DORIS as a potential part of a Global Geodetic Observing System

We have processed all available DORIS data from all available satellites, except Jason-1 over the past 10 years (from January 1993 to April 2003). Weekly solutions have been produced for stations positions coordinates, geocenter motion and scale factor stability. We present here accuracy presently achievable for all types of potential geodetic products. Typically weekly stations positions can be derived with a repeatability of 1.0–1.5 cm using data from 5 satellites simultaneously, showing the significant improvement in precision that has been gained recently using the additional new DORIS satellites. As an example, we show how such new results can detect displacement from large magnitude earthquakes, such as the 2003 Denali fault earthquake in Alaska. Displacements of −5 cm in latitude and +2 cm in longitude were easily detected using the DORIS data and are confirmed by recent GPS determination. The terrestrial reference frame was also well be monitored with DORIS during this 10-year period. Other geodetic products, such as tropospheric corrections for atmospheric studies are also analyzed. Finally, we discuss here the possible advantages and weaknesses of the DORIS system as additional geodetic tool, in conjunction with the already existing GPS, VLBI and SLR services, to participate in an Global Geodetic Observing System (GGOS).     

Tsunami Wave Analysis and Possibility of Splay Fault Rupture During the 2004 Indian Ocean Earthquake

The 2004 Indian Ocean tsunami was observed by two satellites, close in space and time, that traversed the Indian Ocean 2?h after the Sumatra–Andaman earthquake, but which observed different tsunami lead wave morphologies. The earlier satellite, Jason-1, recorded a lead wave with two peaks of similar amplitude and wavelength, while the later satellite, TOPEX/Poseidon, recorded a lead wave with only one longer wavelength uplift. To resolve this disparity, we examine the travel paths of long wavelength waves over the seafloor bathymetry. Waves traveling from the margin will traverse significantly different paths to arrive at the two satellite transects. The result is that the satellites are sensitive to different parts of the margin; Jason-1 is highly sensitive to the margin in the area of the epicenter, while TOPEX is sensitive to a more northerly section. By developing solutions of the ocean gravity wave equations, accounting for dispersion, we show that the double peak of the Jason-1 satellite observations are consistent with coseismic rupture of a splay fault of limited along-strike extent, located north of Simeulue Island. The doubly peaked morphology can be reproduced with co-activation of the subduction zone interface and the splay fault, which creates a seafloor uplift pattern with two distinct areas of uplift. The Jason-1 satellite is sensitive to a splay fault in this portion of the margin, whereas the TOPEX satellite would not be significantly affected by this uplift pattern. By back-projecting satellite observation points to the margin, we constrain the location of the proposed splay fault and find that it correlates with a bathymetric high. The aftershock locations, uplift of corals on Simeulue Island and a fault scarp on Pulau Salaut Besar are also consistent with the activation of a splay fault in the area delimited by the back-projection. Our work also shows that it is critical to fully capture gravity wave dispersion in order to represent features of the lead wave profile that may not be as well characterized by the shallow water (long-wavelength) model. It is also necessary to account for dispersion so as to precisely assess wavefront travel times; this leads us to conclude that the rupture must have reached very near to the trench and propagated with an updip rupture velocity of order 2.0?km/s or more.     

Contributions of Satellite Laser Ranging to Past and Future Radar Altimetry Missions

Satellite laser ranging (SLR) has proven avery efficient method for contributingto the tracking of altimetric satellites anddetermining accurately their orbitalthough hampered by the non-worldwide coverageand the meteorologicalconditions. Indeed, in some cases it is the onlymethod available to determinethe satellite orbit (e.g., the orbits of the ERS-1and Geosat-Follow-On missions).Moreover, any operational and non-weather dependenttechniques, like GPS,DORIS, PRARE, can exhibit systematic errors inpositioning and orbitography. Acomparison with SLR results allows to evidence sucherrors and vice versa. Fordoing that, two different approaches for determiningprecise orbits can beconsidered: one based on global orbit determination,the other on a short-arctechnique used to locally improve a global orbitdetermined by another trackingtechniques, such as DORIS or GPS. We can thusvalidate a global orbit andachieve orbit quality control to a level of2 to 3 centimeters at present and expectto achieve a level of 1 to 2 centimeters inthe near future. Errors induced bystation coordinates or by the gravity field(geographically correlated errors, forexample) can be estimated from SLR tracking data.Colocation experiments withdifferent techniques in the same geodetic siteplay also a key role to ensure preciserelationships between the geodetic referenceframes linked to each technique. Inparticular, the role of the SLR technique is tostrengthen the vertical component(including velocity) of the positioning, whichis crucial for altimetry missions.The role of SLR data in the modelling of the firstterms of the gravity field has finally to be emphasized,which is of primary importance in orbitography,whatever the tracking technique used.Another application of SLR technology is thesatellite altimeter calibration. Examples of past calibrationand future experiments are given, including theaccuracy we can expect from the Jason-1 and EnviSatspace oceanography missions.     

由TOPEX/Poseidon和Jason-1/2探测的1993—2012中国海海平面时空变化

海平面变化是社会经济发展和科学研究的重要内容.利用1993年1月至2012年12月共20年的TOPEX/Poseidon、Jason-1和Jason-2卫星测高数据,研究中国海海平面的时空变化.首先通过三颗卫星伴飞阶段数据得到三颗卫星之间的逐点海面高系统偏差,进行逐点海面高改正,建立了20年的中国海海面高异常时间序列.分析了中国海海面高异常空间分布,给出了1月到12月月均平均海平面异常的空间变化规律.分析了中国海海面高异常的时变规律,分别给出了年、季度和月的海面上升速率.利用小波分析研究了中国海海面高异常周期变化规律,分别给出了渤海、黄海、东海和南海的海面高变化周期.讨论了ENSO对海面高异常的影响.     

Centimeter-level precise orbit determination for the HY-2A satellite using DORIS and SLR tracking data

The HY-2A satellite is the first ocean dynamic environment monitoring satellite of China. Centimeter-level radial accuracy is a fundamental requirement for its scientific research and applications. To achieve this goal, we designed the strategies of precise orbit determination (POD) in detail. To achieve the relative optimal orbit for HY-2A, we carried out POD using DORIS-only, SLR-only, and DORIS + SLR tracking data, respectively. POD tests demonstrated that the consistency level of DORIS-only and SLR-only orbits with respect to the CNES orbits were about 1.81 cm and 3.34 cm in radial direction in the dynamic sense, respectively. We designed 6 cases of different weight combinations for DORIS and SLR data, and found that the optimal relative weight group was 0.2 mm/s for DORIS and 15.0 cm for SLR, and RMS of orbit differences with respect to the CNES orbits in radial direction and three-dimensional (3D) were 1.37 cm and 5.87 cm, respectively. These tests indicated that the relative radial and 3D accuracies computed using DORIS + SLR data with the optimal relative weight set were obviously higher than those computed using DORIS-only and SLR-only data, and satisfied the requirement of designed precision. The POD for HY-2A will provide the invaluable experience for the following HY-2B, HY-2C, and HY-2D satellites.     

The application of satellite systems for crustal deformation studies

Measurements of crustal deformations provide important quantitative data for use in the study of the properties of materials and processes in the Earth's interior, and also for the purposes of delineation of active faults by which oil and gas deposits might be confined.Only the usage of satellite systems makes it possible to conduct deformometric studies over large areas and to accomplish measurements within reasonable time periods. The satellite system discussed here is based on the following concepts: pairs of satellites exchange phase information as the result of which the paired satellites emit coherent signal observed at ground sites by the interferometric method. In this process, the measured value is an increment of the difference of distances from the ground site to the two satellites.     

Development of environmental monitoring satellite systems in China

With the increase in global environmental problems, the necessity and urgency of remote sensing technology being applied to environmental monitoring has been widely recognized around the world. China has launched the environment and disaster monitoring and forecasting small satellite constellation HJ-1A/B and the FY3 atmosphere and environmental satellite, but they still cannot fully satisfy requirements for environmental monitoring. This paper summarizes the current status of satellite environmental monitoring in China and the existing problems of inadequate load design and low data utilization efficiency, and discusses the demand for environmental monitoring satellites. Based on the development of foreign satellite systems for environmental monitoring, the future development and key tasks of the environmental monitoring satellite system in China is discussed, as are some related initiatives.     

Satellite gravity – enhancements from new satellites and new altimeter technology

This paper reviews the impacts of new satellite altimeter data sets and new technology on the production of satellite gravity. It considers the contribution of the increased data volume, the application of new altimeter acquisition technology and the potential for future developments. Satellite altimeter derived gravity has provided gravity maps of the world's seas since the 1980s, but, from 1995 to 2010, virtually all improvements were in the processing as there were no new satellite data with closely spaced tracks. In recent years, new data from CryoSat-2 (launched in 2010) and the geodetic mission of Jason-1 (2012–2013) have provided a wealth of additional coverage and new technology allows further improvements. The synthetic aperture radar mode of CryoSat-2 uses a scanning approach to limit the size of the altimeter sea surface footprint in the along-track direction. Tests indicate that this allows reliable data to be acquired closer to coastlines. The synthetic aperture radar interferometric mode of CryoSat-2 uses two altimeters to locate sea-surface reflection points laterally away from the satellite track. In a study to generate gravity for freshwater lakes, this mode is found to be valuable in extending the available satellite coverage. The AltiKa altimeter uses higher frequency radar to provide less noisy sea-surface signals and its new orbit mode gives potential for further improvements in satellite gravity. Future developments include the potential for swath mapping to provide further gravity improvements.     

青藏高原色林错流域区冰川消融对湖泊水量变化的影响

青藏高原大部分湖泊近年来持续扩张,湖泊水位和水量明显增加.冰川消融是流域水量平衡和水循环的重要影响因素,直接导致湖泊水量变化.由于缺乏大范围的冰川质量平衡观测结果,青藏高原冰川消融对湖泊水量变化的影响仍存在较大争议.本文选择青藏高原内流区的色林错流域区（水系编号5Z2）作为研究对象,利用SRTM DEM和TanDEM-X双站InSAR数据,精确估算该流域三个主要冰川区（普若岗日、格拉丹东和西念青唐古拉）2000-2012年的冰川质量平衡,依次为：-0.020±0.030、-0.128±0.049、-0.143±0.032 m·w.e.·a^-1.并据此采用面积加权法准确推估出5Z2流域的冰川质量变化为：-0.166±0.021 Gt·a^-1.综合ICESat和Cryosat-2卫星测高数据,计算该流域2003-2012年湖泊水量变化速率（3.006±0.202 Gt·a^-1）,并定量评估冰川质量变化对5Z2流域湖泊水量增加的贡献为：5.52%±1.07%,因此在青藏高原色林错流域区,冰川消融不是导致21世纪初期湖泊水位上升的主要因素.

     

COSMO-SkyMed an existing opportunity for observing the Earth

COnstellation of small Satellites for Mediterranean basin Observation (COSMO-SkyMed) is the largest Italian investment in Space Systems for Earth Observation, commissioned and funded by Italian Space Agency (ASI) and Italian Ministry of Defence (MoD). COSMO-SkyMed is a Dual-Use (Civilian and Defence) end-to-end Earth Observation System aimed at establishing a global service supplying provision of data, products and services compliant with well-established international standards and relevant to a wide range of applications, such as Risk Management, Scientific and Commercial Applications and Defence Applications. The system consists of a constellation of four LEO mid-sized satellites, each equipped with a multi-mode high-resolution SAR operating at X-band. Three out of four COSMO-SkyMed satellites have been successfully launched the 8th of June, the 9th of December 2007 and the 25th of October 2008 respectively, while the remaining satellite will be deployed within 2010. COSMO-SkyMed 1 and 2 completed their commissioning phase to test, verify and qualify the overall system and from the 1st of August 2008 both satellites are in the operational phase. The third satellite is still performing its commissioning and it is expected to enter in operation in the second half of 2009. The results of the commissioning phase of COSMO-SkyMed 1 and 2 are presented together with the Scientific Mission Exploitation strategy (i.e. Announcement of Opportunity, Background Mission).     

磁宁静期磁尾爆发性整体流持续时间多点卫星研究

本文利用星簇CLUSTER的三颗卫星数据分析了磁宁静期磁尾爆发性整体流（BBFs, Bursty Bulk Flows）的时间尺度, 并与单个卫星的结果做了比较. 事例研究表明, 利用三颗卫星观测数据判断的BBFs的时间尺度比单个卫星的大一倍左右. 对于三颗卫星观测到的同一个BBFs, BBFs在晨昏方向上的摆动决定了CLUSTER的三个卫星观测到BBFs的先后次序. 三颗卫星的观测也显示了BBFs的高度局域化特征. 磁宁静期磁尾BBFs寿命的增大, 使得BBFs携带的质量和能量的地向输运增加. 这种地向输运增加的结果是: 磁尾储存的能量得到较为平稳的释放, 改变了亚暴起始产生的时间, 为解决磁层压力平衡矛盾（PBI, Pressure Balance Inconsistency）问题提供了新的思路.     

Resolving sea level contributions by identifying fingerprints in time-variable gravity and altimetry

We have studied the ability of the GRACE gravimetry mission and Jason-1 altimetry to resolve ice and glacier induced contributions to sea level rise, by means of a fingerprint method. Here, the signals from ice sheet and land glacier changes, steric changes, glacial isostatic adjustment and terrestrial hydrology are assumed to have fixed spatial patterns. In a joint inversion using GRACE and Jason-1 data the unknown temporal components can then be estimated by least-squares. In total, we estimate temporal components for up to ∼ 80 individual patterns. From a propagation of the full error-covariance from GRACE and a diagonal error-covariance from Jason-1 altimetry we find that: (1) GRACE almost entirely explains the mass related parameters in the joint inversion, (2) an inversion using only Jason-1 data has a marginal ability to estimate the mass related parameters, while the steric parameters have much better formal accuracy. In terms of mean sea level rise the steric patterns have a maximum formal accuracy of 0.01 mm for an 11 week running mean. In general, strong negative error correlations (ρ < − 0.9) exists between the high and low elevation parts of the ice sheet drainage basins, when those are estimated independently. The largest formal errors found are in the order of 40 Gton for small high elevation subbasins in the southern Greenland ice sheet, which are difficult to separate. In a simplified joint inversion, merging high and low elevation basins, we have investigated the ability of the GRACE and Jason-1 data to separate the geocenter motion into a present-day contribution and a contribution from glacial isostatic adjustment (GIA). We find that the GIA related signal is larger than the present-day component with a maximum of −0.71 mm/year in the Z direction. Total geocenter motion rates are found to be −0.28, 0.43, −1.08 mm/year for the X, Y and Z components, respectively. The inversion results have been propagated to the Jason-1 along-track measurements. Over the time period considered, we see that a large part of the variability in the Pacific, Atlantic and Indian ocean can be explained by our inversion results. The applied inversion method therefore seems a feasible way to separate steric from mass induced sea level changes. At the same time, the joint inversion would benefit from more advanced parameterizations, which may aid in fitting remaining signal from altimetry.     

反演地表质量变化的附有空间约束的三维加速度点质量模型法

重力卫星可以在相同误差尺度下对全球质量变化进行连续重复观测,并在近十余年来取得了巨大成功,探索重力卫星数据精化处理方法和相关应用研究具有重要意义.本文基于三维加速度点质量模型法的基本原理,进一步发展建立了时变重力场模型球谐位系数的变化和地面点质量变化的关系,可有效考虑地表质量变化导致的负荷形变的影响;引入等权形式、线性形式、指数形式和高斯形式的空间约束方法处理南北条带噪声和向下延拓导致的病态问题,并与零阶Tikhonov正则化方法进行对比分析.采用模拟数据和一个月的实测GRACE时变重力场模型计算全球质量变化,对三维加速度点质量模型法和几种空间约束方法进行对比分析验证.计算结果表明,对于3°等面积的全球格网质量点,高斯和指数形式空间约束方法的最优相关距离约为500 km,等权和线性形式空间约束方法的最优相关距离约为600 km,各方法均可有效处理条带噪声的影响,四种空间约束方法的计算效果优于零阶Tikhonov正则化方法,本文的相关方法为进一步利用三维加速度点质量模型法监测全球质量变化提供了借鉴.

     

Improvement of ERS-1 orbits using along-track accelerations from DORIS data on SPOT2

In long-arc precise orbit determinations of altimetric satellites such as ERS-1, large errors may occur from mismodelling of aerodynamic drag and solar radiation pressure. Such surface forces for non-spherical satellites require accurate modelling of the effective area and particle-surface interactions, but the dominant source of error is neutral air density as derived from thermospheric models for aerodynamic drag. Several techniques can be employed to alleviate air-drag mismodelling but all require the solution of additional parameters from the tracking data. However, for ERS-1 the sparsity of laser range data limits the application of such empirical techniques. To overcome this, use can be made of the dense DORIS Doppler tracking for SPOT2 which is in a similar orbit to ERS-1. A recent investigation by CNES examined the use of drag scale factors from SPOT2 to constrain the ERS-1 orbit. An improvement to that methodology is to consider along-track mismodelling as observed by timing errors in the Doppler data for each pass of SPOT2. The along-track correction to the acceleration as derived from SPOT2 can then be applied to ERS-1 orbits, solving for a scale factor to absorb systematic errors - particularly that arising from the 50 km altitude difference. Results are presented of the associated improvement in ERS-1 orbits as derived from concurrent SPOT2 arcs. It will be seen that the procedure not only improves the laser range fit, but more importantly, leads to more precise radial positioning as evident in the altimeter and crossover residuals.     

基于多颗低轨卫星的SLR台站坐标几何解算方法

卫星激光测距（SLR）技术作为卫星精密定轨手段和轨道检核重要方法,激光反射器已经成为重力卫星和测高卫星等低轨卫星的基本载荷.经典的SLR台站坐标是使用动力学方法计算的,本文根据多颗低轨卫星（LEO）多历元的激光观测数据,采用几何方法开展地面SLR测站坐标计算.通过组建低轨卫星群实现对全球激光站的动态观测,为了合理配置不同低轨卫星间观测值权重,削弱低轨卫星群可能存在的系统性偏差,提出采用方差分量估计组合的最小二乘法进行解算.实测结果显示,解算出SLR台站坐标框架解与SLRF2014差异平均值在25.1 mm,外符合精度达到1~2 cm.该方法避免了复杂动力学模型,SLR台站坐标的几何计算方法既可以作为激光测站框架解算手段之一,同时将LEO卫星群作为空间并址站实现不同技术地球参考框架间的融合.     

基于Swarm卫星数据的一次地震电离层现象辨识

为进一步探索基于多颗卫星观测数据的地震电离层现象识别,利用Swarm星座三颗卫星观测的电子密度数据和磁场数据,对已报道的2017年11月12日伊朗MW7.3地震震前第9天震中附近的一次地震电离层扰动现象进行辨识。通过分析三颗卫星相邻轨道的电离层扰动特征,获得了异常扰动存在的空间范围;利用Swarm星座三颗卫星轨道的时间和空间差异,计算出异常扰动在空间中可能的传播特征;使用同步观测的磁场数据判断其电磁辐射特性。最终根据现有对地震电离层耦合的认知,并结合分析的结果,认为该扰动为非震源发出的声重波扰动,非沿纬向传播的电离层行波扰动,非同步电磁辐射引发扰动,而是与伊朗MW7.3地震孕育活动无关的一次高纬度强烈电离层活动所引起的扰动变化。     

A survey of studies into errors in large scale space-time averages of rainfall, cloud cover, sea surface processes and the earth's radiation budget as derived from low earth orbit satellite instruments because of their incomplete temporal and spatial coverage

This survey considers those studies conducted into estimating errors in satellite derived large scale space-time means (of the order of 250 km by 250 km by a month) for rainfall, cloud cover, sea surface processes and the Earth's radiation budget, resulting from their incomplete coverage of the space-time volume over which the mean is evaluated. Many of these studies have focused on estimating the errors in space-time means post satellite launch and compare mean data derived from such satellites with that from an independent data set. Pre-launch studies tend to involve computer simulations of a satellite overflying and sampling from an existing data set and hence the two approaches give values for sampling errors for specific cases. However, more generic sampling papers exist that allow the exact evaluation of sampling errors for any instrument or combination of instruments if their sampling characteristics and the auto-correlation of the parameter field are known. These generic and simulation techniques have been used together on the same data sets and are found to give very similar values for the sampling error and are presented. Also considered are studies in which data from several satellites, or satellite and ground based measurements are combined to improve estimates in the above means. This improvement being brought about not only by increased spatial and temporal coverage but also by a reduction in retrieval error.     

Ice and Snow Cover of Continental Water Bodies from Simultaneous Radar Altimetry and Radiometry Observations

We show how the studies of ice and snow cover of continental water bodies can benefit from the synergy of more than 15 years-long simultaneous active (radar altimeter) and passive (radiometer) observations from radar altimetric satellites (TOPEX/Poseidon, Jason-1, ENVISAT and Geosat Follow-On) and how this approach can be complemented by SSM/I passive microwave data to improve spatial and temporal coverage. Five largest Eurasian continental water bodies—Caspian and Aral seas, Baikal, Ladoga and Onega lakes are selected as examples. First we provide an overview of ice regime and history of ice studies for these seas and lakes. Then a summary of the existing state of the art of ice discrimination methodology from altimetric observations and SSM/I is given. The drawbacks and benefits of each type of sensor and particularities of radiometric properties for each of the chosen water bodies are discussed. Influence of sensor footprint size, ice roughness and snow cover on satellite measurements is also addressed. A step-by-step ice discrimination approach based on a combined use of the data from the four altimetric missions and SSM/I is presented, as well as validation of this approach using in situ and independent satellite data in the visible range. The potential for measurement of snow depth on ice from passive microwave observations using both altimeters and SSM/I is addressed and a qualitative comparison of in situ snow depth observations and satellite-derived estimates is made.     

Modeling the ionospheric electron content for the correction of altimetric measurements

The TOPEX-POSEIDON oceanographic satellite (due to be launched in 1992) will proceed to high accuracy altimetric measurements of the sea surface. Since the altimeter signals will propagate through the ionosphere, they will be retarded with respect to their free-space propagation delay. As a result, the measured altitude will exhibit an apparent lengthening which must be considered. In order to correct this effect, the ionosphere total electron content (TEC) beneath the satellite has to be known. This paper addresses the problem of determining the TEC form Doppler measurements performed on telemetric signals propagating between the satellite and the ground stations of the DORIS positioning system. This is an inverse problem which, in general, does not admit a single-valued solution. Physical observations of the ionophere lead us to assume that the TEC along each half-revolution is regular such that we can select an appropriate solution. This solution is approximated by cubic splines. The computed results are compared to simulation results, based on the Bent ionospheric model and seem to be particularly promising.     

Preliminary assessment of the navigation and positioning performance of BeiDou regional navigation satellite system

BeiDou regional navigation satellite system(BDS)also called BeiDou-2 has been in full operation since December 27,2012.It consists of 14 satellites,including 5 satellites in Geostationary Orbit(GEO),5 satellites in Inclined Geosynchronous Orbit(IGSO),and 4 satellites in Medium Earth Orbit(MEO).In this paper,its basic navigation and positioning performance are evaluated preliminarily by the real data collected in Beijing,including satellite visibility,Position Dilution of Precision(PDOP)value,the precision of code and carrier phase measurements,the accuracy of single point positioning and differential positioning and ambiguity resolution(AR)performance,which are also compared with those of GPS.It is shown that the precision of BDS code and carrier phase measurements are about 33 cm and 2 mm,respectively,which are comparable to those of GPS,and the accuracy of BDS single point positioning has satisfied the design requirement.The real-time kinematic positioning is also feasible by BDS alone in the opening condition,since its fixed rate and reliability of single-epoch dual-frequency AR is comparable to those of GPS.The accuracy of BDS carrier phase differential positioning is better than 1 cm for a very short baseline of 4.2 m and 3 cm for a short baseline of 8.2 km,which is on the same level with that of GPS.For the combined BDS and GPS,the fixed rate and reliability of single-epoch AR and the positioning accuracy are improved significantly.The accuracy of BDS/GPS carrier phase differential positioning is about 35 and 20%better than that of GPS for two short baseline tests in this study.The accuracy of BDS code differential positioning is better than 2.5 m.However it is worse than that of GPS,which may result from large code multipath errors of BDS GEO satellite measurements.