Metadata Report for BODC Series Reference Number 648385

• Metadata Summary

• Problem Reports

• Data Access Policy

• Narrative Documents

• Project Information

• Data Activity or Cruise Information

• Fixed Station Information

• BODC Quality Flags

• SeaDataNet Quality Flags

Metadata Summary

Problem Reports

No Problem Report Found in the Database

Data Access Policy

Open Data supplied by Natural Environment Research Council (NERC)

You must always use the following attribution statement to acknowledge the source of the information: "Contains data supplied by Natural Environment Research Council."

Narrative Documents

RD Instruments- Ocean Surveyor 150kHz Vessel mounted ADCP.

¹ Ranges at 1 to 5 knots ship speed are typical and vary with situation.
² Single-ping standard deviation.
³ User's choice of depth cell size is not limited to the typical values specified.

Profile Parameters

• Velocity long-term accuracy (typical): ±1.0%, ±0.5cm/s
• Velocity range: -5 to 9m/s
• # of depth cells: 1 - 128
• Max ping rate: 1.5

Bottom Track

Maximum altitude (precision <2cm/s): 600m

Echo Intensity Profile

Dynamic range: 80dB
Precision: ±1.5dB

Transducer & Hardware

Beam angle: 30°
Configuration: 4-beam phased array
Communications: RS-232 or RS-422 hex-ASCII or binary output at 1200 - 115,200 baud
Output power: 1000W

Standard Sensors

Temperature (mounted on transducer)

• Range: -5° to 45°C
• Precision: ±0.1°C
• Resolution: 0.03°

Environmental

Operating temperature: -5° to 40°C (-5° to 45°C)*
Storage temperature: -30° to 50°C (-30° to 60°C)*

*later instruments have greater range.

Web Page

Further details can be found in the manufacturer's website or in the specification sheet.

James Clark Ross 097 VMADCP Calibration

The following is adapted from the cruise report:

Calibration data can be acquired (and even processed) before getting the main processing suite set up. Since bottom tracking data are particularly useful for calibration, and since this works in shallower water (e.g. in the neighbourhood of the starting port) it is useful to start collecting these data from the very beginning of the cruise. We ran the ADCP in bottom tracking mode from the time we left port until the depth was too great for bottom tracking to work properly (about 600 m).

The ADCP data need to be corrected for magnitude and direction. The corrections for magnitude are more straightforward and should be done whenever possible, while those for direction are more complicated and (if simply using the normal pstar processing suite) are less likely to be necessary (since the effective orientation of the ADCP within the ship is unlikely to change substantially). Calibrations can be calculated by running the usual pstar processing sequence, but editing adpexec3 (which makes the calibration corrections) to have dummy corrections of scale and direction of 1 and 0 respectively. By comparing the ship velocity (ve, vn) with the calibrated bottom track velocity (ebotcal, nbotcal) as given in the final output files of the form 097bot[jday]d.abs, one can find the necessary corrections to the magnitude and direction (which should then be inserted into adpexec3).

Scale

Errors in the magnitude of the measured velocities are caused by variations in sound speed, made more complicated by the ADCP being mounted in a sea chest filled with fluid at a different temperature from the sea water. The changes in scale factor observed in this cruise were of the order of 2%. This is unlikely to be important for measurements taken at stations, but for underway measurements when moving at speeds of ~ 5 m/s (~ 10 knots) this will give an error in the water velocity component in the direction the ship is moving of the order of 10 cm/s.

While the usual processing route will give the scale factors required, it is much simpler to directly compare the speed over the ground calculated from the bottom track ADCP data with the speed over the ground derived from the gps. Note that most of the main processing sequence is designed to correct for direction errors, which is not necessary when comparing speeds. A matlab calibration script (adcpcalg.m) was created in the directory ~jr097/adp/calibrate (more on the directory structure below). First a pair of files containing the adcp and navigation data need to be created using listit:

listit -s starttime -e endtime -i 60 adcp depth bottew bottns > adcp_start_bott
listit -s starttime -e endtime gps_ash lat lon > ash_start_bott

where starttime and endtime are in the usual pstar format YYDDDHHMM, e.g. 050341335 for 13:35 (GMT) on Julian day 034 (3^rd Feb) 2005.

These instructions are repeated as comments in the matlab script. You should also edit adcpcalg.m to select the lowest depth you want to use data for (we used mindepth = 50 m). Note bad data are given a depth of -1, so this also acts as a useful filter. The script could easily be modified to also discard data above a certain depth (on the grounds that the results are less reliable at large depths). The script uses the file sw_dist.m (to find distances between points given in lon, lat form) and a single estimate of the clock difference is also required. The array 'scale' then contains the necessary scaling needed for the calibration (see below): e.g. use mean (or median) and std, perhaps after discarding further 'bad' points, to find the average scale factor.

The scale factor from the initial data near the Falkland Islands was 1.0265, and this value was used for days 34 to 37 on the southward journey and days 66 onward on the return journey (nominally 'warm' waters). For the bulk of the cruise a value of 1.0409 was used, based on measurements around the Fimbul Ice Shelf. This was checked periodically, including in shallower waters around the Filchner Depression, and was not found to change significantly (though the four decimal places in the value exaggerates the accuracy: changes of ±0.002 or so over the course of a single day are not uncommon). The values used here are comparable with the range of 1.021 to 1.0426 found by Gwyn Griffiths on a cruise in the Greenland area (JR106).

Direction

The absolute water velocity is calculated by adding the ship's velocity calculated from the Ashtec gps system to the water velocity calculated using the ADCP. When steaming at 10 knots, small errors in the relative velocity directions would give significant errors in the absolute water velocities.

The ADCP is not perfectly aligned with the centre-line of the ship, so that even if the ship's heading were known perfectly, the velocities returned by the ADCP need to be rotated to give the true velocities. This offset can be calculated using the processed bottom track files of the form 097bot[jday]d.abs, as discussed earlier. We found an offset of phi = -1.78° (i.e. the ADCP velocities need to be rotated by 1.78° anticlockwise). This is within 1/10° of the value found by Gwyn Griffiths (-1.69°).

It is important to note that this offset is not the same as the offset returned by the matlab script adcpcalg.m in the array coursediff. This latter offset is the difference between the course calculated from the ADCP bottom track and the course calculated using the Ashtec gps positions. To determine the ship's heading, the ADCP uses the ship's gyrocompass, not the Ashtec gps heading.

The difference between the Ashtec heading (generally taken to be the true heading) and the gyro heading varies with time, affected by the direction the ship is heading in, sharp turns, and a latitude dependent error. The variation with latitude was particularly noticeable on this cruise, with errors increasing at high latitudes.

The usual pstar processing sequence attempts to eliminate the gyro error, and information on this error is returned in several of the processed files as 'a-ghdg' (Ashtec -; gyro heading). Thus it is only the misalignment angle, phi, that is needed in the final calibration of the ADCP velocities (script adpexec3 ). These various offsets are not independent: the mean value of the difference between the Ashtec and ADCP headings (returned as 'coursediff') is equal to the sum of the mean Ashtec - gyro heading error (a-ghdg) plus the ADCP misalignment offset, phi.

For the processing at CTD stations the usual processing scripts were not used, so a correction based on the difference between the Ashtec and ADCP headings was required. This was either based on summing mean values of a-ghdg and phi, or by direct comparison of ADCP bottom track and Ashtec position data (using the adcpcalg.m script). Errors here are less important, since the ship's speed is very small, and so we used a fixed offset value of -3.9° for all the stations in the Fimbul Ice Shelf area and -5.2° for all the stations in the Filchner Depression area. The difference is accounted for by an increase in the mean Ashtec-gyro error from approximately - 2.1° in the Fimbul area (latitude 70°S) to-3.4° in the Filchner Depression (latitude 75°S). While the information supplied by the Ashtec system is generally good, it is prone to occasional glitches. The Ashtec data are usually smoother than the gyro compass at short time scales, and the gyro compass also has latitude and other errors, but the Ashtec heading has periods where it is significantly wrong. Ideally these should be corrected during the processing, though no attempt to do so was made during the cruise.

James Clark Ross 106 150 kHz VMADCP processing

The following is adapted from the cruise report:

The RD Instruments 150 kHz shipboard ADCP (VM-150) was used throughout the cruise for current profile measurements. No technical problems were encountered. Key configuration parameters were:

The instrument was set to use the computed sound velocity for correcting its velocity data. The usual clock drift within the PC was noted each day for later correction. Data processing used the 'pstar' route, via the Level C 'adcp' data stream, through a series of unix scripts on jrua. Ancillary measurements from the ship's gyrocompass, the Ashtech ADU GPS-based heading reference and GPS position were acquired, processed and merged with the ADCP data as described in sequence below:

gyroexec0

reads in gyro data, performs checks and appends to a gyro data file for the cruise (106gyr01).

ashexec0

reads in Ashtech ADU data and edits the data based on limits set on the quality parameters mrms and brms.

ashexec1

merges the Ashtech and gyro data, to give the heading difference a-ghdg. This is an especially important correction in high latitudes where the gyrocompass error is magnified by the secant of latitude (errors at 80° being nearly three times those at 60°). In addition transient errors of several degrees due to damped Schuler oscillations caused by sharp changes of course needed to be corrected.

ashexec2

despikes the a-ghdg data and forms an average over 5 minutes.

navexec0

reads in the 'bestnav' navigation data and calculates ship speed and distance run over the ground from successive fixes.

navexec1

despikes the data and averages to 2 minutes.

adpexec0

reads in the adcp data, splits the (ungridded) bottom track data from the gridded water column profiles.

adpexec1

corrects the timing based on manual records of clock drift

adpexec2

merges in the heading correction from the ashexec2 file

adpexec3

applies the calibration to the velocity data

adpexec4

merges in the smoothed navigation data and calculates absolute currents. These files (styled 106adp'jday'd.abs) are then appended to form cruise files, which are then averaged to 10 minutes. From these files velocity profiles or single depth maps can be produced. ASCII copies of these averaged concatenated data files were also made available for processing using Matlab.

Calibration

The initial calibration of the VM-150 was checked against GPS during two runs, each of over two hours, on the NW Icelandic shelf. The scale factor (A) was found to be 1.021 +/- 0.001 and the offset angle (phi) was -1.65°. That is, the ADCP vector needed to be rotated anticlockwise by 1.65 degrees. This calibration was at an indicated temperature of 15.8°C. Later, it became clear that this calibration was in error when operating in water temperatures of less than 0°. At these times, the ADCP transducer temperature was in the region of 6°C, but it is not immediately obvious that this is not the correct temperature within the transducer well. Nevertheless, a calibration on the Greenland shelf showed that while the offset angle was no different, the scaling factor should be 1.0426. ADCP data for day 230 onwards was recalibrated. This calibration was used until the end of day 240.

The calibration procedure is detailed in the JR097 cruise report, and the relevant document is attached here.

Range and acoustic backscatter

The range performance of the instrument was impressive. During this cruise, with calm conditions, in excess of 400 m range was usual. With the ship locked into an ice edge, the indicated currents were reasonable, although bottom track was lost and the backscatter profile was anomalous in showing a faster decay than would be expected from spherical spreading.

An approximate calibration was applied to the acoustic backscatter from the VM-150 in order to assess the likely profiling range of the ADCPs on Autosub and for future reference against which the performance of Autosub's ADCPs could be checked.

BODC Processing

The originator's data files for transfer to the BODC QXF format were in the binary P* format. The variables within the P* files were mapped to the following BODC parameter codes:

The data were transferred to QXF format, a BODC-defined subset of NetCDF and BODC's format for 2 dimensional datacycle storage using transfer process 339.

James Clark Ross 106 VMADCP Data Quality

Data Originator Comments

The data originators were pleased with the quality of the data. The range of the VMADCP is quoted at greater than 400m in calm conditions during this cruise. The velocities given by the instrument with the ship locked into an ice edge were reasonable, although in these conditions the bottom track was lost and the backscatter profile was anomalous in showing a faster decay than would be expected from spherical acoustic spreading.

BODC Quality control

Post transfer to BODC internal format (QXF) the data where manually screened using BODC XERPLO visualisation and quality control software. The data were viewed as a continuous time series.

On the whole, the near surface data appear to be of good quality. The positional data have been compared with the underway data in order to identify any positional / timing errors within the VMADCP data. None were found.

Project Information

AutoSub Under Ice (AUI) Programme

AutoSub was an interdisciplinary Natural Environment Research Council (NERC) thematic programme conceived to investigate the marine environment of floating ice shelves with a view to advancing the understanding of their role in the climate system.

The AUI programme had the following aims:

• To attain the programme's scientific objectives through an integrated programme based on interdisciplinary collaborations and an international perspective
• To develop a data management system for the archiving and collation of data collected by the programme, and to facilitate the eventual exploitation of this record by the community
• To provide high-quality training to develop national expertise in the use of autonomous vehicles in the collection of data from remote environments and the integration of such tools in wider programmes of research
• To stimulate and facilitate the parameterising of sub-ice shelf processes in climate models, and to further demonstrate the value of autonomous vehicles as platforms for data collection among the wider oceanographic and polar community

Following the invitation of outline bids and peer review of fully developed proposals, eight research threads were funded as part of AUI:

Physical Oceanography

• ISOTOPE: Ice Shelf Oceanography: Transports, Oxygen-18 and Physical Exchanges.
• Evolution and impact of Circumpolar Deep Water on the Antarctic continental shelf.
• Oceanographic conditions and processes beneath Ronne Ice Shelf (OPRIS).

Glaciology and Sea Ice

• Autosub investigation of ice sheet boundary conditions beneath Pine Island Glacier.
• Observations and modelling of coastal polynya and sea ice processes in the Arctic and Antarctic.
• Sea ice thickness distribution in the Bellingshausen Sea.

Geology and Geophysics

• Marine geological processes and sediments beneath floating ice shelves in Greenland and Antarctica: investigations using the Autosub AUV.

Biology

• Controls on marine benthic biodiversity and standing stock in ice-covered environments.

The National Oceanography Centre Southampton (NOCS) hosted the AUI programme with ten further institutions collaborating in the project. The project ran from April 2000 until the end of March 2005, with some extensions to projects beyond this date because of research cruise delays. The following cruises were the fieldwork component of the AUI project:

Table 1: Details of the RRS James Clark Ross AUI cruises.

All the cruises utilised the AutoSub autonomous, unmanned and untethered underwater vehicle to collect observations beneath sea-ice and floating ice shelves. AutoSub can be fitted with a range of oceanographic sensors such as:

• Conductivity Temperature Depth (CTD) instruments
• Acoustic Doppler Current Profillers (ADCP)
• A water sampler
• Swath bathymetry systems
• Cameras

In addition to use of AutoSub during each cruise measurements were taken from ship. These varied by cruise but included:

• Ship underway measurements and sampling for parameters such as:
  • Salinity
  • Temperature
  • Fluorescence
  • Oxygen 18 isotope enrichment in water
  • Bathymetry using a swath bathymetry system
• Full-depth CTD casts for with observations of samples taken for parameters such as:
  • Salinity
  • Temperature
  • Fluorescence
  • Optical transmissivity
  • Dissolved oxygen
  • Oxygen 18 isotope enrichment in water
  • Water CFC content
• Sea floor photography and video using the WASP system
• Sea floor sampling with trawls/rock dredges
• Sea ice observations (ASPeCt), drifters and sampling

The AutoSub project also included numerical modelling work undertaken at University College London, UK.

The project included several firsts including the first along-track observations beneath an ice shelf using an autonomous underwater vehicle. The AutoSub vehicle was developed and enhanced throughout this programme and has now become part of the NERC equipment pool for general use by the scientific community. Further information for each cruise can be found in the respective cruise reports (links in Table 1).

Data Activity or Cruise Information

Cruise

Complete Cruise Metadata Report is available here

Fixed Station Information

No Fixed Station Information held for the Series

BODC Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:

SeaDataNet Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle: