Underway data processing
Underway instruments take continuous measurements of sea surface data (e.g. salinity, temperature, transmissivity and fluorescence), meteorology (wind speed and direction; air temperature, humidity and pressure; irradiance), navigation (latitude, longitude, speed, course and heading)) and bathymetry. Sea surface data are usually obtained from a continuously-pumped surface seawater supply available on most reserach vessels.
The continuous underway data set includes the following:
- Navigation from the ship
- Data from the meteorological package
- Data from the thermosalinograph
- Data from other instruments attached to the pumped water supply (e.g. fluorometer and transmissometer)
The current procedure for processing underway data is:
1. Archive original data. When the data are first received they go through our accession procedure. The data are securely archived in their original form along with any associated documentation.
2. Merge data into a single binary file of standard format using time as the common variable. The sampling interval is usually set to 60 seconds, with the resulting file comprising 60 second averages.
3. Calibrate data. Manufacturer's calibrations are applied to convert the data to scientific units using instrument calibration coefficients. Some channels are calibrated against simple data or other calibrated instruments such as CTD.
4. Automated corrections.Clean bathymetry data using in-house script and correct relative wind speed and direction to absolutes using Shipboard Automated Meteorological and Oceanographic System (SAMOS) method. This is achieved by using the data for the ship's velocity in the North-South and East-West directions and the ship's heading.
5. Check navigation record for gaps and improbable ship speeds (>15-19 knots depending on vessel). Gaps are filled with log sheet information where they are longer than 15 minutes, or by interpolation for shorter periods.
6. Screen data. Reference streams (e.g. GEBCO bathymetric grid) are added to aid screening. Suspect values are flagged as suspect, missing values as null. Where there are particular problems (e.g. poor quality bathymetry or wind speed data) a warning is given in the accompanying documentation in the form of quality or problem reports depending on the severity.
7. Document data. Documentation is written containing information about instruments, calibration, sampling techniques, parameters, and sea areas.
8. Distribution and delivery. Data are distributed to project participants on request. Data can be extracted from the binary file using BODC's software.