| Summary: | Statement: Initial Processing Incoming data undergoes gross range checks and removal of any corrupted or unrealistic data. Some CTD data is subject to a pressure offset error generally less than 1 decibar. A pressure offset error occurs when the pressure sensor does not read 0 dbar in air. In cases where the pressure offset error was observed before the CTD cast the pressure readings have been corrected for the offset which is stored in the global netCDF attribute named 'CTD_pressure_offset'. Salinity on the PSS-78 scale is computed from conductivity, temperature and pressure using the UNESCO (1983) algorithm with temperature corrected from the ITS-90 scale to the ITS-68 scale used by the algorithm. For the MVP200 CTD model the response time (time constant) of the temperature sensor (100 ms) is slower than the conductivity cell (25ms). This leads to a phenomenon called 'salinity spiking' in the salinity data. Spiking is caused by the fact that calculated salinity is a function of conductivity, temperature and pressure [S=f(C,T,z)]. The temperature response lags the conductivity response resulting an error in the calculation of salinity. This is evident as noticeable spikes in the salinity profiles especially at depths where temperature is rapidly changing e.g in the ocean thermocline. The spiking was reduced reduced by applying a 2.5 sample time shift (0.1 s) to the temperature values and using the shifted temperature value in the re-computation of the salinity. An array of raw pressure, temperature and de-spiked salinity is then passed to the next processing stage. The HS_CTD samples at a lower rate of 1.83 Hz compared to the MVP200 CTD at 25 Hz and is not subject to the salinity spiking issue. The raw CTD data contains a non-monotonic sequence of pressures due to up/down motion of ship/winch. A sequence of unique monotonic pressures up to the maximum value is extracted and then linearly interpolated to 1 decibar pressure levels with the corresponding interpolated temperature and salinity. Finally the ...
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