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POLinSAR Workshop 2003

Session: Sea Ice

Chair(s): Mark Drinkwater and Wolfgang Dierking

Summary

Results were presented from three sensors, from five geographic ice regimes (both hemispheres)
­ In spite of this data we are  limited when drawing conclusions about the robustness of classification results.

Only one common dataset was used throughout all presentations ­ Arctic AIRSAR

Three different classification approaches have been made to date:
­ Max. A Posteriori
­ Hierarchical (knowledge driven)
­ Unsupervised Wishart together with Alpha/Entropy/Anisotropy

Assessment of class accuracy limited by lack of independent reference (I.e. in-situ data)

Globally, L-band full polarimetry appears more effective than C-band for ice classification
­ Caveats are that regional advantages appear to exist in employing C-band polarimetric data

The role of phase in polarimetric classification is not clear:
­ It does however appear to be of some practical value in Open Water/Ice classification, and/or discrimination of thin ice types

Spatial resolution (spaceborne vs airborne) has as yet an undetermined effect on polarimetric parameter distributions.
 

 Roundtable Session Questions and Answers

Question 1: What are the applications of sea ice polarimetry (science and operations)?

­ Ice centres (NIC; CIS; DMI, etc.) use radar data operationally in ship routing/logistical & tactical support ­ but image classification performed largely on wide-swath amplitude images with supervised methods.
­ Scientific/climate applications:

· Ocean-atmosphere heat fluxes governed by ice thickesses (<0.5m)
· Ice thickness governs ice dynamics (i.e. knowledge of distribution of leads and ridges)
Question 2: What applications of sea ice polarimetry are viable based on anticipated sampling capabilities associated with polarimetric modes?

­ It is as yet undetermined whether swath-width limitations(i.e. coverage) of future spaceborne polarimetric data will limit their practical value to operational ice services
· Currently Wide-Swath is the standard product, APmode to be evaluated.
· Currently used to viewing single-channel intensity images.
­ It is undetermined whether polarimetry is of any practical benefit (i.e. added value) to existing operational services.
Currently under evaluation by CIS.
­ *We must consider the implications of operational (<3hour) delivery of polarimetric data for established ice services*

Question 3: For classification of sea ice, what accuracies are we hoping to achieve and what ice types are important to classify - are we in a position to make a generic statement about these goals?

­ Operational needs determine that we must:

· discriminate ice/no ice robustly under any wind conditions.
· detect thick ice and robustly discriminate between thick and thin ice
­ Scientifically: polarimetric data are a means to further understand the details of e-m interaction with inhomogeneous ice media
­ Near future spaceborne polarimetric operating modes can likely NOT supplant more standard mode wide swath imaging, due to the time/space coverage limitations, but rather be used for aperiodic tactical/logistical support as/when more precise information is available.

Question 4: What are the primary limitations of the existing work on polarimetric classification of sea ice?

­ Limitations of existing data sets include:

· Season; region; radar configuration specificity.
A more systematic comparison & evaluation of existing approaches is needed An independent reference is needed for quantitative evaluation of classification accuracy.

Question 5: Which polarimetric parameters or frequencies which govern our ability to classify sea ice?

­ No conclusive answer regarding which polarimetric parameters are indispensible, but to first order (including all data analysed to date) quad-pol amplitudes appear to govern overall discriminatory and thus classification ability.

Question 6: In what way do the future choices of satellite frequencies and channels limit our current capability?

No conclusive answer for single frequency polarimetric data until ALOS and RADARSAT-2 fly
­ The jury is still out on whether L- or C-band is better overall. Existing data limitations prevent any judgement,and examples indicate that C- and L-bands are better under certain circumstances.
­ For sea­ice there appear several advantages of multi- frequency data multi-parameter data over single-frequency polarimetry
­ Multi-frequency polarimetry is undoubdedly superior to single frequency polarimetry
­ Perhaps some opportunities exist for time/space collocation of PALSAR and RADARSAT-2 polarimetry over sea ice to demonstrate the benefits of multi-frequency
­ Some merit to the idea of tandem operations of TerraSAR-L and ­X for multi-freq. data combinations for sea-ice applications.

Question 7: What incremental improvement shall we expect wrt Envisat alternating polarisation mode? Where are the primary benefits at C-band?

­ ASAR APmode will undoubtedly make an improvement over single channel data alone
­ Early examples seem to indicate effective discriminatory capability between thick and thin ice. But the specific advantages over single channel SAR are yet to be established.

Question 8: What are the recommended actions for future work?

­ Further studies needed on common datasets which are available to all study participants.
*This currently limits existing intercomparison of platform-specific results
­ Independent classification references must be established to quantitatively evaluate performance of different methodologies
­ Archival material from AIRSAR and EMISAR sea ice data must be safeguarded for preparations for ALOS and RADARSAT-2
­ Rather than necessarily acquire new campaign data, new existing examples should be processed from archived unprocessed airborne polarimetric data
­ Initiate new studies to exploit Earthnet ALOS resource
 

Sea Ice (Parallel Session) Presentations

Radar Polarimetry of Sea Ice
Mr. Mark Drinkwater
 

Unsupervised Wishart Classifications of Sea-Ice using Entropy, Alpha and Anisotropy decompositions
Alex Rodrigues
 

SAR Polarimetry for Sea Ice Monitoring
Wolfgang Dierking
 

Classification strategies for fully polarimetric SAR data of sea ice
Bernd Scheuchl
 

 

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