent processing principles in a single-point velocimeter format. Rather than providing vertical profiles, it offers highly localized, three-component velocity data at an extremely high temporal resolution. The following sections describe its unique architecture, performance characteristics, and common use cases.

General information on high resolution measurements can be found here: HR mode.

Bistatic geometry: central transmitter with angled receivers

The Vector employs a bistatic acoustic design. A central vertical transmitter emits acoustic pulses while three symmetrically spaced angled receivers detect echoes from a converging point located approximately 15.7 cm above the transmitter, this is fixed from production. This differs from profiler instruments, which transmit and receive along the same slanted beams. The Vector’s geometry is optimized to capture water velocity at a single, fixed location with high directional accuracy. The sampling volume is a combination of the transmit length and receive window, where the default sampling volume is 12 mm. This tightly constrained beam intersection forms the basis of its fixed-volume measurement approach, allowing reliable three-dimensional velocity reconstruction within a defined region near the sensor as shown in Figure (1).

High-resolution point samping methodolgy

Unlike profiling systems, the Vector samples velocity at a fixed point, with spatial resolution determined by the convergence of its bistatic beams just centimeters above the sensor. This approach makes it an excellent companion to profiler instruments by offering detailed temporal data where spatial profiling is not required or feasible. 

Additionally, the Generation 2 Vector uses a simultaneous pinging scheme (updated from the Gen1 Vector using the multiplexing pinging scheme) across all three receivers, enabling synchronized 3D velocity measurements without temporal skew between components, as shown in Figure (2). This is especially beneficial in rapidly evolving flow conditions, such as boundary layers, breaking waves, or turbulent shear, where accurate time alignment is critical for capturing transient flow structures and computing reliable turbulence metrics.

Figure 3 below visualizes how a sound wave transmitted through the central transducer (shown in blue), hits a particle (red), which in next turn reflects parts of the sound (Doppler shifted - shown in black) back to the receiver arm, which will detect the reflected sound wave.

Processing Technique 

Figure 4 below illustrates how the Vector measures velocity using the pulse-coherent Doppler technique. The central transducer transmits two acoustic pulses (P1 and P2), separated by a time lag ( \( \Delta t \) ). As these pulses travel, they reflect off a moving particle (red dot), which shifts position over time. The phase difference ( \( \phi 1 \) and \( \phi 2 \)) between the received echoes of the two pulses is then used to calculate the particle’s velocity. This method allows the instrument to precisely measure water movement by detecting small changes in phase.

The Vector is capable of sampling at rates up to 64 Hz, with the option to reduce this frequency depending on deployment configuration, experimental objectives, and prevailing scattering conditions.

Coordinate system

The Generation 2 Vector has a different XYZ definition from the Gen1 Vector (see Figure (5) below). This change was made to make the Generation 2 Vector coordinate definitions match the rest of Nortek's ADCP systems.

Phase unwrapping in velocimeters instruments

Phase wrapping in the Vector is governed by the same fundamental limit as in profilers, but its correction is more straightforward. Because the Vector samples at a single fixed location with consistently high correlation and minimal geometric variation, velocity ambiguity can be more reliably detected and resolved. Extended velocity range (EVR) is also available in the Generation 2 Vector, but is generally more effective due to the stable acoustic conditions and reduced complexity of applying corrections across only one measurement point rather than an entire profile.

Acoustic Streaming

Acoustic streaming, also known as secondary flow, occurs when the instrument’s transmitted pulses generate a steady fluid motion in the surrounding water. This effect becomes significant when ambient flow velocities are low, particularly when they are below 8 cm/s, where streaming can introduce a measurable velocity bias.

The effect of acoustic streaming can be observed by placing the instrument in still water and monitoring velocity readings over time. In stationary water, induced flows of 2–3 cm/s have been recorded when the power level is set to HIGH, as shown in Figure (6). This effect is more pronounced in small, confined spaces such as laboratory tanks or flumes, where natural flow is minimal, allowing the acoustic energy to generate movement in the water.

Lowering the transmission power reduces the magnitude of transducer-induced flow. However, this comes at the expense of a lower SNR in the return signal. Deploying the instrument in open water or larger tanks helps minimize the effects of acoustic streaming by allowing natural ambient currents to dominate. When streaming cannot be avoided, it can be accounted for by measuring and subtracting the induced flow from velocity readings using background subtraction techniques.

Minimizing boundary interference with Vector measurements

Due to its proximity-based measurement zone and lack of profiling beams, the Vector is less prone to pulse overlap or weak spots seen in near-boundary profiler deployments. However, deployment setup remains critical:

•  If positioned too close to solid boundaries (e.g., tank walls or seabeds), reflections and echoes may distort signal quality. 

• In lab or flume conditions, acoustic dampening materials (e.g., rubber mats, plexiglass) are recommended to reduce unwanted reflections. 

• In natural environments, soft sediment and flow variability usually help minimize boundary echo artifacts.

 While less vulnerable than a profiler instrument, the Vector's precision requires a stable acoustic environment to ensure consistent measurements. Figure (7) below shows how the Generation 2 Vector can be mounted. 

💡 Note: For side-looking applications, the Vector should be mounted perpendicular to the flow. Current flow traveling directly toward the transmitter, the maximum measurable velocity will be the lowest it can be, while flow toward the connector end can cause housing-induced flow disturbance at the sampling volume.

What are weak spots and how can I avoid them?

The presence of a boundary close to the velocimeter sampling volume may give rise to problems. This is especially the case if the boundary is hard (rocks, metal, concrete, glass etc.) and/or the water echo is weak.

The weak spots (or pulse-to-pulse interference, when discussing profilers) are related to the spatial separation between the pulse pairs transmitted by your velocimeter. To be more precise, a weak spot occurs when the first ping hits the bottom and reflects, and this reflected signal reaches the sampling volume at the same time as the second pulse. The position is thus deterministic and can be calculated.

For each velocity range, there are one or two distances that give rise to problems. The existence of these “weak spots” can be identified in the data record by a decrease in the correlation and an increase in the velocity variance. The problem is mostly seen in flumes with a hard bottom but has also been observed in the field, especially at higher velocity ranges.

The vertical extent of weak spots is a function of the bottom composition. If the bottom is well defined (e.g. sand) the extent is no bigger than the transmit pulse or about 1 cm. If the bottom is rough, the vertical extent can be larger. It is also a matter of the relative strength between the water scattering content and the bottom echo – if the water scattering is high the whole issue goes away.

Keep in mind that weak spot positions are only a general location, and they will vary a bit based on the speed of sound and can extend over a range of about 1 cm. For a standard Vector, the distances from the sampling volume to the boundary that should be avoided are:

The problem is less acute at lower velocity ranges and this is in itself a good reason to avoid higher ranges unless required. Another way out of the problem is to use side-looking probes; the issue goes away if there are no boundaries in the path of the transmit pulse. However, a side-looking probe will have exactly the same problem if there is a wall or other vertical obstruction facing the probe.