Improving resolution and accuracy of mean sea surface from kinematic GPS,Vanuatu subduction zone

Satellite altimetry provides an ocean mean sea surface (MSS) map with global coverage and overall excellent precision. However, in some areas, like the Vanuatu archipelago, the coverage and resolution of this tool are not sufficient to correctly map the short scale undulations of the sea surface, due to numerous islands and to strong lithospheric and mantle heterogeneities. New applications such as seafloor geodesy may require local mean surface representation with improved resolution in specific areas. We used sea surface height collected with kinematic GPS during three different cruises in 2004, 2006 and 2007 to reconstruct a homogeneous sea surface map around Santo Island, Vanuatu. We assess the accuracy of this GPS-derived local sea surface to 6–19 cm and evaluate the quality of existing altimetry and gravity-derived MSSs on the Vanuatu archipelago. Observed short scale undulations are interpreted as due to local geodynamics.     

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。     

南极威德尔海电离层异常的综合观测及分析

威德尔海异常是西南极沿海地区在夏季出现的电离层异常现象。本文用西南极地区的GPS跟踪站数据和测高卫星Jason-2数据,分别提取了陆地和海洋地区大范围的电离层TEC参数。GPS反演结果的优势是获取测站上空高精度的TEC时间序列,测高反演的结果整体与GPS的结果精度相当,虽然测高的时间分辨率较低,但其优势是获取海洋广大区域的TEC值。两种观测手段的研究区域互补,可以充分观测威德尔海异常在西南极的变化特征,从空间上来看,威德尔海异常出现在以别林斯高晋海为中心的广大区域,而威德尔海异常也是覆盖了西南极的别林斯高晋海、威德尔海以及可达80°S的西南极陆地区域。从时间上来看,出现时段在每年的10月底到次年3月初,夜晚电子密度增加,白天电子密度降低,随着太阳活动的增强,其异常程度也变大。     

Analysis of some systematic errors affecting altimeter-derived sea surface gradient with application to geoid determination over Taiwan

This paper analyzes several systematic errors affecting sea surface gradients derived from Seasat, Geosat/ERM, Geosat/GM, ERS-1/35d, ERS-1/GM and TOPEX/POSEIDON altimetry. Considering the data noises, the conclusion is: (1) only Seasat needs to correct for the non-geocentricity induced error, (2) only Seasat and Geosat/GM need to correct for the one cycle per revolution error, (3) only Seasat, ERS-1/GM and Geosat/GM need to correct for the tide model error; over shallow waters it is suggested to use a local tide model not solely from altimetry. The effects of the sea surface topography on gravity and geoid computations from altimetry are significant over areas with major oceanographic phenomena. In conclusion, sea surface gradient is a better data type than sea surface height. Sea surface gradients from altimetry, land gravity anomalies, ship gravity anomalies and elevation data were then used to calculate the geoid over Taiwan by least-squares collocation. The inclusion of sea surface gradients improves the geoid prediction by 27% when comparing the GPS-derived and the predicted geoidal heights, and by 30% when comparing the observed and the geoid-derived deflections of the vertical. The predicted geoid along coastal areas is accurate to 2 cm and can help GPS to do the third-order leveling. Received 22 January 1996; Accepted 13 September 1996     

Mission design,operation and exploitation of the gravity field and steady-state ocean circulation explorer mission

The European Space Agency’s Gravity field and steady-state ocean circulation explorer mission (GOCE) was launched on 17 March 2009. As the first of the Earth Explorer family of satellites within the Agency’s Living Planet Programme, it is aiming at a better understanding of the Earth system. The mission objective of GOCE is the determination of the Earth’s gravity field and geoid with high accuracy and maximum spatial resolution. The geoid, combined with the de facto mean ocean surface derived from twenty-odd years of satellite radar altimetry, yields the global dynamic ocean topography. It serves ocean circulation and ocean transport studies and sea level research. GOCE geoid heights allow the conversion of global positioning system (GPS) heights to high precision heights above sea level. Gravity anomalies and also gravity gradients from GOCE are used for gravity-to-density inversion and in particular for studies of the Earth’s lithosphere and upper mantle. GOCE is the first-ever satellite to carry a gravitational gradiometer, and in order to achieve its challenging mission objectives the satellite embarks a number of world-first technologies. In essence the spacecraft together with its sensors can be regarded as a spaceborne gravimeter. In this work, we describe the mission and the way it is operated and exploited in order to make available the best-possible measurements of the Earth gravity field. The main lessons learned from the first 19 months in orbit are also provided, in as far as they affect the quality of the science data products and therefore are of specific interest for GOCE data users.     

Jason-2近海海面高的最优高斯低通滤波半径选择

使用测高卫星Jason-2传感器地球物理数据记录（sensor geophysical data record,SGDR）的近海海面高观测数据,基于最大似然估计4参数法,对波形数据进行重跟踪。考虑到星下点沿轨方向前后相邻海面高观测值中高频改正信号具有相关性的特征,提出了确定海面高的最优高斯低通滤波半径选择的技术方法,即对星下点沿轨方向观测值进行差分计算,形成差分数据序列,根据序列的相关系数性来确定滤波半径。对于SGDR数据,若对其进行低通滤波,建议滤波半径选为2 km,既可抑制沿轨海面高数据的高频误差,又可保证该数据没有被过度平滑。研究成果可为充分利用卫星测高数据建立高精度近海海面高模型提供参考,进而促进高精度陆海无缝垂直基准的技术体系建设。     

Absolute seafloor vertical positioning using combined pressure gauge and kinematic GPS data

Knowledge of the position and motion of points on the seafloor can be critically important in both fundamental research (for example, global geodesy and plate tectonics) and for more practical applications such as seismic risk evaluation, off-shore construction and pipeline monitoring. In the Vanuatu subduction zone, for example, measuring deformation underwater could provide valuable information for modeling deformation and understanding the seismic cycle. We report a shallow water experiment in Vanuatu to measure the relative and absolute depth of seafloor points. The experiment differs from previous efforts mainly in that it uses the height of the sea surface determined by kinematic GPS, allowing us to locate the points in a global reference frame. The ITRF2005 ellipsoidal height of a seafloor benchmark was determined with a 1-sigma uncertainty of 0.7–2.1 cm. The estimated ellipsoidal height differs only by a few tenths of a centimeter between measurements made in 2004 and another set made in 2006. These results are encouraging and open new perspectives for vertical underwater deformation monitoring in shallow water areas. Sea-surface GPS measurements can also help to reduce the uncertainty in depth difference determination for relative measurements.     

近海海平面变化成因分析

近海海平面上升直接威胁人类生存,分析其成因不仅具有重要科学意义,而且能够为应对海平面上升提供相应策略。使用卫星测高、时变重力以及浮标观测研究2002—2020年近海300 km内海平面变化成因。由于时变重力在近海受到较为严重的泄漏误差影响,使用时变重力约束解模拟陆地质量变化对海洋质量的泄漏,其线性趋势约为0.68 mm/a。在季节和非季节尺度上,时变重力和浮标观测解释了卫星测高的结果,证明近海海平面平衡方程在季节和非季节时间尺度上能够被闭合。在长期变化趋势方面,卫星测高显示近海海平面上升速率为3.32±0.45 mm/a,而时变重力与浮标观测之和的速率为2.25±0.51 mm/a,两者之间存在约1 mm/a的速率差。鉴于该速率差高于估计的不确定度,认为当前在闭合近海海平面平衡方程长期趋势方面仍存在不小挑战,一方面近海比容实测数据较为稀少,很可能低估了长期趋势变化;另一方面,不完善的泄漏误差改正和陆地垂直运动改正也会影响近海海平面长期趋势变化。     

海洋卫星测高及其反演全球海洋重力场和海底地形模型研究进展

海洋卫星测高在全球和区域大地水准面建模、全球海洋重力场反演、海底地形探测、海平面变化监测、构造板块运动研究等大地测量领域至关重要。本文概述了海洋微波测高卫星的简要发展历程,重点梳理了卫星测高在全球海洋重力场和全球海底地形建模方面取得的丰硕成果,对比分析了主流的海洋重力场和海底地形模型;介绍了合成孔径雷达高度计、Ka频段雷达高度计、合成孔径雷达干涉仪3种先进微波测高技术,并分析了其各自的优缺点,表明它们将在未来若干年呈并驱发展趋势;较为系统地阐述了海洋卫星测高的另一新型技术,即GNSS反射信号测量技术的研究动态,给出了GNSS-R(GNSS reflectometry)类(试验)卫星的发展脉络和发展前景。卫星测高的发展趋势之一是多颗测高卫星的组网观测,本文概括了曾经提出的和拟议中的若干组网测高计划,扼要介绍了由我国提出并正在实施的双星跟飞测高模式;最后指出了卫星测高发展的几个主要关注点,包括双星跟飞测高和SWOT(surface water ocean topography)任务的2维海面高(差)测量、卫星测高反演海底地形与高级地形激光高度计观测数据及遥感卫星图像的结合、星载GNSS-R厘米级海面高的载波相位测量、人工智能技术在卫星测高中的潜在应用等。     

Assessment of long-range kinematic GPS positioning errors by comparison with airborne laser altimetry and satellite altimetry

Long-range airborne laser altimetry and laser scanning (LIDAR) or airborne gravity surveys in, for example, polar or oceanic areas require airborne kinematic GPS baselines of many hundreds of kilometers in length. In such instances, with the complications of ionospheric biases, it can be a real challenge for traditional differential kinematic GPS software to obtain reasonable solutions. In this paper, we will describe attempts to validate an implementation of the precise point positioning (PPP) technique on an aircraft without the use of a local GPS reference station. We will compare PPP solutions with other conventional GPS solutions, as well as with independent data by comparison of airborne laser data with “ground truth” heights. The comparisons involve two flights: A July 5, 2003, airborne laser flight line across the North Atlantic from Iceland to Scotland, and a May 24, 2004, flight in an area of the Arctic Ocean north of Greenland, near-coincident in time and space with the ICESat satellite laser altimeter. Both of these flights were more than 800 km long. Comparisons between different GPS methods and four different software packages do not suggest a clear preference for any one, with the heights generally showing decimeter-level agreement. For the comparison with the independent ICESat- and LIDAR-derived “ground truth” of ocean or sea-ice heights, the statistics of comparison show a typical fit of around 10 cm RMS in the North Atlantic, and 30 cm in the sea-ice region north of Greenland. Part of the latter 30 cm error is likely due to errors in the airborne LIDAR measurement and calibration, as well as errors in the “ground truth” ocean surfaces due to drifting sea-ice. Nevertheless, the potential of the PPP method for generating 10 cm level kinematic height positioning over long baselines is illustrated.     

Removing attitude-related variations in the line-of-sight for kinematic GPS positioning

We are presenting a method for removing attitude-related variations (ATTRVs) in dynamic 1 Hz GPS positioning. The ATTRVs are separated into vertical and horizontal components. These result from the translational and rotational motions of the vehicle, which is a marine research vessel in our case. We have developed new observation equations that use corrected pseudoranges and carrier phases to account for ATTRVs. In the present contribution, we are only focusing on the vertical signals. These modeled ATTRVs are included as corrections in the line-of-sight (LoS) to each GPS satellite in the RINEX data sets using a new software called RNXATTCOR. Precise IGS sp3-orbits are used as inputs together with precise lever arm coordinates of the onboard GPS antennas, observations from the marine inertial navigation system and a priori 3D position of the vessel. The corrected RINEX data sets are then processed using kinematic processing or sequential processing in Precise Point Positioning (PPP) mode. The reduction of the standard deviation from a running mean in the final processed GPS time series is about 95%. The current method is being proposed for marine geodesy science applications such as along-track calibration/validation of altimetry products, and also for applications related to offshore kinematic high precision GPS positioning such as drilling, offshore platforms stability, marine pipeline positioning, earthquake monitoring and tsunami detection.     

潮汐模型对利用卫星测高数据研究海平面变化的影响

利用T/P测高数据反演了中国近海及西北太平洋海域的潮汐参数,构造了CSR3.0、FES95.2和T/P反演的海潮模型改正下的海面高时间序列,通过海面高变化曲线及功率谱的比较确定了利用测高数据发现的海平面季节内变化主要是潮汐模型误差的贡献.     

Long-term vertical land motion from double-differenced tide gauge and satellite altimetry data

We present a new approach to estimate precise long-term vertical land motion (VLM) based on double-differences of long tide gauge (TG) and short altimetry data. We identify and difference rates of pairs of highly correlated sea level records providing relative VLM estimates that are less dependent on record length and benefit from reduced uncertainty and mitigated biases (e.g. altimeter drift). This approach also overcomes the key limitation of previous techniques in that it is not geographically limited to semi-enclosed seas and can thus be applied to estimate VLM at TGs along any coast, provided data of sufficient quality are available. Using this approach, we have estimated VLM at a global set of 86 TGs with a median precision of 0.7 mm/year in a conventional reference frame. These estimates were compared to previous VLM estimates at TGs in the Baltic Sea and to estimates from co-located Global Positioning System (GPS) stations and Glacial Isostatic Adjustment (GIA) predictions. Differences with respect to the GPS and VLM estimates from previous studies resulted in a scatter of around 0.6 mm/year. Differences with respect to GIA predictions had a larger scatter in excess of 1 mm/year. Until satellite altimetry records reach enough length to estimate precise VLM at each TG, this new approach constitutes a substantial advance in the geodetic monitoring of TGs with major applications in long-term sea level change and climate change studies.     

Precise orbit determination of the Sentinel-3A altimetry satellite using ambiguity-fixed GPS carrier phase observations

The Sentinel-3 mission takes routine measurements of sea surface heights and depends crucially on accurate and precise knowledge of the spacecraft. Orbit determination with a targeted uncertainty of less than 2 cm in radial direction is supported through an onboard Global Positioning System (GPS) receiver, a Doppler Orbitography and Radiopositioning Integrated by Satellite instrument, and a complementary laser retroreflector for satellite laser ranging. Within this study, the potential of ambiguity fixing for GPS-only precise orbit determination (POD) of the Sentinel-3 spacecraft is assessed. A refined strategy for carrier phase generation out of low-level measurements is employed to cope with half-cycle ambiguities in the tracking of the Sentinel-3 GPS receiver that have so far inhibited ambiguity-fixed POD solutions. Rather than explicitly fixing double-difference phase ambiguities with respect to a network of terrestrial reference stations, a single-receiver ambiguity resolution concept is employed that builds on dedicated GPS orbit, clock, and wide-lane bias products provided by the CNES/CLS (Centre National d’Études Spatiales/Collecte Localisation Satellites) analysis center of the International GNSS Service. Compared to float ambiguity solutions, a notably improved precision can be inferred from laser ranging residuals. These decrease from roughly 9 mm down to 5 mm standard deviation for high-grade stations on average over low and high elevations. Furthermore, the ambiguity-fixed orbits offer a substantially improved cross-track accuracy and help to identify lateral offsets in the GPS antenna or center-of-mass (CoM) location. With respect to altimetry, the improved orbit precision also benefits the global consistency of sea surface measurements. However, modeling of the absolute height continues to rely on proper dynamical models for the spacecraft motion as well as ground calibrations for the relative position of the altimeter reference point and the CoM.     

The value of ocean reflections of GPS signals to enhance satellite altimetry: data distribution and error analysis

The time and space distribution of general reflection altimetry from two satellites (senders and receivers) via the the oceans surface is examined with specific reference to GPS senders and two current receiving satellites. While a considerable enhancement of conventional altimeter coverage is possible in all configurations if the reflection signals can be used, repeating passes of these (with GPS senders) having reasonably small cycle times (days to tens of days) occur only if the receiving orbit is nearly polar. Results of an analysis of the fundamental geometry show that over a large range of reflection angles the error of recovered sea heights depends almost entirely on the errors in the delay signal and the radial error of the receiving satellite (using current estimates of GPS orbit accuracies). The most critical element is the precision of the delay measurement. Both it and the accuracy of the receiving orbit should be below the decimeter level for the technique to achieve its full potential.     

联合卫星测高、GRACE与温盐数据分析红海海平面变化季节性特征

本文利用卫星测高、GRACE与温盐数据监测2003-2014年红海海平面变化,并分析了蒸发降水以及亚丁湾-红海质量交换对红海质量变化的影响。红海地区单一的温盐数据存在覆盖不全或质量不佳的问题,综合CORA、SODA与ORAS4温盐数据估算结果得到平均比容海平面变化,以改善比容信号的精度。针对GRACE数据处理过程中截断与空间平滑滤波引起的泄漏误差,提出改进尺度因子纠正泄漏误差,利用卫星测高数据进行模拟实验验证了改进尺度因子的有效性。利用传统尺度因子和改进尺度因子反演的红海质量变化周年振幅分别为16.1±1.3 cm和20.5±1.7 cm,利用卫星测高和温盐数据估算的质量变化周年振幅为20.2±1.0 cm,表明改进尺度因子可有效减小泄漏误差的影响,改善GRACE模型反演红海质量变化的精度。卫星测高、GRACE卫星重力数据以及平均温盐数据具有较好的一致性,联合GRACE和温盐数据估算的红海综合海平面变化周年振幅为16.6±1.7 cm,与卫星测高估算的总海平面变化周年振幅(16.2±0.9 cm)基本一致,表明多源数据可构成完整的红海海平面监测手段。相比于降水-蒸发作用,红海质量变化受红海与亚丁湾的海水质量交换的影响更为显著,其主导了红海质量的季节性变化。     

On the spectral consistency of the altimetric ocean and geoid surface: a one-dimensional example

Geoid models from the new generation of satellite gravity missions, such as GRACE and GOCE, in combination with sea surface from satellite altimetry allow to obtain absolute dynamic ocean topography with rather high spatial resolution and accuracy. However, this implies combination of data with fundamentally different characteristics and different spatial resolutions. Spectral consistency would imply the removal of the short-scale features of the altimetric sea surface height by filtering, to provide altimetric data consistent with the resolution of the geoid field. The goal must be to lose as little as possible from the high precision of the altimetric signal. Using a one-dimensional example we show how the spectrum is changing when a function defined only on a limited domain (ocean in the real case) is extended or not as to cover the complete domain (the whole sphere in the real case). The results depend on the spectral characteristics of the altimetric signal and of the applied filter. Referring to the periodicity condition, as it is requested in the case of Fourier analysis, the action of the two classical filters (Ideal Low Pass and Gauss filter) and of two alternative procedures (wavelets and Slepian) is studied.     

The absolute value of the geopotential

It is suggested that it would be worthwhile to determine the absolute value of the geopotential on the geopotential surface which corresponds to mean sea level. This number would replace the earth’s semi-major axis as the parameter which fixes the earth’s size; but slight variations in the parameter might be employed to study the dynamics of the sea. Fixing this number involves knowing the geopotential for a point on the orbit of a satellite whose true gravitational potential is also known.     

Coherent GPS reflections from the sea surface

In this letter, we explore a method to obtain accurate ocean heights using measurements of the global positioning system (GPS) carrier phase after reflection on the sea surface. A carrier tracking algorithm is employed in measuring travel path differences between GPS direct and reflected signals collected from antennas suspended over a marine estuary, when roughness guarantees partially coherent reflections at the GPS L1 frequency. This technique proves to be sensitive to surface roughness and able to follow tide variations with a precision better than 5 cm (1-sigma) for sea states with significant wave heights below 10 cm. It is expected that this technique could be further extended to rougher sea states using GPS frequency combinations with longer synthetic wavelengths.     

南极Dome A地区高精度DEM的建立——顾及波形重定、坡度改正及数据融合

介绍了CryoSat-2LRM模式下优化后的OCOG、CFI和LIRT的3种波形重定算法。基于南极Dome-A昆仑站区的实测GPS数据,比较了CryoSat-2LRM模式下的3种波形重定方法,对不同波形重定算法下的测高数据进行了精度评估。结果表明优化后的OCOG算法精度最好,与GPS结果平均差值约为-0.07m,标准差约为0.60m,明显优于其他两种算法。通过比较坡度与卫星数据精度的关系,发现坡度因素对于卫星测高数据精度的影响不可忽视,在Dome A地区验证结果表明,坡度改正可使得卫星测高精度提高约38%。最后联合GPS和CryoSat-2OCOG数据,建立了南极Dome-A地区300m分辨率DEM,其精度约为0.24m。