Global mean sea surface and marine gravity anomaly from multi-satellite altimetry: applications of deflection-geoid and inverse Vening Meinesz formulae

 Global mean sea surface heights (SSHs) and gravity anomalies on a 2^′×2^′ grid were determined from Seasat, Geosat (Exact Repeat Mission and Geodetic Mission), ERS-1 (1.5-year mean of 35-day, and GM), TOPEX/POSEIDON (T/P) (5.6-year mean) and ERS-2 (2-year mean) altimeter data over the region 0^∘–360^∘ longitude and –80^∘–80^∘ latitude. To reduce ocean variabilities and data noises, SSHs from non-repeat missions were filtered by Gaussian filters of various wavelengths. A Levitus oceanic dynamic topography was subtracted from the altimeter-derived SSHs, and the resulting heights were used to compute along-track deflection of the vertical (DOV). Geoidal heights and gravity anomalies were then computed from DOV using the deflection-geoid and inverse Vening Meinesz formulae. The Levitus oceanic dynamic topography was added back to the geoidal heights to obtain a preliminary sea surface grid. The difference between the T/P mean sea surface and the preliminary sea surface was computed on a grid by a minimum curvature method and then was added to the preliminary grid. The comparison of the NCTU01 mean sea surface height (MSSH) with the T/P and the ERS-1 MSSH result in overall root-mean-square (RMS) differences of 5.0 and 3.1 cm in SSH, respectively, and 7.1 and 3.2 μrad in SSH gradient, respectively. The RMS differences between the predicted and shipborne gravity anomalies range from 3.0 to 13.4 mGal in 12 areas of the world's oceans. Received: 26 September 2001 / Accepted: 3 April 2002 Correspondence to: C. Hwang Acknowledgements. This research is partly supported by the National Science Council of ROC, under grants NSC89-2611-M-009-003-OP2 and NSC89-2211-E-009-095. This is a contribution to the IAG Special Study Group 3.186. The Geosat and ERS1/2 data are from NOAA and CERSAT/France, respectively. The T/P data were provided by AVISO. The CLS and GSFC00 MSS models were kindly provided by NASA/GSFC and CLS, respectively. Drs. Levitus, Monterey, and Boyer are thanked for providing the SST model. Dr. T. Gruber and two anonymous reviewers provided very detailed reviews that improved the quality of this paper.     

The deviations of the sea surface from the geoid and their effect on geoid computation

The effects of the deviations of sea surface topography from the geoid are estimated for terrestrial geoid computations as obtained from Stokes' formula. The results are based on an equal-area expansion of Lisitzin's sea surface topography data in a spherical harmonic series. It is realized that those data affect mainly the harmonics of degree n≤10. Consequently, in geoids obtained from combination solutions (where low harmonics are dominated by harmonics as obtained from differential orbit improvement) the sea surface topography effects are relatively small.     

联合卫星测高和海洋物理数据计算近海稳态海面地形的可行性分析

在系统论的指导下 ,以测量平差中一个浅显的例子作对比 ,巧妙地阐述了联合卫星测高和海洋物理数据计算近海稳态海面地形的可行性 ,进而将可行的计算方案归结为具有本质联系的三类。     

中国近海及邻近海域高精度高分辨率海面高的确定

利用新的海潮模型(NAO99B)更为合理的共线方法和交叠平差技术,有效改善了ERM数据的径向轨道误差和时变海面高的影响,提高了浅海海域测高数据的测高精度。     

大坡度水下地形的SAR遥感模拟仿真

根据星载合成孔径雷达(SAR)的成像机理,建立了浅海水下地形的数学物理模型,提出了利用该模型进行水下地形计算的数值方法.利用该模型和方法对大坡度水下地形进行数值模拟,分析了大坡度水下地形的地形坡度与SAR探测程度的关系,发现在水下地形坡度较大的情况下,SAR可测量水深达100～200 m.通过对曾文溪附近海域水下地形的数值模拟,证明了SAR对大坡度水下地形的探测深度.     

Applications of generalized additive model (GAM) to satellite-derived variables and fishery data for prediction of fishery resources distributions in the Arabian Sea

This study aims to illustrate how remotely sensed oceanic variables and fishing operations data can be used to predict suitable habitat of fishery resources in Geographic Information System. We used sea surface height anomaly (SSHa), sea surface temperature (SST), chlorophyll concentration (CC), photosynthetically active radiation (PAR) and fishing depth as predictor variables. Fishery data of Indian squid (Loligo spp.) and catfish (Tachysurus spp.) for study period (1998–2004) were segregated randomly to create training and validation. Catch was normalized into Catch per unit Effort (kg h^?1). Generalized additive modelling was performed on training data and then tested on validation data. Suitable ranges of SST, CC, SSHa and PAR for different species distributions were derived and integrated to predict their spatial distributions. Results indicated good match between predicted and actual catch. Monthly probability maps of predicted habitat areas coincide with high catch of the particular month for the study period.     

利用卫星测高资料推求西北太平洋海域的海洋大地水准面

本文根据卫星测高数据和海面动力地形资料，绘制了不相上下北太平洋海域局部大地水准面的精细结构图，对解决卫星测高技术中大地水准面积和海面地形的可分性问题作了初步尝试。     

Investigating Effects of Seafloor Topography on Sea Surface Currents in the Caspian Sea and Northern Indian Ocean

Seafloor topography certainly has an impact on ocean circulation in different ways. Due to this assumption, the sea surface currents calculated by optical flow (Horn–Schunck) and geostrophic currents methods are analyzed to observe this impact. Pair of sea surface temperature imageries, calculated sea surface height and sea level anomaly are showed beside depth map in areas with meaningful bathymetric features such as underwater mountains and pools. The reason for the formation of some eddies in the Caspian Sea and Indian Ocean is concluded from the location of pools and knolls. In this study, in addition to introducing new time span for calculating geostrophic currents, Ocean Surface Current Analyses Real-Time (OSCAR) currents are applied to validate our estimated currents. Variety of products such as sea surface temperature imageries, OSCAR currents, depth map, calculated results like sea level anomaly and absolute dynamic topography and estimated currents via optical flow and geostrophic currents have been collected in this paper to make very detailed investigation on depth effect on mentioned water parameters. Results show that impacts of knolls and pools are meaningfully clear in optical flow and geostrophic currents in shaping and rationing water motions.     

Models for Fitting Gravimetric Geoids and GPS Results

The aim of this investigation is to study how to use a gravimetric(quasi) geoid for levelling by GPS data in an optimal way.The advent of precise geodetic GPS has made the use of a technique possible,which might be called GPS- gravimetric geoid determination.In this approach,GPS heights above the reference ellipsoid are determined for points whose levelled (orthometric) height H is above sea level people have already surveyed;for these points,we thus have the values of the geoid undulation N.These values are then used to constrain the geoid undulations N‘ obtained from the gravimetric solution.     

Is the levelling datum for a continental levelling network so stable that it would permit the determination of secular movements as accurate as modern precise levellings may be observed?

In general the observations within a continental levelling network have been made during day time when the sun is above the horizon. In Northern countries levelling observations have often been made during the summer months and in the morning and the evening, when the sun may be to the North of the prime vertical. This entails that special mean tidal perturbations by the sun on a levelling network may deviate with not quite negligible quantities from general mean tidal influence by the sun on the sea. For the moon the corresponding deviation will be nearly zero. The variance of the levelling (REUN 1960) between the tidal stations M-28 Fredericia and M-48 Genova is (±33 mm x kiloGal)². The author (1965) has found for this line (hypothetical levelling 19/5–2/6 1950 on the Yielding Earth) possiblespecial mean tidal correction by the moon +27.5 mm possiblespecial mean tidal correction by the sun −4.2 mm ― in total +23.3 mm deviating 22.5 mm fromgeneral mean tidal correction of the seafor both moon and sun} +45.8 mm The deviation 22.5 mm between tidal corrections for Mean Sea Level, MSL, and for levelling line is not quite negligible.     

Role of ocean variability and dynamic ocean topography in the recovery of the mean sea surface and gravity anomalies from satellite altimeter data

Procedures to calculate mean sea surface heights and gravity anomalies from altimeter-derived sea surface heights and along-track sea surface slopes using the least-squares collocation procedure are derived. The slope data is used when repeat track averaging is not possible to reduce ocean variability effects. Tests were carried out using Topex, Geosat, ERS-1 [35-day and 168-day (2 cycle)] data. Calculations of gravity anomalies in the Gulf Stream region were made using the sea surface height and slope data. Tests were also made correcting the sea surface heights for dynamic ocean topography calculated from a degree 360 expansion of data from the POCM-4B global ocean circulation model. Comparisons of the anomaly predictions were carried out with ship data using anomalies calculated for this paper as well as others. Received: 19 August 1996 / Accepted: 14 April 1997     

Modelling local GPS/levelling geoid undulations using artificial neural networks

The use of GPS for establishing height control in an area where levelling data are available can involve the so-called GPS/levelling technique. Modelling of the GPS/levelling geoid undulations has usually been carried out using polynomial surface fitting, least-squares collocation (LSC) and finite-element methods. Artificial neural networks (ANNs) have recently been used for many investigations, and proven to be effective in solving complex problems represented by noisy and missing data. In this study, a feed-forward ANN structure, learning the characteristics of the training data through the back-propagation algorithm, is employed to model the local GPS/levelling geoid surface. The GPS/levelling geoid undulations for Istanbul, Turkey, were estimated from GPS and precise levelling measurements obtained during a field study in the period 1998–99. The results are compared to those produced by two well-known conventional methods, namely polynomial fitting and LSC, in terms of root mean square error (RMSE) that ranged from 3.97 to 5.73 cm. The results show that ANNs can produce results that are comparable to polynomial fitting and LSC. The main advantage of the ANN-based surfaces seems to be the low deviations from the GPS/levelling data surface, which is particularly important for distorted levelling networks.     

海洋表面膜特征的SAR图像探测

讨论并分析各种成因类型的海洋表面膜特征及其在海洋ＳＡＲ图像中影像特征。对渤海湾的ＳＡＲ图像中的表面膜特征进行检测并对这些表面膜特征成因进行了分类。     

风速风向对SAR浅海水下地形成像影响的仿真研究

基于SAR浅海水下地形成像机理和M4S海面微波成像程序,建立了SAR浅海水下地形成像仿真模型,改进了传统浅海水下地形成像仿真模型的缺陷.通过仿真研究和分析浅海水下地形SAR图像特征实例,对风速风向与浅海水下地形SAR图像特征的关系提出了新的认识.低风速条件下,浅海水下地形SAR海面后向散射强度整体偏暗,高风速下整体偏亮;sAR图像条带亮暗的程度与风速有一定的关系,但不是主要的影响因素.风向对SAR浅海水下地形成像的影响明显,表现为,在逆风和顺风情况下,浅海水下地形SAR海面后向散射强度整体偏亮,SAR图像分别以亮条带和暗条带为主;侧风情况下,整体偏暗,SAR图像条带亮暗相当;最佳探测风向是逆风向.     

联合卫星重力和卫星测高数据确定稳态海洋动力地形

新一代卫星重力探测任务GRACE大大提高了地球重力场模型中长波分量的精度,使得联合卫星测高平均海面分离更精细稳态海洋动力地形成为可能。本文利用T/P(1994年~2003年)和JASON-1(2003年~2005年)卫星测高数据确定了全球30′×30′平均海面高;基于重力场模型WHU-GM-05,计算得到对应于海面高的GRACE海洋大地水准面格网值;利用“移去-恢复法”和高斯滤波求得全球稳态海面地形。与EGM96、R io05、ECCO和GGM02模型进行比较,检验结果表明GRACE任务有效的改善了海洋大地水准面的精度,使得稳态海洋动力地形能够呈现更多细部。     

How to handle topography in practical geoid determination: three examples

 Three different methods of handling topography in geoid determination were investigated. The first two methods employ the residual terrain model (RTM) remove–restore technique, yielding the quasigeoid, whereas the third method uses the classical Helmert condensation method, yielding the geoid. All three methods were used with the geopotential model Earth Gravity Model (1996) (EGM96) as a reference, and the results were compared to precise global positioning system (GPS) levelling networks in Scandinavia. An investigation of the Helmert method, focusing on the different types of indirect effects and their effects on the geoid, was also carried out. The three different methods used produce almost identical results at the 5-cm level, when compared to the GPS levelling networks. However, small systematic differences existed. Received: 18 March 1999 / Accepted: 21 March 2000     

联合Argo浮标、卫星测高和GRACE数据研究海平面变化

卫星测高、GRACE、Argo等数据为监测海平面变化提供了丰富的观测数据,利用Argo数据计算的比容海平面变化,可以更加深入地理解卫星测高以及卫星重力获得的海平面变化。利用2004年1月至2010年12月间Argo浮标采集的温度和盐度数据,通过数值积分方法计算了65°S~65°N间的比容海平面异常,并通过最小二乘拟合得到比容海平面变化的长期趋势为0.63±0.45 mm/a,与Llovel得到的结果吻合较好。利用卫星测高数据得到该时间段内海平面变化趋势为2.52±0.71 mm/a,GRACE反演得到的海水质量变化引起的海平面趋势为1.84±0.13mm/a,结果表明海水质量变化成为引起海平面变化的主要因素。最后对联合卫星测高、GRACE得到比容海平面变化与相应Argo浮标数据计算结果的空间分布特征进行了比较。     

1985国家高程基准的系统差

基于异常位、高程异常差以及海面地形模型 3种方法 ,分别求出了 1985国家高程基准相对于全球大地水准面的垂直偏差 ,并取得了一致的结果     

近海多种卫星测高联合数据处理技术

海域海况复杂多变 ,潮汐不但受外海能量输入的控制 ,而且在复杂的海岸线、浅海海底和内陆河流运输的作用下变得异常复杂。卫星测高由于受复杂的海洋动力环境和陆地反射的影响 ,数据质量普遍较深海差。将多种卫星测高数据联合处理 ,可以大大提高近海海域平均海面高的精度与分辨率 ,增强测高卫星监测近海复杂动力现象与反演近海复杂动力机制的能力。本文讨论近海多种卫星联合数据处理的技术与方法 ,分别从提取海平面稳态和时变信息的角度较系统地研究了多种卫星测高联合数据处理方法 ,通过提取不同测高卫星海平面观测数据中与时间无关的系统偏差 ,建立多种卫星测高数据的海平面时变基准 ,从而将多种测高卫星海面监测数据融合到一个动力系统中。大大提高测高卫星海平面监测的时空分辨率 ,为联合多种卫星测高数据在大地测量与近海海洋动力学研究中的应用创造基本条件。     

联合大地水准面、海面地形和潮波运动数值模拟的长江口陆海垂直基准转换关系

长江口水域地形地貌研究对河道治理建设、水上交通运输等人类社会经济活动具有重要意义,而建立高精度的无缝深度基准面及其与其他垂直基准间转换模型将直接影响到水陆交界区高精度地形地貌数据的获取及统一综合管理与分析。为此着重研究了基于三维潮波运动数值模拟、海面地形和大地水准面3种手段联合的河口水域无缝深度基准面构建及其与其他垂直基准间转换模型,并在长江口南支这一典型河口水域进行了建模实验和模型精度评估分析。结果显示,垂直基准转换模型中误差为12.4 cm,与现场长期潮位站实际观测结果比对分析得垂直基准转换模型误差绝对值均值为24.2 cm,尽管大于模型中误差估值,但仍满足国际水道测量规范对测深中垂向最大不确定度的要求。