OCI-200 Processing
Southern Ocean Moorings
Oct 97 - Mar 98

Jasmine S. Bartlett and Mark Abbott (PI)
Oregon State University
July 23 1998

Investigators: Mark Abbott (PI), Ricardo Letelier, Jim Richman and Jasmine S. 
Bartlett

Instrument Deployment

12 OCI-200 sensor heads (S/N 52-57, 59-63 and 66) were deployed on individual 
moorings in the Southern Ocean in October and November 1997.  The moorings were 
arranged in a grid pattern centered at (60.5oS, 170oW), with the moorings placed 
approximately 30 km apart.  Each mooring was anchored to the ocean bottom, 
placing the sensor heads at approximately 50 m depth.  A buoyant sphere beneath 
the sensor head was used to limit the range of tilt of the sensors.  
Measurements were made every 6 minutes.  The moorings were left unattended until 
their removal in March 1998.  Of the 12 sensors, 9 provided data for analysis; 2 
of the remaining sensors were found flooded upon recovery and the third sensor 
was not found.  The moorings from which data are available are moorings: 2, 4, 
5, 6, 7, 8, 9, 10 and 12.  All but one of these data sets covers the entire 
period of interest.  The remaining sensor head (that on mooring 10) collected 
data for only 10 days and then ceased to operate.


The Instruments

The measured parameters of each OCI-200 sensor head include downwelling 
irradiance at seven wavebands with bandpasses of approximately 10 nm.  Six of 
the wavebands were the same for all 12 sensor heads (412, 443, 490, 555, 665 and 
683 nm center wavelengths).  The seventh waveband (channel 4 on all sensor 
heads) was centered at 511 nm for six of the sensor heads (moorings 4, 5, 7, 10, 
11 and 12) and 700 nm for the remaining six sensor heads (moorings 1, 2, 3, 6, 8 
and 9). 

Calibrations of the sensor heads were made by Satlantic, Inc. before and after 
the mooring deployment.  The pre-deployment calibrations were made at room 
temperature (20oC), whereas the post-deployment calibrations were made at 
approximately 0oC (closer to the water temperature near the moorings).  Note 
that the calibration values provided in the manual for each sensor head are in 
different units (in terms of millivolts) than required for processing (in terms 
of counts); these values were converted to appropriate units before use.


The Data Processing

Both sets of calibration files (20oC and 0oC) yielded negative irradiances when 
used to calibrate the measurements.  To overcome this problem, the following 
procedure was used:

The data was first converted from binary to ASCII format (as raw counts) using 
the Satlantic, Inc. program asciicon.exe.  The measured dark values (found 
between 23:00 and 01:00 solar time, a total of 20 measurements/day) were then 
averaged for each day, yielding a set of daily dark values.  The raw irradiance 
data (in counts) was then converted to irradiance units by first subtracting the 
appropriate dark value for the day, and then multiplying by a calibration 
coefficient from the post-calibration file and by an immersion correction 
factor, as provided by Satlantic, Inc.  

Daily dark values were used rather than an average dark value for the entire 
time period, since variations in the dark values with time were found for one of 
the sensors.  This sensor (mooring 6) showed a peak in dark values between days 
350 and 380, and otherwise remained relatively constant.  The dark values for 
the other moorings all remained relatively constant throughout the period of 
deployment.  However, mooring 9 showed anomalous dark values on day 430. The 
irradiance data from this day were removed from the mooring 9 data set.

The data files were then truncated to contain only data between the start 
deployment time and end recovery time.  Anomalous data caused by measurements 
made during the actual mooring deployment and recovery were then removed by 
ignoring the first and last 100 data points (10 hours of data at each end). 

Note that the resolution of measurements made by the higher wavelength 
measurements (665, 683 and 700 nm) is much lower than that of the shorter 
wavelengths.  For example, the range in counts of measurements made at 412 nm in 
one case is 0 - 14000, whereas the range in counts for the 665 nm sensor in the 
same data set is only 0 -70, a factor of 200 lower.


Data Time

The timestamp of the data is in GMT time for 1997.  Note that days in the year 
1998 are listed as day(1998) + 365 for continuity.

The Data

The processed data is in the following format:
Column 1: day of year 1997 (GMT time)  (1998 is day of year + 365)  
Column 2: channel 1 (412 nm)
Column 3: channel 2 (443 nm)
Column 4: channel 3 (490 nm)
Column 5: channel 4 ( 511 nm or 700 nm)
Column 6: channel 5 (555 nm)
Column 7: channel 6 (665 nm)
Column 8: channel 7 (683 nm)
Each of the columns contains downwelling irradiance data with units of 
uW/cm^2/nm.

The Calibration Files

The calibration files have the following format:
Rows 1-14: header lines
Rows 15-28: calibration values
Rows 29-34: header lines

An example set of calibration values from one of these files is shown below:
ED 411.7 'uW/cm^2/nm' 2 BU 1 OPTIC2
17.3 2.8992e-003 1.52

The first number, 411.7, is the center wavelength of that channel.  The 
appropriate units for the calibrated channel are shown in quotes (uW/cm^2/nm).  
The calibrated dark value (counts) is the first number on the second line 
(17.3).  The next number (2.8992e-003) is the calibration factor (F) to convert 
units of counts to irradiance (uW/cm^2/nm).  The final number (1.52) is the 
immersion correction (Im).  There are seven pairs of lines in this format, one 
for each channel.  The appropriate equation for converting raw (count) 
irradiance values to irradiance values in units of uW/cm^2/nm is:
Ed(calibrated) = [Ed(counts)-dark(counts)] * F * Im.
Note that the data presented here have already been calibrated and do not need 
to have these corrections applied.  The calibration files are included here for 
completeness.