Summer Lead Data  (SBE25 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 October 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-25 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-25 CTD, atmospheric pressure collected at the main camp is also used. 
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-25 Conductivity-Temperature-Pressure (CTD) sensor was used to make physical measurements of uppermost 60 m of the water column. The instrument was normally mounted to profile from within about 40 cm of the surface to 15 m depth with occasional profiles to 60 m.  
The sensors were calibrated before and after the experiment.
Data was logged internally during each sampling period at a rate of 8 Hz. The data was downloaded after 2 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-25 data was processed using the processing software supplied by the manufacturer (seasoft). The calibration file used is sheba25p.con from post calibrations. No significant differences were found between the pre and post calibrations of the temperature and conductivity sensors. 
For the ctd processing we used the following seasoft routines: 
Datcnv - to insert calibrations output (T, C, P,time(julian date),fluorometer voltage)
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 position data was then linearly interpolated to the time of the CTD to merge the two data streams together. Variations in atmospheric pressure wer subtracted from the CTD pressure record.
 
IMPORTANT NOTES
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.  There is some evidence of drift in the pressure sensor calibration over the duration of the experiment, this is evident by the residual pressure when the sensors are removed from the water.   Extreme care must be used when interpreting the fluorometer signal.  We found that false subsurface maximums were evident because of changes in the fluorescence efficiency as the phytoplankton and instruments went under the ice.
 
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, sigma-T, and fluorometer voltage. 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.