3.2.13.1 ADCP Observations - The purpose of the 150-KHz vessel mounted ADCP is to measure the ocean current velocity continuously over the upper 300 meters of the water column, usually in 8-meter depth increments.  Current velocities relative to the earth at this spatial and temporal resolution cannot be measured by other methods: CTD sections, current-meter moorings, or drifting buoys.  Additionally, ADCP data are used to estimate the abundance and distribution of biological scatterers over the same depth range and in the same depth increments.

3.2.13.2 ADCP Data Collection - ADCP measurement requires four instruments working in concert:

1. The ADCP,
2. The ship's gyrocompass,
3. A Global Positioning System (GPS) receiver, and
4. A GPS Attitude Determination Unit (ADU), presently the TSS Position and Orientation System for Marine Vessels (POS/MV).

The ADCP is connected to a dedicated Personal Computer (PC) and controlled by RD Instruments' (RDI) Data Acquisition System (DAS) version 2.48 software.  DAS shall be configured to use the user-exit programs AGCAVE.COM and UE4.EXE.  Separate written instructions detailing the ADCP setup and configuration files are kept in the large, black ADCP notebook in a filing cabinet drawer of the ship’s DataPlot compartment.

The ADCP PC is interfaced to the ship's gyrocompass, primary scientific GPS receiver, and ADU.  The navigation GPS receiver shall be configured to send NMEA-0183 $GPGGA and $GPVTG sentence sets to the PC’s COM2 serial port at the maximum fix update rate for that receiver (usually a 1- or 2-second rate) and with the maximum number of decimal places for position precision (optimally 4).  The ADU shall be configured to send the NMEA-0183 $PASHR sentence set to the PC’s COM1 serial port once per second.  The user-exit program UE4.EXE shall be configured to control acquisition and processing of GPS and ADU sentence sets, and to synchronize the PC clock with the time reported by the primary GPS receiver.

The ADCP PC logs data from the profiler to Iomega Zip disks.  No more than one Iomega Zip disk will be required for each cruise.  At the end of the cruise, a backup of the Iomega Zip disk should be made to a unique subdirectory on another disk, maintained by the ship for this purpose, until the original data are certified at PMEL.

3.2.13.3 Scientific Computer System (SCS) Data Logging - Detailed post-cruise processing of ADCP data can take advantage of a larger quantity of navigation data than is retained by the ADCP acquisition software.  Thus, the ship's SCS is relied on to log GPS and ADU navigation data at high rates.  An SCS ADCP Event Log should be set up to do this.

ADCP analysis requires the input from navigation, heading, and ADCP electronics box sensors on the NOAA Ship MILLER FREEMAN SCS parent sensors, only, need be logged; SCS child sensor logging is not required for ADCP analysis.  The required SCS parent sensors and logging rates are as follows:

 

SCS Sensor	Logging Rate (sec)
Trimble P-code GPS $GPGGA	1
Trimble P-code GPS $GPVTG	1
Northstar DGPS $GPGGA	10
Northstar DGPS $GPVTG	10
Ship’s Gyrocompass $HEHDT	10
Raw RDI box temperature	60
Raw RDI box voltage	60
POS/MV $PASHR	10
MX-412 DGPS $GPGGA	10
MX-412 DGPS $GPVTG	10

The required SCS parent sensors and logging rates are as for ADCP analysis, no other SCS sensors need be logged, but other users may have their own SCS logging needs.  If those conflict with the ADCP needs for the above sensors, then SCS should be configured to record these at the fastest logging rate required by all users.

In the above table it is assumed that for ADCP purposes, the primary position sensor is the Trimble P-code GPS receiver, and that the Northstar and Leica MX-412 DGPS receivers are secondary.  If the primary GPS receiver should malfunction during a cruise, then the Northstar should be made the primary ADCP navigation device.  This is accomplished by connecting the Northstar to the ADCP’s COM2 serial port, and setting SCS to record the North star’s $GPGGA and $GPVTG sentence sets at 1-second rates.  If the Northstar also fails, then the Leica MX-412 would be made the primary sensor in an analogous manner.  Changes in the availability of GPS equipment shall be communicated to Pacific Marine Environmental Laboratory (PMEL) to allow the above list to remain current.  It is the responsibility of the ship to install and enable the appropriate encryption key for use in the P-code receiver should GPS Selective Availability be turned on again by Department of Defense (DoD).

3.2.13.4 ADCP Underway Operations - The ADCP operates continuously during the entire cruise.  At the start of a cruise, the system shall be configured and started according to the provided checklists Before Leaving Port and Underway to Operations Area.  The ADCP and its interface to the gyro and navigation must be checked daily by completing the ADCP Daily Log and at the end of the cruise with the ship tied to the pier.

The centerboard height affects the depth of sampling; therefore, the centerboard shall be lowered as soon as practical upon leaving port and remain lowered throughout the cruise.  If it is necessary to raise the centerboard during the cruise, the times of raising and lowering must be logged in the Electronic Marine Operations Abstract (E-MOA).  In case of problems, please describe the problem, error message numbers, flashing lights, etc., and contact Dr. Ned Cokelet either by telephone (206) 526-6820 or by e-mail to Edward.D.Cokelet@noaa.gov at PMEL as soon as possible.

Dedicated ADCP transects should be run at constant heading (not constant course-over-ground) if practical, thus minimizing gyrocompass lag; however, transects along lines of current-meter moorings should remain on the line with the ship's heading gradually adjusted to accomplish this.  Sharp turns should be avoided and the ship's speed should be constant.  A speed of 12 knots is often satisfactory, but the ship may have to slow down if the ADCP's "percent good pings" decreases below 75% in the upper 200-250 meters due to sea state.

The ADCP should operate in bottom-track mode when the water depth is less than about 500 meters for more than a few hours.  This gives currents that are better compensated for transducer misalignment but somewhat lower in statistical significance because the number of pings is reduced.  For extended periods in deeper water, an ADCP configuration without bottom tracking should be used.

3.2.13.5 ADCP Underway Operations - The ADCP operates continuously during the entire cruise.  At the start of a cruise, the system shall be configured and started according to the provided checklists Before Leaving Port and Underway to Operations Area.  The ADCP and its interface to the gyro and navigation must be checked daily by completing the ADCP Daily Log and at the end of the cruise with the ship tied to the pier.

The centerboard height affects the depth of sampling; therefore, the centerboard shall be lowered as soon as practical upon leaving port and remain lowered throughout the cruise.  If it is necessary to raise the centerboard during the cruise, the times of raising and lowering must be logged in the Electronic Marine Operations Abstract (E-MOA).

In case of problems, please describe the problem, error message numbers, flashing lights, etc. on the log sheets.  Also, contact Dr. Ned Cokelet either by telephone (206) 526-6820 or by e-mail to Edward.D.Cokelet@noaa.gov at PMEL as soon as possible.

3.2.13.6 ADCP Configurations - Several ADCP DAS configuration (.CNF) files are provided in the C:\ADCP248 directory on the ADCP PC.  For system checkout before acquiring current data, use CHECK02.CNF or CHECK02X.CNF.  (The X-suffix is appended to all configurations that enable the ADCP to be controlled by an external trigger pulse as required when the ADCP is used in conjunction with an Alaska Fisheries Science Center (AFSC) hydroacoustics fish-stock-assessment echo sounder such as the SIMRAD EK 500 Scientific Echosounder.  External triggering makes the ADCP and the echo sounder ping and listen in concert, reducing interference.)  For ADCP current measurements, use 02WBT.CNF (or 02WBTX.CNF) when the water depth is less than 500 meters for over two hours.  WBT stands for With Bottom Track, which means the ADCP alternately measures the water and sea-bottom velocities and produces the best accuracy.  If working in water depths greater than 500 m for more than 2 hours, use 02NBT.CNF (or 02NBTX.CNF) where NBT stands for No Bottom Track.  This suspends bottom searching and concentrates all pings in the water for the best reduction in variance.

3.2.13.7 Data Dispensation - At the end of each cruise, a copy of the ADCP ping data logged by the ADCP’s PC and the SCS files for the above sensors only should be sent to:

Dr. E. D. Cokelet
NOAA/PMEL
7600 Sand Point Way NE
Seattle, Washington 98115

Telephone:  (206) 526-6820
E-mail:  Edward.D.Cokelet@noaa.gov

3.2.13.8 Dedicated ADCP Transects - One or more dedicated ADCP transects may be requested during a cruise.  Each should be run at constant heading (not constant course-over-ground) if practical, thus minimizing gyrocompass lag.  However, transects along lines of current-meter moorings should remain on the line with the ship's heading gradually adjusted to accomplish this.  Sharp turns should be avoided and the ship's speed should be constant.  A speed of twelve knots is often satisfactory, but the ship may have to slow down if the ADCP's "percent good pings" decreases below 75% in the upper 200-250 meters due to sea state.

3.2.13.9 ADCP Backtrack-L Calibration - Occasionally, the ship may be requested to execute a backtrack-L calibration maneuver to test the instruments and to calibrate the transducer misalignment angle for which a 0.5° error can seriously bias the measurements.  The "misalignment angle" may change with the ship's trim as well as with remounting the ADCP transducers.  The basic idea is to measure the current twice on closely spaced parallel tracks of opposite heading when the ADCP and GPS are working well.  The maneuver consists of four legs (north, south, east and west headings) connected by simple U-turns forming an L shape.  Each leg should be 30 minutes long; the first 10 minutes are to allow the ship and instruments to stabilize on the new heading.  The entire calibration should require about 2.5 hours with 5 minutes allowed for each turn.  The following should be considered:

1. Negligible currents are best; however, stronger currents are acceptable as long as they are reasonably uniform and steady.  Avoid regions of strong horizontal shear due to topography, flow through passes, eddies, and current boundaries.  In tidal currents, calibrate when the current is steadiest, often at maximum flood and ebb rather than at slack water.
2. Calibration legs can be done in any order provided opposite-headed legs are sequential.
3. Opposite-headed legs should be parallel and closely spaced, but not retraced.  Use U-turns to minimize gyrocompass oscillations.  Avoid Williamson and hairpin turns.
4. The ADCP's PC screen should show at least 75%-good pings down to 250 meters.
5. The ship should go fast enough to detect a misalignment error (over five knots), but slow enough to satisfy condition 4.  This depends on sea conditions; however, ten to twelve knots is often satisfactory.
6. Choose a time when GPS is navigating and is expected to remain so over the next two hours.

3.2.13.10 ADCP Absolute Backscatter Calibration - A test to calibrate the absolute backscatter strength and to determine the background noise level of the ship’s ADCP system may be performed once per cruise at the discretion of the Chief Scientist.  Specific instructions in such event will be provided by PMEL personnel aboard, and cannot be anticipated in advance of the cruise.  Typically, such a test will be attempted in conditions when weather is relatively calm and the water depth exceeds 250 meters.  This test may require that the main power plant, pumps, sonars, and other sources of acoustic and electronic noise be shut down.  If conducted in the course of normal operations, the work will require about one hour.  There may be opportunities for variations of the test at other times when the ship is at anchor, requiring the cooperation of the ship's officers and engineering watch.

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allen.macklin@noaa.gov