Atmospheric modelling in GPS analysis and its effect on the estimated geodetic parameters

Permanently operating Global Positioning System (GPS) receivers are used today, for example, in precise positioning and determination of atmospheric water vapour content. The GPS signals are delayed by various gases when traversing the atmosphere. The delay due to water vapour, the wet delay, is difficult to model using ground surface data and is thus often estimated from the GPS data. In order to obtain the most accurate results from the GPS processing, a modelling of the horizontal distribution of the wet delay may be necessary. Through simulations, three such models are evaluated, one of which is developed in this paper. In the first model the water vapour is assumed to be horizontally stratified, thus the wet delay can be described by only one zenith parameter. The second model gives the wet delay with one zenith and two horizontal gradient parameters. The third model uses the correlation between the wet-delay values in different directions. It is found that for large gradients and strong turbulence the two latter models yield lower errors in the estimated vertical coordinate and wet-delay parameters. For large gradients this improvement is up to 7 mm in the zenith wet-delay parameter, from 9 mm down to 2 and 4 mm for the second and third models, respectively. Received: 7 May 1998 / Accepted: 1 March 1999     

Neotectonics and seismic hazard assessment in Hengchun Peninsula,southern Taiwan

Neotectonic data may indicate whether stored elastic energy will be dissipated or released destructively. The Hengchun Peninsula of Taiwan is the southern extremity of the Central Range of Taiwan and thus an emergent part of the accretionary wedge resulting from subduction of Eurasia beneath the Philippine Sea plate at the Manila Trench. Radiocarbon dating of fossil shorelines on the peninsula shows that it has been uplifted at an average rate of 3.8 mm/yr during the Holocene. About $1/3$ of the uplift is due to deformation along the Hengchun reverse fault but, in contrast with the Chelungpu and other low angle reverse faults west of the Central Range, it accommodates strain principally by aseismic creep. To cite this article: C. Vita-Finzi, J.-C. Lin, C. R. Geoscience 337 (2005).     

The continental margin in Iceland — A snapshot derived from combined GPS networks

The tectonical setting in Iceland is quite complex due to the interaction of the Iceland hot spot and the Mid Atlantic Ridge. While in the north of the island one active spreading zone exists, the divergent motion in the centre and the south is distributed over at least two volcanic rift zones. The spreading rate increases linearly along the Western Volcanic Zone from north to south up to 8 mm/yr at the Hengill triple junction. On the contrary, the spreading rate of the parallel Eastern Volcanic Zone decreases from 16 mm/yr down to 6 mm/yr at the island's southern coast. The Hreppar microplate between the two predominant rift zones has an independent motion, which is distinct from that of the Eurasian and North American plates. A new detected feature is the spreading activity around the Hofsjökull volcanic zone located in the centre of Iceland with a significant rate of 6 mm/yr. During this investigation the coordinate sets of nearly 20 years of GPS data acquisition on Iceland were combined to get a velocity field for the surface of Iceland. This velocity field is based on a linear kinematic model with the consideration of local non-linear effects like volcano up-doming and displacements due to major earthquakes.     

Geodesy and relativity

Relativity, or gravitational physics, has widely entered geodetic modelling and parameter determination. This concerns, first of all, the fundamental reference systems used. The Barycentric Celestial Reference System (BCRS) has to be distinguished carefully from the Geocentric Celestial Reference System (GCRS), which is the basic theoretical system for geodetic modelling with a direct link to the International Terrestrial Reference System (ITRS), simply given by a rotation matrix. The relation to the International Celestial Reference System (ICRS) is discussed, as well as various properties and relevance of these systems. Then the representation of the gravitational field is discussed when relativity comes into play. Presently, the so-called post-Newtonian approximation to GRT (general relativity theory) including relativistic effects to lowest order is sufficient for practically all geodetic applications. At the present level of accuracy, space-geodetic techniques like VLBI (Very Long Baseline Interferometry), GPS (Global Positioning System) and SLR/LLR (Satellite/Lunar Laser Ranging) have to be modelled and analysed in the context of a post-Newtonian formalism. In fact, all reference and time frames involved, satellite and planetary orbits, signal propagation and the various observables (frequencies, pulse travel times, phase and travel-time differences) are treated within relativity. This paper reviews to what extent the space-geodetic techniques are affected by such a relativistic treatment and where—vice versa—relativistic parameters can be determined by the analysis of geodetic measurements. At the end, we give a brief outlook on how new or improved measurement techniques (e.g., optical clocks, Galileo) may further push relativistic parameter determination and allow for refined geodetic measurements.     

测绘学科发展综述

本文综述了测绘学科的最新进展,给出了传统测绘学的概念和特征,主要论述了测绘学科新技术的发展现状,对全球定位系统、卫星重力测量、遥感、地理信息系统、宽带网络和虚拟现实这几项新技术分别给出其定义、学科特征和最新进展动态。文中最后给出了测绘学的现代概念,阐述了测绘学科正在向地球空间信息科学跨越和融合。     

Future Satellite Gravimetry for Geodesy

After GRACE and GOCE there will still be need and room for improvement of the knowledge (1) of the static gravity field at spatial scales between 40 km and 100 km, and (2) of the time varying gravity field at scales smaller than 500 km. This is shown based on the analysis of spectral signal power of various gravity field components and on the comparison with current knowledge and expected performance of GRACE and GOCE. Both, accuracy and resolution can be improved by future dedicated gravity satellite missions. For applications in geodesy, the spectral omission error due to the limited spatial resolution of a gravity satellite mission is a limiting factor. The recommended strategy is to extend as far as possible the spatial resolution of future missions, and to improve at the same time the modelling of the very small scale components using terrestrial gravity information and topographic models.We discuss the geodetic needs in improved gravity models in the areas of precise height systems, GNSS levelling, inertial navigation and precise orbit determination. Today global height systems with a 1 cm accuracy are required for sea level and ocean circulation studies. This can be achieved by a future satellite mission with higher spatial resolution in combination with improved local and regional gravity field modelling. A similar strategy could improve the very economic method of determination of physical heights by GNSS levelling from the decimeter to the centimeter level. In inertial vehicle navigation, in particular in sub-marine, aircraft and missile guidance, any improvement of global gravity field models would help to improve reliability and the radius of operation.     

国际大地测量和地球物理联合会2003年日本大会札记

简要介绍了IUGG2003年日本札幌大会的概况,IAG(2003-2007)的组织机构和它准备实施的“全球组合观测系统”(IGGOS)的情况,并将参加学术会议的几点收获做扼要介绍。其中主要包括:邻近国家大地坐标系统和坐标框架的更新和现代化;卫星重力学,如CHAMP,GRACE所取得的进展;和GPS数据处理方法的改善等。     

Pre-seismic, co-seismic and post-seismic displacements associated with the Bhuj 2001 earthquake derived from recent and historic geodetic data

The 26th January 2001 Bhuj earthquake occurred in the Kachchh Rift Basin which has a long history of major earthquakes. Great Triangulation Survey points (GTS) were first installed in the area in 1856–60 and some of these were measured using Global Positioning System (GPS) in the months of February and July 2001. Despite uncertainties associated with repairs and possible reconstruction of points in the past century, the re-measurements reveal pre-seismic, co-seismic and post-seismic deformation related to Bhuj earthquake. More than 25 Μ-strain contraction north of the epicenter appears to have occurred in the past 140 years corresponding to a linear convergence rate of approximately 10 mm/yr across the Rann of Kachchh. Motion of a single point at Jamnagar 150 km south of the epicenter in the 4 years prior to the earthquake, and GTS-GPS displacements in Kathiawar suggests that pre-seismic strain south of the epicenter was small and differs insignificantly from that measured elsewhere in India. Of the 20 points measured within 150 km of the epicenter, 12 were made at existing GTS points which revealed epicentral displacements of up to 1 m, and strain changes exceeding 30 Μ-strain. Observed displacements are consistent with reverse co-seismic slip. Re-measurements in July 2001 of one GTS point (Hathria) and eight new points established in February reveal post-seismic deformation consistent with continued slip on the Bhuj rupture zone.     

用大地测量资料反演青藏高原构造应力场的初步尝试

本文讨论了大地测量反演构造应力场的理论与方法。结合青藏高原地质、地球物理资料、利用限单元法，初步建立了青藏高原构造力场三维弹性数值分析模型。用地表水准和ＧＰＳ资料提供的位移值作为地表边界约束条件，反演计算了青藏高原变场和应力场，数值结果表明，青藏高原构造应力场以南北向挤压为主，东西向拉张为辅，欧亚板块与印度板块相撞作用仍控制着高原现今构造应力场。     

第8届现代地壳运动国际会议概况

国际大地测量学协会第８届现代地壳运动国际会议于１９９３年１２月６－１１日在日本神户举行，有２３个国家的近１６０位专家学者出席了会议，中国共有１２位学者参加。本文综合介绍了这次会议的概况和学术内容。