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ASL Shallow Water Ice Profiler (SWIP)

SWIP Information

The SWIP measures ice draft by comparing the distance from the SWIP to bottom of the ice to the height of the water column. The range to the bottom of the ice is determined by the travel time of an acoustic pulse and the sound speed of the water (determined by the water quality meter). The height of the water column is determined by the SWIP's onboard pressure sensor, corrected by water density (determined by the water quality meter) and barometric pressure. The exact calculation may be found in the sensor tab under the device listing page (linked above). The infographic below illustrates the calculation (image courtesy ASL Environmental Sciences). For more information, see the ASL SWIP webpage http://www.aslenv.com/SWIP.html

The SWIP includes several sensors parsed from the raw ascii data: "Target 1 Range" for instance. All of these sensors starting with "Target" are used for determining the ice draft. Users will be interested primarily in the "Ice Draft" sensors, of which there are two versions; corrected and uncorrected. The uncorrected version is based on the SWIP's data only, without corrections for sound speed, density or barometric pressure, whereas the corrected version accounts for these variables, as described above.

Normal operation for Ice Profilers is to deploy over a winter, recover and post-process the data. However, post-processing is not an option here as the ice draft measurements are calculated in real time. Quality control is done for out of range values (persistence less than 10, elimination of multipath echoes, errors in the absolute pressure sensor). The system has been optimized for reliability at its location. An offset is of 13 cm is applied as the difference in height been the pressure sensor and the acoustic transducer for the uncorrected ice draft. The corrected ice draft has an additional offset to make the ice draft zero under open water conditions. The SWIP may measure an ice draft larger than expected as slush ice on the underside of the ice will contribute to the ice draft in some conditions. Verification of ice draft by in situ measurements has not been done at this time.