Please note that the detailed information in this section is included only for completeness. It is not necessary to have this information in order to use the data.
At AWI, processing takes place using the following data files and executable files.
|Binary data file from the instrument
|Calibration data, with pre-deployment coefficients
|Air pressure data are 6 hourly data from the European Centre for Medium-Range Weather Forecasts (ECMWF)
|Program to translate binary data to hexadecimal. The hexadecimal output format is useful in quality control of start time, time to water and time marks and in looking for gaps in the data.
|Program to change binary data to ASCII (written by APL). Modified by H. Witte for each mooring to skip gaps and change the output from ice draft to travel time. Outputs four files: ulsxx.highdraft, ulsxx.lowdraft, ulsxx.temperature and ulsxx.hourly (ulsxx.hourly is the measured pressure every hour and is not used for further work).
|Puts linearly interpolated temperature data in the lowdraft (tindraftl.for) and, if necessary, highdraft (tindrafth.for) data files.
|Program to find sound speeds
|Program to calculate ice draft using travel time and sound speed data files. The program eislauftest.for flags the drafts as ice or water, and outputs monthly files.
|A MATLAB ® program (written by H. Witte) that splits the output of eislauftest.for into four monthly files: *.
Quality control for binary and ASCII data output files takes place as follows.
Binary-data: Start time, sample time, time to water and time marks are controlled with rawuls.exe. Days with missing time marks are left out and binary data are converted to ASCII data with sepuls.c.
ASCII-data: Start and end time of lowdraftdata (the data acquired at 5 minute intervals) are controlled. Start and end time of highdraftdata (the data acquired at 10 second intervals for 25 minutes at 0000 and 1200 UTC) are controlled. Temperature and pressure data are plotted.
Notes on processing steps
Processing raw upward looking sonar data to produce draft measurements involves accounting for factors that introduce errors. For example, sound velocity depends on temperature and salinity. These parameters vary seasonally and on shorter time scales. The depth of the instrument is measured by a pressure sensor, and this measurement must be corrected for variations in pressure caused by variations in water density and air pressure. These factors and others are accounted for in the following processing steps:
- Recalibrating the instrument crystal frequency parameter if necessary for reducing error in time and temperature measurements. Adding temperature and pressure along with travel time to the output files.
- Calculating sound velocity.
- Refining the sound velocity estimate by calibrating it based on distance to level water.
- Calculating ice draft for 5 minutes measurements.
- Recalculating drafts where needed by adjusting sound velocity estimates.
1. Recalibrating the instrument crystal frequency parameter if necessary.
Each instrument has a set of associated calibration coefficients for temperature, pressure, and time recorded in file paros.dat. The calibration coefficients are set and recorded before deployment, and the clock oscillator frequency is measured at room temperature. When the ULS is operating in the ocean, the temperature is usually not too far from 0 C. This difference in temperature changes the clock crystal frequency by a significant amount. When the mooring is retrieved, recorded temperature is examined for differences with that recorded by other instrumentation on the same mooring (generally the temperature sensor on a nearby current meter). If an offset exists, the clock oscillator frequency constant (ÔxtalÕ) is adjusted until the temperature is corrected. Note that this correction also has an effect on the travel time (pulse reflection times).
Example: If T(ULS)= -2.1 ; T(currentm.)= -1.8 at P(ULS)= 50db and P(currentm.)= 70db, then the adjustment is T(ULS)= T(ULS)+0.3
The adjustments to xtal that were made for the moorings in this data set are given below:
(uls32 1997/1998: T=Tuls-0.15 -> xtal cal = 12004380.0 -> xtal(changed)= 12004350.0)
(uls32 1993/1994: T=Tuls-0.35 -> xtal cal = 12004400.0 -> xtal(changed)= 12004205.0)
(uls49 1994/1995: T=Tuls+0.17 -> xtal cal = 12002560.0 -> xtal(changed)= 12002650.0)
(uls31 1992/1993: not changed xtal=12004400.0)
(uls47 1998/1999: not changed xtal=12002400.0)
(uls25 1999/2000: T=Tuls+0.7 -> xtal cal=12002400.0 -> xtal(changed)= 12002750.0)
(uls32 1999/2000: T=Tuls-0.2 -> xtal cal=12004380.0 -> xtal(changed)= 12004280.0)
(uls26 1991/1992: T=Tuls+0.5 -> xtal cal=12004500.0 -> xtal(changed)= 12004800.0)
(uls48 2000/2001: T=Tuls+3.4 -> xtal cal=12002400.0 -> xtal(changed)= 12004150.0)
(uls47 2001/2002: T=Tuls-0.05 -> xtal cal= 12002400.0 -> xtal(changed)= 12002360.0)
(uls31 2000/2001: T=Tuls+3.7 -> xtal cal= 12002400.0 -> xtal(changed)= 12004300.0)
2. Adding temperature and pressure
The air pressure from ulsxx.presSea for every measurement was written to the output file in order to more accurately estimate instrument depth. Pressure at the instrument depth must be corrected for variations in air pressure using 6-hourly surface pressure analyses from the European Centre for Medium Range Weather Forecasting (ECMWF). These were used directly for the ’highdraftdata’ and interpolated for the ’lowdraftdata.’
The temperature for every measurement was written to the output file along with pressure and travel time. Temperature recorded by the instrument was linearly interpolated to match the acquisition times for ’lowdraftdata’. For ’highdraftdata’ the recorded midnight temperature was used, while the noon temperature was interpolated between 11.00 a.m. and 1.00 p.m. This interpolation is necessary, since the temperature is measured only 23 times a day.
3. Calculating the sound velocity ’first guess’
To calculate ice draft from travel time the sound velocity must be known. A constant velocity of 1442 meters per second was used as a starting point. It was assumed that in areas of open water the difference of two sequentially measured pulse travel times must be zero. A program was used (trickyt.for) to look in the highdraftdata for times where t1-t2 ~ 0 in any 25 minutes sample and to calculate sound velocity by c=depth/travel time. This estimate of sound velocities was used to calculate the ice draft.
4. Refining the sound velocity estimate by determination of distance to surface
Ice draft was calculated for the ’highdraftdata’c acquired in the burst sample mode (10 second intervals for 25 minutes) and the results were plotted on a screen. The plot should show areas of open water (which are usually identifiable by their chaotic profile, in contrast to the definite profile of ice) at a draft of zero. The offset of the open water level from zero was used to refine the sound velocity estimate. The program limits velocity to values between 1428 and 1467 meters per second. Calculated velocities falling outside of that range were fixed to an assumed value. The sound velocity was plotted to check for obviously erroneous values (spikes) that were then removed.
5. Calculating the ice draft for the 5 minute interval measurements.
The program ’eislauf.for’ uses pressure recorded by the instrument (for the instrument depth), the air pressure (form ECMWF) and the sound velocity value in the output files of ’trickyt.for’ to calculate the ice draft. (The refined sound velocity values from the ’highdraftdata’ measurements were linearly interpolated to get one for every ’lowdraftdata’ measurement)
6. Recalculating drafts based on adjusted sound velocity estimates
Potential errors in the ice drafts resulting from incorrect sound velocity estimates which were evident by unrealistic jumps in the ice draft profile were eliminated by adjusting the sound velocity to reestablish continuity.