We present results of combining occultation refractivity profiles from GPS/MET with ECMWF global analyses in a 1DVar framework in order to separate the wet and dry contributions to refractivity and assess their impact on the analyzed temperature, surface pressure and specific humidity fields. We find significant zonal mean temperature, surface pressure and humidity differences between the 1DVar solutions and the ECMWF analyses reflecting biases between the GPS refractivities and ECMWF analyses. Large profile-to-profile temperature discrepancies in the tropical lower stratosphere are due to waves not represented in the analyses. The 1DVar solution is generally drier than ECMWF particularly in the southern subtropics. Lack of moisture above 300 hPa in the present model caused the solution to make large adjustments in low latitude surface pressure and tropospheric temperatures to increase upper troposphere densities and compensate for the missing upper level moisture. The discrepancies between the solution and the background and observational data sets represent roughly a 2-sigma level of agreement rather than the 1-sigma level desired in a 1DVar solution. Given the simplicity of our error covariances, our results are promising as a first step. In the future, the error covariances need to be refined and, in particular, to vary with location.

View full abstract

The radio holographic principle is briefly described and tested by using radio occultation data of the GPS/MET and MIR/GEO experiments. Sub-Fresnel spatial resolution -12 m/pixel was achieved using focused synthetic aperture radio holographic approach, and direct evidence of multibeam propagation effects in the atmosphere was obtained. The achieved instrumental accuracy in angular distance measurements was near 0.004 milliradian/pixel, and observed angular distance between different rays was equal to 0.3 milliradians. The angular resolution of the radio holographic method depends on the wavelength as λ¹ compared to λ^1/2 in conventional methods. In general case the principal limit of the vertical resolution may be determined using focused synthetic aperture antenna theory and may achieve a value -20-40 m under assumptions of spherical symmetry and quiet atmospheric conditions. Wave structures were discovered in the altitude distribution of the gradient electron density at a height interval of 60-95 km with spatial period 1-2 km and vertical resolution 300-500 m. Good correspondence was found between the temperature profiles revealed by radio holographic analysis and those obtained by traditional retrieval using UCAR GPS/MET data.

View full abstract

Real-time national Global Positioning System (GPS) networks are being established in a number of countries for atmospheric sensing. UCAR, in collaboration with participating universities, is developing one of these networks in the United States. The network, named “SuomiNet” to honor meteorological satellite pioneer Verner Suomi, is funded by the U.S. National Science Foundation. SuomiNet will exploit the recently-shown ability of groundbased GPS receivers to make thousands of accurate upper and lower atmospheric measurements per day. Phase delays induced in GPS signals by the ionosphere and neutral atmosphere can be measured with high precision simultaneously along up to a dozen GPS ray paths in the field of view. These delays can be converted into total electron content (TEC), and integrated water vapor (if surface pressure data or estimates are available), along each GPS ray path. The resulting continuous, accurate, all-weather, real-time upper and lower atmospheric data create a variety of opportunities for atmospheric research. In this letter we describe SuomiNet, its applications, and the opportunity to coordinate national real-time GPS networks to create a global network with larger scientific and operational potential.

View full abstract

Simultaneous GPS and water vapor radiometer (WVR) observations were carried out in Tsukuba during May-June 1998, for the validation of precipitable water content (PWC) gradients estimated from single-site GPS data. Slant path PWC observed by WVR were fitted into hourly PWC gradients (WVR gradients) using the least-square method. GPS PWC gradients were retrieved from tropospheric delay gradients that were estimated with GIPSY OASYS 2 package (GIPSY gradients). The results indicate that GIPSY gradients had good, linear correlation with WVR gradients, especially for a large gradient range. Both gradients had spike-shaped, short time-scale (-3 hours) peaks which were mostly associated with synoptic fronts. The GIPSY gradients were also compared with mesoscale PWC gradients calculated from zenith wet delay data of GPS network (NET gradients). The results show that GIPSY gradients did not have very good correlation with NET gradients, and that significant meso-scale discrepancy existed between the two gradients for a cold frontal case on 19 June 1998. One possible reason for this discrepancy is vertical differences in RH gradients, because GIPSY gradients are sensitive to RH gradients around the scale height of humidity (-2500 m) while RH gradients in lowermost level have largest weights for NET gradients. To study PWC gradients associated with the fronts, GPS gradients were compared with other meteorological data over the Kanto Plain for two frontal cases. The results indicate that large PWC gradient zones with horizontal scale of about several tens kilometers in cross-frontal directions were collocated with the surface wind shear zones of the fronts. This suggests that the large PWC gradients were due to humidity discontinuity around the fronts.

View full abstract

We report the delineation of the onset of the Asian Monsoon based on GPS sensing of water vapor in Thailand. We conducted GPS observations at five sites in Thailand since March 1998 under the hydrological project called GAME-T. The objective of the project is to clarify the water and energy cycle system in the Asian Monsoon area. As a preliminary analysis, we used data from March to June 1998 and estimated the water vapor content in the zenith direction (PWV) every 30 minutes using GIPSY software (GPS-PWV). A comparison of the resultant PWV with those estimated from rawinsonde data (Sonde-PWV) suggested that, generally, the long term trends of both GPS-PWV and Sonde-PWV are consistent and a rapid increase of water vapor content is visible in May, which corresponds to the onset of the Monsoon. However, systematic differences between GPS-PWV and Sonde-PWV are eminent. The RMS of the difference (RMSD) between Sonde-PWV and GPS-PWV reaches about 8.7 mm. This large RMSD can be reduced to about 5 mm by removing some unreliable sonde data and making a linear correction to Sonde-PWV. In addition, a comparison of GPS-PWV with other meteorological data (temperature, humidity, and rainfall) showed that there is a strong correlation between a rapid increase of GPS-PWV and heavy rainfall in Bangkok and in Chiang Mai, which may be used to judge the onset of the Monsoon in the area accurately.

View full abstract

The NOAA Forecast Systems Laboratory (FSL) has been continuously calculating integrated atmospheric precipitable water (IPW) from GPS signal delays since 1994. Using rapid orbit information, these data have the accuracy required for use in a numerical weather prediction model through data assimilation. Parallel cycles with and without GPS-IPW data have been running at FSL since November 1997 using the 60-km Rapid Update Cycle (RUC). Verification of the analysis and the 3, 6, and 12-h forecasts against rawinsondes has been ongoing throughout the experiment. Results from these statistics showa consistent improvement in short-range forecasts of relative humidity when the GPS data are included. Precipitation verification has also been calculated for this experiment, and results show that GPS data also improve these forecasts. Recently, the average number of available GPS observations jumped from 18 to 56, and results for November-December 1999 show that the previous slight positive signal is now amplified to a stronger positive impact on the short-range moisture forecasts.

View full abstract

Three-dimensional distributions of water vapor in a mesoscale precipitation system, which developed on 7 July 1996 in the Baiu front, were estimated directly from the GPS data of ‘GPS Earth Observation Network’ (GEONET) of the Geographical Survey Institute. In estimating the three-dimensional distribution of water vapor from the GPS data, we used the tropospheric delays along each path from GPS satellites to GPS receivers on the ground. The result showed that the moist air extended up to the height of 6 km in the precipitation region in the early stage of the precipitation system and that a dry air intruded into the precipitation system from the northwest in the middle-level (from 3 km to 5 km altitude) in the later stage. This dry air intrusion in the middle-level was supported by numerical simulations.

View full abstract

A formulation of local GPS tropospheric tomography for determining 4-D distribution of refractivity in the troposphere is presented together with a preliminary analysis of local dense GPS campaign data. Dividing the modeling space up to a height of 10 km above GPS receivers into cells, the refractivity in each cell is estimated in a successive time window by a tomographic reconstruction method in a quite similar manner like the seismic velocity in each cell in Earth's interior is estimated in seismic tomography. The basic data for tomography are GPS slant delays for respective pairs of station and satellite, which are the sum of postfit phase residual, hydrostatic and wet slant delay. On the other hand, the slant delay from a station to a satellite is expressed by the summation of the product of path length and refractivity in each cell along the ray path. In a given time window, we have numerous observed slant delays corresponding to different ray trajectories, and the refractivity in each cell can be estimated by discrete inversion and least squares methods. The observational equations are usually singular so that we use a damped least squares method popular in seismic tomography. An example of real data analysis is given for the 1995 Shigaraki GPS campaign data, which reveals the spatial and temporal change of refractivity corresponding to the passage of ‘cold front’.

View full abstract

Tropospheric tomography using data from local networks of Global Positioning System (GPS) receivers is producing encouraging spatio-temporal representations of the wet refractivity field. In this work we present the results from a small-scale geodetic experiment that we carried out at the Onsala Space Observatory. Seven GPS receivers distributed within a radius of 3 km from the center, were deployed during 21 days in the summer 1998. The limited number of sites and their spatial configuration present a challenge for tropospheric tomography. Using novel GPS techniques to determine the vertical structure of the atmosphere, we observed, for one session, a strong horizontal water-vapor gradient with a leading edge at higher altitude than the trailing edge, entering from the north. The vertical structure obtained independently using tomographic techniques matched such situation. These results suggest tomography is a promising technique for the determination of the spatio-temporal structure of the atmosphere. We will present preliminary results of the tropospheric tomography, using simulations and experimental data, together with some comparisons with radiosonde data.

View full abstract

The 69 colocated sites in the area of the West Germany are used to estimate the transformation parameters with the seven parameter similarity transformation model. According to the solutions of transformation parameters and corresponding residuals between DHDN and ETRF, detail analyses for global and regional transformation are performed, which show that the global accuracy of the transformation between DHDN and ETRF is ±5 m with WRMS (weighted root mean square) of±0.64mand regional transformations have much better fitting. Through the comparisons of estimated parameters of each state, the systematic differences in definition and observation among regional systems of DHDN have been confirmed. Practical usage of 7 parameter similarity transformation and some suggestions are pointed out.

View full abstract

A working group of the International GPS Service (IGS) was created to look after Reference Frame (RF) issues and contribute to the densification and improvement of the International Terrestrial Reference Frame (ITRF). One important objective of the Reference Frame Working Group is to generate consistent IGS station coordinates and velocities, Earth Rotation Parameters (ERP) and geocenter estimates along with the appropriate covariance information. These parameters have a direct impact on other IGS products such as the estimation of GPS satellite ephemerides, as well as satellite and station clocks. The information required is available weekly from the Analysis Centers (AC) (cod, emr, esa, gfz, jpl, ngs, sio) and from the Global Network Associate Analysis Centers (GNAAC) (JPL, mit, ncl) using a “Software Independent Exchange Format” (SINEX). The AC are also contributing daily ERPs as part of their weekly submission. The procedure in place simultaneously combines the weekly station coordinates, geocenter and daily ERP estimates. A cumulative solution containing station coordinates and velocity is also updated with each weekly combination. This provides a convenient way to closely monitor the quality of the estimated station coordinates and to have an up to date cumulative solution available at all times. To provide some necessary redundancy, the weekly station coordinates solution is compared against the GNAAC solutions. Each of the 3 GNAAC uses its own software, allowing independent verification of the combination process. The RMS of the coordinate differences in the north, east and up components between the AC/GNAAC and the ITRF97 Reference Frame Stations are 4-10 mm, 5-20 mm and 6-25 mm. The station velocities within continental plates are compared to the NNR-NUVEL1A plate motion model (DeMets et al., 1994). The north, east and up velocity RMS are 2 mm/y, 3 mm/y and 8 mm/y. Note that NNR-NUVEL1A assumes a zero vertical velocity.

View full abstract

The purpose of this research is to provide a new algorithm for fixing the three-dimensional kinematic reference frame of space geodetic stations in which only vertical components of quasi-stable site velocities for every X, Y, Z direction are used. This KRF does not depend upon any geological model and, thus, is free from the hidden errors coming from the uncertainties of the reference plate motion model, errors in survey data of selected sites as reference stations and the misfit between the measurements and the model predictions. The method has been applied to the VLBI data collected during the period from 1979 to 1997 analyzed by NASA. We have used for the analysis the estimated rate of change of 340 baseline vectors between 59 VLBI sites. As it is clear from our numerous experiments, this algorithm gives a fairly stable results which are in a good agreement with the NUVEL-1 NNR plate motion model if we include for fixing of KRF even a few sites (15-20 points). In the worst case our results obtained using various criteria for selection of quasi-stable sites have 1 mm/yr level agreement for all sites in the horizontal and vertical directions. The agreement of our results and the NUVEL-1 NNR model is on the order of a few millimeters per year in each coordinate. The largest discrepancies reach 20 mm/yr in the sites close to the plate boundaries. The uncertainties of vertical direction do not exceed 1 mm/yr for half of the sites. As a final result, we do not find any clear evidence suggesting the change of Earth's radius, which is considerably less than 1 mm/yr.

View full abstract

Global Positioning System (GPS) measurements were made to estimate the crustal deformation strain rate for the Deccan trap region, western India. Estimated horizontal velocity vectors in ITRF96 are in the range of 40-60 mm/yr with an average of 51 mm/yr in N47°E. Using the horizontal components of the velocity vectors, strain field has been estimated by the Least Square Prediction (LSP) method. An extension is observed along the west coast and south of Koyna andWarna reservoirs, transcending in to a region of compressive regime towards the interior of the shield area. The west coast geothermal province coincides with the extension regime. The collision between India and Eurasia is likely cause for compression regime. The results are discussed in conjunction with the seismo-tectonics of the study region.

View full abstract

In order to monitor the crustal deformation in the area of eastern Kanto, central Japan, Ibaraki University has started the high precision geodetic studies with GPS measurements in collaboration with Geographical Survey Institute of Japan (GSI). The first measurements were taken in 1991 with a small network of 8 sites. Four more sites were included since 1998 for a better coverage in the region. In the present study, we analyzed the data collected since 1998 by using the GAMIT/GLOBK software for estimating the displacement vectors in the ITRF96 reference frame. The data from the continuous GPS network of GSI and the GPS sites of IGS close to the studied region (Tsukuba and Usuda) were included for obtaining the primary solutions. The initial results by constraining the co-ordinates of the IGS stations nearest to the network confirmed the repeatability of the baseline components. The results presented here were obtained by assuming the 4 fiducial stations around the network, namely, Irkutsks (IRKT), Fairbanks (FAIR), Sheshan (SHAO) and Dededo (GUAM). The average uncertainties of the horizontal velocities from three consecutive campaigns, each separated by six months, were estimated to be ±4.6 mm/yr and ±3.5 mm/yr for the northward and eastward components, respectively.

View full abstract

Although the Kozu Island Volcano, one of the Izu Islands Volcanoes in the south part Central Japan, is an active volcano, there is no record of the eruption for about 1100 years since the last eruption in 833 A.D. Since 1988, frequent earthquake swarms are observed around the Kozu Island, and the uplift of 2-4 cm/yr is observed on the island by tidal observations. Station velocities detected by GPS measurements since 1989 show velocities that differ from the convergent velocity of the Philippine Sea plate calculated from plate motion models. A local GPS network with 12 stations is occupied around the volcano, and the GPS measurements are repeated every about six month since July 1996. Inflated deformation of 2-4 cm/yr are detected from the GPS measurements and the pressure source is estimated to be located in the northeastern part of the island at a depth of 2.1 km using Mogi solution. Negative gravity changes of more than 30 microgal are also measured above the pressure source in the period November 1998 to July 1999, consistent with uplift.

View full abstract

In order to detect the crustal movement in the Beppu Bay area, where the highest intra-plate strain rates in Japan is expected, precise GPS measurements have been repeatedly conducted since 1994 at one-year intervals. The result shows N-S or NNE-SSE extension taking place at a rate of more than 10 mm/yr around Beppu Bay. This is essentially consistent with the result obtained by the eodetic survey conducted over the last 100 years, and it suggests a constant rate of crustal motion over the period. The result gives a strong constraint for future modeling of the stress field recovery in the area. However, we failed to identify whether the movement occurred at any specific fault. A denser array of GPS points is needed in the area, and especially near the major faults, in order to investigate possible fault-related motion.

View full abstract

This paper deals with the optimal design of GPS geodynamic geodetic networks. A simple strike-slip fault-model is used to investigate the design of networks and to decide where to place geodetic stations in order to satisfy the accuracy of fault-model parameter estimates, which sites should be selected from the initial mass of permissible points which are bounded by topography and other conditions.

View full abstract

Recently, in comparing with Earth rotation data, we found some exciting correlation phenomena between lengthof-day (LOD) and crustal deformations as well as stress observations in China, the correlation appears most typically on decade time scales and seems to be direction-dependent. As LOD variation induced stress and deformation is far less than the observed ones, we infer that decade variations in crustal deformation and stress are very possibly attributed to the core-mantle coupling process, just likeLODdecade variations are attributed to core-mantle coupling. More detailed and extended investigations are going on, and we believe that the similar correlation phenomena should also exist in other part of the world. The LOD-correlated crustal deformations are expected to provide a better constraint on verifying the real core-mantle coupling mechanism, topographic, electromagnetic or gravitational coupling.

View full abstract

Historical geodetic measurements have been used to infer on the displacement and strain states locally or regionally. They are also often used to invert for other geophysical parameters. However, historical geodetic measurements have been known to contain significant scaling and orientation errors, which may even be different in different parts of a network. These significant error sources may result in producing a wrong (or at least, a misleading) displacement or strain field. When such a displacement or strain field is further used to invert certain geophysical parameters, mis-interpretations may be expected. Thus, in this paper, we will perform a theoretical analysis to answer the following three questions: (i) are displacements obtainable from historical geodetic data? (ii) are strains obtainable from historical geodetic data? and (iii) what geodynamical value do historical geodetic data have?

View full abstract

Continuous GPS networks, typically with a station spacing of about 30 km, are still not dense enough to accurately characteristise the dynamics of active faults. Interpolation of these GPS results can improve our understanding of active faults and hence promote related studies. Moreover, even when the networks are densified in order to recover the signature of active faults, the station configuration design may not be ideal. Interpolation at these points, based on the GPS results from a well-designed station network, can provide a good quality control measure. As a first step in the interpolation process an irregular grid pattern is formed, based on the locations of the GPS stations, by using the indexed sorting algorithm. In order to interpolate objectively, the GPS stations and the intended interpolating points are classified into different sub-regions according to their positions in relation to the faults, which are expressed by open- and closed-curve models. GPS results from stations in the same sub-region are used to derive a dynamic model for interpolation at grid points in the same sub-region. A deformation distribution model based on GPS and differential Synthetic Aperture Radar Interferometry (InSAR) results is used as constraints to scale the time series generated using the dynamic model.

View full abstract

Mt. Iwate (2, 038 m) is an active volcano located in northeastern Japan. Unrest of the volcano started in September, 1995 with intermediate-depth tremors. The shallow seismicity gradually became active in February, 1998, accompanying the notable crustal deformation observed by a dense GPS network. The pattern of the horizontal displacements is characterized by radially directing outward from the volcano. We estimated the source position by inversion analyses for every two-months period, assuming two models; a point pressure source (Mogi model) and a tensile fault. The comparison of AIC's for the two models indicates that the latter is proper from February to April, while the former is preferable afterward. The tensile fault was located at about 5 km WSW of the summit and 3 km in depth, then a Mogi source was estimated at the western neighbor of the tensile fault in the successive period and moved westward as far as 10 kmWof the summit with shallowing its depth. It should be noted that the seismic area also expanded westward in the same period. This synchronicity suggests that the both phenomena were caused by a movement of magma from the deeper part beneath the summit to the western shallower part.

View full abstract

To better understanding the observation data, we attempt to assess the detectability for the crustal deformation observation network of the Hot Springs Research Institute of Kanagawa prefecture (HSRI). The HSRI has carried out continuous monitoring of crustal deformation with GPS, EDM, borehole tiltmeters, groundwater leveling since 1989. Our observation purpose is to detect the pre-, co- and post-seismic deformations of the hypothetical M7 class earthquake designated as the “Western Kanagawa Prefecture Earthquake (WKE)”. In order to estimate the detectability, we apply a simple calculation method, that is comparing between co-seismic deformation (displacement and tilt) and the accuracy of observation instruments. Co-seismic deformation is calculated using the Okada's method, which is a formulation of the expected crustal deformation due to seismic faulting as a function of an earthquake magnitude and a hypocentral distance without assuming focal mechanism. It is indicated that the highest detectability area is located in the center of our HSRI observation network. Comparing detectability contour maps for several kinds of networks, we can find that the detectability of the co-seismic deformation by the tiltmeter network is higher than those by the GPS and EDM networks in the case of the M5 class earthquake with a focal depth of 10 km, and that the detectability by the GPS network is almost equal to that by the EDM network in the case of the M6 class earthquake with a focal depth of shallower than 15 km. For M4 class earthquakes, it is difficult for the GPS, EDM and tiltmeter networks to detect co-seismic deformation. The present result shows advantageous characteristics of the HSRI geodetic network and an important constraint on investigating the pre-, co- and post-seismic deformation of the WKE. To monitor the pre-, co- and post-seismic deformation of the WKE, it is necessary to combine these observations in the most effective way.

View full abstract

Following 198 years of dormancy, an eruption started at Mt. Fugen, the main peak of Unzen volcano, in Kyushu, Japan, in November 1990. A dacite lava dome began to grow in May 1991. We installed the surveying points of GPS in 1992 around the lava dome in order to observe the ground deformation that accompanied the growth of the lava dome. In the winters of 1993 and 1994, we observed swift ground deformations that radiated from the vent of the volcano. It was presumed that rising magma accumulated and expanded the volcano body. After the lava effusion stopped in 1995, we also installed surveying points on the lava dome. EDM mirrors were permanently fixed to the large rocks with bolts. A GPS survey was carried out 2 or 3 times each year to estimate the 3-dimensional displacement. The result of the EDM survey showed that the baselines from the flank of the volcano were shortening 5 mm per day, and the result of the GPS survey showed that the displacement vector of the dome was parallel to the direction of the steepest slope of the old volcano body. This indicates that the inside of the lava dome is still very hot, and that deformation of the dome is viscous.

View full abstract

To investigate the precise crustal deformation in the Taisetsu mountains, Central Hokkaido, Japan, we have conducted GPS survey in 1998 and 1999. The coordinates at summit peaks of the mountain chain, horizontal displacements and strain are evaluated in this period. Major results are summarized as (1) horizontal displacements at the summit peaks are about 2 cm in the direction of north, (2) horizontal strain is estimated to be compressional in the NW-SE direction. Although the precision of the estimates awaits for the future survey, this results are consistent with the direction of the compressional axes of focal mechanism solutions of local earthquakes in the surrounding region and with the deformation vector by the triangulation in central Hokkaido.

View full abstract

Assessment of Antarctic rebound is complicated by two issues: (1) The total ice volume at Last Glacial Maximum is contentious, with estimates ranging from just a few meters to several tens of meters of equivalent eustatic sea level rise. (2) The late-Holocene mass budget is also uncertain. Space-based geodesy may provide important data in the coming years for estimating the recent ice mass balance state of Antarctica. Toward this end, GPS has an important role for isolating the solid earth movements that are associated with postglacial rebound. Here we provide numerical examples of vertical motions that are predicted by coupling realistic glacial load histories to 20th century ice mass imbalance estimates for the Antarctic Peninsula. The main complexity revealed by these examples is the striking difference among predictions that have an oscillatory mass change during the last 5000 to 50 years, as opposed to those having a continuous (non-oscillatory) mass drawdown of the grounded ice sheet.

View full abstract

Between 1988 and 1995, five seasons of Global Positioning System (GPS) data were collected at seventy-three locations near the 2500 metre contour of the Lambert Glacier Drainage Basin (LGB). These data have been processed using GAMIT/GlobK software to determine surface velocities at the surveyed sites. Results show that velocities along the traverse route vary between 0.5 ma-1 and 63 ma-1 with the location of the major outlet glaciers inside the LGB clearly identifiable within the data. A subset of the LGB GPS data has been studied to investigate an efficient method for determination of ice velocities at remote sites within a single field season. Results from this study have shown that ice velocities within 2% of the long-term estimate may be determined using two to four hour GPS site occupations separated in time by a minimum of forty days.

View full abstract

A GPS network has been installed to monitor vertical crustal movement in the Lambert Glacier region, East Antarctica. The program commenced in January 1998 with a solar-powered GPS system installed at Beaver Lake. Solar-powered observations were also made late in the Antarctic summer of 1999. In January 2000, two new solarpowered sites will be installed to expand the monitoring network. In addition, we will be installing a hydrogen fuel cell power system at Beaver Lake to enable the equipment to operate throughout the winter months when solar power is not available. In this paper we outline the equipment which has been developed in order to operate remote GPS equipment in Antarctica, provide predictions of the expected rate of rebound and comment on preliminary results from the data collected to date.

View full abstract

Knowledge of the surface velocity and strain of ice shelves is important in determining their present kinematic state and detecting any change in that state. Data collected using the Global Positioning System (GPS) often plays an important role in determing these parameters, either directly, or as ground-truthing to other techniques such as InSAR. The processing of GPS data on floating ice shelves is complicated by the presence of a distinct vertical tidal signal and large horizontal motions in the data. Over a one hour period, vertical and horizontal movements can be as much as 0.3 metres and 0.1 metres respectively. For such GPS data to be processed using conventional static methods would require the observation period to be split into small (-1 hour) segments, and the segments processed separately. Other processing options may include kinematic processing or sequential processing, although these techniques have their own drawbacks. Instead, we have developed software to remove signals based on a priori knowledge of the ice shelf motion. The tidal signal is removed using a local tide model and the horizontal velocity effect is corrected to a specific time epoch. This allows us to process our GPS data in a tide-free, velocity-free environment for a given day using conventional GPS processing software. The corrected GPS data, now largely free from the effects of ice shelf motion, may then be combined to produce high precision velocity and strain rate models of the ice shelf.

View full abstract

Repeated GPS measurements have been performed at the centre of the Greenland Ice Sheet since 1992. Results have shown that the ice sheet is essentially stable at this location, with GPS-determined strain and elevation change rates in good accordance with yearly snow accumulation and glaciological flow models. In a local ice cap in East Greenland (Geikie Plateau) repeated GPS, airborne laser altimetry and SAR interferometry have been used to study ice movements in the more climatically variable coastal zone, where meter-level annual elevation changes are possible due to the high precipitation. The paper outlines the surveys carried out, some preliminary results, and intercomparisons of GPS, laser altimetry and SAR techniques.

View full abstract

The paper outlines a method allowing to compute the TEC with a precision of about 2-3 TECU and to detect Travelling Ionospheric Disturbances using GPS measurements. We describe the solar cycle dependance of the TEC and TIDs. Since the beginning of 1998, we have observed a stronger ionospheric activity due to the increasing solar activity. This ionospheric activity is characterized by larger TEC values which are regularly reaching the level of 60 TECU and by a larger number of Travelling Ionospheric Disturbances. During the winter 1999-2000, the mean daily TEC was above 45 TECU; at solar minimum the mean daily TEC is ranging from 4 TECU to 12 TECU. In January 2000 (close to solar maximum) more than 1300 events due to TID's were detected: it is 6.5 more than in January 1996 (at solar minimum).

View full abstract

In this paper we investigate the form and dynamics of shock acoustic waves (SAW) generated during the rocket Proton launching from the Baikonur cosmodrome in 1998 and 1999. In spite of the difference of geophysical conditions, the ionospheric response for all launchings has a period of about 300 s and the amplitude exceeding background fluctuations under quiet and moderate geomagnetic conditions by factors of 2 to 5 as a minimum. The angle of elevation of the SAW wave vector varies from 45° to 60°, and the SAW phase velocity (900-1200 m/s) approaches the sound velocity at heights of the ionospheric F region maximum. The position of the SAW source, inferred by neglecting refraction corrections, corresponds to the segment of the rockets path at a distance no less than 700-900 km from the launch pad, which is consistent with the estimated delay time of SAW source triggering (250-300 s).

View full abstract

During the current period of solar maximum, there is concern within the GPS community regarding GPS receiver performance during periods of intense geomagnetic substorms. Such storms are common in the high latitude auroral region, and are associated with small-scale scintillation effects, which can cause receiver tracking errors and loss of phase lock. The auroral oval can extend many degrees equatorward under active ionospheric conditions, with an impact on precise positioning applications in Canada, the United States and Northern Europe. In this paper, a study of receiver tracking performance is conducted during periods of auroral substorm activity. Dual frequency observations are obtained using codeless and semicodeless GPS receivers (Trimble 4000SSi, NovAtel MiLLennium and Ashtech Z-12), and performance comparisons are established and interpreted with respect to GPS availability at solar maximum and the years beyond.

View full abstract

Since the middle of 1996, we have made a routine observation of TEC using GPS in Chofu (35.65°N, 139.54°E), Tokyo. We examine in this paper the relationship between the perturbation components of TEC and geomagnetic field variations during 11 storm events for 1997-1998 to clarify the mid-latitude characteristics of TEC variations at the time of magnetic storms. The perturbation components of TEC were derived every 20 seconds by subtracting quiet-time TEC values which are estimated as the average of both 3 days just before and after the storm period. The magnetic field data from Memambetsu Magnetic Observatory (45.92°N, 144.20°E) are used for the identification of the storm-time variations. Our results show that (1) the amplitude level of the TEC variation tends to increase during the first 24 hr of storm and then decrease below its usual-day level with recovery in one or two days later for the typical magnetic storm, and (2) during a negative-value phase of the TEC variation, which follows the initial positive hump structure, the perturbation amplitude of TEC shows a remarkable reduction in summer compared to in winter.

View full abstract

We developed a new technology for global detection of ionospheric disturbances, on the basis of phase measurements of the total electron content (TEC) along the line-of-sight (LOS) between the receiver on the ground and transmitters on the GPS satellites using an international GPS networks. Temporal dependencies of TEC with the time resolution of 30 s are obtained for a set of spaced receivers of the GPS network simultaneously for the entire set of visible satellites. These series are subjected to filtering in the selected range of oscillation periods using known algorithms for spatio-temporal analysis of signals. An analysis is made of the possibilities of using the GLOBDET when detecting the ionospheric response of solar flares. In this case it is best to make the coherent summation of the filtered series of TEC. Because of a statistical independence of the background fluctuations, the signal/noise ratio, when the flare effect is detected, is increased due to a coherent processing by at least the √N times, where N is the number of LOS.

View full abstract

In the absence of the selective availability, which was turned off on May 1, 2000, the ionosphere can be the largest source of error in GPS positioning and navigation. Its effects on GPS observable cause a code delays and phase advances. The magnitude of this error is affected by the local time of the day, season, solar cycle, geographical location of the receiver and Earth's magnetic field. As it is well known, the ionosphere is the main drawback for high accuracy positioning, when using single frequency receivers, either for point positioning or relative positioning of medium and long baselines. The ionosphere effects were investigated in the determination of point positioning and relative positioning using single frequency data. A model represented by a Fourier series type was implemented and the parameters were estimated from data collected at the active stations of RBMC (Brazilian Network for Continuous Monitoring of GPS satellites). The data input were the pseudorange observables filtered by the carrier phase. Quality control was implemented in order to analyse the adjustment and to validate the significance of the estimated parameters. Experiments were carried out in the equatorial region, using data collected from dual frequency receivers. In order to validate the model, the estimated values were compared with “ground truth”. For point and relative positioning of baselines of approximately 100 km, the values of the discrepancies indicated an error reduction better than 80% and 50% respectively, compared to the processing without the ionospheric model. These results give an indication that more research has to be done in order to provide support to the L1 GPS users in the Equatorial region.

View full abstract

Following a scheme developed for a subduction zone by Savage in 1983, which was successfully applied to a transform plate boundary by Matsu'ura and others in 1986, a dislocation model for a plate collision zone is formulated. The solution consists of a rigid plate motion and a tensile dislocation. In addition to this, a strike-slip dislocation is needed when the collision boundary is not perpendicular to the plate converging direction. Theoretically predicted gradual change in horizontal displacements over the collision zone well explains the results of GPS continuous observation in central Japan where the Eurasian and Okhotsk plates are thought to be colliding. The maximum uplift rate is predicted as 1/π times that of converging velocity, however the observed uplift rate is much smaller than that, although vertical movements observed by GPS network is much less accurate than horizontal movements. A comparison of the theoretical results with the observation suggests relatively thin elastic plates, whose thickness is about 30 km. The obtained dislocation model has a close connection with a horizontal detachment fault. The displacement fields above the advancing and retreating edges of a horizontal rectangular detachment fault are mathematically equivalent to those of collision and rift zones, respectively, and lateral edges to a transform fault boundary. The displacement fields at a colliding and a rifting boundaries are the same except for their signs.

View full abstract

We interpret the continuous GPS data for the Japanese islands from 1996 to 1999 using the model of Hashimoto and Jackson (1993). In this model crustal deformation is represented by a combination of rigid block motions and deformation due to slip deficits along the fault-bounding blocks. Hashimoto and Jackson used 19 blocks, 104 faults, and geodetic data spanning 100 years. In the present work we assume the same fault and block geometry, and we use only the continuous GPS data. Compared to the previous study, the motions of the major blocks are a bit larger: 107 ± 8 mm/yr for the Pacific plate, 47 ± 2 mm/yr for the Philippine Sea plate, and 24 ± 2 mm/yr for the Izu block, all relative to the Amurian plate. The estimated slip directions on active inland faults are now more consistent with geological estimates. Slip deficit rates exceed 10 mm/yr along the Itoigawa-Shizuoka Tectonic Line, Shinanogawa Seismic Zone, Atotsugawa fault, Hanaori fault, Arima-Takatsuki Tectonic Line, Rokko faults, Median Tectonic Line, and southern boundary of the Beppu-Shimabara graben. Some interplate faults along the Japan and Kurile trenches have slip deficit rates larger than 100 mm/yr, although postseismic deformation from the 1994 Far Off Sanriku earthquake may contaminate these estimates. An interplate fault off Kyushu has a negative slip deficit, possibly due to post-seismic movements from the 1996 events in Hyuganada.

View full abstract

The applicability of a new inversion method to the Japanese Islands is examined. This method can compute a self-equilibrating stress increment from a strain increment, and the validity of the method is verified for metal-likematerials. Some modifications will be needed in applying the new inversion method to the Japanese Islands when a strain increment measured by the GPS array is used as input data. In this paper, we try to compute the stress increment associated with a measured displacement increment. It is shown that the inversion works and the stress increment is computed. The validity of the results, however, cannot be verified right now. Some information on regional constitutive relations is obtained from the measured strain increment and the predicted stress increment. We discuss the applicability of the inversion method, and clarify modifications that are needed for more reliable prediction.

View full abstract

To estimate the elastic and inelastic strains in the Japanese Islands, we used horizontal velocity vectors at 917 sites of the nation-wide GPS network for the period April 1996 to March 1998. To segregate the signal and noise in the velocity vectors, we employed the Least-Squares Prediction technique (LSP). Estimated signals (displacement vectors) were differentiated in space to reconstruct the total strains (elastic plus inelastic). Then, we estimated inelastic strains of the islands using the inversion method introduced by Hori et al. (1999). The estimated rate of inelastic strains is of the order of 10^-3 μstrains/yr. Compared this with the rate of total strains of the order of 0.1μstrains/yr, inelastic part was found to be negligible. Finally, shear stresses at the surface of the Japanese Islands have been estimated, based on the elasticity theory.

View full abstract

GPS observables taken from light-weight GPS buoys more than 80 km from the GPS reference stations have been analysed using different noise models in the parameter estimation process. The time series solution of the GPS buoy positioning have been used to extract values of oceanic variables (sea level, tides and waves) and tropospheric information. These variables are compared with data from models and with measurements from a meteorological buoy.

View full abstract