برآورد ارتفاع درختان جنگل با استفاده از مد DCP داده­ های Compact PolInSAR

نوع مقاله: مقاله پژوهشی

نویسندگان

1 دانشجوی دکتری، دانشکده مهندسی نقشه برداری، دانشگاه صنعتی خواجه نصیرالدین طوسی

2 استاد، دانشکده مهندسی نقشه برداری، دانشگاه صنعتی خواجه نصیرالدین طوسی

3 دانشیار، دانشکده مهندسی نقشه برداری، دانشگاه صنعتی خواجه نصیرالدین طوسی

10.22131/sepehr.2019.37503

چکیده

چندین مطالعه انجام شده در دهه اخیر نشان داده است که سامانه­های تصویربرداری رادار با روزنه مجازی (SAR) در مد Compact پلاریمتری (CP) می­توانند بر معایب سامانه­های تصویربرداری SAR در مد تمام پلاریمتریک (FP) غلبه کرده و عملکرد قابل ­قبولی را در کاربردهای مختلف سنجش­ از دور مانند مدیریت و پایش منابع مهم طبیعی از جمله جنگل­ها ارائه دهند. در این راستا، فناوری نوینی به ­نام تداخل ­سنجی پلاریمتریک SAR (PolInSAR)، به­ عنوان ابزاری توانمند در این حوزه، بسیار مورد توجه قرار گرفته است. در این مقاله، عملکرد داده­های C-PollnSAR) Compact PollnSAR)در مد ارسال و دریافت قطبش دایروی (DCP) جهت برآورد ارتفاع درختان جنگل مورد بحث ­و بررسی قرار گرفته است. برای این منظور، روش­های مرسوم جهت بازیابی ارتفاع درختان در مناطق جنگلی، شامل روش تفاضلی مدل رقومی ارتفاعی (DEM)، روش اندازه دامنه کوهرنسی و نیز روش ترکیبی (فاز و کوهرنسی)، بر روی این داده­ها پیاده­سازی شد. به ­منظور ارزیابی عملکرد داده­های C-PolInSAR، نتایج حاصل از این داده­ها با نتایج به­ دست آمده از داده­های Full PolInSAR) F-PollanSADR)
مقایسه و ارزیابی گردید. نتایج تجربی به­ دست آمده در این تحقیق بر دو مجموعه داده شبیه­سازی شده از نرم­افزار PolSARProSim در باندهای L و P نشان دادند که داده­های C-PolInSAR در مد DCP، عملکرد و نتایج یکسانی نسبت به داده­های F-PolInSAR با در نظر گرفتن HH+VV به ­عنوان قطبش پس­پراکنش شده از زمین، در برآورد ارتفاع دارند. به ­ویژه آن­که، داده­های C-PolInSAR در مد DCP بهبود 78/0 متری و 55/0 متری را به ­ترتیب در باندهای L و P نسبت به دادههای F-PolInSAR با انتخاب HH-VV به­ عنوان قطبش زمین، در برآورد ارتفاع درختان حاصل کردند. علاوه ­براین، به ­کارگیری داده­های C-PolInSAR هنگامی­ که منابع سامانه­های تصویربرداری پلاریمتریک محدود هستند، در دسترس نیستند، و نیز در طول موج­های بلند، که قطبش ارسالی متأثر از چرخش فارادی است، می­تواند یک راه­کار مؤثر باشد.

کلیدواژه‌ها


عنوان مقاله [English]

Forest height estimation using DCP mode of compact PolInSAR data

نویسندگان [English]

  • Amir Aghabalaei 1
  • Hamid Ebadi 2
  • Yasser Maghsoudi 3
1 Ph.D. Student, Faculty of geodesy and geomatics engineering, K. N. Toosi University of Technology
2 Professor, Faculty of geodesy and geomatics engineering, K. N. Toosi University of Technology
3 Associate Professor, Faculty of geodesy and geomatics engineering, K. N. Toosi University of Technology
چکیده [English]

Extended Abstract
Introduction
Monitoring and assessment of the biosphere are two essential tasks at any scale. Based on this, forests play an important role in controlling the climate and the global carbon cycle. For this reason, biomass and consequently forest height are known as the key information for forest monitoring. In the recent decade, several studies have shown that the Synthetic Aperture RADAR (SAR) imaging systems in Compact Polarimetry (CP) mode can overcome the disadvantages of Full Polarimetric (FP) SAR imaging systems and provide a good performance in various remote sensing applications such as monitoring and managing the important natural resources like forests. In this regard, a novel technique named Polarimetric Interferometry SAR (PolInSAR) has been further considered as a powerful tool for forest height estimation.
 
Materials & Methods
In this research, the performance of the Compact PolInSAR (C-PolInSAR) data in Dual Circular Polarization (DCP) mode has been investigated in order to retrieve the forest height. For this reason, the common methods which are used for forest height estimation including Digital Elevation Model (DEM) differential method, coherence amplitude inversion, and phase & coherence inversion methods were applied and implemented on these data. In all of the aforementioned methods, LL+RR and LR polarizations were considered as the selected channels for estimating the volumetric and ground coherences, respectively. Then, the estimated coherences were considered as the input parameters for all of the mentioned methods.
 
Results & Discussion
To evaluate the performance and the efficiency of C-PolInSAR data in DCP mode, the results obtained from these data were compared with those obtained from Full PolInSAR (F-PolInSAR) data. The results obtained in this study in two datasets simulated from PolSARProSim software in both L and P bands showed that the C-PolInSAR data in DCP mode yielded a similar result compared to the F-PolInSAR data for forest height estimation (when the HH+VV polarization is adopted as the ground backscattering), because, in this case the LL+RR and the LR polarizations are equal to the HV and the HH+VV polarizations, respectively, particularly, the C-PolInSAR data in DCP mode yielded 0.78 m and 0.55 m improvements for forest height estimation in L and P bands, respectively. In addition, all of the employed methods provided better and closer results compared to the real forest height (i.e. 18 m) in L band compared to P band, because the electromagnetic (EM) waves have a more penetration into the canopy in L band compared to P band. Thus, the attenuation of these waves is low and consequently the height estimation is more accurate. Without considering the used bands, the DEM method provided the lowest precision compared to other methods, because the HV (or LL+RR) phase center can lie anywhere between half the tree height and top of the canopy. The exact location of this phase depends on two vegetation parameters which are the wave mean attenuation and the vertical canopy structure variations. In this case, the trees have very thin canopies, and consequently, the attenuation is small, but the phase center is high due to the structure. In other words, when the canopy extends over the entire forest height, then the phase center can be at half the true height for low density (low attenuation), through to the top of the canopy for dense vegetation (high attenuation). This ambiguity is inherent in single baseline methods, and in order to overcome this, model-based correction methods need to be employed.
It was also observed that the coherence amplitude method is among the weak algorithms due to ignoring the phase and its sensitivity to the attenuation and structural variations but it can be used as a backup solution when other approaches fail. Finally, the phase and the coherence inversion method had better results than two aforementioned methods for the forest height estimation. In this method, selecting the factor ‘’ is very important and it should be selected in a way to be strong towards the attenuation changes. In this study, = 0.4 was adopted to maintain the height error variations.
 
Conclusion
As the final result, the C-PolInSAR data can be an efficient strategy due to its performance, when the full polarimetric imaging systems are either limited or not available. Moreover, utilizing these data in long wavelengths (e.g. P band) is more appropriate due to the effect of the Faraday rotation on the transmitted polarization.

کلیدواژه‌ها [English]

  • Compact PolInSAR
  • Dual circular polarization
  • Forest height estimation
  • Digital Elevation Model (DEM) differential method
  • Coherence amplitude inversion method
  • Phase and coherence inversion method
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