On the mechanism for imaging ocean waves by synthetic aperture radar

Based on the analogies between spectral analysis of synthetic aperture radar (SAR) imagery, the two-frequency technique, and the technique of self-mixing of Doppler microwave backscatter for surfaces with periodically varying reflectivity, the SAR imaging of ocean waves is considered as a process of measuring correlation between the values of backscattered SAR radiation in different azimuthal positions of the platform. In this way a relatively simple explanation is given for the focusing effect and for the wave image contrast dependence on the integration time. Results of this approach are in agreement with those obtained in previous works by another (much more complex) manner: the wave image focus shift is proportional to half of the wave phase speed (the wave traveling along the line of flight), and the image contrast does not decrease with an increase the integration time.     

Polarimetric radar remote sensing of ocean surface wind

Experimental data are presented to support the development of a new concept for ocean wind velocity measurement (speed and direction) with the polarimetric microwave radar technology. This new concept has strong potential for improving the wind direction accuracy and extending the useful swath width by up to 30% for follow-on NASA spaceborne scatterometer mission to SeaWinds series. The key issue is whether there is a relationship between the polarization state of ocean backscatter and surface wind velocity at NASA scatterometer frequencies (13 GHz). An airborne Ku-band polarimetric scatterometer (POLSCAT) was developed for proof-of-concept measurements. A set of aircraft flights indicated repeatable wind direction signals in the POLSCAT observations of sea surfaces at 9-11 m/s wind speed. The correlation coefficients between co- and cross-polarized radar response of ocean surfaces have a peak-to-peak amplitude of about 0.4 and are shown to have an odd-symmetry with respect to the wind direction, unlike the normalized radar cross sections     

光与雷达遥感协作及其农业应用

用遥感影像提取农作物信息是精细农业的基础.传统的光学遥感局限于大气无云的优质影像,而微波遥感在提取农作物信息方面尚未得到充分的利用.寻找一种能够克服这些局限的农作物遥感信息提取的方法是有意义的.探讨了一种光和雷达遥感协作的方法,并应用于土壤和植被信息的提取,验证其应用潜力.用Karam等(1992)的微波散射模型,并作了相应修改:1)考虑粗糙地表面后向散射;2)光学遥感作为模型变量直接输入.将这种协作方法应用到两个数据集的处理,一个来自亚利桑那Maricopa农业中心,一个来自日本国家农业-环境科学协会的Tsukuba试验基地.这些数据包括一些实测的土壤和植被的数据以及陆地卫星Landsat和欧洲遥感卫星ERS遥感影像.结果初步表明雷达影像和光学影像在提取土壤和植被信息时可以有效地结合起来,在特定地方的农业管理中有着较好的应用前景.     

FOPAIR: a focused array imaging radar for ocean remote sensing

FOPAIR, a FOcused Phased Array Imaging Radar, provides high-resolution X-band images of the ocean surface. The system is designed to provide high-speed imagery (up to 180 frames/s) for short range applications (50-400 m) from a fixed platform such as a pier or tower. FOPAIR employs a fast, sequentially sampled antenna array and uses a software-based beamforming technique to generate high resolution imagery without the need for multiple radar receivers or beamforming hardware typical of active phased arrays. A summary of the principles of operation and the design of the instrument is given, followed by examples of FOPAIR's imaging capability. To the authors' knowledge, these examples include the highest-resolution, highest-speed microwave images of the ocean surface produced to date. A brief comparison between FOPAIR and synthetic aperture radar techniques is also included     

The effect of orbital motions on synthetic aperture radar imagery of ocean waves

The formation of wave-like patterns in synthetic aperture radar (SAR) images of the ocean surface caused by orbital motions is investigated. Furthermore, the degradation in azimuthal resolution due to these motions is calculated by applying a least square fit to the phase history. Formulas are given which describe the variation of intensity in azimuthal direction in the image plane as well as the degradation in azimuthal resolution as a function of ocean wave amplitude, wave frequency, direction of wave propagation, and radar wavelength, incidence angle, and integration time.     

Space-time processing for multichannel synthetic aperture radar

Synthetic aperture radar (SAR) provides high-resolution images of a non-moving ground scene, but fails to indicate the presence and position of moving objects. As in airborne MTI (moving-target indication) systems the solution to this problem is to use an array of antennas or subapertures and several receiving channels (`MSAR', or multichannel SAR), and to apply multichannel clutter suppression. One of the most efficient methods is adaptive space-time processing (STAP), which can be simplified to frequency-dependent spatial processing in the Doppler domain. Some of these techniques applied to SAR are reviewed and illustrated with data gathered by the German experimental multichannel SAR system `AER-II'     

Electronic processing for synthetic aperture array radar

A simple method of electronically processing synthetic aperture arrays is presented. The processing technique is based on the fact that synthetic aperture recorded signals are one-dimensional Fresnel zone-plate lenses. By essentially employing unfocussed processing methods on individual Fresnel zone-plates and then appropriately summing the results, a narrow beam "semifocussed" aperture may be formed, using only binary electronics.     

Multibeam synthetic aperture radar for global oceanography

A single-frequency multibeam synthetic aperture radar concept for large swath imaging desired for global oceanography is evaluated. Each beam illuminates a separate range and azimuth interval, and images for different beams may be separated on the basis of the Doppler spectrum of the beams or their spatial azimuth separation in the image plane of the radar processor. The azimuth resolution of the radar system is selected so that the Doppler spectrum of each beam does not interfere with the Doppler foldover due to the finite pulse repetition frequency of the radar system.     

An optical processor for synthetic aperture radar

In this paper a 3-lens optical processor is described. It has the features of quick and easy adjustment, wide adaptability, easy assembly and disassembly. It is a good optical processor for processing synthetic aperture radar signals while working in the field.     

Compact acousto-optic processor for synthetic aperture radar imageformation

A compact implementation of a real-time acousto-optic synthetic aperture radar (SAR) imager is described. The architecture generates SAR imagery by decomposing the required 2D integration into a cascade of two 1D integrations, in a manner similar to that used in computed tomography for medical imaging applications. To achieve low power consumption, the required integrations are performed in the analog optical domain, with a crossed Bragg cell configuration, using a combination of spatial and temporal integration of optical signals. The time integrating and space integrating modules of the architecture are coupled via a common path interferometer. Subimages are formed on a 2D CCD detector array that is rotated during image formation to avoid a computationally difficult digital interpolation operation. The complex-valued subimages are combined in a digital frame buffer for dynamic range enhancement before being displayed. To accommodate needed flexibility, the required filter functions are calculated in a digital controller and downloaded through the Bragg cells. Results of a laboratory demonstration are presented. Performance projections suggest that the architecture may offer an advantage over an all-electronic approach for high-resolution applications which are severely constrained in power consumption     

Synthetic aperture radar imaging of moving ocean waves

A theory for the radar imaging of ocean waves is presented under the assumptions that a swell propagates through an ensemble of Bragg scatterers and that the integration time of the synthetic aperture radar (SAR) is small compared to the angular velocity of the swell. Results are prsented which show image development and distortions caused by the radial velocities and accelerations of the swell. Neglecting small wave bunching and tilts due to the longer underlying waves, and considering only one-dimensional geometries, the mechanism of wave motions are considered and their efforts on the production of the usual intensity Pattern representing the wave image are studied. The analysis shows that in certain situations a processed image can appear which has twice the spatial period of the actual long wave on the ocean, which can confuse the interpretation of ocean wave analysis.     

真实孔径雷达海洋图像的分形特征分析

结合海洋雷达图像的电磁散射机理,采用小波分析方法对真实孔径雷达海洋图像的分形维数和特征进行分析,发现虽然海洋表面波具有分形的特征,但是海洋雷达图像在布拉格散射机制主导的情况下,不具备分形特征.为了准确计算雷达图像的分形维数,还对差分盒计数法、数学形态学方法和小波分析方法三种分形维数的计算方法进行了评估,通过对已知维数的模拟图像进行分形维数的计算,分析了三种方法的优缺点.     

干扰合成孔径雷达的统一方程

合成孔径雷达(SAR)是当前的一种新型雷达体制,它既能测定目标的座标位置又能对目标成像,不论在军用还是在民用都有广泛的应用和广阔的发展前景,当然也就成了雷达干扰研究的重点对象。我们应用了几年的时间,分析了对SAR的干扰原理;推导出对SAR的干扰方程;做了多种仿真干扰试验,室内小功率模拟干扰试验和实际飞行干扰试验,获得大量干扰数据和干扰图像,证明了推导的干扰方程的正确性。还证明了常规脉冲雷达的干扰方程与SAR的干扰方程可以统一成一种方程。只是在计算干扰等效功率时,取不同的干扰压制系数即可。     

Segmentation of polarimetric synthetic aperture radar data

A statistical image model is proposed for segmenting polarimetric synthetic aperture radar (SAR) data into regions of homogeneous and similar polarimetric backscatter characteristics. A model for the conditional distribution of the polarimetric complex data is combined with a Markov random field representation for the distribution of the region labels to obtain the posterior distribution. Optimal region labeling of the data is then defined as maximizing the posterior distribution of the region labels given the polarimetric SAR complex data (maximum a posteriori (MAP) estimate). Two procedures for selecting the characteristics of the regions are then discussed. Results using real multilook polarimetric SAR complex data are given to illustrate the potential of the two selection procedures and evaluate the performance of the MAP segmentation technique. It is also shown that dual polarization SAR data can yield segmentation resultS similar to those obtained with fully polarimetric SAR data     

多视角聚束合成孔径雷达成像

在介绍聚束合成孔径雷达高分辨率成像机理、其获得更高分辨率的难点所在、以及空间分辨率与观测空间频谱关系原理的基础上,提出利用多视角观测增加对目标的观测角,从而增加目标观测的空间频率范围,进而提高空间分辨率的思想.通过对每一视角观测采用波数域成像算法获得每次观测的频谱,利用空间频谱与空间域的对应关系进行频谱融合,对融合后的频谱进行逆傅立叶变换获得方位向分辨率提高的图像.计算机模拟验证了多视角聚束合成孔径雷达成像提高成像分辨率的有效性.     

Parallel implementation of synthetic aperture radar algorithms

Two sequences of operations necessary for implementation of high resolution image formation in strip and spotlight modes of the synthetic aperture radar (SAR) are presented. The sequences are mapped onto a mesh-connected SIMD architecture. The mapping includes not only parallel implementation of all the basic computation steps, but also all the necessary data transformation and communication operations. Detailed estimates of the processing times are provided for Hughes Research Laboratories Systolic/Cellular architecture.This work was partially supported by NSF grant No MIP-8714689     

An analytical presentation of the monochromatic ocean wave image bya regular or an interferometric synthetic aperture radar

An asymptotic expansion of the theoretical model for simulation of a monochromatic ocean wave imaging by either regular synthetic aperture radar (SAR), or by along-track interferometric SAR (INSAR), and its analytical approximation are considered. The model takes into account the so-called velocity bunching imaging mechanism. The domains of validity of the simplified asymptotic and analytical expressions for both SAR and INSAR imaging of the ocean waves are established by comparing those solutions with the corresponding full numerical simulations. The analytical presentations of the SAR and the INSAR wave imaging obtained in the present study are used to understand better the limitations and the relative advantages of both techniques and to define the dimensionless parameters which govern the imaging process     

Bistatic synthetic aperture radar imaging for arbitrary flight trajectories

In this paper, we present an analytic, filtered backprojection (FBP) type inversion method for bistatic synthetic aperture radar (BISAR). We consider a BISAR system where a scene of interest is illuminated by electromagnetic waves that are transmitted, at known times, from positions along an arbitrary, but known, flight trajectory and the scattered waves are measured from positions along a different flight trajectory which is also arbitrary, but known. We assume a single-scattering model for the radar data, and we assume that the ground topography is known but not necessarily flat. We use microlocal analysis to develop the FBP-type reconstruction method. We analyze the computational complexity of the numerical implementation of the method and present numerical simulations to demonstrate its performance.