Friday, January 7, 2011

Polar Glacier Motion Monitoring

Glaciers in Polar Regions are accelerating to melt due to complicated global climate change. It is of great importance to monitor glacier motion since glaciers play an important role in the global climate system and sea level rise, interacting in a complex fashion with atmosphere and ocean. The atrocious geographical and natural conditions, however, make it quite difficult and often dangerous to carry out field expeditions to in situ velocity measurement in the remote Polar Regions.

Satellite imagery is suitable for studying Polar Regions. Cross-correlation of sequential optical images can be used to measure the ice flow velocity. Two methods that are widely used and proved to be of good accuracy are Normalized Cross-Correlation (NCC) and Orientation Correlation (OC). Scientists around the world have done such work using remote sensing data such as MODIS, Landsat MSS/TM/ETM+, ASTER, SPOT and EO-1 ALI etc. Optical data are main data sources at the beginning of polar research. Unfortunately, optical data coverage is “spotty”, being severely limited by long polar night, frequent cloud cover, failures of data transmission links, and featureless ice surface. In our team’s Antarctic Mapping Project we collected over 1100 ETM+ images between 1999 and 2003 from thousands against the odds to map the mosaic image of Antarctica in 15m resolution with 6 bands. It is a quite tough job. Nevertheless, optical data are essential and important to glacier motion monitoring since there are large amount of archived data which are valuable for research on earth’s long-term surface change.

Compared with optical data, imaging radars are more suitable for research on glaciers because of their all-weather, day-night imaging capacity. Spaceborne Synthetic Aperture Radar (SAR) is a powerful research tool by virtue of its ability to penetrate the shroud of darkness and cloud cover and acquire high-resolution images in different microwave band frequency and polarizations. SAR interferometry (InSAR) has now become a well-established means of measuring ice velocity in high accuracies. InSAR is of great advantage in monitoring glacier motion since it can detect subtle changes of glacier surface. Additionally, methods based on speckle and fringe tracking are developed. For loss of coherence, i.e., in the case of rapid and incoherent flow and of large acquisition time intervals between the two SAR images, methods of processing optical data are also adopted to process SAR intensity images, which could serve as an alternative to differential InSAR for the measurement of glacier motion.

The main problems of research on glacier motion are data acquisition and accuracy verification. With more and more multi-source data becoming available, the emerging Light Detection and Ranging (LiDAR) data included, the first problem will be gradually solved. Multi-spectral data of high qualities, however, are still difficult to obtain. As for the second problem, historical observational data and field work are essential to verify research results. NASA’s Operation IceBridge aircraft missions are really worthy looking forward to.

[1] Robert Massom. Dan Lubin, Polar Remote Sensing [ M ]. Vol II. UK. 2006
[2] W. Gareth Rees. Remote Sensing of Snow and Ice [ M ]. 2006
[3] Tazio Strozzi, et al. Glacier Motion Estimation Using SAR Offset-Tracking Procedures [ J ]. Geosci & Rem Sens, 2002, 40 (11): 2384~2391.

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