Metadata Report for BODC Series Reference Number 624985


Metadata Summary

Data Description

Data Category Currents -subsurface Eulerian
Instrument Type
NameCategories
Teledyne RDI Ocean Surveyor 75kHz vessel-mounted ADCP  current profilers
Instrument Mounting research vessel
Originating Country United Kingdom
Originator -
Originating Organization Southampton Oceanography Centre (now National Oceanography Centre, Southampton)
Processing Status banked
Project(s) Marine Productivity
 

Data Identifiers

Originator's Identifier SUR25813
BODC Series Reference 624985
 

Time Co-ordinates(UT)

Start Time (yyyy-mm-dd hh:mm) 2001-11-16 18:32
End Time (yyyy-mm-dd hh:mm) 2001-11-17 17:22
Nominal Cycle Interval 120.01 seconds
 

Spatial Co-ordinates

Southernmost Latitude 55.86100 N ( 55° 51.7' N )
Northernmost Latitude 56.68770 N ( 56° 41.3' N )
Westernmost Longitude 39.14730 W ( 39° 8.8' W )
Easternmost Longitude 38.71680 W ( 38° 43.0' W )
Positional Uncertainty 0.0 to 0.01 n.miles
Minimum Sensor Depth 21.0 m
Maximum Sensor Depth 965.0 m
Minimum Sensor Height -
Maximum Sensor Height -
Sea Floor Depth -
Sensor Distribution Sensor fixed, measurements made at fixed depths - The sensor is at a fixed depth, but measurements are made remotely from the sensor over a range of depths (e.g. ADCP measurements)
Sensor Depth Datum Instantaneous - Depth measured below water line or instantaneous water body surface
Sea Floor Depth Datum -
 

Parameters

BODC CODE Rank Units Short Title Title
DBINAA01 0 Metres BinDep Depth below sea surface (ADCP bin)
AADYAA01 1 Days Date(Loch_Day) Date (time from 00:00 01/01/1760 to 00:00 UT on day)
AAFDZZ01 1 Days Time(Day_Fract) Time (time between 00:00 UT and timestamp)
ALATAS01 1 Degrees Lat_AshGPS Latitude north (WGS84) by Ashtech GPS
ALONAS01 1 Degrees Lon_AshGPS Longitude east (WGS84) by Ashtech GPS
ASAMAS01 2 Decibels ADCPSA Signal return amplitude from the water body by shipborne acoustic doppler current profiler (ADCP)
LCEWAS01 2 Centimetres per second CurrVelE_ADCP Eastward current velocity (Eulerian) in the water body by shipborne acoustic doppler current profiler (ADCP)
LCNSAS01 2 Centimetres per second CurrVelN_ADCP Northward current velocity (Eulerian) in the water body by shipborne acoustic doppler current profiler (ADCP)
LERRAS01 2 Centimetres per second ADCPER Current velocity error in the water body by shipborne acoustic doppler current profiler (ADCP)
LRZAAS01 2 Centimetres per second ADCPVC Upward current velocity in the water body by shipborne acoustic doppler current profiler (ADCP)
PCGDAP01 2 Percent Percent_Good_(ADCP) Acceptable proportion of signal returns by acoustic doppler current profiler (ADCP)
 

Definition of Rank

  • Rank 1 is a one-dimensional parameter
  • Rank 2 is a two-dimensional parameter
  • Rank 0 is a one-dimensional parameter describing the second dimension of a two-dimensional parameter (e.g. bin depths for moored ADCP data)

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

in addition

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 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 75kHz Vessel mounted ADCP.

Long-Range Mode
Vertical Resolution Cell Size 3 Max. Range (m) 1 Precision (cm/s) 2
8m 520 - 650 30
16m 560 - 700 17
High-Precision Mode
Vertical Resolution Cell Size 3 Max. Range (m) 1 Precision (cm/s) 2
8m 310 - 430 12
16m 350 - 450 9

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

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)

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

BODC code Pstar variable description units comments
DBINAA01 bindepth Bin depth m none
LCEWAS01 absve E-W current velocity (VMADCP) cm/s none
LCNSAS01 absvn N-S current velocity (VMADCP) cm/s none
LRZAAS01 velvert VMADCP vertical current velocity (+ve up) cm/s none
LERRAS01 velerr Error velocity (VMADCP) cm/s none
ASAMAS01 various VMADCP signal return amplitude db see transfer
PCGDAP01 good4bm Percent Good Signal Return (Shipborne ADCP) % see transfer
ALATAS01 lat Latitude north (Ashtech GPS) ° none
ALONAS01 lon Longitude east (Ashtech GPS) ° none

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

Mean latitude ° N SD Mean longitude ° W SD RMS pos error
Govan Ashtech G12 55.86633 0.00002° (2.16 m) 4.35270 0.00003° (1.86 m) 2.85 m
Trimble GPS 4000 55.86634 0.00001° (1.08 m) 4.35266 0.00001° (0.62 m) 1.25 m
GPS GLOS 55.86629 0.00004° (4.32 m) 4.35274 0.00005° (3.10 m) 5.32 m
Reykjavik Ashtech G12 64.15010 0.00004° (2.39 m) 21.93873 0.00003° (1.89 m) 3.05 m
Trimble GPS 4000 64.15014 0.00002° (2.33 m) 21.93879 0.00003° (1.53 m) 2.79 m
GPS GLOS 64.15005 0.00003° 21.93870 0.00006°

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

Time gap (yr, JD, hr, min, sec) Duration
Leg 1 01 307 21:01:00 to 01 307 21:02:07 ~1 min
01 313 13:52:54 to 01 313 13:54:08 ~1 min
01 313 13:56:54 to 01 313 13:59:04 ~2 min
01 317 14:27:26 to 01 317 14:31:03 ~4 min
01 317 14:33:34 to 01 317 14:36:53 ~3 min
01 324 23:52:23 to 01 324 23:56:24 ~4 min
01 325 08:48:00 to 01 325 08:49:05 ~1 min
01 325 09:22:33 to 01 325 09:23:36 ~1 min
01 326 09:39:29 to 01 326 10:15:54 ~36 min
01 326 21:42:40 to 01 326 21:44:14 ~2 min
Leg 2 01 329 18:59:10 to 01 329 19:00:11 61 s
01 330 19:02:54 to 01 330 19:04:00 66s
01 331 20:42:00 to 01 331 20:43:04 64s
01 333 09:20:17 to 01 333 09:33:25 13.1 min
01 337 09:04:44 to 01 337 09:54:55 50.2 min
01 340 00:44:54 to 01 340 00:46:28 94 s
01 340 17:26:49 to 01 340 17:33:52 7.0 min
01 341 08:30:45 to 01 341 08:31:50 65 s
01 342 08:25:41 to 01 342 08:26:44 63 s
01 345 20:22:13 to 01 345 20:23:47 94 s

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:

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:

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

Cruise Name D258
Departure Date 2001-11-01
Arrival Date 2001-12-18
Principal Scientist(s)Raymond T Pollard (Southampton Oceanography Centre), Steven J Hay (Fisheries Research Services Aberdeen Marine Laboratory)
Ship RRS Discovery

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:

Flag Description
Blank Unqualified
< Below detection limit
> In excess of quoted value
A Taxonomic flag for affinis (aff.)
B Beginning of CTD Down/Up Cast
C Taxonomic flag for confer (cf.)
D Thermometric depth
E End of CTD Down/Up Cast
G Non-taxonomic biological characteristic uncertainty
H Extrapolated value
I Taxonomic flag for single species (sp.)
K Improbable value - unknown quality control source
L Improbable value - originator's quality control
M Improbable value - BODC quality control
N Null value
O Improbable value - user quality control
P Trace/calm
Q Indeterminate
R Replacement value
S Estimated value
T Interpolated value
U Uncalibrated
W Control value
X Excessive difference

SeaDataNet Quality Control Flags

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

Flag Description
0 no quality control
1 good value
2 probably good value
3 probably bad value
4 bad value
5 changed value
6 value below detection
7 value in excess
8 interpolated value
9 missing value
A value phenomenon uncertain