ADCP documentation

[ruxandra-valcu]

Add to The Great Google Doc some more info on ADCP functionality.

Some initial sources (we probably need more):

• https://github.com/topics/adcp
• https://dankelley.github.io/oce/articles/C_adp.html (R library, check for other useful stuff for echosounders too maybe)
• https://www.whoi.edu/what-we-do/explore/instruments/instruments-sensors-samplers/acoustic-doppler-current-profiler-adcp/
• https://support.nortekgroup.com/hc/en-us/articles/360030966632-Introduction-to-AD2CP-turbulence-measurements and connected files
• https://github.com/callumrollo/adcp-glider-guide

[simedroniraluca]

Nortek ADCPs

Main reference: HERE

Types of Nortek ADCPs

Current Meters (Aquadopp and Aquadopp Deep Water (DW))

• Type of Instrument: Current Meter
• What it Measures: Water currents at one level, temperature, compass orientation, tilt, and pressure.
• Range: Designed for deployments down to 3000/6000 meters.
• Frequency: Operates at 2 MHz.

Current Profilers (Aquadopp Profiler (AquaPro) and Acoustic Wave and Current Profiler (AWAC))

• Type of Instrument: Current Profiler
• What it Measures: Current and direction in multiple layers through the water column, temperature, compass orientation, tilt, and pressure.
• Range: Varies from few cm to several hundred meters.
• Frequency: Available in 2 MHz for the AquaPro; 1, 0.6, or 0.4 MHz for the (AWAC); and 2/1 and 2/0.6 MHz for Z-cell profiler (Aquadopp Z-cell is an extension of the AquaPro; it has the same specifications, but with the added capability of measuring the current at the level of the instrument (“Cell Zero”))

Wave Sensors (AWAC, AquaPro, Aquadopp)

• Type of Instrument: Wave Gauge and Current Meter
• What it Measures: AWAC - Wave height and direction
• Range: AWAC is deployable up to 100 m below the surface;

Doppler Effect

When a reflecting particle moves towards the instrument (like an Acoustic Doppler Current Profiler), the received pulse has a higher frequency than the transmitted pulse due to compression - the wavefronts are 'bunched up'. If the reflecting particle moves away from the instrument, the received pulse has a lower frequency than the transmitted pulse. This is due to a stretching effect - the wavefronts are 'spread out'.

Narrowband Profilers

Narrowband acoustic Doppler instruments emit a constant-frequency sound pulse into the water to measure its velocity. The sound pulse reflects off small particles suspended in the water, assumed to move at the water's speed (key assumption). The Doppler Shift, or change in frequency between the emitted and received pulses, is used to calculate water velocity. Movements along the path of the acoustic pulse are measured. The speed of sound in water is factored into the velocity calculation.

Pulse Coherent Profilers (AquaPro HR)

These transmit at least two pulses in order to obtain a measure of the current velocity. The pulses bounce off particles suspended in the water and return to the device. This is the backscattered signal. The signal is then 'range-gated', or separated into different segments for individual processing.

For each range cell or segment, the phase difference between the two backscattered signals is measured. The water current velocity can be calculated using the transmitted frequency, time difference between the pulses, and the phase difference itself.

The Doppler phase shift is computed using the covariance method. This involves calculating the complex covariance of the return signals and taking the four-quadrant arctangent (atan2) of the real and imaginary parts of the covariance function.

The method accounts for velocity ambiguity, which occurs if the phase difference falls outside the range of ±π (plus or minus pi). This ambiguity means that certain phase shifts can yield identical results, leading to potential errors in velocity measurement.

Pulse coherent systems are typically used for difficult measurement situations such as turbulence measurements, very slow or low energy flows, and rapidly varying flows requiring high single ping accuracy.

Doppler current sensors

Doppler Beams: These sensors use large transducers in relation to the sound's wavelength to produce narrow acoustic beams. Narrow beams are important for collecting accurate data. The beamwidth differs for each instrument and can be found in their respective technical specifications.

To measure the three components of velocity (east, north, and up), at least three beams are necessary. Each beam measures the velocity of particles at three different locations, and the device combines these three along-beam velocities. Using the relative orientation of the transducers and basic trigonometry, the instrument calculates the 3-dimensional water velocity.

There are two types of systems - mono-static and bi-static. Mono-static systems use the same transducer to transmit and receive signals. On the other hand, bi-static systems use one central transducer to emit pulses and three or four outer transducers to receive the echo.

Sensors

• Pressure
• Temperature (The instruments use the speed of sound to convert time to distance, and since speed of sound depends on temperature, this information is of high importance)
• Magnetometer and Tilt

Measurement Area

The instruments measure the velocity in three dimensions. Each transducer generates a sound pulse and measures the Doppler shift of the returned signal. The Doppler shifts from all the three beams are combined to calculate the 3D velocity in a specified depth layer, a sampling volume or in a profile covering a defined depth. The measurement area varies according to the transducer geometry, the frequency used, blanking distance, deployment method etc.