Shadow Enhancement in Synthetic Aperture Sonar Using Fixed Focusing

A shadow cast by an object on the seafloor is important information for target recognition in synthetic aperture sonar (SAS) images. Synthetic aperture imaging causes a fundamental limitation to shadow clarity because the illuminator is moved during the data collection. This leads to a blend of echo and shadow, or geometrical fill-in in the shadow region. The fill-in is most dominant for widebeam synthetic aperture imaging systems. By treating the shadow as a moving target and compensating for the motion during the synthetic aperture imagery, we avoid the geometrical shadow fill-in. We show this to be equivalent to fixing the focus at the range of the shadow caster. This novel technique, referred to as fixed focus shadow enhancement (FFSE) can be used directly as an imaging method on hydrophone data or as a postprocessing technique on the complex SAS image. We demonstrate the FFSE technique on simulated data and on real data from a rail-based SAS, and on two different SAS systems operated on a HUGIN autonomous underwater vehicle.      

Azimuthal ambiguities in synthetic aperture sonar and syntheticaperture radar imagery

It is shown that azimuthal ambiguities are not eliminated by the nulls of a sonar (or radar) beam pattern and have a definite influence on image quality. In synthetic aperture systems that are strongly limited in spatial sampling, particularly in ocean borne synthetic aperture sonar (SAS) and spaceborne synthetic aperture radar (SAR), azimuthal ambiguities will corrupt the images unless special measures are taken. These azimuthal aliases may be reduced by emphasizing the centermost portion of the available synthetic aperture length, and deemphasizing the endmost portions. This minimizes the effects from synthetic array elements that most strongly contribute to aliases     

A synthetic aperture sonar system capable of operating at high speed and in turbulent media

Despite their potential ability to produce highly resolved images of the seabed, synthetic aperture sonars are not widely used. The primary reason for this restricted use is that most synthetic aperture systems are based on the radiation and detection of short-duration modulated pulses. Due to the low speed of sound in water, the pulse repetition frequency is low and so it has been difficult to maintain the required pulse-to-pulse phase coherence. This paper describes a new approach to synthetic aperture sonars based on continuous transmission with some form of frequency modulation. That is, a sonar that transmits and receives continuously but uses some form of frequency coding (in this case a linear frequency sweep) to determine range. Using a continuous transmission frequency modulated sonar it is possible to make a synthetic aperture sonar that can produce coherent apertures many wavelengths long. In addition to the combination of synthetic apertures and continuous transmission frequency modulation, further modifications are suggested to reduce the effect of lateral towfish movement and the effects of medium turbulence resulting in random path-length variations.     

SHADOWS合成孔径声纳系统及性能测试

与传统的侧扫声纳相比,合成孔径声纳系统具有全覆盖无遗漏扫测,测量范围大、速度快,图像分辨率高等优点。本文介绍我国引进第一套SHADOWS合成孔径声纳系统的技术性能,分析SHADOWS合成孔径声纳系统性能测试情况及实际测量中的应用价值,提出了存在的问题及其对策。     

三维成像雷达高度计的工作机理及其可能的海洋学应用

三维成像雷达高度计(China Imaging Altimeter,CIALT)是中科院空间中心在传统高度计的基础上,结合偏离天底点观测技术与高度跟踪测量技术、合成孔径技术和干涉技术发展起来的新型雷达高度计.其中,偏离天底点技术是为了增大刈幅宽度,高度跟踪技术是为了兼容海陆信息的观测,合成孔径技术是为了增加方位向分辨率,干涉技术是为了测量像素点的高程值.CIALT相当于是一部Ku波段、小于5°入射角的SAR,它的成像机理与SAR和InSAR基本相似,同时也能够达到传统高度计那样的厘米量级的精确度.但是,CIALT的入射角小于5°,它发射的电磁波不满足发生Bragg散射和二尺度散射的条件,只能发生准镜面散射.因此,CIALT不能完全应用SAR的一些遥感机理和研究算法,但是可以在SAR的研究基础上发展自己的遥感机理和研究算法.CIALT技术主要是解决增加有效测量点和减小时间间隔的问题,它满足了现代海洋研究与应用对于海洋参数采样空间间隔和时间间隔的要求.它在海洋三维立体成像、海洋大地水准面、洋流、中尺度涡、海面风场、海浪和潮汐等方面有良好的应用前景.     

Analysis of Phase Error Effects on Stripmap SAS

It is often of interest to consider how uncompensated platform motion can degrade the ideal point scatterer response (PSR) of a synthetic aperture sonar (SAS). This information can be used to shape the design of the sonar itself as well as that of the platform carrying it. Also, knowledge of how certain types of motion affect a SAS image can reduce the time spent in troubleshooting motion estimation and compensation schemes. In the field of spotlight-mode synthetic aperture radar (SAR), the effects of phase errors across the synthetic aperture are well documented (for example, Chapter 5 of Carrara , 1995). The counterpart problem for the stripmap mode is less well developed in the literature. This paper explores the effects of uncompensated phase errors on stripmap imagery and shows that, under certain conditions, they are similar to those for spotlight mode processing.      

一维综合孔径微波辐射计遥感海面温度的敏感性分析

一维综合孔径微波辐射计能够有效提高观测的空间分辨率,其观测入射角通常在0°~55°范围内变化。为了开发适用于一维综合孔径微波辐射计的海面温度反演算法,需要评估其观测亮温对海洋大气环境要素的敏感性。利用海面发射率模型和大气辐射传输模型,构建了适用于一维综合孔径微波辐射计的微波海洋大气辐射传输模式,研究了C波段垂直和水平极化微波辐射亮温在不同入射角下对海洋大气环境要素的敏感性变化情况,并定量计算了相应的敏感系数。结果表明:垂直和水平极化亮温对海洋大气环境要素的敏感性表现出不同的特性。随着入射角的增大,垂直极化亮温对海面温度的敏感性增强,对海面风场的敏感性相对减弱;水平极化亮温则相反。由大气水汽含量和云液态水含量误差引入的垂直和水平极化亮温误差随入射角增大而增大,但是,即使在55°的大入射角下垂直和水平极化亮温误差仍小于0.12 K。对于海面温度反演精度优于1 K的要求,一维综合孔径微波辐射计的测温精度需优于0.6 K。研究结果对于一维综合孔径微波辐射计海面温度反演算法的研究和载荷设计具有一定的理论指导意义。     

Synthetic Aperture Sonar: A Review of Current Status

This is a review paper that surveys past work in, and the recent status of, active synthetic aperture sonar (SAS). It covers the early historical development of SAS with its provenance in synthetic aperture radar (SAR) and flows through into what work has been published in the open literature up to early 2007. The list of references is sufficiently complete to include most past and recent SAS publications in the open refereed literature.      

A deconvolution algorithm for broadband synthetic aperture dataprocessing

Conventional processing of synthetic aperture sonar (SAS) data is equivalent to a two-dimensional matched filter operation. In principle, two-dimensional deconvolution improves the resolution of the processed image. However, its direct implementation is generally impractical, due to numerical problems. The paper discusses the development of iterative algorithms that efficiently perform the deconvolution of broadband synthetic aperture data and gives examples of their application. It is concluded that, in many cases, the proposed approach is preferable to more classical solutions     

Effects of random phase changes on the formation of synthetic aperture radar imagery

The effects of Gaussian random and linear phase change on the response of the matched azimuth processor of a synthetic aperture imaging radar is analyzed numerically.     

Sea surface temperature retrieval based on simulated microwave polarimetric measurements of a one-dimensional synthetic aperture microwave radiometer

Compared with traditional real aperture microwave radiometers, one-dimensional synthetic aperture microwave radiometers have higher spatial resolution. In this paper, we proposed to retrieve sea surface temperature using a one-dimensional synthetic aperture microwave radiometer that operates at frequencies of 6.9 GHz, 10.65 GHz,18.7 GHz and 23.8 GHz at multiple incidence angles. We used the ERA5 reanalysis data provided by the European Centre for Medium-Range Weather Forecasts and a radiation transmission forward model to calculate the model brightness temperature. The brightness temperature measured by the spaceborne one-dimensional synthetic aperture microwave radiometer was simulated by adding Gaussian noise to the model brightness temperature.Then, a backpropagation(BP) neural network algorithm, a random forest(RF) algorithm and two multiple linear regression algorithms(RE1 and RE2) were developed to retrieve sea surface temperature from the measured brightness temperature within the incidence angle range of 0°–65°. The results show that the retrieval errors of the four algorithms increase with the increasing Gaussian noise. The BP achieves the lowest retrieval errors at all incidence angles. The retrieval error of the RE1 and RE2 decrease first and then increase with the incidence angle and the retrieval error of the RF is contrary to that of RE1 and RE2.     

Synthetic aperture sonar for seabed imaging: relative merits ofnarrow-band and wide-band approached

The advantages of using wideband sonar systems in underwater acoustical imaging by means of synthetic aperture (side-looking) sonars are described and illustrated through simulation examples. The simulations are conducted for two cases of sonar platform motion: perfect trajectory and disturbed trajectory. Several schemes used for wideband synthetic aperture processing are investigated and their relative merits (resolution and complexity) in the case of both disturbed and perfect trajectories are evaluated. Quantitative image evaluation is initially achieved through the evaluation of performance as regards resolution. The problems involved in the definition of the image quality are discussed     

Inversion for the compressional wave speed profile of the bottomfrom synthetic aperture experiments conducted in the Hudson Canyon area

In September 1988, a series of acoustic propagation experiments were conducted in the Hudson Canyon area. These included synthetic aperture experiments in which a source transmitting a set of four pure tones was towed toward/away from a vertical array of 24 receivers. Data obtained at 50 Hz during one of the synthetic aperture experiments are used to obtain a model for the compressional wave speed profile in the bottom using a modal inverse method. This model is further refined using 175 Hz data. The ability of the inferred model to predict the field at 50 Hz and higher frequencies is examined     

高精度和高分辨率水下地形地貌探测技术综述

为提高我国水下地形地貌探测技术水平,促进对海洋的科学认知和高效开发利用,文章综述高精度和高分辨率水下地形地貌探测技术研发进展,并分析关键技术发展方向。研究结果表明:采用机载激光、多波束、侧扫声呐、浅地层剖面、双频识别声呐、合成孔径声呐和水下三维扫描声呐等探测技术以及无人船、水下机器人和海底观测网等探测平台,可获取高精度和高分辨率水下地形地貌信息;应在提高设备性能、减小探测误差和完善数据算法等方面加大研究力度,重点发展综合探测技术,从而全面和清晰地反映水下地形地貌。     

The Seasat microwave instruments in historical perspective

The history of research and development that led to the successful demonstration of the four microwave remote sensing instruments on Seasat (the scatterometer, radiometer, altimeter, and synthetic aperture radar (SAR)) is briefly reviewed in this paper.     

Passive acoustic synthetic aperture processing techniques

Various parameters associated with the track of a stable CW source moving with constant velocity are estimated using synthetic aperture and Doppler processing techniques. These include the source frequency before Doppler distortion by its motion, the relative speed between the source and a constant velocity receiver, the range at closest approach to the source track, and the relative bearing to the source. Different processing techniques are suggested for a range of signal stabilities and observation times. Frequency analysis, or Doppler processing, supplements conventional synthetic aperture processing, and for relatively unstable signals a synthetic Doppler method is recommended. This method makes use of a rapid scan of signals from a succession of sensors in a horizontal line array to stimulate a higher speed motion of the array     

Condor-E Spacecraft with a Synthetic Aperture Antenna and Its Capabilities

Izvestiya, Atmospheric and Oceanic Physics - The main parameters of a small spacecraft (SSC) (Condor-E) and its onboard radar with a synthetic aperture antenna (SAR) are presented in the paper. Two...     

Anew oil spill detection algorithm based on Dempster-Shafer evidence theory

Features of oil spills and look-alikes in polarimetric synthetic aperture radar(SAR) images always play an important role in oil spill detection. Many oil spill detection algorithms have been implemented based on these features. Although environmental factors such as wind speed are important to distinguish oil spills and look-alikes, some oil spill detection algorithms do not consider the environmental factors. To distinguish oil spills and look-alikes more accurately based on environmental fact...     

Comments on "A sea surface height estimator using synthetic aperture radar complex imagery"

It is shown that sea-surface height may not be estimated directly from the phase of complex synthetic aperture radar (SAR) imagery.