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

Forest height estimation using DCP mode of compact PolInSAR data

چندين مطالعه انجام شده در دهه اخير نشان داده است كه سامانه­‌هاي تصويربرداري رادار با روزنه مجازي (SAR) در مد Compact پلاريمتري (CP) مي­‌توانند بر معايب سامانه­‌هاي تصويربرداري SAR در مد تمام پلاريمتريك (FP) غلبه كرده و عملكرد قابل ­قبولي را در كاربردهاي مختلف سنجش­ از دور مانند مديريت و پايش منابع مهم طبيعي از جمله جنگل­‌ها ارائه دهند. در اين راستا، فناوري نويني به ­نام تداخل ­سنجي پلاريمتريك SAR (PolInSAR)، به­ عنوان ابزاري توانمند در اين حوزه، بسيار مورد توجه قرار گرفته است. در اين مقاله، عملكرد داده‌­هاي C-PollnSAR) Compact PollnSAR)در مد ارسال و دريافت قطبش دايروي (DCP) جهت برآورد ارتفاع درختان جنگل مورد بحث ­و بررسي قرار گرفته است. براي اين منظور، روش­‌هاي مرسوم جهت بازيابي ارتفاع درختان در مناطق جنگلي، شامل روش تفاضلي مدل رقومي ارتفاعي (DEM)، روش اندازه دامنه كوهرنسي و نيز روش تركيبي (فاز و كوهرنسي)، بر روي اين داده‌­ها پياده‌­سازي شد. به ­منظور ارزيابي عملكرد داده­‌هاي C-PolInSAR، نتايج حاصل از اين داده­‌ها با نتايج به­ دست آمده از داده‌­هاي Full PolInSAR) F-PollanSADR) مقايسه و ارزيابي گرديد. نتايج تجربي به­ دست آمده در اين تحقيق بر دو مجموعه داده شبيه­‌سازي شده از نرم­‌افزار PolSARProSim در باندهاي L و P نشان دادند كه داده­‌هاي C-PolInSAR در مد DCP، عملكرد و نتايج يكساني نسبت به داده­‌هاي F-PolInSAR با در نظر گرفتن HH+VV به ­عنوان قطبش پس‌­پراكنش شده از زمين، در برآورد ارتفاع دارند. به ­ويژه آن­كه، داده‌­هاي C-PolInSAR در مد DCP بهبود 0/78 متري و 0/55 متري را به ­ترتيب در باندهاي L و P نسبت به داده‌هاي F-PolInSAR با انتخاب HH-VV به­ عنوان قطبش زمين، در برآورد ارتفاع درختان حاصل كردند. علاوه ­براين، به ­كارگيري داده‌­هاي C-PolInSAR هنگامي­ كه منابع سامانه­‌هاي تصويربرداري پلاريمتريك محدود هستند، در دسترس نيستند، و نيز در طول موج­‌هاي بلند، كه قطبش ارسالي متأثر از چرخش فارادي است، مي‌­تواند يك راه­كار مؤثر باشد.

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.