Magnetic Field Design of the Dipole for Super-FRS at FAIR

The Super-FRS (Super FRagment Separator) is a part of FAIR (Facility for Antiproton and Ion Research), which will be constructed at GSI, Germany by 17 countries. The Super-FRS comprises 24 superferric dipole magnets. The 2D and 3D magnetic field simulations of the prototype magnet are described in this paper. A passive trim slot and four chamfered removable poles are used to satisfy the required field homogeneity which is better than +/-3 x 10(-4) at 1.6 T, 0.8 T and 0.16 T in a wide elliptical useable aperture of 380 mm x 140 mm. Measurement results at various field levels are shown in this paper as well. It can be seen from the comparison of calculation and measurement results that the magnetic designs of the magnet fulfils the requirements.

Morphologies of metallocene-catalyzed short chain branched polyethylene single crystals formed from the melt at higher temperature

Metallocene-catalyzed short chain branched polyethylene single crystals, formed from the melt at a higher crystallization temperature of 114 degreesC, were obtained. Highly elongated lamellae were formed, which are different from truncated lozenge or lenticular shaped single crystals formed at a lower crystallization temperature. It was found that there existed a definite line in the lamellae along the longitudinal growth direction and two regions were separated by the definite line. The lateral habits of both the regions were asymmetrical about the b-axis due to the chain tilting, which was the same as that at a lower crystallization temperature. Generally, the highly elongated lamellae were not straight, but curved towards the opposite direction with chain tilting direction due to a series of edge dislocation within a lamella. The inner side of a lamella was serrated and the outer side was smooth due to the lamellar curvature. The thickness of both regions of a lamella was different, the broader region was thicker than the narrower region, which was different from the uniform thickness of the lamellae formed at a lower crystallization temperature. The different thicknesses within a lamella were considered as the result of the initial thickness difference and the impact of isothermal thickening. (C) 2001 Elsevier Science Ltd. All rights reserved.

Axial growth of hexactinellid spicules: Formation of cone-like structural units in the giant basal spicules of the hexactinellid Monorhaphis

The glass sponge Monorhaphis chuni (Porifera: Hexactinellida) forms the largest bio-silica structures on Earth; their giant basal spicules reach sizes of up to 3 m and diameters of 8.5 mm. Previously, it had been shown that the thickness growth proceeds by appositional layering of individual lamellae; however, the mechanism for the longitudinal growth remained unstudied. Now we show, that the surface of the spicules have towards the tip serrated relief structures that are consistent in size and form with the protrusions on the surface of the spicules. These protrusions fit into the collagen net that surrounds the spicules. The widths of the individual lamellae do not show a pronounced size tendency. The apical elongation of the spicule proceeds by piling up cone-like structural units formed from silica. As a support of the assumption that in the extracellular space silicatein(-like) molecules exist that associate with the external surface of the respective spicule immunogold electron microscopic analyses were performed. With the primmorph system from Suberites domuncula we show that silicatein(-like) molecules assemble as string- and net-like arrangements around the spicules. At their tips the silicatein(-like) molecules are initially stacked and at a later stay also organized into net-like structures. Silicatein(-like) molecules have been extracted from the giant basal spicule of Monorhaphis. Applying the SDS-PAGE technique it could be shown that silicatein molecules associate to dimers and trimers. Higher complexes (filaments) are formed from silicatein(-like) molecules, as can be visualized by electron microscopy (SEM). In the presence of ortho-silicate these filaments become covered with 30-60 nm long small rod-like/cuboid particles of silica. From these data we conclude that the apical elongation of the spicules of Monorhaphis proceeds by piling up cone-like silica structural units, whose synthesis is mediated by silicatein(-like) molecules. (C) 2008 Elsevier Inc. All rights reserved.

Validation of RADARSAT-2 fully polarimetric SAR measurements of ocean surface waves

C band RADARSAT-2 fully polarimetric (fine quad-polarization mode, HH+VV+HV+VH) synthetic aperture radar (SAR) images are used to validate ocean surface waves measurements using the polarimetric SAR wave retrieval algorithm, without estimating the complex hydrodynamic modulation transfer function, even under large radar incidence angles. The linearly polarized radar backscatter cross sections (RBCS) are first calculated with the copolarization (HH, VV) and cross-polarization (HV, VH) RBCS and the polarization orientation angle. Subsequently, in the azimuth direction, the vertically and linearly polarized RBCS are used to measure the wave slopes. In the range direction, we combine horizontally and vertically polarized RBCS to estimate wave slopes. Taken together, wave slope spectra can be derived using estimated wave slopes in azimuth and range directions. Wave parameters extracted from the resultant wave slope spectra are validated with colocated National Data Buoy Center (NDBC) buoy measurements (wave periods, wavelengths, wave directions, and significant wave heights) and are shown to be in good agreement.

On SAR wind speed ambiguities and related geophysical model functions

Recent investigations show that normalized radar cross sections for C-band microwave sensors decrease under high wind conditions with certain incident angles instead of increase, as is the case for low to moderate wind speeds. This creates the problem of ambiguities in high wind speed retrievals from synthetic aperture radar (SAR). In the present work, four geophysical model functions (GMFs) are studied, namely the high wind C-band model 4 (CMOD4HW), C-band model 5 (CMOD5), the high wind vertical polarized GMF (HWGMF_VV), and the high wind horizontal polarized GMF (HWGMF_HH). Our focus is on model behaviours relative to wind speed ambiguities. We show that, except for CMOD4HW, the other GMFs exhibit the wind speed ambiguity problem. To consider this problem in high wind speed retrievals from SAR, we focus on hurricanes and propose a method to remove the speed ambiguity using the dominant hurricane wind structure.

Speed ambiguity in hurricane wind retrieval from SAR imagery

Under strong ocean surface wind conditions, the normalized radar cross section of synthetic aperture radar (SAR) is dampened at certain incident angles, compared with the signals under moderate winds. This causes a wind speed ambiguity problem in wind speed retrievals from SAR, because two solutions may exist for each backscattered signal. This study shows that the problem is ubiquitous in the images acquired by operational space-borne SAR sensors. Moreover, the problem is more severe for the near range and range travelling winds. To remove this ambiguity, a method was developed based on characteristics of the hurricane wind structure. A SAR image of Hurricane Rita (2005) was analysed to demonstrate the wind speed ambiguity problem and the method to improve the wind speed retrievals. Our conclusions suggest that a speed ambiguity removal algorithm must be used for wind retrievals from SAR in intense storms and hurricanes.

Near-Space Wide-Swath Radar Imaging With Multiaperture Antenna

Near-space, defined as the altitude region between 20 and 100 km, offers many capabilities that are not accessible for low Earth-orbit (LEO) satellites or airplanes because it is above storm and not constrained by orbital mechanics and high fuel consumption. Hence, a high flying speed can be obtained for the maneuvering vehicles operating in near-space. This offers a promising solution to simultaneous high-resolution and wide-swath synthetic aperture radar (SAR) imaging. As such, one near-space wide-swath SAR imaging technique is presented in this letter. The system configuration, signal model, and imaging scheme are described. An example near-space SAR system is designed, and its imaging performance is analyzed. Simulation results show that near-space maneuvering vehicle SAR indeed seems to be a promising solution to wide-swath SAR imaging.

A new wind vector algorithm for C-band SAR

Ocean wind speed and wind direction are estimated simultaneously using the normalized radar cross sections or' corresponding to two neighboring (25-km) blocks, within a given synthetic aperture radar (SAR) image, having slightly different incidence angles. This method is motivated by the methodology used for scatterometer data. The wind direction ambiguity is removed by using the direction closest to that given by a buoy or some other source of information. We demonstrate this method with 11 EN-VISAT Advanced SAR sensor images of the Gulf of Mexico and coastal waters of the North Atlantic. Estimated wind vectors are compared with wind measurements from buoys and scatterometer data. We show that this method can surpass other methods in some cases, even those with insufficient visible wind-induced streaks in the SAR images, to extract wind vectors.

Ocean wave imaging mechanism by imaging radar

Analytical representations of the high frequency spectra of ocean wave and its variation due to the variation of ocean surface current are derived from the wave-number spectrum balance equation. The ocean surface imaging formulation of real aperture radar (RAR) is given using electromagnetic wave backscattering theory of ocean surface and the modulations of ocean surface winds, currents and their variations to RAR are described. A general representation of the phase modulation induced by the ocean surface motion is derived according to standard synthetic aperture radar (SAR) imaging theory. The detectability of ocean current and sea bottom topography by imaging radar is discussed. The results constitute the theoretical basis for detecting ocean wave fields, ocean surface winds, ocean surface current fields, sea bottom topography, internal wave and so on.

Depth inversion in coastal water based on SAR image of waves

Wave-number spectrum technique is proposed to retrieve coastal water depths by means of Synthetic Aperture Radar (SAR) image of waves. Based on the general dispersion relation of ocean waves, the wavelength changes of a surface wave over varying water depths can be derived from SAR. Approaching the analysis of SAR images of waves and using the general dispersion relation of ocean waves, this indirect technique of remote sensing bathymetry has been applied to a coastal region of Xiapu in Fujian Province, China. Results show that this technique is suitable for the coastal waters especially for the near-shore regions with variable water depths.

Remote sensing of ocean waves by polarimetric SAR

A new method to measure ocean wave slope spectra using fully polarimetric synthetic aperture radar (POLSAR) data was developed without the need for a complex hydrodynamic modulation transform function. There is no explicit use of a hydrodynamic modulation transfer function. This function is not clearly known and is based on hydrodynamic assumptions. The method is different from those developed by Schuler and colleagues or Pottier but complements their methods. The results estimated from NASA Jet Propulsion Laboratory (JPL) Airborne Synthetic Aperture Radar (AIRSAR) C-band polarimetric SAR data show that the ocean wavelength, wave direction, and significant wave height are in agreement with buoy measurements. The proposed method can be employed by future satellite missions such as RADARSAT-2.

Wind-vector estimation for RADARSAT-1 SAR images: Validation of wind-direction estimates based upon geometry diversity

In this letter, a new wind-vector algorithm is presented that uses radar backscatter sigma(0) measurements at two adjacent subscenes of RADARSAT-1 synthetic aperture radar (SAR) images, with each subscene having slightly different geometry. Resultant wind vectors are validated using in situ buoy measurements and compared with wind vectors determined from a hybrid wind-retrieval model using wind directions determined by spectral analysis of wind-induced image streaks and observed by colocated QuikSCAT measurements. The hybrid wind-retrieval model consists of CMOD-IFR2 [applicable to C-band vertical-vertical (W) polarization] and a C-band copolarization ratio according to Kirchhoff scattering. The new algorithm displays improved skill in wind-vector estimation for RADARSAT-1 SAR data when compared to conventional wind-retrieval methodology. In addition, unlike conventional methods, the present method is applicable to RADARSAT-1 images both with and without visible streaks. However, this method requires ancillary data such as buoy measurements to resolve the ambiguity in retrieved wind direction.

合成孔径雷达提取海面风、浪信息研究

传统海洋测量方法例如岸基观测站、船只和浮标等方式只能对海面进行单点观测，存在很多不足之处：观测点少，只能获得有限的点的资料，无法得到大范围的海面信息；费用较高，无法大范围的密集观测；传统方法受天气影响较大，无法长时间的连续观测；星载合成孔径雷达能全天候、全天时、高分辨率对海面成像，能实现多波段、多极化、多视角得观测海面，提供大范围、高精度的实时动态海面信息。同时，这种全天候、全天时和高分辨率观测海洋的优势是可见光和红外传感器所没有的。总之研究如何从SAR影像中有效地获取海面信息具有重要的科学和实用意义。 自从1978年Seasat卫星发射以来SAR图像就广泛的应用于海洋要素的反演如海面风场、波高、平均周期。其中对海面风场的反演研究的最多，一般的方法是首先对SAR图像进行快速傅立叶变换得到SAR图像谱，通过图像谱的峰值信息能够得到具有180°模糊的风向，利用SAR图像的条纹或者浮标等外部信息来消除180°模糊确定风向，其次把风向和图像的正交后向散射截面数值(NRCS)带入经验函数CMOD4通过迭代计算得到风速。这种方法得到的海面风场需要外部信息的辅助，不利于大范围的业务化的反演海面风场。因此本文试图直接通过SAR图像来反演海面风场而不利用其他外部信息的帮助。2002年3月1日发射的ENVISAT卫星所获得的ASAR图像具有不同于ERS系列的SAR图像的新特点：不同入射角的情况下获得ASAR图片、双极化数据以及可以获得更宽的带幅的图像。双极化数据可以同时地提供同极化和交错极化的影像，两种极化的数据能够增加分辨目标的能力和提供目标更多的信息，这方面的优势也使得利用双极化的数据来消除SAR图像反演风场中的180º模糊问题提供了可能。本文推导了利用双极化的ASAR图像反演海面风场的新函数，该函数是在同极化和交错极化函数的基础上推导出来的，有效的消除了只用一副ASAR图片反演风场所固有的180º模糊现象。风速和风向的反演结果与Quikscat数据之间的均方根误差分别为0.53 m/s和2.21º。该方法与传统方法的比较可以看出新方法与浮标数据以及Quikscat数据符合的更好。 利用SAR图像来反演海浪的波高也是一个研究的比较多的领域。比较传统的方法是由SAR图像得到的海浪谱计算出波高，目前国际上比较流行的SAR图像反演海浪谱的模式有两种：一种是Hasselmann（1991）提出的在Max－Planck Institute（MPI）发展起来的方法，Hasselmann（1996）进行了改进和完善。另一种是Mastenbroek和de Valk（2000）提出的半参数化反演方法。这两种方法的最主要的缺点就是需要引进外部信息(WAM模式结果或者散射计的风的信息)来消除图像所固有的180°方向模糊问题。而本文应用的经验函数方法(CWAVE)不需要引进任何外部信息而直接得到海浪的重要参数－有效波高(Hs)，输入该经验函数的参数主要有：波模式图像的雷达截面、图像方差以及由SAR谱得到的20个参数。这些参数的选择是通过逐步回归方法进行筛选的。CWAVE经验函数的系数是通过6000幅全球分布的ERS-2波模式的图像谱拟合同时同地点的WAM模式结果得到的。利用CWAVE经验函数反演了1998年9月到2000年11月两年多的全球接近一百万的ERS-2 SAR图像的有效波高，利用NOAA浮标数据对反演结果进行了验证，他们之间的相关系数为0.83, 均方根误差为0.61m，偏差为0.02 m；反演结果也与欧洲中长期预报中心(ECMWF)的ERA-40有效波高资料和高度计资料进行了比较，结果表明该方法是通过SAR图像反演Hs的一种有效方法。 SAR的反演结果也应用于有效波高的非线性统计分布的研究。结合动力学和随机统计学原理推导了海面高度、波高和有效波高的非线性统计分布，同时利用了NOAA浮标数据、华师大的波高数据以及SAR的有效波高数据分别对推导出的非线性统计分布函数进行了检验。 SAR的结果也用来反演新的风浪成长关系： ，与已有的风浪成长关系的比较表明该成长关系与已有的结果比较一致。

干涉合成孔径雷达海浪遥感理论与应用研究

干涉合成孔径雷达 (Interferometric Synthetic Aperture Radar，InSAR) 是以合成孔径雷达复数据提取的相位为信息源获取地表三维信息和海表散射体运动信息的新型微波成像雷达。 InSAR通过两幅天线同时观测 (单轨模式)，或两次近平行的观测 (重复轨道模式)，获取地面或海面同一景观的复图像对。20世纪90年代以来，InSAR陆地和海洋研究成为微波遥感的热点，广泛应用于地表变形监测、南极冰流测量、地面或海面慢速运动目标检测等领域。 近年来，国际上逐渐应用机载顺轨或交轨干涉合成孔径雷达进行海表面流速测量以及海面波成像机制研究。相对于传统单天线合成孔径雷达 (Synthetic Aperture Radar, SAR)，双天线干涉合成孔径雷达 (InSAR) 测量海表面波有着独特的优势： (1) InSAR复图像的相位近似正比于海面散射体的径向速度，这种内在的成像机制提供了直接测量海表面动态运动的机会。 (2) 真实孔径雷达调制传递函数几乎对InSAR相位图像没有影响，而对传统SAR图像影响较大。 基于干涉合成孔径雷达测量海浪的优势，本文做了一些干涉合成孔径雷达海浪遥感理论与应用研究工作，主要内容大致可归纳如下： (1)基于新建立的顺轨干涉合成孔径雷达 (Along-Track Interferometric Synthetic Aperture Radar，ATI-SAR) 相位谱与海浪谱之间的非线性映射关系，通过数值模拟研究了不同雷达参数和海况参数对应的ATI-SAR相位谱。数值模拟结果表明：距离速度比率、雷达入射角、天线间距和有效波高和波长比率是影响ATI-SAR海浪成像的重要因素。进一步，利用机载X波段水平极化相位图像和机载C波段水平极化相位图像谱结合方向波骑士浮标测量的海浪方向谱验证了ATI-SAR相位谱与海浪谱之间的非线性映射关系。结果显示用前向映射关系计算的相位谱与实际观测的相位谱较为一致，二者相关系数总体大于0.6，而且对成像非线性不敏感. (2)建立了包含海表面高度和速度聚束的交轨干涉合成孔径雷达 (Across-Track Interferometric Synthetic Aperture Radar，XTI-SAR) 涌浪干涉相位模型，得到了涌浪成像的解析表达式，进一步研究了XTI-SAR沿方位向传播的涌浪成像机制。定义二次谐波振幅与基波振幅比率来表征成像非线性，通过比较XTI-SAR和ATI-SAR相位的二阶调和分量，分析不同海况和干涉SAR参数情况下的数值模拟，结果表明：当速度聚束弱时，XTI-SAR相位比ATI-SAR相位具有较强的非线性，ATI-SAR比XTI-SAR更适合测量海浪。当速度聚束强时，XTI-SAR相位比ATI-SAR相位具有较弱的非线性，XTI-SAR比ATI-SAR更适合测量海浪。 (3)基于包含海表面高度和速度聚束的交轨干涉合成孔径雷达 (XTI-SAR) 涌浪干涉相位模型，结合多维高斯变量的特征函数方法建立了新的XTI-SAR相位谱与海浪谱非线性积分变换。新积分变换不同于Bao (1999) 建立的积分变换，两者形式上区别在于新积分变换中包含了长波径向轨道速度一阶倒数项。数值模拟显示：通常情况下，长波径向轨道速度一阶倒数项不能忽略。进一步，我们针对不同雷达参数和海况结合新非线积分变换对XTI-SAR海浪成像进行了数值模拟，结果表明：同顺轨干涉合成孔径雷达 (ATI-SAR) 海浪成像一致，距离速度比率 和有效波高与波长比率 是影响XTI-SAR海浪成像的重要因子。 (4)基于新的ATI-SAR相位谱与海浪谱之间的非线性映射关系，发展了利用ATI-SAR相位图像反演海浪方向谱的参数化反演模式，并由此得到海浪波长、波向和有效波高。反演结果与现场浮标观测结果比较一致。相对于其它反演模式，参数化反演模式的优点在于：(1) 不需要任何附加信息如初猜海浪谱、散射计提供的风速风向等信息。 (2) 不需要对相位图像进行辐射定标，可以由反演的海浪谱直接计算有效波高。(3) 反演结束后还可以得到成像区域的局地风速信息。因此，参数化反演模式可以实现风、浪信息的联合反演。