Overview

Nortek VM Acquisition is designed to collect high-quality ADCP data from moving vessels with minimal configuration effort. For most deployments, the default ADCP configuration is the recommended option and should be used.

In special cases, typically for scientific measurements or dynamic platforms, advanced users may need to adjust individual deployment parameters. The custom deployment feature allows VM Acquisition to load a comma-separated deployment text file and send those parameters to the VM ADCP when the measurement starts.

How VM ensembles are organized 

The Nortek VM system measures using ensembles of pings. Depending on the instrument maximum ping rate, the available pings can be grouped into one or more ensembles per second. Optional pings, such as echosounder, altimeter, and bottom-track pings, are placed at the end of the ensemble when enabled. The remaining slots are filled with velocity measurements from the slanted beams. 

Figure 1. Example VM1000 ensemble configurations. The VM1000 has a 16 Hz maximum ping rate; other VM ADCP systems follow the same principle with a lower maximum ping rate.

The ensemble update rate must leave enough slots for the configured ping types. The default VM ensemble update rate is selected to support the optional supplementary pings in addition to the current profiling ping or pings.

Parameters commonly adjusted in custom deployments

The parameters below are the ones most commonly considered for custom VM deployments. Adjust only the parameters required for the measurement objective, and verify the resulting deployment file before use.

SR - sampling rate

• Increasing the ensemble update rate may be useful when a bottom-track ping must be closer in time to the velocity pings, or when the objective is to resolve shorter time-scale variations in turbulent water. This can be particularly useful on dynamic platforms such as USVs or small boats operating in choppy conditions.
• Confirm that the selected ensemble length can accommodate all enabled ping types. Higher update rates reduce the number of slots available in each ensemble.

BT - bottom track

• Use BT=1 to enable bottom track and BT=0 to disable it. Bottom-track speed-over-ground measurements are made in the ADCP platform time and coordinate system, which helps avoid spatial, rotational, or timing offsets between separate devices and normally gives the best results.
• Disabling bottom track may be useful when bottom track is out of range, invalid because of a moving riverbed, or intermittent because of a very soft bed. The freed slot is filled with an additional velocity ping.
• If bottom track is disabled, use a PTP or NTP time server and enable GNSS angular motion correction in VM Acquisition when applicable for the best qualitative outcome.

ALTI - altimeter

• Use ALTI=1 to enable the altimeter and ALTI=0 to disable it. On VM1000 and VM500 systems, the altimeter uses the vertical beam to measure depth directly under the vessel.
• Disabling the altimeter may be useful when direct depth under the vessel is not critical and the slot is better used for an additional velocity ping. Bottom track also estimates depth from the four slanted beams, but with lower accuracy.

VR - velocity range

• The default velocity range is 5.0 m/s along the beams. Lowering the range, for example from 5.0 to 2.5 m/s, can improve single-ping precision when operating in smooth water with low vessel speed and low background current.
• Lower velocity range improves precision but reduces tolerance for high velocities. If velocity exceeds the selected range, the ADCP data will contain an unresolvable ambiguity.

NB - number of beams

• NB=5; enable the fifth beam when direct vertical velocity from the vertical beam is required. This consumes an additional separate ping, so the minimum ensemble size is 2 pings when this option is enabled.

Example scenarios

• A USV or small boat operating in choppy water may benefit from a higher ensemble update rate and fewer averaged pings.
• A river deployment with invalid or intermittent bottom track may benefit from disabling bottom track and using the freed ping slot for additional velocity measurements.
• A study focused on small spatial or time-scale phenomena from a moving vessel may benefit from single-ping ensembles or minimal averaging, provided the motion correction and time synchronization are robust.

Procedure: create and start a custom VM deployment

1. Connect to the instrument using Nortek VM Acquisition.
2. Configure the instrument as close as possible to the desired setup using the standard VM Acquisition controls.

3. Open Deployment details and select Save to File to create a deployment file based on the current configuration.

   Figure 2. Save a deployment file from the closest standard configuration before editing individual parameters.

    

4. Open the saved file in a text editor such as Notepad. Modify only the required parameters, then save the file.

5. In VM Acquisition, clear the Use default deployment checkbox. When this option is cleared, a custom deployment file can be selected at the start of the measurement.

   Figure 3. Clear Use default deployment to allow selection of a custom deployment file when starting the measurement. 

    

6. Set the required basic settings, then start the measurement and select the custom deployment file when prompted.
7. If the custom deployment is valid, VM Acquisition starts the measurement. Some unsupported data types may not be visualized in VM Acquisition but are stored in the raw AD2CP file for manual post-processing.

8. If the deployment is invalid, VM Acquisition displays a message describing the first detected error. Correct the file and repeat the validation process. Several iterations may be required, because only one error may be shown at a time.

   Figure 4. Error message for an invalid configuration. The set sampling rate is invalid and should be one of the suggested instead.

Operational notes and troubleshooting