1. INTRODUCTION

The analysis of SAR time series data (or data stacks) comprised of multiple scenes (n > 15) was first introduced through the development of PS-INSAR analysis [1A. Ferretti, C. Prati, and F. Rocca, "Permanent scatterers in SAR interferometry", IEEE Trans. Geosci. Rem. Sens., vol. 39, no. 1, pp. 8-20, 2001.
[http://dx.doi.org/10.1109/36.898661] ]. Single polarization data stacks from the European ERS-1/2 missions were readily available and greatly aided method development. Few second-generation SAR satellites are capable of collecting polarimetric data operationally, so there are not many polarimetric SAR data stacks available to date. As a consequence, the analysis of polarimetric time series data is not wide spread. Cloude et al. investigate time series decomposition analysis for compact polarimetry and employ a mixed space/time average to help reduce speckle and highlight key land-use features [2S.R. Cloude, D.G. Goodenough, H. Chen, D. Leckie, and D. Hill, "Time series decomposition analysis for compact polarimetry", In: Proceedings of PolInSAR, 6^th International Workshop on Science and Applications of SAR Polarimetry and Polarimetric Interferometry: Frascati, Italy: European Space Agency, 2013.]. Conradsen et al. present an improved method to analyze polarimetric time series data for changes instead of pairwise investigation of the data [3K. Conradsen, A.A. Nielsen, and H. Skriver, "Change detection in a time series of polarimetric SAR data", In: Proceedings of PolInSAR, 7^th International Workshop on Science and Applications of SAR Polarimetry and Polarimetric Interferometry, Frascati, Italy: European Space Agency, 2015.]. The concept of point-like coherent scatterers in urban areas is introduced in [4R.Z. Schneider, K.P. Papathanassiou, I. Hajnsek, and A. Moreira, "Polarimetric and interferometric characterization of coherent scatterers in urban areas", IEEE Trans. Geosci. Rem. Sens., vol. 44, no. 4, pp. 971-984, 2006.
[http://dx.doi.org/10.1109/TGRS.2005.860950] ]. The authors investigate airborne L-band high-resolution data for interferometric and polarimetric properties, but only have a short time series available (3 scenes). An approach to utilize polarimetric time series data for land cover monitoring is presented in [5J.W. Cable, J.M. Kovacs, J. Shang, and X. Jiao, "Multi-temporal polarimetric RADARSAT-2 for land cover monitoring in northeastern ontario, Canada", Remote Sens., vol. 6, no. 3, pp. 2372-2392, 2014.
[http://dx.doi.org/10.3390/rs6032372] ]. The authors perform traditional polarimetric analysis methods on a per acquisition basis.

Here, a novel way to evaluate polarimetric persistent point targets in data stacks is presented. The approach is an extension of a traditional method to analyze distributed targets that is used under consideration of a set of restrictions. For improved accuracy, the method is combined with acoherent decomposition applied to select targets only. Tests on two polarimetric data stacks acquired by RADARSAT-2 were performed.

2. BACKGROUND

A number of methods were developed that allow the decomposition of polarimetric data for a more detailed characterization of the scattering.

3. DATA

Two RADARSAT-2 Fine-Quad (FQ) data stacks were available for this study and are described in Table 1. To facilitate time series analysis, all scenes for each of the stacks were co-registered to a common master with sub-pixel accuracy. Only sub-images were investigated for both stacks for easier data handling. Due to the larger stack size, a smaller spatial subset was processed for the Vancouver data set to reduce the size of the data set.

4. ANALYSIS OF POLARIMETRIC DATA STACKS

Four guiding principles in the development of a method for the analysis of point targets in polarimetric data stacks can be defined:

• Coherent target decomposition methods used on polarimetric data stacks need to be used on each level separately.
• Distributed target analysis methods can be applied to data averaged through time thus preserving resolution.
• The temporal component of a data stack adds a level of complexity to the interpretation of the analysis results.
• This work focuses on stable targets only. No attempt is made to interpret changes between acquisitions.

Using these guidelines, three methods to derive information from the polarimetric data stack for the purpose of identifying and analyzing stable polarimetric targets were used.

1. Implementation of the Touzi criterion [8R. Touzi, and F. Charbonneau, "Characterization of target symmetric scattering using polarimetric SARs", IEEE Trans. Geosci. Rem. Sens., vol. 40, no. 11, pp. 2507-2516, 2002.
   [http://dx.doi.org/10.1109/TGRS.2002.805070] ] to identify coherent targets for each layer of the stack separately.
2. Stack Cameron Decomposition. Implementation of the Cameron decomposition and application on each layer of the data stack separately resulting in a stack of Cameron classes. (SSCM was implemented in a similar fashion, but given the similarity of the methods this avenue was not further investigated.)
3. Calculate the time average coherency matrix T3_average through the stack for each pixel (x,y). Based on T3_average, calculate H_stack, A_stack, and α_stack for the time-averaged data. These parameters retain the resolution of the original data. Because the focus here is on targets with one dominant scattering mechanism, A_stack for these points is based on small values and likely strongly affected by noise. A_stack is therefore not used for result interpretation.

The implementation described above leads to three different results that require careful interpretation. Interferometric analysis of data stacks for the evaluation of target displacement requires a careful selection of target candidates that meet certain criteria [16A. Hooper, H. Zebker, P. Segall, and B. Kampes, "A new method for measuring deformation on volcanoes and other natural terrains using InSAR persistent scatterers", Geophys. Res. Lett., vol. 31, p. L23611, 2014.]. The same concept is true when looking at polarimetric analysis of a data stack for the purpose of target identification, though a different set of criteria needs to be met. Here, we define Polarimetric Persistence, which essentially requires a level of consistency of analysis results through the data stack:

5. RESULTS AND DISCUSSION

Fig. (2) shows the time averaged span image, H_stack, A_stack, and α_stack for a subset of the FQ Montreal data set. The span image is presented for reference only, to allow easier interpretation of the other parameters. The scene shows generally high Entropy. This is also evident in the color density scatter plot of the H_stack,α_stack-space (Fig. (3), top image). Similar to spatially averaging Entropy, H_stack represents a mix of scattering mechanisms in the resolution element; however, here the mixing occurs on two levels: 1) spatially, within the resolution element, and 2) temporally, through changes between the acquisitions of the data stack. This second dimension of change renders high entropy pixels ambiguous and unsuitable for a detailed polarimetric analysis through the data stack, unless additional information is available. Therefore, we focus here on low entropy portions of the scene. Of particular interest are areas with H_stack<0.5. Lower H_stack means that the pixel can be interpreted with higher confidence.

It should be noted that a stable target can result in high H_stack. Such a target will have multiple scattering mechanisms within the resolution cell equally contributing to the final signal. The proposed threshold will therefore not catch all stable targets, but pure ones.

Fig. (3) (lower 6 plots) shows a comparison of the stack Cameron class results and the H_stack, α_stack-space (a confidence threshold of 70% was chosen for this case, i.e. the same Cameron result was obtained in more than 70% of layers). Each stack Cameron class is presented in a separate plot. The plots clearly show the relation between each stack Cameron class and the α_stack-angle. Both decompositions describe the scattering type of the resolution element and, more importantly, both provide the same information. Dihedral and Trihedral scattering are well separated (α_stack~90° and α_stack~0° respectively, with about 10° range) and consistent with the definition of the α-angle for a single scene. Low α_stack is equated with rough surface scattering (single bounce). A Trihedral (triple bounce) shows the same polarimetric behavior as a single bounce return, except for the backscatter strength. Narrow Diplane (α_stack > 57.5°) scattering is more closely related to dihedral scattering than it is to trihedral scattering. Dipole scattering shows α-angles between 35° and 57.5°. Quarter wave scattering also roughly lies in this center band, although higher H_stack values can be observed for the latter case. Cylinder scattering is best described with α_stack-angles smaller than 35°. While there are exceptions, particularly noted are the quarter wave results, the majority of the stack Cameron results show a low H_stack. The vast majority of stable targets identified using the Touzi criterion also show H_stack<0.5. Higher confidence thresholds for stack Cameron and the Touzi criterion lead to fewer points identified in both cases that also show a lower average H_stack. The plots for the Vancouver data set (see Fig. 4) are consistent with the Montreal results. These results are in agreement with the hypothesis that time averaged entropy for a pixel can be interpreted as an indication of a stable target for the data stack.

Fig. (2) Montreal data set stack averaged span, Hstack and Astack (black=0, white=1), as well as αstack (black=0 degrees, white=90 degrees). The scene encompasses downtown Montreal (on the top left of the scene) with surroundings, the St. Lawrence River, and several bridges.

Fig. (3) Hstack/αstack density scatter plot of the Montreal data stack (black=low density to yellow=high densitly; top image). Cameron stack results with a 70% confidence threshold are plotted in the Hstack/αstack space (rest).

Fig. (5) shows the classification result of the combined approach for the Montreal scene. Only polarimetric persistent points are classified into elemental scatterers. A more detailed view is shown in Fig. (6). Classified targets are predominantly located in the downtown core and in other built up areas. This is to be expected as these areas have a high likelihood for point targets.

One potential application of the method is the classification of point targets identified using other means. A method to identify persistent targets in the InSAR sense (PTIn) is to select them using the phase stability method [16A. Hooper, H. Zebker, P. Segall, and B. Kampes, "A new method for measuring deformation on volcanoes and other natural terrains using InSAR persistent scatterers", Geophys. Res. Lett., vol. 31, p. L23611, 2014.]. The approach seeks to identify targets with a stable interferometric phase in a single-polarization data stack for PS-INSAR analysis. Target characterization is not provided. If a polarimetric data stack is available, the method proposed here can be used for providing information about the type of pure elemental scatterers with high confidence. H_stack, α_stack decomposition and stack based Cameron decomposition can also be used to provide polarimetric information for all PTIn. In this case the detection is provided through the phase stability method only. H_stack and/or the number of scenes with consistent Cameron class provide a measure for the classification confidence, instead of being used for detection.

Fig. (4) Hstack/αstack scatter plot of the Vancouver data stack. Cameron stack results with a 70% confidence threshold are plotted in the Hstack/αstack space.

For the Montreal data set, about 20% of PTIn in the scene are directly classified with the proposed method. The difference is likely because PTIn are selected based on phase stability only, a criterion which can be satisfied for pure targets as well as for targets with a mix of scatterers. Polarimetric persistent targets are pure targets by definition, which requires a single dominant scattering mechanism.

In comparison, for the Vancouver data set, only 6.2% of PTIn are directly classified using the proposed method (see Fig. 7). This lower number is likely due to the steeper incidence angle for the acquisitions. The resulting lower ground range resolution leads to a larger area covered by a pixel. This leads to a higher potential for more scattering mix in a single resolution cell.

For both data sets, all PTIn can be interpreted as discussed above. The confidence in the classification result will be lower, as these points likely experience a greater degree of scattering mix, which means targets are no longer pure. The resulting stack entropy serves as an indicator of the classification confidence. The stable phase of the PTIn indicates little change over time and the time average coherency matrix can in fact be used to interpret the target. The interpretation of α_stack is similar to the original interpretation suggested in [12S.R. Cloude, and E. Pottier, "An entropy based classification scheme for land applications of polarimetric SAR", IEEE Trans. Geosci. Rem. Sens., vol. 35, no. 1, pp. 68-78, 1997.
[http://dx.doi.org/10.1109/36.551935] ], indicating an average dominant scattering mechanism, whereas H_stack describes the scattering mix.

Fig. (5) Combined Hstack/αstack - stack Cameron classification of the Montreal data stack overlaid on the time averaged span image. The inset shows the detail presented in Fig. (6).

Fig. (6) Detail of the combined Hstack/αstack - stack Cameron classification presented in Fig. (5). Orange points indicate unclassified PTIn.

SUMMARY AND CONCLUSION

A novel way to find and evaluate stable targets in polarimetric SAR data stacks is introduced. The approach is an extension of well-established polarimetric analysis methods for single scenes. It retains the full resolution of the data set, though temporal changes between acquisitions add additional complexity. Result interpretation is therefore performed under consideration of a set of boundary conditions.

Traditional point target analysis methods like the Cameron decomposition allow a rigorous evaluation of polarimetric point targets; however, they need to be applied on every layer of a polarimetric data stack separately. Proof is provided that distributed target decomposition methods can be applied on a data stack while preserving resolution. The proposed H_stack, α_stack analysis allows a fast and reliable identification and evaluation of stable targets in polarimetric data stacks. H_stack can be used to identify polarimetric persistent scatterers. It can also be used as a degree of confidence in the subsequent analysis. The α_stack–angle describes the type of scatterer; its output for polarimetric persistent targets is consistent with stack Cameron results. The recommended thresholds for the α_stack-angle are driven by a comparison with the stack Cameron results.

• α_stack < 35°: Trihedral scattering (α~0°, with a 10° band) or cylinder scattering (rest). A clear separation of these two scattering types solely based on α_stack is not possible with a simple threshold, though trends can be observed.
• 35° < α_stack < 57.5°: Dipole or quarter wave scattering (further separation not possible solely in the H_stack/α_stack-space)
• α_stack > 57.5°: double bounce scattering (α_stack~90°, with a 10 ° band), or narrow diplane scattering (rest)

The proposed approach uses the H_stack, α_stack method in combination with a stack Cameron decomposition (the latter can be used more efficiently on select points only). The result is a classification of polarimetric stable points in a scene to pure canonical targets (i.e. trihedral, cylinder, dipole, narrow diplane, dihedral, quarter wave, and helix) with a high degree of confidence.

Fig. (7) Vancouver data set stack averaged span, Hstack (black=0, white=1), as well as αstack (black=0 degrees, white=90 degrees). The scene shows a portion of downtown Vancouver and the False Creek region. The Hstack/αstack - stack Cameron classification is shown overlaid on the time averaged span image. Unclassified PTIn are shown in orange.

The polarimetric persistence criteria lead to the identification of fewer points compared to persistent interferometric points. The percentage of persistent interferometric scatterers that also meet the proposed polarimetric persistence criteria appears to be dependent on the incidence angle, with more overlap for shallower incidence angles (with corresponding higher ground range resolution).

The proposed method can also be used to interpret targets identified through other means (like interferometric persistence) and provide a measure of confidence for the analysis results; however, interpretation uncertainties will increase in this case. For targets with high stack Entropy, it is better to interpret α_stack as is recommended for distributed targets.

Polarimetric data stacks will likely remain rare given that SAR satellites like RADARSAT-2 have many possible acquisition modes and users may often prefer higher resolution or larger area coverage at single or dual polarization over polarimetric data. This work shows the potential of polarimetric data stacks for the identification and interpretation of targets that meet polarimetric persistence criteria.

CONFLICT OF INTEREST

The author confirms that this article content has no conflict of interest.