Metadata Report for BODC Series Reference Number 624924

• 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

RRS Discovery 258 VMADCP Data Quality

Data originator comments

The following are data originators comments, on both the 75 and 150kHz VMADCPs, adapted from the D258 cruise report.

The on-station data tend to be the best quality ADCP data, penetrating deepest into the water column. The on-station data for the CTD stations were selected and averaged into u and v profiles for each ADCP. The data were merged together and the differences in u and v calculated (75 minus 150). As on Discovery 253, the results were very encouraging, suggesting the ADCPs agreed within the expected noise level of the instruments:

U (east) Mean = 0.129 cm/s, sd = 1.973 (n = 1356)
V (north) Mean = 0.167 cm/s, sd = 2.161 (n = 1356)

However, it was clear from the %good values that the 150 kHz data appeared considerably poorer. This may be a result of the 4 m bin length chosen for the 150 kHz setup and sparse winter populations of zooplankton. Certainly the %good contour plots look like they show biological layering, but only a thorough investigation of the amplitude backscatter will confirm this.

Depth of penetration

The main potential advantage of the 75 kHz ADCP is that the lower frequency means greater depth penetration, though at reduced vertical resolution (16m bins vs 4m). During Discovery 258 the 75kHz ADCP managed to reach 700-750m on station, and 400-500 m steaming. In contrast, typical maximum depths for the 150 kHz are 350-400 m under the same conditions. It is noticeable though that the 75 kHz depth penetration during steaming suffered very readily with the onset of anything other than calm conditions. It was postulated on Discovery 253 that the forward well is more prone to contamination by bubbles than the aft well, and if the 75 kHz ADCP is to become the standard ADCP for Discovery it may be appropriate to move the 75 kHz to the aft well. However, the underway data during cruise 258 were generally poor as a result of poor weather and high steaming speeds when weather windows occasionally permitted them. The mid-depth spiking in the 75 kHz data at ~330 m, discussed on Discovery 253, was not obvious during cruise 258, however the small amount of good underway data available may make this observation unreliable.

BODC Quality control

Post transfer to BODC internal format (QXF) the data were manually screened using BODC XERPLO visualisation and quality control software. The data were viewed as a continuous time series with the 75 and 150 kHz data streams displayed simultaneously. Visual inspection confirmed the originators findings in that

• the on station data tend to be the best quality
• data quality suffers greatly during poor weather conditions and/or high steaming speeds (>2m/s)

in addition

• notable spikes occur in both series when the ship is maneuvering on and off station
• when on station the 150 kHz data show less variability c.f. the 75kHz instrument

Because of the poor quality of the underway data no attempt has been made at quality control. Users are advised to use these data with caution. Attention was restricted to the periods when the vessel was on station. Data in these periods all data appeared to be within acceptable limits and without acceptable justification no flagging was attempted.

One period of note is 29/11/2001 06:30 (approx.) to 17:00, where the 75 and 150 kHz E-W velocity signals diverge in all bins. The difference is approximately 100cm/s when the 75kHz trace ends. As expected the two series initially overlay one another and the 150 kHz series remains changes little while the eastward component of 75 khz series grows steadily until the difference is 100cm/s. It was not obvious why the series diverge and without further investigation not possible to ascertain which series is correct.

Data Access Policy

Open Data

These data have no specific confidentiality restrictions for users. However, users must acknowledge data sources as it is not ethical to publish data without proper attribution. Any publication or other output resulting from usage of the data should include an acknowledgment.

If the Information Provider does not provide a specific attribution statement, or if you are using Information from several Information Providers and multiple attributions are not practical in your product or application, you may consider using the following:

"Contains public sector information licensed under the Open Government Licence v1.0."

Narrative Documents

RD Instruments- Ocean Surveyor 75kHz 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: 0.7

Bottom Track

Maximum altitude (precision <2cm/s): 950m

Echo Intensity Profile

Dynamic range: 80dB
Precision: ±1.5dB

Transducer and 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 on the manufacturer's website or in the specification sheet

RRS Discovery 258 75kHz VMADCP processing

Introduction

The following is adapted from the D258 Cruise report.

Two RDI vessel-mounted Acoustic Doppler Current Profilers (VM-ADCPs) were operated on Discovery 258; the 150 kHz VM-ADCP and a new 75 kHz Phased Array instrument (Ocean Surveyor) that had been fitted immediately prior to Discovery 253 (FISHES; May/June 2001).

The 150 kHz ADCP was mounted in the hull 1.75 m to port of the keel, 33 m aft of the bow at the waterline and at an approximate depth of 5 m. The 75 kHz ADCP was also mounted in a second well in the hull, but 4.15 m forward and 2.5 m to starboard of the 150 kHz well. The following section describes the operation and data processing paths for the 75kHz ADCP.

Narrative

Discovery 253 was the first scientific cruise on which the new RDI Ocean Surveyor 75 kHz Phased Array ADCP was used and thus a new processing path was written. No significant changes were made to this on Discovery 258. The instrument was configured to sample over 120 sec intervals with 60 bins of 16m depth, pulse length 16m and a blank beyond transmit of 8m. The instrument is a narrow band phased array ADCP with 76.8 kHz frequency and a 30° beam angle. The PC was running RDI software VmDAS v1.2.012 and WinADCP v1.1.0. Gyro heading, and GPS Ashtech heading, location and time were fed as NMEA messages into the software which was configured to use the Gyro heading for co-ordinate transformation. The software logs the PC clock time, stamps the data (start of each ensemble) with that time, and records the offset of the PC clock from GPS time. This offset was applied to the data in the processing path before merging with navigation. The ADCP was fitted in the forward well previously occupied by the unsuccessful ACCP and before that the ADCP prior to the 1992 re-fit. During fitting a nominal offset of 45° was intended, but the April 2001 trials cruise ascertained that the offset was in fact 60°, and this offset was accounted for in the RDI software. Bottom tracking was switched on early in the cruise and at the end of the leg 1 for calibration purposes.

The 2 minute averaged data were written to the PC hard disk in files with a .LTA extension, e.g. D258005_000000.LTA, D258006_00000.LTA. Sequentially numbered files were created whenever data logging was stopped and re-started. The software closes the file once it reaches 48MB in size (a user-specified size), though on Discovery 258 files were closed after ~24 hr, so they never became that large. The .LTA and .ENX files were transferred to a networked Mac for ftp to a Unix directory; .ENX files contain the raw ping by ping profiles ready for averaging and were recorded in case they could be useful for looking at deep acoustic backscatter signals.

No decent calibration was obtained at the beginning of the cruise, and therefore the values derived during Discovery 253 were used instead. That calibration was established from bottom tracking data collected on long straight SeaSoar runs of Fine Scale Survey 2. The values were:

phi = 1.3578 (sd = 0.078) A = 1.0050 (sd = 0.0031).

SOC PEXEC processing

In the following script description, '##' indicates the daily file number.

surexec0 data read into Pstar format from RDI binary file (psurvey, new program written on Discovery 253 by S Alderson). Water track velocities written into 'sur' file, bottom track into 'sbt' files if in bottom track mode. Velocities were scaled to cm/s and amplitude by 0.45 to db. The time variable was corrected to GPS time by combining the PC clock time and the PC-GPS offset. The depth of each bin was determined from the user-supplied information. Output files: sur258##.raw, sbt258##.raw.

surexec1 data edited according to status flags (flag of 1 indicated bad data). Velocity data replaced with absent data if variable '2+bmbad' was greater than 25% (% of pings where >1 beam bad therefore no velocity computed). Time of ensemble moved to the end of the ensemble period (120 sec added with pcalib). Output files: sur258##, sbt258##.

surexec2 this merged the ADCP data (both files) with the Ashtech a-ghdg created by ashexec2. The ADCP velocities were converted to speed and direction so that the heading correction could be applied and then returned to east and north. Note the renaming and ordering of variables. Output files: sur258##.true, sbt258##.true.

surexec3 applied the misalignment angle, phi, and scaling factor, A, to both files. Variables were renamed and re-ordered to preserve the original raw data. Output files: sur258##.cal, sbt258##.cal.

surexec4 merged the ADCP data (both files) with the Trimble GPS 4000 navigation file (gp42581) created by gp4exec0 and the bestnav navigation file (abnv2581) created by navexec0. Ship's velocity was calculated from 2 minute spot positions taken from the gp42581 file and applied to the ADCP velocities. The end product is the absolute velocity of the water. The time base of the ADCP profiles was then shifted to the centre of the 2 minute ensemble by subtracting 60 seconds and new positions were taken from abnv2581, this last stage was not done in the processing scripts on Discovery 253. Output files: sur258##.abs, sbt258##.abs.

BODC processing

The originators files used for transfer were SOC Pstar format calibrated, gridded depth dependent data, filenames sur258##.abs. Where ## is the number of the day file

Transfer mapping

The varibles within the Pstar files where mapped to BODC parameter codes

Pstar variables present in the files but not mapped for transfer included: evelcal, nvelcal, ve, vn, a-ghdg, good3bm, 2+bmbad, verr and distrun. These where excluded on the grounds that that they were either intermediated processing infomation or available elsewhere (i.e. underway navigation)

Transfer

The data were trnasferred to QXF format, a BODC-defined subset of NetCDF and BODC's format for 2 dimensionsal datacycle storage. Pstar null data were set to the appropriate absent data values for the code in the BODC parameter dictionary.

The parameter code mapping outlined in the table above was not straightforward for percent good or acoustic return intensity. For percent good, channel 14 (good4bm) was chosen as the most likely match from those available. The P* files did not appear to contain a single channel for the acoustic signal return amplitude therefore the acoustic signal return was averaged during transfer, from four channels labled intense1, intense2, intense3 and intense4.

Aside from the averaging of the signal intensity not transformations or unit conversions where untaken during transfer.

RRS Discovery 258 Navigation

The following is adapted from the Discovery 258 cruise report.

Discovery's best determined position was calculated by the process "bestnav". The main data source was a newly purchased Ashtech G12 positioning system. Thus the GPS Trimble 4000 system, used for most recent cruises was recorded separately. In fact the Ashtech G12 electronics board replaced the Sea Star Mark III Differential GPS system and continued to provide differential corrections to the GPS 4000 system. Thus an examination of positional accuracy, whilst tied up alongside in Govan and Reykjavik, showed that the corrected GPS 4000 system provided higher positional accuracy than the new Ashtech G12 system (calculated with Pstar program gpsrms).

However, results from all three of these systems (Table 1, below) indicate sufficient precision to enable a calculation of ship's velocities to better than 1 cms^-1, and therefore below the instrumental limits of the RDI ADCP systems.

Table 1. Comparison of port-based positional accuracy determinations

If there were gaps in the G12 data, the bestnav process used other inputs as necessary. These were turned to in the strict preference order: GPS Trimble 4000 data, GPS Ashtech 3D, GPS Glonass (which uses a combination of Russian and American satellite networks). Or, as a last resort, if no GPS was available the Chernikeef electo-magnetic log velocity data and gyro heading were used to dead-reckon the ship's position.

Data were transferred daily from the Level C bestnav stream to the Pstar absolute navigation files, abnv2581 (leg 1) and abnv2582 (leg 2) The G12, gps-4000, gps_glos and gyro (gyronmea) data streams were also transferred daily. Processing scripts nav-, gyro-, gps-exec0 etc are summarized below

Heading

The ship's attitude was determined every second with the ultra short baseline 3D GPS Ashtech ADU2 navigation system. Configuration settings from previous calibrations (Trials cruise in April 2001) were used throughout the cruise. Four antenna, two on the boat deck, two on the bridge top, measured the phase difference between incoming satellite signals from which the ship's heading, pitch and roll were determined. The data were used to calibrate the gyro heading information using the ashexecs listed below

Ashtech 3D GPS coverage was generally good. Dropouts occurred several times; but on only one occasion was it necessary to reset the Ashtech Unit in the Comms Room. Gaps over 1 min in the data stream are listed in table 2.

Table 2. Gaps in Ashtech 3D GPS coverage

SOC PEXEC navigation processing

The text in bold refers to SOC PEXEC processing scripts. Details of the PEXEC data processing format (Pstar) and programs can be found at:
http://www.soc.soton.ac.uk/JRD/PEXEC/pexec.html

navexec0 transferred data from the RVS bestnav stream to Pstar, calculated the ships velocity, appended onto the absolute (master) navigation file and calculated the distance run from the start of the master file. Output: abnv2581 and abnv2582.

gyroexec0 transferred data from the RVS gyronmea stream to Pstar, a nominal edit was made for directions between 0-360° before the file was appended to daily master files.

gp4exec0 transferred data from the RVS gps_4000 stream to Pstar, edited out pdop (position dilution of precision) greater than 5 and appended the new 24 hour file to master files gp42581 and gp2582.

glosexec0 this was identical to gp4exec0 but transferred the RVS gps_glos data stream to Pstar in master files gls2581 and gls2582.

gpsexec0 this was identical to gp4exec0 but transferred the RVS gps_g12 data stream to Pstar in master files gps2581 and gps2582.

ashexec0 transferred data from the RVS gps_ash stream to Pstar.

ashexec1 merged the Ashtech data from ashexec0 with the gyro data from gyroexec0 and calculated the difference in headings (hdg and gyroHdg); ashtech-gyro (a-ghdg) (daily files).

ashexec2 edited the data from ashexec1 using the following criteria:

• heading 0 < hdg < 360 (degrees)
• pitch -5 < pitch < 5 (degrees)
• roll -7 < roll < 7 (degrees)
• attitude flag -0.5 < attf < 0.5
• measurement RMS error 0.00001 < mrms < 0.01
• baseline RMS error 0.00001 < brms < 0.1
• ashtech-gyro heading -10 < a-ghdg < 10 (degrees)

The heading difference (a-ghdg) was then filtered with a running mean based on 5 data cycles and a maximum difference between median and data of 1 degree. The data were then averaged to 2 minutes and further edited for:

-2 < pitch <2
0 < mrms < 0.004
-10 < a-ghdg < 10

The 2 minute averages were merged with the gyro data files to obtain spot gyro values. The ships velocity was calculated from position and time, and converted to speed and direction. The resulting heading difference should be a smoothly varying trace that can be merged with ADCP data to correct the gyro heading. do.plotash was the script used to produce diagnostic plots to check this. During ship manoeuvres, bad weather or around data gaps, there were spikes which were edited out manually (plxyed).

ashexec3 appended daily Ashtech files to a master file (ash258smt.ave) after removing any overlapping time steps. The master file was subsequently used in VMADCP and surfmet data processing.

Project Information

Marine Productivity programme (MarProd)

The Marine Productivity programme (MarProd) was a Thematic Programme of the Natural Environment Research Council. It was funded for a period of five years starting in 2000. Its main goal was "to develop coupled modelling and observation systems for the pelagic ecosystem, with emphasis on physical factors affecting zooplankton dynamics" with the following specific objectives:

• To identify the dominant spatial and temporal scales of physical parameters and zooplankton population dynamics, by observation, modelling and retrospective analysis

• To parameterise the critical processes governing zooplankton dynamics by observations and experiments

• To construct and validate spatially explicit models of zooplankton and their food and predators, capable of resolving short term changes in population structure

• To provide data for model validation by developing and applying new interdisciplinary techniques to a wide spectrum of biological and physical parameters

• To develop a database and information system for historic and new data and models.

The programme was composed of two phases: Phase 1 projects (2000-2002) focused on the use of historical datasets and existing biological models, complemented by laboratory experiments and remote-sensing analyses to gain a better understanding of the dynamics of zooplankton populations in shelf seas. The main, field-based Phase 2 of the programme (2001-2005) focused on the open ocean. The fieldwork phase took place between November 2001 and December 2002 and consisted of four surveys in the northern North Atlantic in early winter 2001 and 2002, and in spring and summer 2002.

MarProd was a major UK contribution to the international Global Ocean Ecosystem Dynamics project (GLOBEC).

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: