EIGEN-GL04C重力场模型的精度分析

本文对GFZ发布的GRACE EIGEN-GL04C重力场模型从球谐系数分析、误差阶方差分析两方面进行了精度评价。研究表明,相较于以往的重力场模型,EIGEN-GL04C重力场模型精度对120阶以下(未包含J2项)的地球中长波部分具有明显的改善。GRACE重力卫星测量已经成为获取地球重力场信息的重要手段。     

利用GRACE资料构造大尺度时变重力场统一模型

基于2003~2012年的GRACE卫星重力资料,采用最小二乘拟合的方法,构建了时变重力场统一模型IGG-TVG2013。该模型以球谐系数的形式表达,在考虑趋势项和周期项等经验参数的基础上,还考虑了加速度项和潮汐模型误差、大地震等因素的影响。将IGG-TVG2013模型与GRACE资料进行了比较分析,在全球92%以上的区域二者符合精度优于±1ugal;利用该模型外推预测了2013年1~6月的重力场变化,结果与GRACE实测数据符合较好。这表明IGG-TVG2013模型不但能较好地描述重力场的连续时空变化,而且具有一定的短期预测能力。     

利用GRACE RL05模型监测长江流域水储量变化

本文基于GRACE最新重力场模型RL05序列研究了高斯滤波、Wiener滤波、各向异性滤波三种方法在长江流域水储量变化监测中的适用性,计算了应用三种方法得到的水储量变化速率。通过与长江流域水文模型的比较,高斯滤波平滑半径为430km时所得的结果与Wiener滤波基本一致,但各向异性滤波反演的结果与水文模型更为接近,并且优于前面两种方法。研究结果表明GRACE RL05时变重力场球谐系数误差存在各向异性的分布特征,因此各向异性滤波更适用于GRACE区域水储量变化的研究。     

各向异性组合滤波法反演陆地水储量变化

地球时变重力场模型反演陆地水储量变化已为全球气候变化研究作出巨大贡献,考虑到时变重力场模型球谐系数中存在相关性,其高阶次项具有较大的误差,需采用最优的滤波方法进行空间平滑。本文提出一种新的各向异性组合滤波方法,其基本思想是将改进的高斯滤波法与均方根(root mean square,RMS)滤波法组合,即对球谐系数的低阶次采用改进的高斯滤波法,而高阶次采用RMS滤波法。首先分析了最新的GRACE RL05系列时变重力场模型系数误差特性,基于全球水储量变化反演结果,分析比较了高斯滤波、改进的高斯滤波、RMS滤波和DDK滤波与本文提出的组合滤波法的有效性及精度,并利用模型结果进行了验证,计算结果表明,组合滤波法的中误差最小。研究结果表明,本文提出的组合法相比于先前的滤波方法,可有效地过滤高阶次的噪声,消除南北条带误差,同时减少信号泄漏,提高信噪比,保留更多有效的地球物理信号,进而提高反演精度。     

根据人卫激光测距、GRACE和地球物理模型求解地球低阶重力场季节变化

本文利用2002年4月至2010年10月的Lageos1和Lageos2两颗激光卫星观测数据、GRACE以及地球物理模型三种独立的方法计算地球低阶重力场系数J 的变化,根据大气压强数据计算 J 时分别按反变气压计（IB）和非反变气压计(NIB)两种假设进行计算。通过分析 J 的季节特性表明,大气在NIB假设下得到的周年振幅比在IB假设下得到的振幅大3倍左右,相位相差47°;大气和陆地水的质量变化对 J 周年变化的贡献占主导地位,海洋的影响最小;大气、海洋和陆地水得到 J 半年振幅和相位值与SLR得到的振幅和相位值吻合较差,尤其是在IB假设下大气得到的结果与SLR结果相差最大; SLR、GRACE和地球物理模型三种独立方法得到的 J 周年项之间吻合相对较好,GRACE得到的周年振幅比SLR得到的周年振幅大50%左右, SLR观测得到的 J 周年振幅介于在NIB和IB两种假设下地球物理模型得到的结果之间;GRACE与SLR得到的 J 半年项的振幅相同,在IB假设下AOW得到的 J 半年振幅和相位与SLR结果差异最大。     

重力梯度张量的球谐分析

深入研究了利用卫星重力梯度数据确定地球重力场模型的球谐分析方法，导出了由重力梯度张量的球函数展开系数确定扰动位球谐系数的实用解算模型。模拟试算结果验证了本文算法的有效性和实用性。     

大气积分方法对GRACE大气去混频模型的影响

讨论利用表面压力积分方法以及3种不同大气垂直积分方法并结合去平滑球谐分析方法,计算出4种不同的GRACE大气去混频模型。从谱域、空域以及主成分分析来比较4种GRACE去混频模型的差距。结果表明:4种大气积分方法信号相近,但也存在较小的差距。用于现阶段时变重力场模型计算这些差距时影响可忽略,但是如果要达到GRACE模拟精度,以及下一代卫星重力场模型的精度,需要考虑大气积分方法间差距带来的影响。     

采用点质量模型方法反演中国大陆及周边地区陆地水储量变化

研究了反演区域陆地水储量变化的点质量模型方法,采用Tikhonov正则化方法解决了反演过程中参数估计病态问题。利用GRACE（gravity recovery and climate experiment）时变重力场模型数据,用点质量模型方法反演了中国大陆及其周边地区陆地水储量变化,将点质量模型反演结果与球谐系数法反演结果、GLDAS（global land data assimilation）水文模型数据进行了验证分析,并选取了4个特征点计算了陆地水储量变化时间序列。实验结果表明,由于点质量模型方法将研究区域内不同网格质量变化对地球重力场的影响分离开来,所得区域陆地水储量变化局部信号更明显,并且点质量模型方法反演结果与GLDAS水文模型数据相关性更强。     

利用GOCE卫星轨道反演地球重力场模型

根据积分方程法反演地球重力场的数学模型,利用GOCE卫星2009-11-02～2010-01-02共61d的精密轨道数据反演了几组地球重力场模型。结果表明,GOCE卫星轨道能有效提取地球重力场的长波信息,弥补了GOCE卫星重力梯度带宽的限制,在106阶次的大地水准面误差为±9.6cm,该阶次精度优于EIGEN-CHAMP03S及GRACE卫星两个月轨道反演地球重力场的精度,但由于两极空白,反演的带谐位系数精度偏低。联合GOCE及GRACE卫星轨道反演的模型在106阶次的大地水准面误差为±6.9cm,弥补了GOCE卫星轨道的缺陷。     

基于低轨卫星跟踪数据的地球重力场反演

地球重力场是大地测量学、固体地球物理学和海洋学研究中不可缺少的信息源。因此,对地球重力场的研究成为现代大地测量学及其相关地学学科发展中最活跃的领域之一。本世纪,新一代卫星重力探测技术有了重大的进展,CHAMP和CGRACE重力卫星能够以很高的精度恢复重力场的中、长波信息,主要包括精密轨道数据、K波段观测数据以及加速度计数据。基于这类数据可得到高精度的地球重力场模型。在此背景下,本文利用卫星跟踪数据反演重力场模型的理论和方法,提出了改进算法,充分挖掘出卫星跟踪数据中的重力场信息。主要研究内容和创新点如下:(1)在阅读大量文献基础上,阐述了地球重力场理论的研究现状,重点研究了混合边值问题。首先讨论了极区球冠情况下混和边值问题的解法;然后借助于坐标旋转变换将任意球冠下的混和边值问题变换成可解算的极区球冠的情况,并研究了不同坐标系下球谐系数之间的变换关系;利用EGM08重力场模型对建立的求解方法进行模拟计算,结果表明:给出的解算方法能达到亚毫米量级的精度。作为混和边值问题的应用拓展,用于精化局部大地水准面,结果表明:若不顾及边界效应,计算精度至少达到5 cm。因此建立的混和边值问题解法不仅理论上可行,而且能保持足够的计算精度,适用于多种类型重力观测数据的综合处理。(2)基于新思路推导了线性化卫星轨道扰动方程组,该方程组可估算线性化误差。并从数学公式和数值计算上分析了线性化误差的影响,得出结论:舍去项M(t,r_1,ρ)的量级与ρ的二次项相关,舍去项gradT(t,r_2)-gradT(t,r_1)的量级与ρ的一次项有关。(3)基于卫星在地固系的运动公式推导了更加严密的能量守恒方程,通过模拟数据分析改进的地固系能量守恒方程有足够的精度去恢复地球重力场模型。(4)GRACE卫星位差中存在2CPR(two cycles per revolution)误差。利用一个月的GRACE模拟数据分析得出2CPR误差的移除可使3阶以上的重力场系数精度提高1～2个量级以上。     

The US Gravimetric Geoid of 2009 (USGG2009): model development and evaluation

A new gravimetric geoid model, USGG2009 (see Abbreviations), has been developed for the United States and its territories including the Conterminous US (CONUS), Alaska, Hawaii, Guam, the Commonwealth of the Northern Mariana Islands, American Samoa, Puerto Rico and the US Virgin Islands. USGG2009 is based on a 1′ × 1′ gravity grid derived from the NGS surface gravity data and the DNSC08 altimetry-derived anomalies, the SRTM-DTED1 3′′ DEM for its topographic reductions, and the global geopotential model EGM08 as a reference model. USGG2009 geoid heights are compared with control values determined at 18,398 Bench Marks over CONUS, where both the ellipsoidal height above NAD 83 and the Helmert orthometric height above NAVD 88 are known. Correcting for the ellipsoidal datum difference, this permits a comparison of the geoid heights to independent data. The standard deviation of the differences is 6.3 cm in contrast to 8.4 cm for its immediate predecessor— USGG2003. To minimize the effect of long-wavelength errors that are known to exist in NAVD88, these comparisons were made on a state-by-state basis. The standard deviations of the differences range from 3–5 cm in eastern states to about 6–9 cm in the more mountainous western states. If the GPS/Bench Marks-derived geoid heights are corrected by removing a GRACE-derived estimate of the long-wavelength NAVD88 errors before the comparison, the standard deviation of their differences from USGG2009 drops to 4.3 cm nationally and 2–4 cm in eastern states and 4–8 in states with a maximum error of 26.4 cm in California and minimum of −32.1 cm in Washington. USGG2009 is also compared with geoid heights derived from 40 tide-gauges and a physical dynamic ocean topography model in the Gulf of Mexico; the mean of the differences is 3.3 cm and their standard deviation is 5.0 cm. When USGG2009-derived deflections of the vertical are compared with 3,415 observed surface astro-geodetic deflections, the standard deviation of the differences in the N–S and E–W components are 0.87′′ and 0.94′′, respectively.     

Combining EGM2008 and SRTM/DTM2006.0 residual terrain model data to improve quasigeoid computations in mountainous areas devoid of gravity data

A global geopotential model, like EGM2008, is not capable of representing the high-frequency components of Earth’s gravity field. This is known as the omission error. In mountainous terrain, omission errors in EGM2008, even when expanded to degree 2,190, may reach amplitudes of 10 cm and more for height anomalies. The present paper proposes the utilisation of high-resolution residual terrain model (RTM) data for computing estimates of the omission error in rugged terrain. RTM elevations may be constructed as the difference between the SRTM (Shuttle Radar Topography Mission) elevation model and the DTM2006.0 spherical harmonic topographic expansion. Numerical tests, carried out in the German Alps with a precise gravimetric quasigeoid model (GCG05) and GPS/levelling data as references, demonstrate that RTM-based omission error estimates improve EGM2008 height anomaly differences by 10 cm in many cases. The comparisons of EGM2008-only height anomalies and the GCG05 model showed 3.7 cm standard deviation after a bias-fit. Applying RTM omission error estimates to EGM2008 reduces the standard deviation to 1.9 cm which equates to a significant improvement rate of 47%. Using GPS/levelling data strongly corroborates these findings with an improvement rate of 49%. The proposed RTM approach may be of practical value to improve quasigeoid determination in mountainous areas without sufficient regional gravity data coverage, e.g., in parts of Asia, South America or Africa. As a further application, RTM omission error estimates will allow refined validation of global gravity field models like EGM2008 from GPS/levelling data.     

函数模型误差弹性自适应滤波修正

多源导航信息融合过程中,观测模型和动力学模型随时间和空间变化复杂,高精度的动态载体导航与定位需要观测模型和动力学模型具有准实时或实时修正的能力。针对包含观测模型误差以及动力学模型误差的滤波系统,提出了一种基于信息滤波的弹性自适应滤波算法。所提算法以不含模型误差的标准信息滤波器为主滤波器,分别构造了观测函数模型及动力学函数模型误差补偿滤波器,对两类模型误差进行补偿。所提方法强调模型补偿项的弹性自适应估计和状态参数的弹性组合,提高了时变模型误差估计的稳定性。半物理仿真实验结果表明,基于函数模型补偿的弹性自适应滤波算法可以有效地估计观测模型和载体动力学模型误差项,水下拖体的三维位置偏差在0.2 m以内,两类模型误差的影响基本消除,明显提高了载体动态参数的估计精度。     

联合SAR相位和振幅信息校正电离层延迟相位

电离层延迟是合成孔径雷达差分干涉测量(DInSAR)的误差源之一,对长波段SAR数据影响尤为严重。文中提出联合SAR相位和振幅信息精确测量方位向偏移,并基于方位向偏移构建模型校正DInSAR电离层延迟相位。选取覆盖2008年汶川地震的ALOS PALSAR(轨道号471)同震干涉对为实验数据,进行电离层延迟校正实验和精度分析。结果表明,实验同震干涉对中电离层贡献相位达17rad,对应约32cm的视线向形变误差,经过校正后地震远场形变均方根减小59%。对比传统方法校正结果,所提方法可有效提高电离层延迟相位校正精度。     

Temporal variability of global Topex/Poseidon altimetry: tide and annual signals?

Summary The global power of the differences of altimetric sea heights at collocated points in the first 44 cycles of the Topex/Poseidon (T/P) mission, apparently shows the influence of errors in the major tides and long period ocean signals. Results show the principal semidiurnal tides (M2/S2 lumped together) in the Cartwright and Ray (1991) model are probably in error globally by 3–4 cm (rms). The dominant fluctuation in the differences over the 44 cycles (14 months total) arises from an annual signal of 4–6 cm (rms), significantly greater than long term climate data suggests (but with considerable uncertainty due to unresolved semiannual effects).     

有偏迭代估计在短基线集形变模型中的应用

进行SBAS技术时,选择的干涉对一般会出现一景影像和其他几景影像干涉的现象,导致构建的线性形变模型病态。在这种情况下,最小二乘法求解SBAS线性形变模型已不太适合。针对这一问题,本文结合Liu估计的有偏迭代法在求解病态线性模型时能够克服病态性对结果的影响,提出将此方法应用到求解SBAS线性形变模型中。以覆盖济宁地区的13景ENVISAT ASAR数据展开了SBAS技术在城市地面沉降监测中的应用实验,分别利用LS法、基于Liu估计的有偏迭代法解算形变模型。结果表明,基于Liu估计的有偏迭代法获得研究区的年平均沉降速率标准差比LS法获得的低1.4,且其求解的结果相较于LS法获得的结果更准确、稳定。     

北斗三号卫星FCB估计及其模糊度固定

随着北斗卫星导航系统(BeiDou navigation satellite system, BDS)的建设和完善,更多的北斗在轨卫星开始提供全球性的定位、导航、授时服务。为了验证北斗系统整体精密单点定位-模糊度固定(precise point positioning-ambiguity resolution, PPP-AR)的效果,基于全球分布的测站2020-08-01—2020-08-31共31 d的观测数据进行GPS、BDS双系统相位小数周偏差(fractional cycle bi?as, FCB)估计, 并对其中BDS-3卫星FCB产品时变特性进行分析。结果表明,大部分BDS-3卫星宽巷FCB在31 d内保持相对稳定,变化小于0.2周, 窄巷FCB在1 d之内的变化小于0.1周。利用估计的FCB产品进行动态和静态PPP-AR解算。单BDS-3静态PPP?-AR的历元固定率可以达到89.8%,东、北、天3个方向的均方根误差（root mean square error,RMSE）分别为0.94 cm、0.73 cm和1.39 cm,动态PPP-AR的历元固定率为83.9%,东、北、天3个方向的RMSE分别为1.99 ?cm、1.70 cm和3.28 cm。     

Formulation of Time Series Vegetation Index from Indian Geostationary Satellite and Comparison with Global Product

To study impact of climate change on vegetation time series vegetation index has a vital role to know the behaviour of vegetation dynamics over a time period. INSAT 3A CCD (Charged Couple Device) is the only geostationary sensor to acquire regular coverage of Asia continent at 1 km × 1 km spatial resolution with high temporal frequency (half-an-hour). A formulation of surface reflectances in red, near infrared (NIR), short wave infrared (SWIR) and NDVI from INSAT 3A CCD has been defined and integrated in the operational chain. The atmospheric correction of at-sensor reflectances using SMAC (Simple Model for Atmospheric Correction) model improved the NDVI by 5–40% and also increased its dynamic range. The temporal dynamics of 16-day NDVI composite at 0500 GMT for a growing year (June 2008–March 2009) showed matching profiles with reference to global products (MODIS TERRA) over known land targets. The root mean square deviation (RMSD) between the two was 0.14 with correlation coefficient (r) 0.84 from 200 paired datasets. This inter-sensor cross-correlation would help in NDVI calibration to add continuity in long term NDVI database for climate change studies.     

利用新型C波段雷达卫星研究南伊内里切克冰川运动特征

利用新型C波段Sentinel-1卫星获取的2015年2月至2017年2月期间的影像数据,研究分析了天山中部南伊内里切克冰川不同时段的运动特征。利用偏移量追踪技术计算不同时间段冰川位移,首先采用三步配准的方法进行主辅影像高精度整体配准,然后基于归一化互相关（normalized cross correlation,NCC）算法通过调整窗口参数精确估算局部偏移量,进一步分离得到冰川移动信息。监测结果表明:（1）在空间分布上,狭长的冰舌区是冰川主要的高速流动区域,冰舌区底部流速小于上部,两侧流速小于中间,末端流速明显减缓。（2）在季节变化上,冰川运动速率与温度变化趋势一致,在5月至8月期间运动速率最快,沿剖面线的最高速率达49 cm/d;在11月至次年2月期间运动最为缓慢,速率为25~30 cm/d左右。（3）在年度变化上,2015年夏季的运动速率比2016年整体高约1~3 cm/d,其他季节则没有明显差异。与高分辨率L波段PALSAR-2影像的监测结果进行定量对比分析时,将冰舌区的像元进行抽稀后统计,得到两种数据获取的运动速率之差的均值为3.48 cm/d,标准差为±3.78 cm/d,证实了南伊内里切克冰川运动监测结果的可靠性。     

Separation between reference surfaces of selected vertical datums

This paper discusses the separation between the reference surface of several vertical datums and the geoid. The data used includes a set of Doppler positioned stations, transformation parameters to convert the Doppler positions to ITRF90, and a potential coefficient model composed of the JGM-2 (NASA model) from degree 2 to 70 plus the OSU91A model from degree 71 to 360. The basic method of analysis is the comparison of a geometric geoid undulation derived from an ellipsoidal height and an orthometric height with the undulation computed from the potential coefficient model The mean difference can imply a bias of the datum reference surface with respect to the geoid. Vertical datums in the following countries were considered: England, Germany, United States, and Australia. The following numbers represent the bias values of each datum after adopting an equatorial radius of 6378136.3m: England (-87 cm), Germany (4 cm), United States (NGVD29 (-26 cm)), NAVD88 (-72 cm), Australia AHD (mainland, -68 cm); AHD (Tasmania, -98 cm). A negative sign indicates the datum reference surface is below the geoid. The 91 cm difference between the datums in England and Germany has been independently estimated as 80 cm. The 30 cm difference between AHD (mainland) and AHD (Tasmania) has been independently estimated as 40 cm. These bias values have been estimated from data where the geometric/ gravimetric geoid undulation difference standard deviation, at one station, is typically ±100 cm, although the mean difference is determined more accurately.The results of this paper can be improved and expanded with more accurate geocentric station positions, more accurate and consistent heights with respect to the local vertical datum, and a more accurate gravity field for the Earth. The ideas developed here provide insight on the determination of a world height system.