Underway data processing
Underway instruments take continuous measurements of sea surface data (e.g. salinity, temperature, attenuance, chlorophyll and nutrients), meteorology, navigation and bathymetry. They usually measure a continuously pumped surface sea water supply available on most research 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, pCO2 system, nutrient analysers)
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 30 seconds for short cruises, or 60 seconds for longer cruises.
3. 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.
4. Screen data. Suspect values are flagged as suspect, missing values as null. Where there are particular problems (e.g. poor quality of bathymetry data or wind speed data) a warning is given in the accompanying documentation.
5. Calibrate data. Manufacturer's calibrations are applied to convert the data to scientific units. Some channels are calibrated against sample data or other calibrated instruments such as the CTD. Relative wind speed and direction are corrected to absolutes. This is achieved by using the data for the ship's velocity in the North-South and East-West direction and the ship's heading.
6. Document data. Documentation is written containing information about instruments, calibration, sampling techniques, parameters and sea areas.
7. Distribution and delivery. Data are distributed to project participants on request. Data can be extracted from the binary file using BODC's software.