G33A-0887
Feasibility of gravity field recovery from GRACE line-of-sight (LOS) gradiometry observations using the torus approach
The dedicated satellite gravity field mission \textsc{Grace} employs the concept of satellite-to-satellite tracking in low-low mode (\textsc{sst}-ll). The ratio between the precise inter-satellite K-band range and the range acceleration (ρ/\ddot{ρ}) can be treated approximately as a line-of-sight (\textsc{los}) gradiometry observable. Theoretically, this observable has a similar pattern as the gravity gradient tensor along-track component Vxx. The \textsc{Grace} \textsc{los} gradiometry data from both real and simulated range and range acceleration observations are processed by the torus-based semi-analytical approach to recover the gravity field. The torus-based approach shows its efficiency in gravity field determination in terms of the time and storage requirements and its flexibility of dealing with any geopotential functional. Our preliminary results show that the estimated spherical harmonic coefficients are not accurate enough compared to a reference model. The major reason would be that the ≈ 220 km baseline of \textsc{Grace} does not fulfill the \textsc{los} gradiometry assumption of a sufficiently small baseline. Another possible reason is the additional error from interpolation, which can be improved by iteration.
G33A-0888
Hydrological Modeling of Groundwater Disturbance to Gravity Signal for High-accuracy Monitoring of Volcanic Activity
Gravity observation is one of the effective methods to detect magma movements in volcanic eruptions [e.g., Furuya et al., J. Geoph. Res., 2003]. Groundwater-derived disturbances have to be corrected from gravity variations for highly accurate monitoring of volcanic activities. They have been corrected with empirical methods, such as tank models and regression curves [e.g., Imanishi et al., J. Geodyn., 2006]. These methods, however, are not based on hydrological background, and are very likely to eliminate volcanic signals excessively. The correction method of groundwater disturbance has to be developed with hydrological and quantitative approach. We thus estimate the gravity disturbance arising from groundwater as follows. (1) Groundwater distributions are simulated on a hydrological model, utilizing groundwater flow equations. (2) Groundwater-derived gravity value is estimated for each instant of time, by integrating groundwater distributions spatially. (3) The groundwater-derived gravity, as the correction value, is subtracted from observed gravity data. In this study, we simulated groundwater flow and groundwater-derived gravity value on the east part of the Asama volcano, central Japan. A simple hydrological model was supposed, consisting of homogeneous soil, lying on a flat impermeable basement. Hydraulic conductivity, which defines groundwater velocity, was set as 2.0×10-6[m/s], which is consistent with typical volcanic soils. We also observed time variations of watertable height, soil moisture and gravity simultaneously during the summer of 2006 at Asama volcano, and compared the observations with the theoretical values. Both simulated groundwater distributions and gravity changes agree fairly well with observed values. On variations of water level and moisture content, rapid increase at the time of rainfalls and exponential decrease after rainfalls were illustrated. Theoretical gravity changes explained 90% of the observed gravity increase (+20μgals) for the heavy rainfall (200mm) of mid-July 2006. These facts showed that even a simple hydrological model can reproduce characteristic variations of groundwater and gravity at the same time. We believe that hydrological simulation with more sophisticated model (such as 3D inhomogeneous soil lying on a curved basement) will enable us to estimate groundwater disturbance more accurately. Improved groundwater correction will reveal detailed magma movements in volcanic eruptions.
G33A-0889
Modeling Hydrological Gravity Effect Using Stochastically Characterized Groundwater Flow Equations - A Case Study for Taiwan's Hsinchu Superconducting Gravimeter Site) * Cheng, C (chengstark@faculty.nctu.edu.tw) Hwang, C (cheinway@mail.nctu.edu.tw)
Hydrological effect on gravity data is one of the most important corrections for measurements by a superconducting gravimeter. In pursuing a proper local-scale hydrogeology model referred to a gravimeter site, the accurate determination of water table in the vicinity certainly is an important issue since it not only serves as the basis to calculate the gravitational effect from mass beneath, but also provide a boundary condition to assess the water absorbed in the aeration above. This study elaborates on the groundwater flow equation which involves the piezometric head, transmissivity of media in aquifer, and the flow. The former two are characterized as stochastic field indexed by 2-D planar coordinates, and can be expressed in the form of either forward problem or inverse problem ¡Vbased on what the boundary conditions are offered. The piezometric head observations and meteorology data provided by various institutes are supplied as boundary conditions. The nationwide groundwater monitoring program provides plenty of head data. As far as the transimissivity is concerned, a nationwide rough database maintained by the Water Resource Agency of Taiwan is applied as the approximation for transimissivity. The solution of the stochastic flow equation takes the form of co-Kriging predictors where the variogram and co-variogram come from careful structure analysis of transimissivity and piezometric head. Alternatively, extra piezometric head data and gravity variation from superconducting gravimeter can be served to validate the outcome of reconstructed water table model.
G33A-0890 [WITHDRAWN]
Sea-Level Variations Due To Current Changes In The Cryosphere Observed By GRACE
Currently huge ice mass loss over the Polar Regions is observed by the Gravity Recovery and Climate Experiment (GRACE) satellite mission considerably contributing to global sea level change. However, mostly due to the self- gravitational and elastic feedback effect global sea level change is not uniform but varies depending on location, most notably over the Polar Regions. While GRACE is able to monitor significant ice mass changes over the cryosphere, the rather small gravity signal of global sea change induced by ice melting is hard to detect and highly obstructed by continental leakage. Forward gravity modelling of the detected ice mass changes can help to reveal the spatial sea level change. Here we use ice mass changes over Greenland and Antarctic ice sheets and the Alaskan glaciers obtained from analysing five years of GRACE temporal gravity observations (release four) to model global sea level change induced by ice melting. We present GRACE-derived ice mass changes as well as forward modelled global sea level change patterns.
G33A-0891
GOCE gradiometer: estimation of bias and scale factors of all 6 individual accelerometers by precise orbit determination
The GOCE gradiometer ideally consists of an orthogonal triad of three pairs of accelerometers. A method has been developed and implemented for the estimation of absolute calibration parameters, such as accelerometer biases, bias drifts and scale factors by precise orbit determination. The method was applied to a test data set of simulated GOCE observations. Concerning the accelerometers, the following error sources were taken into account: frequency dependent observation noise, misalignments of the axes of the individual accelerometers, quadratic terms, non-orthogonalities, biases and bias drifts, and scale factor errors. The orientation of the accelerometers is derived from star tracker observations, that were affected by realistic errors as well. It was found that the observations taken by individual accelerometers needed to be corrected for local satellite gravity gradient and rotational terms (i.e. caused by centrifugal and angular accelerations) due to their offset location with respect to the satellite center of mass. These corrections were found to be sufficiently adequate when using a simple reference gravity field model and derive the rotational terms from the star tracker observations. However, for an accurate determination of the accelerometer scale factors is was found that a high-precision gravity field is required in the orbit determination itself or that gravity field coefficients need to be co-estimated (making the procedure computationally demanding, but still feasible). It was found that with a high-quality a priori gravity field model and with the co-estimation of not only biases but also bias drifts, scale factors can be determined with an accuracy better than 0.01 for two of the three axes of each accelerometer, the exception being the axis pointing along the long axis of the satellite (more or less coinciding with the flight direction). This axis coincides with the axis of drag-free control, which results in a small variance of the signal to be calibrated and thus an inaccurate determination of its scale factor in the presence of relatively large (colored) accelerometer observation errors.
G33A-0892
Is the Earth Expanding ?
Whether the Earth is expanding or contracting is an interesting problem in geoscience. The information of the secular change of the Earth's gravity field supports the conclusion that the Earth is expanding at least in recent years. The gravitational potential could be expressed as a spherical harmonic series outside the Earth. In practical applications, it could be approximately realized by a truncated series, e.g., EGM96 (the Earth Gravity Model 96), EIGEN-GL04C, etc. The principal moments of inertia of the Earth are related to the second-order coeffients of the spherical harmonic series. Based on EGM96 as well as EIGEN-GL04C, the principal moments of inertia and especially their temporal variations are determined. All the three principal moments of inertia are gradually increasing at almost the same rate in recent 10 years, which might be due to the following three causes: 1) the rise of the sea level; 2) the increase of the total mass of the Earth; 3) the expansion of the Earth in the general sense. Calculations show that the rise of the sea level has too week influence on the variation of the principal moments of inertia comparing with the actual observations, and consequently the cause 1) could be given up. As to the increase of the total mass of the Earth, there does not exist any definite evidence, especially, it has not been found that the geocentric constant GM (where G and M are gravitational constant and the total mass of the Earth respectively) varies with time by various satellite approaches. Hence, it is reasonable to assume that the increase of the principal moments of inertia is caused by the expansion of the interior of the Earth or at least the whole mantle. This clearly demonstrates that the Earth is expanding at least in recent 10 years. Preliminary numerical calculations show that the Earth is expanding at the rate around 0.6 mm/yr at least in recent 10 years. Taking into account the Dirac's large number postulate, it could be concluded that the Earth expansion has and will have happened continuously in a long geological history.
G33A-0893
Gravity Field Parameter Estimation Using QR Factorization
This study compares the accuracy of the estimated geopotential coefficients when QR factorization is used instead of the classical method applied at our institute, namely the generation of normal equations that are solved by means of Cholesky decomposition. The objective is to evaluate the gain in numerical precision, which is obtained at considerable extra cost in terms of computer resources. Therefore, a significant increase in precision must be realized in order to justify the additional cost. Numerical simulations were done in order to examine the performance of both solution methods. Reference gravity gradients were simulated, using the EIGEN-GL04C gravity field model to degree and order 300, every 3 seconds along a near-circular, polar orbit at 250 km altitude. The simulation spanned a total of 60 days. A polar orbit was selected in this simulation in order to avoid the ‘polar gap' problem, which causes inaccurate estimation of the low-order spherical harmonic coefficients. Regularization is required in that case (e.g., the GOCE mission), which is not the subject of the present study. The simulated gravity gradients, to which white noise was added, were then processed with the GINS software package, applying EIGEN-CG03 as the background gravity field model, followed either by the usual normal equation computation or using the QR approach for incremental linear least squares. The accuracy assessment of the gravity field recovery consists in computing the median error degree-variance spectra, accumulated geoid errors, geoid errors due to individual coefficients, and geoid errors calculated on a global grid. The performance, in terms of memory usage, required disk space, and CPU time, of the QR versus the normal equation approach is also evaluated.
G33A-0894
In-flight GOCE Gradiometer Calibration And Validation
One of the key observables of the European Space Agency's Gravity field and steady-state Ocean Circulation Explorer (GOCE) Mission will be the gradiometer measurements. The GOCE gradiometer consists of three pairs of identical ultra-sensitive accelerometers, mounted on three mutually orthogonal arms. To meet the mission goals - determine the Earth's gravity field and its anomalies with an accuracy exceeding 1 mGal, and the global geoid with an accuracy of 1-2 cm at a spatial resolution of 100 km - the calibration and validation of the gradiometer is of utmost importance. The instrument will undergo a series of calibration procedures before launch. In flight, the gradiometer will be calibrated prior to the scientific measurement phases by employing a unique satellite operating mode comprising satellite and proof mass shaking and calibration techniques. Ultimately, the gradiometric measurements are externally calibrated using external gravity information over well- surveyed areas and global gravity field models. In this paper, the calibration schemes and methods are described in detail. It highlights the benefits of using a series of calibration and validation steps. Specific focus will be put on the description of the in-flight calibration and validation part and numerical results using data from the GOCE End-to-End simulator will be presented. The paper concludes with an outlook and a discussion on possible future developments.
G33A-0895
Analysis of the Characteristics of the GRACE Dual One-Way Ranging System
The GRACE (Gravity Recovery and Climate Experiment) is a dedicated space-borne mission to map the Earth's gravity field with unprecedented accuracy. The primary measurement is the range change between the two satellites, which reflects the gravity perturbation difference between the two locations. The range change is measured by a high-accuracy microwave ranging system based on the Dual One-Way Ranging (DOWR) concept. In this paper, we analyze the characteristics of the DOWR and develop possible improvements to its computation from the raw observables. In particular, we investigate the high frequency noise that appears to exceed what we would expect from the design specifications. Possible causes are a higher than expected frequency instability in the ultra-stable oscillator, or undesirable effects from other instrument components or external environmental influences. Based on an improved understanding of the instrument characteristics, we investigate possible improvements to the DOWR processing, especially addressing the attenuation of the excessive high frequency noise in the data.
G33A-0896
GRACE: a Couple of Geodetic Satellites for Ocean Tide Determination ?
The GRACE mission has proved its pertinence for monitoring time variations of surface masses. Five years of geoid models from CNES/GRGS are now available at 10-day intervals on the BGI web site. They are delivered in terms of geoid and of equivalent water mass variation as well at a realistic spatial scale of 500 km and are used in the new global gravity models named EIGEN-5S and -5C produced in cooperation with GFZ. But if gravitational variations over 10 to 30 days can be relatively well modelled from GRACE GPS and K-band range-rate (inter-satellite) data, a new question is raised: can GRACE help improving long wavelength ocean tides models ? A positive answer would also help reducing aliasing effects already detected in geoid models. We have performed new computation of GRACE data over the year 2006, using a time variable a priori gravity model and adjusting along orbit and instrumental parameters some ocean tide spherical harmonic coefficients of main semi-diurnal and diurnal waves from the a priori FES2004 model. This GRACE ocean tide solution has been evaluated particularly in conjunction with altimeter crossover data from Jason1 and ENVISAT missions. Context, method and results are presented on poster.
G33A-0897
Changes in accuracy of gravity recovery due to ground track variability: GRACE, CHAMP, and GOCE Changes in accuracy of gravity recovery due to ground track variability: GRACE, CHAMP, and GOCE
Following an earlier recognition of degraded monthly geopotential recovery from GRACE (Gravity Recovery And Climate Experiment) due to prolonged passage through a short repeat (low order resonant) orbit, we extend these insights to the whole of this and other current and future continuous tracking missions, in particular to CHAMP (CHAllenging Minisatellite Payload) and GOCE (Gravity field and steady state Ocean Circulation Explorer). The extension from geopotential results for exact repeat missions to the general case for non-repeat free-falling orbits is made through the proxy of the satellite ground track density, in particular its maximum spacing in longitude over a given period of time. We illustrate wide track-density variations over time for these orbits in both latitude and longitude and estimate geopotential recovery will be as widely affected as well within these regimes; lesser density of the tracks leading to poorer recoveries. We have used recent models of atmospheric density to estimate minimum and maximum drag and the orbit decrease of GRACE and provide a warning of future degraded performance as other low order resonances are encountered. We have also found the variations of the densities of the ground tracks of CHAMP correlate well with quality of geopotential parameters derived from this satellite. Finally, implications for the GOCE orbit are also discussed, with some suggestions for its orbit in the scheduled measuring phases of this mission, to avoid low-order resonances.
G33A-0898
The optimal flight path for airborne gravimetry
The most important cost driver for any airborne gravimetry campaing is the flight time. Therefore, it is important to investigate the possiblies of designing a flight path that will minimize the needed time while at the same time ensuring that the research objectives are met. In our case, the objective is to derive the local geoid from observed gravity values. Unfortunately, only observations along the flight path can be made, leaving areas between the flight lines uncovered. When one tries to fit a surface function (such as spherical harmonics or a set of sines and cosines) to the observations, this function tends to produce spurious oscillations in these particular areas. The advantage of fitting spherical harmonics to gravity data is that once the Stokes coefficients are known, the geoid can be computed directly. To avoid the oscillation problem, one normally uses another approach called Least-Squares Collocation (LSC) to predict the most probable gravity values in the unobserved areas. Afterwards, the gravity values are converted into geoid undulations by applying the Stokes integral. A possible problem with this approach is that the spatial covariance function used could be incorrect for the surveyed area, producing an incorrect geoid. In practise, the accuracy of the obtained geoid is estimated by comparing the result at points where GPS and levelling values are available. Another validation approach is to use different programs to compute the geoid with the same observations and see if their results correspond. For both approaches (fitting of spatial functions to observations and the LSC/Stokes method) we will present a full propagation of errors that will help to predict the accuracy of the geoid before the observations have been made. Taking the Azores as a test case, we will use the instrumental noise observed during the AGMASCO campaign and state the required spacing between the flight paths.
G33A-0899
GOCE Quick-Look Gravity Field Analysis
The goal of the GOCE satellite mission (steady state Gravity field and Ocean Circulation Explorer), which will be launched by ESA (European Space Agency) in spring 2008, is to observe the Earth's gravity field with high resolution and global coverage. It is based on a sensor fusion concept which will combine SST (satellite-to- satellite tracking) measurements and SGG (satellite gravity gradiometry). While SST is more sensitive to long wavelength gravity signal, SGG will be able to observe shorter wavelengths down to 100-80 km. Due to the large number of observations and unknown parameters the analysis is a large computational task and final results will only be available several months after the end of the mission. In order to monitor the data quality during the flight time of the satellite a Quick-Look gravity field analysis is performed regularly on partial data sets with a latency of only a few days. This analysis is based on a semi- analytic approach which uses several simplifying assumptions: 1) a circular orbit with constant orbit height and inclination, 2) a perfect repeat pattern with regular global data coverage and 3) a perfect alignment of the satellite to its flight direction. Recent simulations using actual parameters of the mission show, that none of the 3 assumptions will be met, which will affect the quality of Quick-Look gravity field solutions. A strategy how to deal with these deficiencies will be shown. This includes the use of the so-called torus-approach, which is less vulnerable to an irregular data distribution. Furthermore a strategy to choose the weights between SST and SGG observations using variance component estimation will be shown.
G33A-0900
Investigation of an integrable approximation of the problem of motion of a material point in the gravitational field of a rigid body
This research is focus in the integrable approximation of the gravity potencial of a new model that can be interpreted as a new alternative of solution in the problem of the two centers fixed. The solution of the diferential equations that describe the motion of a particle in a specific gravity field was reduced to the form of quadratures by using the Jacobi method in ellipsoidal coordinates. Circular trayectories of the particle were determined and also an investigation was performed based on the particle stability according to Liapunov theory. Bifurcation flows of Poincare-Chetaev and Smale were also constructed (in the plane of the integration constants) concerning different types of possible orbits according to Alekseev´s theory for the case of reduced systems as a function of the constant of areas and in terms of the energy constant. In this matter the following types of trayectories were investigated: 1 Motion among surfaces of an hyperboloid and an ellipsoid; 2 Motion among surfaces of ellipsoids and the interior of a hyperboloid; 3 Motion (not restricted) in the interior of a hyperboloid; 4 Motion (not restricted) in the interior of a hyperboloid and in the exterior of an ellipsoid; 5 Motion (not restricted) in the exterior of an ellipsoid and in the interior o a hyperboloid.
G33A-0901
Modelling Individual Sources Of Mass Distribution And Transport In The Earth System By Means Of Satellites
It has been generally acknowledged that practically all mass-motions within and on the Earth generate a signature in the gravitational field. Current satellite gravity missions, e.g. GRACE, have improved our understanding of the interactions and dynamics of the various components of the Earth's fluids. Future missions such as GOCE promise to enhance our understanding of the dynamics of these fluids even more. Nonetheless the task of unambiguously unravelling the different sources of mass distribution and transport from the accumulated gravity signal as it is observed remains nontrivial. In order to optimally benefit from the high- resolution (both in time and space) and high-accuracy gravity data coming from possible future gravity missions, a study has been initiated by ESA to investigate the potential for improving our ability to separate the various contributions. In this study, we will investigate the need for improved geophysical modelling, mission design and scenario, and methodology required to 1) to de-alias noise from real signal as well as to 2) separate the contributions from the various sources (e.g. to separate ocean mass variability from atmospheric mass variability). This study is being performed by a consortium of nine European groups, combining expertise in all relevant geophysical fields and in designing satellite gravity missions. The study started early 2007 and will last until the end of 2008. This paper presents the background of the study, and explains the (simulation) approach that will be taken to address the goals of the study. Major issues that play a role for the latter are the mission concepts, the state-of-the-art geophysical modelling, the use of dedicated spatio-temporal sampling to tackle the separability issue and the use of complementary data and models.
G33A-0902
Igneous Rock and Paleogravity
Due to the Earth has evolved for 4.5 billion years since its birth, it is not reasonable to let scientists to believe that gravity field has never changed. Paleogravity, the ancient gravity field, will become a very important parameter to study Earth's evolution, even the evolution of the solar system, if people can find some rational and feasible way to measure paleogravity. A. D. Steward had made great efforts on that and tried to use the temperature of inclusions and geothermal temperature in mantle to estimate the limits of paleogravity. Unfortunately, in his approach, there are no enough parameters of covering materials above the inclusion, such as density and elastic module, therefore, the physics for measuring paleogravity is not solid, and the result from that is not acceptable. So far as I know, has yet a rational and feasible way to find out the paleogravity. In my approach, without any strong assumption, a feasible way has been derived. We drill out a vertical lava core cylinder from a typical ancient igneous rock mass, from the top part of the core cylinder; we cut out one piece of sample sector 0, and cut out two pieces of samples sectors 1 and 2 from the rest parts, from higher position to lower position. All samples must be unbroken and un-decayed. By measuring their densities d0, d1 and d2 and elastic deformation modulus E1 and E2, respectively, we can find the paleogravity. g=A(B+C), where A=1/(dh), B=E2-E1, C=(E1/d1-E2/d2)d0; And where d is the average density from sector 1 to sector 2, h is the distance between sector 1 to sector 2. Considering the influence on densities and elastic deformation modules caused at different layers by not only the paleogravity, but also the different composition of sectors at different layers£¬we must take a composition normalization on the ratios E2/E1 and d2/d1, by mass spectrum or melting method.
G33A-0903
VLBI Measurements for Time Transfer between Time and Frequency Laboratories
In the usual geodetic VLBI analysis, clock offsets and their rates of change at participating stations except for the reference station are estimated. The averaged formal error (1σ) of the clock offsets is typically about 20 picoseconds in the geodetic VLBI experiments regularly conducted by the International VLBI Service for Geodesy and Astrometry (IVS). This accuracy is better than other techniques like GPS time transfer and TWSTFT (Two-way Satellite Time and Frequency Transfer) which are used to maintain Coordinated Universal Time (UTC). It will become possible to use the geodetic VLBI technique for accurate time transfer if we can collocate the VLBI radio telescopes at Time and Frequency laboratories. For this purpose, we started to develop a compact and transportable VLBI system. In this study, to confirm the potential of VLBI time transfer aiming at the practical use of VLBI time transfer in the future, we compared the results of VLBI time transfer and the results of GPS time transfer (Carrier Phase) by using Kashima-Koganei baseline (109 km). The averaged formal error (1σ) of the clock offsets when they are estimated every one hour was 29 picoseconds. The results of VLBI time transfer were consistent with the results of GPS time transfer. The difference of both results was about ±500 picoseconds and it is considered to be dominated by the uncertainty of the GPS time transfer. In terms of frequency stability, the Allan deviation was evaluated and it showed that VLBI time transfer is more stable than GPS time transfer in the time range from 2000 seconds to 60000 seconds. Based on these results, we will discuss about the possible improvements to the time transfer between Time and Frequency laboratories by collocating the compact VLBI system at the laboratories.
G33A-0904
VLBI-simulations for the estimation of degree-three Love and Shida numbers h3 and l3
For the displacement due to solid Earth tides, the IERS Conventions 2003 recommend several corrections to nominal values. One of these corrections is the in-phase contribution by using the real Love and Shida numbers h3 and l3 at all degree-3 tides, where only the contribution of the moon is relevant. The maximum predicted radial displacement is in the order of 1.7 mm. The nominal values are 0.292 for h3, and 0.015 for l3, respectively. Using realistic station and source catalogues, we simulated VLBI (Very Long Baseline Interferometry) group time delays, with a white noise going up to 2 cm, and taking into account the solid Earth tides displacement. Goal of this study was to investigate whether degree-3 Love and Shida numbers can be unambiguously determined from VLBI observations. Therefore, several setups w.r.t., e.g., station constellation, cutoff angle, time span, sampling interval, and different levels of white noise were tested. Attention was put into the separability and correlation between the degree-2 and degree-3 Love and Shida numbers.
G33A-0905
Precise GPS baseline solutions for the tandem satellites of FORMOSAT- 3/COSMIC
In the early phase of the FORMOSAT-/COSMIC (FC) mission, spacecrafts FM3 and FM4 will fly in tandem for at least 13 months at an altitude of 525 km. The separation between FM3 and FM4 is about 250 km, similar to that of the GEACE-A and B satellites. We experiment with both float and fixed ambiguity solutions using doubled differences of phases to solve for the baseline vectors of FM3 and FM4. The accuracy of baseline vectors from such solutions is better than 1 cm, and outperforms the 2-cm kinematic orbits of FORMOSAT-/COSMIC, the later being based on zero differenced phases. Due to cancellation of common-mode perturbing forces, use of baseline vectors of FM3-FM4 will improve gravity harmonic coefficients at degrees below 40 over current existing gravity models. This paper will present the technique of our baseline solutions, accuracy assessments and prospects of gravimetric applications of FM3-FM4 baseline vectors.
G33A-0906
PBO Facility Construction: Basin and Range and Rocky Mountain Regions Status
The Plate Boundary Observatory (PBO), part of the larger NSF-funded EarthScope project, will study the three- dimensional strain field resulting from active plate boundary deformation across the western United States. PBO is a large construction project involving the reconnaissance, permitting, installation, documentation, and maintenance of 875 permanent GPS stations in five years. 163 of these stations lie within the Basin and Range and Rocky Mountain Regions consisting of the states of Montana, Idaho, Nevada, Utah, Wyoming, Colorado, New Mexico, and Arizona. During the fourth year of the project, the Basin and Range and Rocky Mountain regions of PBO completed reconnaissance and nearly all permitting activities, and maintained a fast pace of station installations. The fall of 2006 and spring of 2007 were devoted to the construction of a large push of 50 stations, most located on Bureau of Land Management controlled public lands in Nevada. This transect is located along Highway 50 and will profile the extension of the Basin and Range province. The Yellowstone area, including surrounding National Parks and Forests was the target of summer 2007, during which time 10 remote stations with difficult logistics were installed. To date, construction is complete for 135 of 163 GPS stations.
G33A-0907
GPS Installation Progress in the Northern California Region of the Plate Boundary Observatory
The Plate Boundary Observatory (PBO) is the geodetic component of the NSF funded EarthScope Project. The final PBO GPS network will comprise 875 continuously operating GPS stations installed throughout the Western US and Alaska. There are 449 Stations planned for California with 232 of these in Northern California (NCA). This poster will present a progress report and highlights of GPS installations in NCA over the past year. At the end of the third year of the project (10/2007), PBO NCA installed 131 GPS stations. In the fourth year of the project we installed 64 stations for a total of 195 stations. This total comprises 80% of the along the active transform margin, 70% of the sites on volcanoes and calderas and 67% of the sites covering the extensional regime of the Basin and Range. Highlights from this year include completing reconnaissance and permitting of GPS networks around the volcanoes at Mount Shasta and Mount Lassen. Six of the eight GPS stations planned for Mount Shasta were built and at varying azimuths and distances from the crater. Mount Lassen has 9 GPS stations, 6 stations in the Lassen Volcanic National Park and 3 more on surrounding National Forest Service property. Data from these stations are available from the UNAVCO archive. We expect to complete the Northern California part of the PBO GPS network by the end of Sept 2008. In order to accomplish this goal, we are working to finalize reconnaissance and permitting activities for sites whose initial permits were rejected. There will likely be a handful of sites that we will have to relocate from their originally proposed locations in order to get a permitted location.
G33A-0908
Update on Plate Boundary Observatory (PBO) Activities in the PNW Region
The Plate Boundary Observatory (PBO), which is part of the larger NSF-funded EarthScope project, is nearing the end of year 3 of the installation phase of 852 continuously operating GPS stations in the Western United States. The Pacific Northwest (PNW) region will install 134 continuous GPS stations by the end of September 2008. The sites are distributed along the fore and back-arc of the Cascadia Subduction Zone and at Mt. St. Helens. At the end of September 2007, the PNW region will be several stations short of its installation goal of 110 GPS stations, mostly due to an unusually early and high danger wildfire season. The scientific priority during this past year was to concentrate installations in the Oregon back arc region, the Southwest Oregon fore arc region and the Idaho panhandle. In the last year UNAVCO has added 10 stations to the Pacific Northwest region, raising the number of stations from 124 to 134. The majority of these stations are located within the fore and back arc regions of Southern Oregon. In addition the UNAVCO installed its first building mounted site within a difficult area along the Southwest Oregon coast. UNAVCO will install its remaining 24 new continuous GPS stations in the Pacific Northwest in year 5. The remaining stations are distributed throughout the region, and comprise a mix of standard monuments, and strainmeter collocations. Our goal is to have all stations installed by August 31 2008. Reconnaissance work for all of the GPS sites have been completed, and have had permits submitted.
G33A-0909
Support of EarthScope GPS Campaigns at the UNAVCO Facility
In order to support portable GPS deployments funded by the NSF's EarthScope Science panel, PBO has purchased 100 campaign GPS systems. Based Topcon GB-1000 equipment, the systems have been designed for stand-alone temporary or semi-permanent deployment that will be used for densifying areas not sufficiently covered by continuous GPS, and responding to volcanic and tectonic crises. UNAVCO provides support for all aspects of these projects, including proposal and budget development, project planning, equipment design, field support, and data archiving. Ten of the 100 systems will be purchased with real-time kinematic (RTK) capability requiring additional radio and data logging equipment. RTK systems can be used to rapidly map fault traces and profile fault escarpments and collect precise position information for GIS based geologic mapping. Each portable self-contained campaign systems include 18 Ah batteries, a regulated 32 watt solar charging system, and a low-power dual frequency GPS receiver and antenna in a waterproof case with security enhancements. The receivers have redundant memory sufficient for storing over a year's worth of data as well as IP and serial communications capabilities for longer-term deployments. Monumentation options are determined on a project-by-project basis, with options including Tech2000 masts, low-profile spike mounts, and traditional tripods and optical tribrachs. Drilled-braced monuments or masts can be installed for "semi-permanent" style occupations. The systems have been used to support several projects to date, including the University of Washington's 30- unit deployment to monitor the Episodic Tremor and Slip event in November, 2005 and the ongoing Rio Grande Rift experiment, run by the Universities of Colorado, Utah State, and New Mexico, which has seen the construction of 25 permanent monuments in 2006 and 2007. http://facility.unavco.org/project_support/es/pbo-camp/pbo-camp.html
G33A-0910
PBO Nucleus Project Status: Integration of 209 Existing GPS Stations into the Plate Boundary Observatory
Tectonic and earthquake research in the US has experienced a quiet revolution over the last decade precipitated by the recognition that slow-motion faulting events can both trigger and be triggered by regular earthquakes. Transient motion has now been found in essentially all tectonic environments, and the detection and analysis of such events is the first-order science target of the EarthScope Project. Because of this and a host of other fundamental tectonics questions that can be answered only with long-duration geodetic time series, the incipient 1100-station EarthScope Plate Boundary Observatory (PBO) network has been designed to leverage 445 existing continuous GPS stations whose measurements extend back over a decade. The irreplaceable recording history of these stations will accelerate EarthScope scientific return by providing the highest possible resolution. This resolution will be used to detect and understand transients, to determine the three-dimensional velocity field (particularly vertical motion), and to improve measurement precision by understanding the complex noise sources inherent in GPS. The PBO Nucleus project supports the operation, maintenance and hardware upgrades of a subset of the six western U.S. geodetic networks until they are subsumed by PBO. Uninterrupted data flow from these stations will effectively double the time-series length of PBO over the expected life of EarthScope, and has created, for the first time, a single GPS-based geodetic network in the US. The other existing sites remain in operation under support from non-NSF sources (e.g. the USGS), and EarthScope continues to benefit from their continued operation On the grounds of relevance to EarthScope science goals, geographic distribution and data quality, 209 of the 432 existing stations were selected as the nucleus upon which to build PBO. Conversion of these stations to a PBO-compatible mode of operation was begun under previous funding, and as a result data now flow directly to PBO archives and processing centers while maintenance, operations, and meta-data requirements are continue to be upgraded to PBO standards. At the end of this project all 209 stations will be fully incorporated into PBO, meeting all standards for new PBO construction including data communications and land use permits. Funds for operation of these stations have been included in planned budgets for PBO after the construction phase ends and PBO begins an operational phase in 2008. At this time work on the project is apporixmately 80% complete, with over 90% of the stations having been upgraded. The data from these stations serve a much larger audience than just the few people who work to keep them operating. This project is now collecting the data that will be used by the next generation of solid-earth researchers for at least two decades. Educational modules are being developed by a team of researchers, educators, and curriculum development professionals, and are being disseminated through regional and national workshops. An interactive website provides the newest developments in tectonics research to K-16 classrooms. http://www.unavco.org/exnet/exnet.html
G33A-0911
The EarthScope Plate Boundary Observatory Akutan Alaskan Volcano Tiltmeter Installation
During August of 2007, the Plate Boundary Observatory (PBO) successfully installed four Applied Geomechanics Lily Self Leveling Borehole Tiltmeters on Akutan Volcano, in the central Aleutian islands of Alaska. All four stations were collocated with existing PBO Global Positioning Systems (GPS) stations installed on the volcano in 2005. The tiltmeters will aid researchers in detecting and measuring flank deformation associated with future magmatic intrusions of the volcano. All four of the tiltmeters were installed by PBO field crews with helicopter support provided by JL Aviation and logistical support from the Trident Seafood Corporation, the City of Akutan, and the Akutan Corporation. Lack of roads and drivable trails on the remote volcanic island required that all drilling equipment be transported to each site from the village of Akutan by slinging gear beneath the helicopter and with internal loads. Each tiltmeter hole was drilled to a depth of approximately 30 feet with a portable hydraulic/pneumatic drill rig. The hole was then cased with splined 2.75 inch PVC. The PVC casing was cemented in place with grout and the tiltmeters were installed and packed with fine grain sand to stabilize the tiltmeters inside the casing. The existing PBO NetRS GPS receivers were configured to collect the tiltmeter data through a spare receiver serial port at one sample per minute and 1 hour files. Data from the GPS receivers and tiltmeters is telemetered directly or through a repeater radio to a base station located in the village of Akutan that transmits the data using satellite based communications to connect to the internet and to the UNAVCO Facility data archive where it is made freely available to the public.
G33A-0912
Global GPS Data Analysis at the National Geodetic Survey
NOAA's National Geodetic Survey (NGS) has been one of the Analysis Centers (ACs) of the International GNSS Service (IGS) since its inception in 1994. Solutions for daily GPS orbits and Earth orientation parameters are regularly contributed to the IGS Rapid and Final products, as well as for weekly station positions. These are combined with those of the other ACs and distributed to users. The IGS realization of the International Terrestrial Reference Frame is derived from the time series of combined AC weekly frames. To perform this task, NGS has developed and refined the Program for the Adjustment of GPS EphemerideS (PAGES) software. Although PAGES has continuously evolved over the past 15 years, recent efforts have focused mostly on updating models and procedures to conform more closely to IGS conventions. Particularly significant was the IGS change in November 2006 to absolute antenna calibrations models, including ground and satellites as well as recognizing antenna radomes. Other modifications include updates from the IERS Conventions (mostly for geophysical effects and tropospheric delay) and adoption of the modified CODE orbit model (six position and velocity parameters, midday velocity breaks, and five nuisance radiation pressure parameters). Some processing strategies have also been revised to simplify and strengthen the least-squares adjustment framework, such as applying Delaunay triangulation to construct the global double-differenced baseline network. Elevation-dependent weighting of phase observations is now employed. The net result of all changes has been a major improvement during the past year in the NGS performance, compared to other IGS ACs, for almost all products generated. Details of our processing updates and demonstrations of the improvements will be provided. Issues still requiring attention and NGS plans for future developments will be reviewed.
G33A-0913
Geodesy, a Bibliometric Approach for 2000-2006
In recent years, bibliometric science has been frequently applied in the development and evaluation of scientific research. This work presents a bibliometric analysis for the research work performed in the field of geodesy "science of the measurement and mapping of the earth surface including its external gravity field". The objective of this work is to present a complete overview of the generated research on this field to assemble and study the most important publications occurred during the past seven years. The analysis was performed including the SCOPUS and WEB OF SCIENCE databases for all the geodetic scientific articles published between 2000 and 2006. The search profile was designed considering a strategy to seek for titles and article descriptors using the terms geodesy and geodetic and some other terms associated with the topics: geodetic surfaces, vertical measurements, reference systems and frames, modern space-geodetic techniques and satellite missions. Some preliminary results had been achieved specifically Bradford law of distribution for journals and education institutes, and Lotka's law for authors that also includes the cooperation between countries in terms of writing together scientific articles. In the particular case of distributions, the model suggested by Egghe (2002) was adopted for determining the cores.