Co-polarization channel imbalance determination by the use of bare soil

This paper describes a novel technique which determines the co-polarization channel imbalance by the use of natural bare soil, instead of a trihedral corner reflector (CR). In polarimetric synthetic aperture radar (PolSAR) remote sensing, the polarimetric calibration (PolCAL) is the key technique in quantitative earth parameter measurement. In general, the current PolCAL process can be separated into two parts. The first part tries to estimate the crosstalk and the cross-polarization (x-pol) channel imbalance components by the reflection symmetry and the reciprocity properties, without a CR. Then, at least one trihedral CR is required to determine the co-polarization (co-pol) channel imbalance; however, it is not always possible to deploy a CR in difficult terrain such as desert. In this paper, we utilize bare soil as a stable reference target, and four common natural constraints of bare soil are evaluated to determine the co-pol channel imbalance, without the use of a CR. It should be mentioned that we do not propose to replace the CR by a natural target, but we utilize the natural target to enhance the PolCAL accuracy when a CR is missing. The four constraints are: (1) the consistency of the polarimetric orientation angle (CPOA) between the PolSAR POA and the digital elevation model (DEM) derived POA; (2) the unitary zero POA (UZPOA) of a flat ground surface; (3) the zero helix (ZHEX) component of the ground surface; and (4) the unitary version of the previous zero helix (UZHEX). In the theoretical part of this paper, we demonstrate that the forth constraint is the most suitable in different scenes. We then propose a multi-scale algorithm to further improve the robustness of the co-pol channel imbalance determination. In the experimental part, we apply our new methods to simulated airborne SAR (AIRSAR) and real uninhabited aerial vehicle SAR (UAVSAR) data. Without the use of any CR, the recovered results show that the estimated amplitude and phase error of the co-pol channel imbalance are less than 0.5 dB and 5°, respectively.     

适用于三维隐式叠前深度偏移中矩阵求逆的混合算法

在深度偏移方法中，把二维隐式方法推广到三维，就会面对一个分块对角矩阵求逆问题. 通常，这种矩阵的求逆将耗费大量计算时间，严重制约了三维隐式方法偏移在实际资料处理中的广泛应用. 在螺旋边界条件下，该矩阵H具有Toeplitz结构的正定厄密矩阵，其快速求逆可由谱法LU分解或直解法快速实现. 本文结合谱法LU分解和直接解法方法的优点，提出了一种混合算法. 文中采用谱分解方法建立起矩阵列元素的谱分解表，并采用直解法的递推公式，可以快速给出矩阵的分解. 通过与谱法分解和直解法在分解精度和分解速度两方面的比较表明，本文方法与谱法相比，在非均匀介质中亥姆霍兹算子矩阵分解时的精度提高10倍；在计算速度方面，混合方法比简化后的直解法快. 因此，该方法的提出，在计算精度许可的条件下，最大限度地减少三维隐式差分偏移中矩阵求逆占用的时间，从而使得该方法能真正用于实际地震资料的处理.     

一种优化的极化SAR图像海面目标检测方法

提出一种优化的极化SAR图像海面目标检测方法,结合改进的极化SAR四分量分解中的螺旋散射分量与Wishart分类器,充分利用极化散射特性、结构特征、统计特性来进行目标的自动检测。同时通过纹理特征相似性克服了Wishart分类器在无目标海域检测时容易将强度值较高的海杂波误认为目标的缺陷。采用美国无人机UAVSAR在Mexico海域和巴拿马Barro Colorado Island海域获取的两组L波段全极化数据进行实验验证。实验结果表明:文中的优化方法能够较准确检测海面目标,很好地降低虚警率;同时解决了Wishart分类器在无目标海域发生错检的问题。     

Gas-phase models for the evolved planetary nebulae NGC 6781, M4-9 and NGC 7293

We have studied the chemistry of the molecular gas in evolved planetary nebulae. Three pseudo-time-dependent gas-phase models have been constructed for dense (10⁴–10⁵ cm⁻³) and cool ( T ∼15 K) clumpy envelopes of the evolved nebulae NGC 6781, M4-9 and NGC 7293. The three nebulae are modelled as carbon-rich stars evolved from the asymptotic giant branch to the late planetary nebula phase. The clumpy neutral envelopes are subjected to ultraviolet radiation from the central star and X-rays that enhance the rate of ionization in the clumps. With the ionization rate enhanced by four orders of magnitude over that of the ISM, we find that resultant abundances of the species HCN, HNC, HC₃N and SiC₂ are in good agreement with observations, while those of CN, HCO⁺, CS and SiO are in rough agreement. The results indicate that molecular species such as CH, CH₂, CH₂⁺ , HCl, OH and H₂O are anticipated to be highly abundant in these objects.     

大比例尺三维地质建模方法研究

当前地下空间大比例尺建模难度较大,少有案例提及。主要难点在于严格按照原始地层开展大比例尺建模,会出现大量的透镜体和螺旋体,给建模工作带来较大难度。6号线地面沉降易发区的试验段研究,共涉及7个站点和6个区间的模型。通过收集整理多方位资料,以横向1:500,纵向1:200比例尺进行大比例建模。选用钻孔537个,剖面299条,格子252个,建模面积为0.6km~2。按照规范对筛选出的钻孔归纳岩性、统一命名,将岩层分为7大类,40层。应用交互拼接建模的方法,将模型分为地质体、地表建筑物两个部分,加入DEM、卫星贴图耦合显示。同时,地层的属性数据也在建模过程中加入到地层属性表中。本次工作解决了透镜体和螺旋体的建模难点,提高了建模精度。