Accurate and reliable measurements of ocean subsurface currents are fundamental to a vast array of scientific disciplines, ranging from advanced climate modelling and predicting marine ecosystem trends. Within the framework of EMSO ERIC, single-point current meters and Acoustic Doppler Current Profilers (ADCPs) are indispensable instruments for quantifying the speed and direction of water currents throughout the water column.

At the recent OCEANS 2025 conference in Brest, France (June 16-19), Dr. Beatrice Tomasi, researcher from NORCE and the University of Bergen, who works in the EMSO Nordic Seas Regional Facility, presented a pivotal paper titled “Pre- and post-deployment recommendations for single-point current meters and Acoustic Doppler Current Profilers”.

 

Fig 2. Beatrice Tomasi during the paper presentation at OCEANS 2025, Brest, June 13-16. Credits: NORCE

The paper directly addresses the critical imperative to mitigate biases that can compromise the accuracy and integrity of data collected by these oceanographic instruments. This work, a collaboration between the EMSO ADCP working group and the French Naval Hydrographic and Oceanographic Service (SHOM) benefiting from the MINKE EU project, addresses critical biases affecting ADCP and single-point current meter data accuracy (read our news for more on this here).

Single-point current meters and ADCPs are mounted on most EMSO ERIC platforms, being the second most used instruments in the consortium, just after the CTD, and allow to capture data on subsurface currents, considered by the Global Ocean Observing System (GOOS) as an Essential Ocean Variable (EOV) for ocean physics.

 

Fig.3: (A) Map of EMSO platforms with one or more current meters/profilers installed. (B) Overall number (47) of single-point current meters (23) and ADCPs (24) used at EMSO sites sorted by deployment depth. Left: Power source of the instruments, Right: Pulse frequency of the instruments. Instruments with frequencies equal or greater than 1000 kHz are single-point current meters, except for one unit which is an ADCP. Below 1000 kHz all units are ADCPs.

 

How ADCPs and single-point current meters work

These instruments emit ultrasound pulses (“pings” of sound) at a constant frequency into the water. As the sound waves travel, they hit particles suspended in the moving water and their frequency changes upon return due to the Doppler effect. By measuring this “Doppler shift,” the instrument calculates particle velocity, and thus water speed. Multiple acoustic beams determine the three-dimensional direction of the current. ADCPs provide measures at different points (cells) along a profile that can reach hundreds of meters (depending on the ADCP frequency) while single-point current meters measure just one cell close to the instrument.

 

Addressing Data quality challenges

The paper outlines a methodology for calibrating Acoustic Doppler Current Profilers (ADCPs) and demonstrates the impact of calibration errors on oceanographic data and autonomous underwater vehicle (AUV) operations.

Three different ADCPs (labelled A, B, and C) with slightly different frequencies and characteristics were evaluated at SHOM: ADCP A operates at 300 kHz, while ADCPs B and C operate at 500 kHz (all have four transducers).

ADCP B and ADCP C were bought in 2022 and deployed in 2023 as part of the EMSO-Mohn observatory at the Fåvne hydrothermal vent in the Nordic Seas EMSO Regional Facility, at a water depth of approximately 2,600 m. Following a one-year deployment, the observatory was recovered in July 2024. ADCP A was purchased in early 2020 and deployed in 2021 at a depth of 190 m for twelve months before being recovered and stored until these tests took place. The compass calibration for ADCP A took place in May 2024, and for ADCPs B and C together, it took place in January 2025.

 

Addressing measurement biases, from laboratory to use cases:

The calibration process, occurred at the specialised SHOM facility, which has a map of the magnetic field of the terrain to identify areas of weak magnetic gradients, crucial for accurate compass calibration. The error measurement experiment was unfolded into two different approaches:

  1. Standalone Calibration (ADCP tested in isolation, to minimise magnetic interference);
  2. Full Mechanical Assembly Calibration (ADCP tested with all associated deployment hardware to replicate deployment conditions).

Three types of errors have been identified by this work, which need to be properly evaluated to ensure data accuracy:

  • compass heading measurement errors (affected by magnetic field anomalies due to nearby metallic instruments or batteries, which can create magnetic anomalies, causing heading errors of up to 10 degrees),
  • tilt-meter (both in pitch and roll directions) measurement error
  • velocity measurement error (affected by the acquisition systems and by the piezo-electric elements receive/transmit sensitivity, which can experience gradual drift over extended deployment periods).

 

 

Fig. 4:  Magnitude of the angular error (in polar coordinates) in the heading measurement for the three tested ADCPs as standalone on the platform. The error magnitude spans from less than 2◦ to about 10◦.

 

Two hypothetical use cases have been presented to demonstrate the practical consequences of not compensating for calibration biases, particularly compass errors:

  1. Oceanographic Example: The first use case explores how compass errors affect horizontal current measurements. Using calibration results for ADCP A, historical horizontal current data from October 2018 to October 2019 collected at the EMSO site E2M3A were recalculated. The results show that the error on current components varies with time (Fig. 6) and can be significant (between 10% and 16%) in modifying current fields during specific periods, even if the general behavior of the time series is maintained (Fig. 5). This highlights that compass errors, particularly those induced by the magnetic environment, can introduce biases when calculating transports or detecting eddies, especially if different instruments with varying magnetic environments are used or if the heading continuously changes. Errors in pitch and roll can also affect vertical velocity estimations, impacting studies of vertical motions like mixing or zooplankton migration.

 

Fig. 5. Measured (black) and corrected (red) current components at 224 m depth for a two months interval spanning February and March 2019 of the southern Adriatic Sea 150 kHz ADCP

 

Fig. 6. Top and center: Difference between measured and corrected current components (Du and Dv) for a two months interval spanning February and March 2019 of the southern Adriatic Sea 150 kHz ADCP; bottom: heading of the instrument: heading angle for which the errors are the biggest are indicated with orange rectangles.
 

2. Operation of Autonomous Underwater Vehicles (AUVs). When the ADCP is integrated into mobile underwater platforms, two situations can occur. Most of the time, the AUV itself has a very accurate system that knows exactly which way it is pointing and how it is tilted. In this case, the ADCP doesn’t need to rely on its own internal compass or tilt sensors. The second possibility is that the AUV relies on the ADCP since the AUV’s own navigation is not as good, so it needs to use the ADCP’s built-in compass and tilt sensors to know its orientation. When this happens, it is absolutely critical to calibrate the ADCP very carefully beforehand because the ADCP is moving with the AUV, and any errors in its compass or tilt readings will directly affect the data it collects and how the AUV navigates.

 

Best Practices

To systematically address these identified challenges, the research proposes a robust set of best practices and detailed recommendations tailored for instrument operators. These comprehensive guidelines span the entire instrument lifecycle:

  • Pre-deployment Calibration: Emphasis is placed on comprehensive calibration procedures, ideally conducted with the full mechanical assembly intended for the actual deployment, to account for potential interference.
  • Performance Monitoring: Recommendations include continuous performance monitoring during deployment, where communication capabilities allow, to detect anomalies in real-time.
  • Post-deployment Error Characterisation: Detailed analysis and characterisation of any observed errors after recovery are crucial for data quality assessment.
  • Meticulous Documentation: The importance of meticulous documentation of all procedures, calibrations, and results is stressed to facilitate future data interpretation, validation, and quality assurance.

“Our paper was well received by the community during the session. It has been highlighted that this work is applicable to any instrumentation that has a compass, and the recommendations could be extended to those as well” explains Tomasi.

This significant research underscores EMSO ERIC’s commitment to delivering the highest quality ocean data from single-point current meters and ADCPs, thereby supporting the scientific integrity and reliability of global ocean observation efforts for years to come.

 

Fig 1. Instrumentation deployment around the ADCP C during the heading error measurements

 

Images adapted from: Tomasi, B., Ursella, L., LAUS HEYNDRICKX, C., Le Menn, M., Lefevre, D., O’Malley, C., Ferreira, H., Martins, A., Cusi, S. (2025). Pre- and post-deployment recommendations for single-point current meters and Acoustic Doppler Current Profilers. Accepted for publication in OCEANS 2025 Brest Proceedings, forthcoming on IEEE Xplore (© IEEE 2025).