This documentation describes the University of Hawaii “currents” group LADCP setup. It is was written for operations on a CLIVAR cruise. Parts may be out of date, but should be a reasonable guide to GO-SHIP and forward. References to CLIVAR are retained to highlight the fact that this is not a document kept current...
An Acoustic Doppler Current Profiler (ADCP) uses the Doppler shift of frequency along its beams to infer the water velocity past the instrument. Using instrument position and attitude (heading, pitch, and roll) the measured velocities can converted to earth coordinates. After motion of the instrument is removed, what remains is the ocean velocity. A lower ADCP (LADCP) is installed on a rosette and lowered down to the bottom to obtain the velocity profile of the full water column. A shipboard ADCP (SADCP) is installed in the hull of the ship and measures the velocity of about the first 1000 m depth of the water column at all time, on and off stations.
These useful references are available on the web:
- RDI “broadband primer” (download from RDI)
- From AMS journals :
- Fischer and Visbeck (1993): Deep Velocity Profiling with Self-contained ADCPs, J. Atm. Ocean. Tech., 10, 764–773
- Visbeck (2002) : Deep Velocity Profiling Using Lowered Acoustic Doppler Current Profilers: Bottom Track and Inverse Solutions, J. Atm. Ocean. Tech., 19, 794–807
See the LADCP CLIVAR Computer Overview section for the location of these files on the UH LADCP computers.
The LADCP application uses an ADCP on a CTD rosette, along with a power supply (usually an internal or external battery pack). Instrument attitude usually come from a magnetic flux-gate compass inside the instrument. Vertical position is estimated using time integral of measured vertical velocity or by calculating depth from the CTD data. Some instruments have an internal pressure sensor.
Acquiring LADCP data during a CTD cast requires that the instrument be given appropriate commands and told to start pinging. Communication is usually done via serial cable. UH LADCP operations use a rechargeable external battery (charged through a cable). When the CTD is ready to be deployed these cables must be detached and secured. The rosette goes down and comes back, with the LADCP pinging and collecting data. After the rosette is secured, the cables are reattached to LADCP and battery, the battery is charged, the LADCP is told to stop pinging, and the LADCP data are downloaded to a computer.
The CLIVAR program was designed to measure the physical and chemical properties of the world’s oceans. This is a monumental task. Sampling an ocean basin requires a set of cruises that cover a grid of representative tracks, usually running north-south or east-west, each crossing the whole basin. A CLIVAR cruise is one (or part of one) such leg. CLIVAR cruises usually last 4-6 weeks, and for most of the cruise, stop every 1/2deg to do a full-depth CTD. The rosette is usually large, to accomodate the demand for water (used by all the chemists). The LADCP and its battery are attached to the rosette and record data for the entire cast. It is the responsibility of the LADCP representative on the cruise to record, retrieve, and process the LADCP data from the CTD casts.
At minimum, the LADCP representative is responsible for the collection and processing of the LADCP data. This person will stand a 12-hour watch and will collect and process data during that time. During the other 12 hours, one person will have been chosen to help with LADCP operations (to acquire and download the data and run a quick sanity check on the data). This opposite watchstander will not process the data. Usually we expect that they will not try to troubleshoot any problems either: their job is to obtain and check the data, and wake up the primary LADCP watchstander if there is a problem. The job of the opposite watchstander accounts for about 20 minutes per cast once they know what they are doing (more like 40 mintues when training).
The LADCP representative has additional responsibilities. The following aspects of the CLIVAR cruise must also be covered (with varying degrees of help from Univ Hawaii “currents” group, depending on the cruise):
- before the cruise - (probably) get training in LADCP operations
- going to the ship - carry a data disk and perhaps a computer
- in port before the cruise
- set up all cables, instruments, computers
- train the opposite watchstander
- every cast
- prepare LADCP for deployment (charge battery, send commands)
- (possibly) help with rosette deployment and recovery
- download LADCP data to the acquisition PC
- sanity check on LADCP data
- copy LADCP data to the processing PC
- download ancillary data
- sanity check on Shipboard ADCP (SADCP) data
- process LADCP data on the processing PC
- make plots and data available to scientists at sea
- back up all data
- in port after the cruise
- (probably) take down and pack all the gear
- (probably) ship the gear
- going home
- carry a data disk and perhaps a computer
The following data are required to process the LADCP data:
(sometimes positions are include in the CTD time-series)
shipboard ADCP (SADCP) – all-time (on and off stations) fully processed upper-ocean velocities
In the future it may be necessary to also save the 24Hz CTD data as this will become part of the processing: the code is under development.
At the present time, only the first three dataset are actually used for processing. The Univ Hawaii “currents” group is responsible for the shipboard ADCP data, so the LADCP representative is asked to keep an eye on the shipboard ADCP system and download the data for post-cruise processing.