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POLInSAR Workshop 2003

Topic : Polarimetry/Interferometry

Chair/s S Cloude, D Corr, H Skriver, J C Souyris

Summary.
There were eight papers presented over two sessions in this topic area. Included were two overview papers, summarising the findings of the two consortia funded under the ESA-ESRIN project “Applications of SAR Polarimetry”. There were two further papers concerned with surface parameter estimation using full polarimetry (surface slope estimation) and polarimetric interferometry (surface roughness and moisture content). Two papers were then presented on the implications of employing full polarimetry and interferometry for space and airborne system design. One paper was concerned with  polarimetric radar classification of a mountainous natural forested area and a comparison with optical classification results. Finally a paper was presented on the use of sub-aperture techniques to study the variation of full polarimetric response across the synthetic aperture.

Conclusions and recommendations.
Arising from these presentations and the round-table discussions, we have identified three important topic areas that we suggest form the basis for future studies in the area of POLSAR and its applications:

Topic 1 : The Importance of Full versus Partial Polarimetry in Radar Remote Sensing

Strong evidence was presented from both consortia that the use of fully polarimetric data provides improved discrimination and classification results for land, sea ice and forestry applications. In addition, fully polarimetric data also provides improved quantitative parameter estimation in surface roughness and moisture studies. The importance of obtaining both polarimetric phase and amplitude information were once again highlighted. This can be expressed as a recognition that there is important information contained along the diagonal of the coherent scattering matrix and not just in one column. Hence a major recommendation of this group is for ESA to continue developments towards fully polarimetric space based sensors.

The question of the best radar centre frequency was also addressed. From the results presented it seemed that L-band provides the best general performance across all terrain types and has less class variability than C-band. There were a few contrary observations demonstrating that for some agricultural applications C-band shows better performance than L-band. With the planned future deployment of a fully polarimetric C-Band sensor (Radarsat 2) already at an advanced stage, it is recommended that ESA set priority in providing a space borne fully polarimetric capability at L-band for land, ice and forestry applications.

Topic 2 : The Implications of Full Polarimetry for Radar System Design

It follows from topic 1 that current partial or ‘light’ polarimetry sensors such as ASAR are not optimally matched to the information content of the polarimetric scattering matrix. Two solutions can be suggested to improve this situation in future sensors. Full quadpol scattering matrix measurements provide one approach and yield the widest range of applications and best accuracy in classification and surface parameter retrieval (and also for calibration issues related to Faraday rotation and system channel cross-talk and amplitude/imbalance correction in the data). However the required increased PRF causes swath coverage limitations, which are critical for certain applications such as sea ice. Hence there is some trade-off between coverage and information extraction in utilisation of this mode. However, a new idea was presented at the workshop, which could provide an alternative strategy for the design of polarimetric sensors. The p/4 mode involves a mixed basis measurement, whereby the polarisation of the transmitter is constant but different to that of the orthogonally polarised receiver. This then maintains lower PRF operation and hence the maximum coverage of the sensor. However, because of the mismatch between transmit and receive basis, there is a mixing of information along the diagonal of the scattering matrix and hence there is improved classification and parameter estimation compared with existing column based switching methods. A detailed analysis of the covariance matrix of this mode was presented and it was shown how extra assumptions must also be used to enable good parameter estimation in this mode. It is not then well suited to point targets but for distributed targets such as surface and volume remote sensing the performance seems good.

It is recommended that further studies be made of this mixed basis mode of operation to further assess the ability to extract surface and volume parameters and to examine more closely the calibration and system cost and design implications.

Topic 3 : Techniques for Classification and Segmentation of POLSAR Images

It was recognised by both consortia that the performance of Maximum Likelihood Wishart based classifiers was sometimes inferior to other methods. This was attributed to the fact that such classifiers employ the full covariance matrix and hence, in addition to information channels, contains some noisy channels, which reduce the reliability compared to some other methods. It was recognised that, while the Wishart approach is statistically optimum in some sense, hybrid methods employing sub-space parameter methods can provide improved performance. Several of these were presented at the workshop, including hierarchical adaptive threshold methods and log-normal intensity channel methods. The last of these showed particularly good results, although only on one data set, and it is recommended that the method be tested on other data sets before deciding on its suitability for recommendation as a good strategy for general classification studies.

The performance of supervised and unsupervised Wishart classifiers will be improved by applying constraints based upon phase characteristics of the covariance data. A validated inter-cluster distance measurement for cluster management is also required.

It was further noted that there now exist a wide range of polarimetric parameters for investigation across the spectrum of applications. To assist users in the selection of parameters for their applications, a set of software tools are being developed. One such tool set was presented by one of the consortia and is to be made available as an output from the workshop. It is recommended that further such tool sets be developed and provided to the user community.

One final topic of interest was the study of statistical fluctuations in the data. There are two main sources of these, speckle and natural class variability. Techniques for handling speckle have been widely considered and there is now a wide choice of mature methods available to the user. However, the studies showed that class variability can sometimes be the dominant source of fluctuations in the data. However this term tends to be frequency dependent with higher variability at C than L bands. This again lends support to the idea of deployment of a future L-band quad-pol sensor.
 

Presentations:

A Review of the Applications of SAR Polarimetry and Polarimetric Interferometry - an ESA-funded study
Dr Douglas Corr

Applications of Synthetic Aperture Radar Polarimetry
Dr Henning Skriver

A Review of Polarization Orientation Estimation from Polarimetric SAR Data
Dr. Jong-Sen Lee

Surface Parameter Estimation Using Interferometric Cohrences between Different Polarisations
Irena Hajnsek

What can be learnt from a half-polarimetric SAR?
Dr Jean-Claude Souyris

Why We Do Need to Place Multi-band Single and Multiple Pass POLinSAR Monitoring Platforms into Space
Professor Wolfgang-Martin Boerner

Polarimetric SAR interferometry classification, resolution estimation and comparison with optical images in Glenaffric radar projrct.
Dr. Parivash Lumsdon

Analysis of SAR Response Anisotropic Behavior Using Sub-Aperture Polarimetric Data
Dr. Laurent Ferro-Famil

 

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