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4. RAPPLS_TotalFreeboard

Description

The dataset submitted here is 'Sea-ice freeboard derived from airborne laser scanner'.

Between 2007 and 2012, the Australian Antarctic program operated a scanning LiDAR system and other scientific instruments for sea-ice geophysical surveys in East Antarctica. For example see Lieser et al. [2013] for the 2012 survey. The dataset here provides the sea-ice freeboard (i.e. elevation above sea level) along various helicopter flight lines of the 2012 survey in the sea-ice zone between 113 degE and 123 degE.

The data collection was based on:
- Riegl LMS Q240i-60 scanning LiDAR, measuring sea ice elevation above the WGS84 reference ellipsoid;
- Hasselblad H3D II 50 camera, taking aerial photographs at about 13 cm resolution every 3-5 seconds (older digital camera used in 2007);
- inertial navigation and global positioning system, OxTS RT-4003.

The following geophysical corrections were applied to the sea-ice elevations to derive the sea-ice freeboard:
- geoid correction (from the EGM2008 Earth gravity model);
- mean ocean dynamic topography correction (from the DTU Space model - DTU10MDT);
- ocean tide correction (from the Earth and Space Research CATS2008 Antarctic tide model);
- atmospheric pressure (inverse barometer effect) correction from ECMWF data (4-year average) and ship-board underway observations.
The geophysical corrections have been validated along selected flight lines by extracting ocean surface elevations from leads between ice floes as identified in the aerial photography.

Contained in this dataset are the following files:
- a netCDF file for 8 selected flights of the 2012 survey containing sea-ice freeboard values;
- a postscript file for 4 of the 8 selected flights showing the residuals from the applied geophysical corrections. These 4 flights were selected on the basis of having a good spread of observable leads along the entire flight line that enabled the extraction of ocean surface elevations.

Purpose

Derive combined sea-ice and snow elevation as intermediate step to sea-ice thickness data.

Quality

The quality of this dataset is affected by the vertical movement of the airframe: the helicopter provides a vibrating mounting point for the laser scanner and associated instrumentation. The Inertial Motion Unit (IMU) was not able to capture the full spectrum of small-scale vibrations. Hence the derived total freeboard is affected by some instrument associated noise.

Consequently:
* The data exhibit approximately 0.2 m vertical scatter due to uncharacterised vibration at the LIDAR head (compared to the same instrument in a fixed-wing aircraft);
* In 2007, a single frequency GPS was used to position the helicopter, limiting absolute point elevation accuracy to no better than 30cm (combined GPS + IMU);
* From 2008 onwards a dual-frequency GPS was used to position the helicopter, enabling nadir point elevation accuracies in the sub-decimetre range.

Access

These data are publicly available for download from the provided URL.

Temporal Coverages

• Start date: 2012-09-25 - Stop date: 2012-11-06

Spatial Coverages

Science Keywords

• EARTH SCIENCE > CRYOSPHERE > SEA ICE > FREEBOARD

Use Constraints

This data set conforms to the CCBY Attribution License (http://creativecommons.org/licenses/by/4.0/).

Please follow instructions listed in the citation reference provided at http://data.aad.gov.au/aadc/metadata/citation.cfm?entry_id=RAPPLS_TotalFreeboard when using these data.