Summer Lead Data  (SBE19 CTD)
CONTACT INFORMATION: 
P.I.s Clayton Paulson and W. Scott Pegau
COAS 104 Ocean Admin Bldg
Corvallis OR 97331 
phone: 541 737-5229
fax : 541 737-2064
email: cpaulson@oce.orst.edu
         spegau@oce.orst.edu 
DATA VERSION: 
The data was initially processed by W. Scott Pegau in August of 1999. The data supplied is version 1. This data is expected to be in its final format. 
This document contains notes on the collection and processing of data related to the SBE-19 CTD used in profiling mode as part of the summer leads portion of SHEBA. Further information and pictures of the equipment is available on the Sheba Main Page. 

Most of the data was collected from a small lead about 1 km WNW from the ship. EQUIPMENT: 
This data set includes data from three instruments 2 Garmin 12XL GPS units and a SBE-19 CTD. 
GPS notes:
One Garmin 12xl gps was left on the ice to provide a reference point. The second unit was place on the port edge of the skiff for determining the position of the boat. 
The gps units were internally recorded at 15 second intervals except on a couple occasions early on in the experiment. The time synchronization between units drifted over time. 
SBE-19 notes:
A SeaBird SBE-19 Conductivity-Temperature-Pressure (CTD) sensor was used to make physical measurements of uppermost 5 to 15 m of the water column. The instrument was normally mounted to profile from within a couple cm of the surface to 15 m depth.  
The sensors were calibrated before and after the experiment.
Data was logged internally during each sampling period at a rate of 2 Hz. The data was downloaded after 2 to 4 hours of sampling because of the limited internal memory. The CTD's internal time stamp was synchronized with the gps time. 
PROCESSING NOTES: 
The position from each gps unit was interpolated to 1 second intervals using a piece-wise cubic spline and the two interpolated records were merged. The position of the gps on the ice was subtracted from the position on the boat to provide a relative position. The relative position was converted from degrees to meters using 111,120 m/degree latitude. The longitude component was multiplied by the cosine of the latitude and then the meters/degree constant. 
The SBE-19 data was processed using the processing software supplied by the manufacturer (seasoft). The calibration file used is sheba19f.con it includes a compensation slope for the conductivity cell that is the average of the slope from pre and post calibrations (1+post)/2. The correction applied changed the slope of the conductivity by <0.2% compared to the pre-cruise calibration. No changes were applied to the temperature cell calibration that drifted by a millidegree between calibrations. The pre-cruise calibration was combined with the conductivity slope to develop the final configuration file (sheba19f.con). 
For the ctd processing we used the following seasoft routines: 
Datcnv - to insert calibrations output (T, C, P,time(seconds from start))
Filter - lowpass conductivity using a 0.5 second filter by the software manual recommendation
Derive - to determine salinity and sigma-t 
Asciiout - change from binary to ascii and remove header except for column headers 
The ascii version of the CTD data was then merged with the gps data. The two records were merged based on time. The logged starting time and date was added to the time interval output from Datcnv. The position data was then linearly interpolated to the time of the CTD to merge the two data streams together. 
 
IMPORTANT NOTES
The pressure records have not been compensated for changes in atmospheric pressure.   The data record includes times when the sensor was not in the water and all up and down casts collected.  Because of the deployment techniques used the downcast generally contains the best quality data.  Rapid retrieval on the upcasts tend to cause the physical structure to be smeared.  The position within the lead should be determined by using the bowctd data to define the outline of the lead.
 
DATA FORMAT: The data is supplied in ascii, tab-delimited, columnar format files. Each file covers a 2 to 4 hour sampling period within a day. The columns include the date as YYYYMMDDHHmmss, the julian date with January 1 being day 1, relative position EW, relative position NS, pressure, temperature, salinity, and sigma-T. The units of position are in meters, pressure in decibar (1 decibar is ~ 1 meter), temperature in degrees C, salinity in PSU, and sigma-T is the density - 1000 with units of kg/m^3.