International GPS Service For Geodynamics Research Articles

ASSESSMENT OF THE INFLUENCE OF MODERN CRUSTAL MOVEMENTS AND THE RECENTLY ACTIVATED PRECAMBRIAN STRUCTURAL PLAN ON THE RELIEF OF THE LAKE LADOGA REGION (THE SOUTHEASTERN BALTIC SHIELD)

This paper describes the influence of modern crustal movements and the recently activated Precambrian structural plan on the relief of theLakeLadogaregion. It presents the results of comprehensive studies, including processing of the regional geological and geomorphological data by the modern methods, as the major novelty of our work. The solutions of earthquake focal mechanisms suggest the current subhorizontal NW compression in the study area. Based on the computer simulation by the Roxar software, we have identified areas wherein new fractures are most likely to occur, determined the dominant directions of such fractures, and revealed the areas of intense vertical movements in the given stress state. The input database included a digital model of the relief and the spatial patterns of ancient faults represented by large-size inhomogeneities influencing the stress field. Strain values were estimated from the horizontal displacements recorded by the International GPS Service for Geodynamics (IGS) and the GPS networks in theRepublicofKareliaand the southeastern regions ofFinland. Using the LESSA software, we have estima­ted the relief orientation characteristics: the density of lineaments, and elongation lines, which are indicative of the changes in the dominant directions of the strike of the lineaments (‘hatches’) in the study area. By interpreting the satellite images and the topographic maps (scale 1:20000), we reveal a number of geological structures, such as gra­nite-gneiss domes and large-size faults, which are directly reflected in the relief. The study results give grounds to establish an indirect relationship between the relief and the modern field of deformation: the areas with high strain values correspond to the areas with steep slopes. The computer simulation data show a NE-trending linear zone with the increased amplitudes of vertical movements. This zone occupies the region along the NW shoreLakeLadoga. In the block divisibility scheme based on the relief analysis, this region is comprised of blocks that are small in area: the Pearson correlation coefficient between the density of the block boundaries and the vertical displacements amounts to 65 %. In addition, this zone is distinguished by an increased permeability of the crust, which was estimated from the helium survey data. Most of the seismic events occurred in the areas characterized by either high or medium probability of new fracturing in case of NW compression. This gives an argument in support of the conclusion that compression takes place in theLadoga Lakeregion at the present stage. According to estimations by the LESSA software, the maximum densities of the linear relief elements correlate with the spatial patterns of the Precambrian faults. A number of faults and granite-gneiss domes are directly manifested in the relief, reflecting the inherited nature of the development of the latter.

Analysis of Global Ionospheric TEC Disturbance During the Magnetic Storm in Nov. 2004

利用全球分布的GPS原始观测数据提取的电离层总电子含量(TEC)分析了2004年11月6日至12日期间全球电离层暴的形态特点与发展过程.结果表明,11月8日磁暴主相期间电离层暴以大范围的强烈正暴为主,在11月10日的恢复相,Dst又一次降到最低值前后期间,电离层再次受到很强的扰动,大范围的正暴和负暴交替出现.这次磁暴期间夏季半球的负暴更加强烈,反映出负暴偏向于在夏季半球发生的季节变化特点.另外,磁暴期间,夜晚TEC值普遍比磁暴前的平静期要低,具体是什么机制导致还需要进一步收集数据和分析.

Study of ionospheric response to the 4B flare on 28 October 2003 using International GPS Service network data

Using the GPS data from as many as 114 GPS stations of the International GPS Service for Geodynamics (IGS), the morphological features of the ionospheric total electron content (TEC) variations on the sunlit hemisphere during the 4B solar flare on 28 October 2003 is studied. It is found that the strongest sudden increase of TEC (SITEC) happened during the flare, and the magnitudes of SITEC vary at regions with different local solar zenith angle (SZA). In the northern hemisphere, the TEC enhancement is approximately symmetrical to the local noon, and its value is usually greater than 14 TECU (1 TECU = 1016/m2) if the SZA is less than 60°. On the whole, as the SZA increases, the value of TEC enhancement in the northern hemisphere decreases. It is worth mentioning that even in the regions of SZA between 90° and 100°, the SITEC was still seen from the temporal TEC curves. Using a photochemical model, the electron production rate over the sunlit boundary region is calculated and some obvious features of SITEC over this region are analyzed. In the polar region, the effect of this flare on the ionosphere exceeds the effect of the ionospheric scintillations and it seems that the ionosphere in the northern polar region responses more sensitively to this flare. In the end, superimposed on the curves of the rate of TEC change, there are some small disturbances (spikes) synchronously appearing on all curves and thus indicating an existence of similar structures in the EUV band of the flare.

Study of the ionospheric total electron content response to the great flare on 15 April 2001 using the International GPS Service network for the whole sunlit hemisphere

The characteristics of the ionospheric response to the solar flare on Apr. 15, 2001 were studied using the total electron content (TEC) obtained at GPS observational stations in the whole sunlit hemisphere under International GPS Service for Geodynamics. It was found that the largest enhancement of the sudden increase of total electron content during this flare is ∼2.6 TECU (1 TECU = 1016 m−2). The effects of solar flare radiation on the ionosphere can be recognized even in the region at 0600 or 1800 LT. Owing to ionospheric scintillation, the TEC enhancement could not be derived from temporal TEC variation curves in high latitudes. On the other hand, the synoptic picture of the sunlit ionospheric response to the flare was obtained, and the results showed the relationship of TEC enhancements with solar zenith angles. The larger the solar zenith angle, the smaller the TEC enhancement. Even minor fast changes in TEC during this flare are revealed in the changing rate of temporal TEC variations in midlatitudes and low latitudes. These small globally synchronous disturbances of TEC were compared with the solar X‐ray fluxes of this flare observed by satellites, and a close correlation between those small ionospheric disturbances and the hard X‐ray flux fluctuations was found.

Geodetically Observed Surface Displacements of the 1999 Chi-Chi Earthquake near Southern Termination of the Chelungpu Fault

Using the Global Positioning System (GPS) we investigated coseismic deformation in southwestern Taiwan for the Rayli (7/7/1998, ML =6.2), Chi-Chi (9/21/1999, ML=7.3) and Chiayi (10/22/1999, ML =6.4) earthquakes based on data collected from 1996 to 2001. The GPS network includes 130 campaign-surveyed, 32 local tracking stations and 13 IGS (International GPS Service for Geodynamics) sites. The geodetic data were processed with GAMIT/GLOBK software. The results indicate that coseismic displacement vectors show opposite moving directions across the surface trace of the Chelungpu-Tachienshan-Chukou thrust fault. In general, the hanging-wall moves toward NW (or W, SW)as opposed to NE (or E, SE) for the footwall. The average amount of displacement is larger in the hanging-wall than in the footwall, with maximum displacement appearing at sties near the mainshock. An array of NW -SE trending left-lateral tear faults distributed in a zone of tens of kilometers wide are interpreted as transfer faults accommodating SW -directed, right-lateral thrusting in the vicinity of southern termination of the Chelungpu fault.

The correlation of flare?s location on solar disc and the sudden increase of total electron content

The correlation of flare’s location on solar disc with the value of sudden increase of total electron content is analyzed with flare’s parameters observed by GOES satellite from 1997 to 1999 and total electron content derived from GPS data observed by international GPS Service for Geodynamics. It is found that, besides the flare’s maximum X-ray flux, flare’s location on solar disc has some relation with the value of SITEC during the flare. The ionospheric response to a flare with a smaller maximum X-ray flux and nearer angular distance to the Sun’s central meridian line may be stronger than that of the flare with a larger maximum X-ray flux and far angular distance. To the flares with the same class of maximum X-ray flux, the nearer the angular distance to the Sun’s meridian, the stronger the ionospheric response.

The application of GPS observations to equatorial aeronomy

Routine observations of the ionospherically imposed propagation effects upon GPS satellite signals are available online from the International GPS Service for Geodynamics. With over 200 ground‐based stations now reporting data, ionospheric studies ranging from the analysis of single‐site observations to the full global network have demonstrated the geophysical science yield possible from this remarkable resource. In this paper we expand on the use of GPS data for comprehensive “regional studies” at low latitudes. Using the GPS observing sites in South America, we show how routine observations are processed to form reliable total electron content (TEC) values capable of describing the latitudinal, longitudinal, local time, and storm time behavior over the full span of the continent. To study the dominant F region structure at low latitudes, the Appleton anomaly, two indices are developed to assess its strength (Is) and asymmetry (Ia). TEC data at 30 s intervals are used to form phase fluctuation indices that capture plasma irregularity patterns at 15‐min (fp) and hourly (Fp) time resolution. Tests of Fp at Atlantic and Pacific sector locations show them to reproduce accurately all known occurrence patterns for equatorial spread F (ESF). The use of the three indices (Is,Ia, and Fp) to formulate predictive capabilities for ESF on the basis of the enhancement or suppression of growth rate indicators was not particularly successful.

Contributions of the international gps service for geodynamics (IGS) to global geodynamics, astronomy and atmosphere science: review and outlook

contributions of the international gps service for geodynamics (IGS) to global geodynamics, astronomy and atmosphere science: review and outlook

Velocity field of around the Sea of Okhotsk and Sea of Japan regions determined from a new continuous GPS network data

To investigate the current crustal movements in and around the Sea of Okhotsk and Sea of Japan regions, we have established a continuous GPS network. By the end of 1997, the network had been expanded to include 12 new stations. Data for the period from July 1995 to November 1997 were analyzed together with data from International GPS Service for Geodynamics (IGS) global stations. To fix the estimated coordinates to the terrestrial reference frame, the Tsukuba IGS station was assumed to be moving westward relative to the stable Eurasian continent at ∼2cm/yr according to Heki's[l996] estimate. We find that: (1) stations in the western margin of the Sea of Japan have eastward velocity vectors, (2) the pole position of the Okhotsk plate is located near Okha, which reconfirms the Okhotsk micro plate, (3) a plate boundary of the Okhotsk and Amurian plates between southen Sakhalin and Hokkaido is suggested.

Geodetic applications of GPS

The Global Positioning System (GPS) has become a widely used tool in geodetic studies of Earth. We review the principles of the applications of GPS to geodetic problems and discuss its applications to problems of global and regional geodesy. A global network of dual-frequency GPS receivers continuously tracks the GPS satellites and the data are distributed to globally accessible on-line archives with delays from a few hours to a few days. Analysis of these data occurs within a few days and the results are made available in the form of satellite ephemerides accurate to a few centimeters and receiver position estimates accurate to a few millimeters for weekly averaged positions. The collection and analysis of these data occurs under the auspices of the International GPS Service for Geodynamics (IGS) which maintains Internet-accessible archives in the United States and Europe. The data and analyzed results are used to study problems of global geodynamics with direct application to global plate tectonics and postglacial rebound and to studies of the the excitation of variations in Earth's rotation. When the global data are combined with data collected in a local region they can be used to study the deformation processes in that region. In the long run, analysis of these regional data will lend to improved dynamical models and to a better understanding of earthquakes and other deformation processes.

The role of GPs in the WEGENER project

In recent years, the Global Positioning System (GPS) has become a major tool for high precision geodesy. The precision is now better than 1 ppb. In this paper, the importance of GPS for the WEGENER project is highlighted. First, an overview of the GPS system is presented including a description of the development of its capabilities. One of the most significant developments of the past few years has been the establishment of the International GPS Service for Geodynamics (IGS). Thanks to its standards and products, the state-of-the-art in high precision GPS has been advanced considerably. Therefore, a summary of the goals and principles of IGS is presented next. This is followed by a compilation of the major GPS projects in Europe and its surrounding areas, since 1988. Most of these are not related to the WEGENER project, but it shows how much work has already been done. It also indicates the possible future role of GPS in WEGENER. Since all these projects are usually quite independent, they might benefit from the existence of a consistent, dense, accurate regional reference frame. This is exemplified by the success of IGS, which is based on an expanding global network of continuously operating permanent GPS stations. It provides a stable global reference frame, which makes it feasible to combine the results from independent field campaigns. In certain regions of the world the coverage is not yet optimal, however, and it is planned to further densify the IGS network. Therefore, it might be interesting to establish a dense regional GPS network in the general research area of WEGENER, which extends from Greenland to mid-Asia. This idea is promoted in Section 5 of the paper, where it is proposed to create such a network under the name of WEGNET, for WEGENER GPS Network. It would seek to establish new sites where none exist yet and to include existing stations that may already have been established for other purposes. Apart from providing a regional densification of IGS and a reference frame for WEGENER, the network may also yield valuable contributions to the monitoring of tectonic motions in the area and for studies concerning ocean and atmospheric loading.

Initial results from WING, the continuous GPS network in the western Pacific area

To investigate tectonic deformation in the western Pacific, a continuous GPS tracking network has been established, and named the Western Pacific Integrated Network of GPS (WING). Between 1995 and March 1997 we establised ten new sites. Data for the period July 1995 to October 1996 were analyzed, together with data from International GPS Service for Geodynamics (IGS) global sites, to estimate daily coordinates. A fiducial‐free approach was used to obtain the most accurate baseline estimates. To fix the estimated coordinates to the terrestrial reference frame, the Tsukuba IGS site is assumed to be moving westward relative to the stable Eurasian continent at ∼2 cm/yr according to Heki's [1996] estimate. We find that: (1) velocities of sites well within oceanic plates are in good agreement with rigid plate motion models; (2) sites close to plate boundaries are all affected by the deformation at those boundaries, among which back‐arc rifting (spreading) is clearly visible at the Mariana and Okinawa troughs; (3) sites in eastern Asia are moving east to east‐southeast relative to the stable Eurasian continent, suggesting long distance effects of the northward collision of India with Asia.

The role of GPS in the study of global change

The Global Positioning System (GPS) may be used today as a mature technique in geodesy and geodynamics. Thanks to the orbits, the Earth orientation parameters, and the coordinates and velocities of about 100 IGS (International GPS Service for Geodynamics) stations, which are made available to the scientific community on a daily basis, GPS is a very powerful and serious contributor to all scientific questions related to high accuracy positioning on and near the Earth's surface. In this article we first give a short characterization of the GPS (as compared to other space-geodetic techniques like VLBI and SLR). The difficulties related to GPS-determined station heights are subsequently discussed, in particular in view of the necessity to model tropospheric refraction. In studies related to Global Change as, e.g., sea level monitoring, and postglacial rebound, station heights and their development in time are of particular interest. It is known for a long time, however, that GPS-derived station heights are not of the same quality as GPS-derived horizontal positions. It is thus much more delicate to extract vertical (as opposed to horizontal) movements from GPS time series. When looking at station heights on a global scale, the motion of the Earth's crust, defined by the tracking sites, relative to the center of mass of the Earth has to be taken into consideration in order to correctly interpret the relative movement of the crust and the sea level. This “motion of the geocenter” may have a size of up to a few centimeters. Solar radiation pressure acting on the GPS satellites is difficult to model due to the complicated shape of the spacecrafts and may affect the geocenter estimates derived from GPS data. GPS may contribute, however, to monitoring the time variations of the geocenter. The influence of the troposphere on the GPS signals is not just a nuisance, it may be used as a signal, too. GPS may be used as an accurate instrument to measure the integrated water vapor above the GPS sites. Long time series of water vapor values from a dense global network may eventually reveal trends in the water vapor content of the atmosphere. Last but not least, we discuss the importance of the IGS, its global products and the densification project, for Global Change investigations.

Ingesting GPS-derived TEC data into the International Reference Ionosphere for single frequency radar altimeter ionospheric delay corrections

At the University of New Brunswick (UNB), we have developed the capability to independently produce hourly global total electron content (TEC) maps from Global Positioning System (GPS) data. UNB's hourly GPS-derived global TEC maps can be ingested directly into a modified version of the International Reference Ionosphere 1995 (IRI-95) model and augmented with a plasmaspheric electron content model to update its coefficient sets. We present a technique to provide improved IRI-95 predictions by using the modified model as a sophisticated interpolator between hourly GPS-derived TEC updates. The updated IRI-95 coefficient sets will make it possible to provide ionospheric delay corrections for various applications including single frequency radar altimeter missions such as the upcoming Geosat Follow-On mission. Since the spacecraft will be orbiting “inside” the ionosphere, our GPS-updated IRI-95 electron density profile will allow us to integrate the electron densities up to the spacecraft altitude to remove the bias imposed by the ionosphere. This would not be possible using GPS-derived TEC alone since it provides integrated electron content up to the altitude of the GPS satellites (20,200 km). In this paper, we present results based on 3 days' worth of global GPS data (33 International GPS Service for Geodynamics (IGS) stations) at a medium solar activity time (year 1993) and 3 days' worth of global GPS data (74 IGS stations) at a low solar activity time (year 1995). We also compare our updated IRI-95 predictions using UNB's global TEC maps and the original IRI-95 predictions, against TOPEX/Poseidon (T/P) dual frequency altimeter-derived TEC data. Based on 3 days' worth of global GPS data during the medium solar activity time in 1993, the results show that there was better than a 9 TECU level (1 sigma) agreement in the total electron content on a global scale with the T/P-derived TEC data using the UNB technique. For the 1995 low solar activity time, our results agreed with the T/P data at better than the 5 TECU level (1 sigma). These results suggest than our method may be viable for providing ionospheric delay corrections for future single frequency altimeter missions.

Satellite test of special relativity using the global positioning system

A test of special relativity has been carried out using data of clock comparisons between hydrogen maser clocks on the ground and cesium and rubidium clocks on board 25 global positioning system (GPS) satellites. The clocks were compared via carrier phase measurements of the GPS signal using geodetic receivers at a number of stations of the International GPS Service for Geodynamics (IGS) spread worldwide. In special relativity, synchronization of distant clocks by slow clock transport and by Einstein synchrony (using the transmission of light signals) is equivalent in any inertial frame. A violation of this equivalence can be modeled using the parameter $\ensuremath{\delta}c/c,$ where $c$ is the round-trip speed of light ($c=299792458\mathrm{m}/\mathrm{s}$ in vacuum) and $\ensuremath{\delta}c$ is the deviation from $c$ of the observed velocity of a light signal traveling one way along a particular spatial direction with the measuring clocks synchronized using slow clock transport. In special relativity $\ensuremath{\delta}c/c=0.$ Experiments can set a limit on the value of $\ensuremath{\delta}c/c$ along a particular spatial direction (henceforth referred to as ``direction of $\ensuremath{\delta}c$''). Within this model our experiment is sensitive to a possible violation of special relativity in any direction of $\ensuremath{\delta}c,$ and on a nonlaboratory scale (baselines $>~20000\mathrm{km}$). The results presented here set an upper limit on the value of $\ensuremath{\delta}c/c<5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ when considering all spatial directions of $\ensuremath{\delta}c$ and $\ensuremath{\delta}c/c<2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ for the component in the equatorial plane.

Integrated satellite interferometry in Southern California

Geodetic studies of crustal deformation and earthquakes in California, which began with the analysis of repeated triangulation measurements after the 1906 San Francisco earthquake, have been revolutionized by new satellite interferometric techniques in space geodesy and remote sensing. Today, space geodetic monitoring of crustal deformation at regional scales relies on a combination of continuous Global Positioning System regional arrays (CGPS), such as the Southern California Integrated GPS Network (SCIGN), and less frequent field GPS surveys. A global continuous GPS array, the International GPS Service for Geodynamics (IGS), provides precise satellite ephemerides and a well‐defined terrestrial reference frame. Data from CGPS arrays can also be used to map tropospheric water vapor and ionospheric disturbances.

High precision GPS network with precise ephemerides and earth body tide model

The permanent GPS network of the International GPS Service for Geodynamics (IGS) provides a global data set of precise GPS measurements, which is invaluable in the development of algorithms and software for very high accuracy GPS positioning. The intensive EPOCH'92 campaign expanded the core network to include densification in many regions of the world. This project has involved the analysis of data from the Brazilian GPS network, with selected connections to the IGS Global Network. In this preliminary report we discuss the initial data processing, demonstrating a level of relative positioning accuracies of better than 10 ppb in height differences, typically the worst positional component. Besides the normal standard used in the processing of a GPS network, it involved an Earth body tide model.

GPS phase fluctuations in the equatorial region during sunspot minimum

GPS multisite and multisatellite observations of phase fluctuations are used to characterize irregularity development in the equatorial region. We use 30‐s samples of total electron content obtained from phase differences between 1.2 and 1.6‐GHz signals as gathered by the International GPS Service for Geodynamics (IGS). These observations from a number of ground receivers allowed for simultaneous studies over a range of latitudes and longitudes in the region near and distant from the magnetic equator. Several storm periods were analyzed in the time period from November 3,1993, to October 2,1995. By examining phase fluctuations at latitudes near the magnetic equator, in the anomaly region, and poleward of the anomaly region, it was found that very high altitudes were reached by plumes of irregularities during seven magnetic storms in solar minimum years. These also occurred during periods of magnetic quiet. The data set allowed the determination of effective altitudes of equatorial irregularities. During low solar flux years, there was a plethora of different altitude regimes from thin layers to altitudes over 2000 km.

A two‐layer model of the ionosphere using Global Positioning System data

We present a new approach to model the Ionosphere based on GPS data. Previous authors have used models with an unique shell. In this case we have included a second shell to account for the distribution of the electrons in the outer part of the Ionosphere. We have analyzed the ionospheric electron content of a region above 30 degrees in declination in different conditions of ionospheric activity using the Kalman filter. The data used has been obtained from the International GPS Service for Geodynamics (IGS) network. Simultaneously we have studied the receiver and transmitter differential biases showing the effects of neglecting the outer part of the Ionosphere in the model. It appears a systematic variations for the receivers—depending on its latitude—not for the satellites.

COSPAR report to united nations 1996: Satellite dynamics

The COSPAR Panel on Satellite Dynamics is concerned with the orbital and attitude motion of artificial satellites and space probes and with techniques for the determination of these motions. Topics in satellite dynamics in which progress has been made in the current year include the further exploitation of the earth observation spacecraft ERS-1, ERS-2 and TOPEX/Poseidon, with their payloads including advanced tracking systems and altimeters; continuing development of the International GPS Service for Geodynamics (IGS); advances in gravity fiel modelling and intensified efforts in the analysis of potential future missions for gravity field improvement; refined modelling of non-gravitational forces and preparation of space missions with sensors able to accurately measure such forces.