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T F Baker - One of the best experts on this subject based on the ideXlab platform.
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Ocean Tide loading displacements in western europe 2 gps observed anelastic dispersion in the asthenosphere
Journal of Geophysical Research, 2015Co-Authors: M S Bos, T F Baker, Nigel T. Penna, P J ClarkeAbstract:GPS-observed vertical Ocean Tide loading displacements show in Cornwall, southwest England, and in Brittany, northwest France, discrepancies of 2–3 mm with predicted values based on the isotropic Preliminary Reference Earth Model for the main tidal harmonic M2, yet in central Europe the agreement is better than 0.5 mm. By comparison of Ocean Tide models and validation with Tide gauge observations, we demonstrate that the uncertainties in the former are too small to cause this disagreement. Furthermore, we find that different local models of the crust and different global elastic reference models derived from seismological observations can only reduce the observed discrepancies to 1–2 mm, which still exceeds the GPS observational uncertainty of 0.2–0.4 mm. It is customary to use the elastic properties of the Earth as given by seismic models. Previously, there has been insufficient evidence to determine how to modify these properties during the transformation from seismic to tidal frequencies to account for possible anelastic dispersion in the asthenosphere, and so this effect has been ignored. If we include this effect, then our discrepancies reduce further to 0.2–0.4 mm. This value is of the same order as the sum of the remaining errors due to uncertainties in the Ocean Tide models and in the GPS observations themselves. This research provides evidence in western Europe of a reduction of around 8–10% of the seismic shear modulus in the asthenosphere at tidal frequencies. In addition, we find that the asthenosphere absorption band frequencies can be represented by a constant quality factor Q.
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Ocean Tide loading displacements in western europe 1 validation of kinematic gps estimates
Journal of Geophysical Research, 2015Co-Authors: Nigel T. Penna, M S Bos, P J Clarke, T F BakerAbstract:GPS has been extensively used to estimate tidal ground displacements, but the accuracy of this has not been systematically verified. Using more than 20 sites distributed across western Europe, we show that postprocessed kinematic precise point positioning GPS with appropriately tuned process noise constraints is capable of recovering synthetic tidal displacements inserted into real data, with a typical accuracy of 0.2 mm depending on the time series noise. The kinematic method does not result in erroneous propagation of signals from one coordinate component to another or to the simultaneously estimated tropospheric delay parameters. It is robust to the likely effects of day-to-day equipment and reference frame changes, and to outages in the data. A minimum data span of 4 years with at least 70% availability is recommended. Finally, we show that the method of reducing apparent coordinate time series noise by constraining the tropospheric delay to values previously estimated in static batch GPS analysis, in fact, results in the suppression of true tidal signals. Using our kinematic GPS analysis approach, periodic displacements can be reliably observed at the 0.2 mm level, which is suitable for the testing and refinement of Ocean Tide and solid Earth response models.
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Assessing the accuracy of predicted Ocean Tide loading displacement values
Journal of Geodesy, 2008Co-Authors: Nigel T. Penna, T F Baker, Machiel Bos, Hans-georg ScherneckAbstract:The accuracy of Ocean Tide loading (OTL) displacement values has long been assumed to be dominated by errors in the Ocean Tide models used, with errors due to the convolution scheme used considered very small (2–5%). However, this paper shows that much larger convolution errors can arise at sites within approximately 150km of the coastline, depending on the method used to refine the discrete regularly spaced grid cells of the Ocean Tide model to better fit the coastline closest to the site of interest. If the local water mass redistribution approach is implemented, as used in the OLFG/OLMPP software recommended in the IERS 2003 conventions, OTL height displacement errors of up to around 20% can arise, depending on the Ocean Tide model used. Bilinear interpolation only, as used in theSPOTL and CARGA softwares for example, is shown from extensive global and regional comparisons of OTL displacement values derived from the different methods and softwares to be more appropriate. This is verified using GPS observations. The coastal refinement approach used in the OLFG/OLMPP software was therefore changed in August 2007 to use bilinear interpolation only. It is shown that with this change, OTL displacement values computed using OLFG/OLMPP, SPOTL and CARGA invariably agree to the millimetre level for coastal sites, and better than 0.2mm for sites more than about 150km inland.
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An estimate of the errors in gravity Ocean Tide loading computations
Journal of Geodesy, 2005Co-Authors: Machiel Bos, T F BakerAbstract:The error contributions within the Ocean Tide loading (OTL) convolution integral computation were determined to be able to estimate the numerical accuracy of the gravity OTL values. First, the comparison of four OTL programs by different authors (CONMODB, GOTIC2, NLOADF and OLFG/OLMPP) at ten globally distributed gravity stations using exactly the same input values shows discrepancies between 2% and 5%. A new program, called CARGA, was written that is able to reproduce the results of these programs to a level of 0.1%. This has given us the ability to state with certainty the cause of the discrepancies among the four programs. It is shown that by choosing an appropriate interpolation of the Green’s function, refinement of the integration mesh and a high-resolution coastline, an accuracy level of better than 1% can be obtained for stations in Europe. Besides this numerical accuracy, there are errors in the Ocean Tide model such as a 1% uncertainty in the mean value of the sea-water density and the lack of conservation of tidal water mass, which can produce offsets of around 0.04 μgal.
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stability of direct gps estimates of Ocean Tide loading
Geophysical Research Letters, 2004Co-Authors: C R Allinson, Matt A King, T F Baker, P J Clarke, Stuart Edwards, P CruddaceAbstract:[1] We observe Ocean Tide loading (OTL) at diurnal and semi-diurnal periods by directly estimating fixed-period harmonic motions within individual daily GPS analyses. Stacking of multiple solutions allows us to isolate the principal near-diurnal and near-semi-diurnal OTL components. Using data from UK sites where predicted OTL for the M2 component varies from 3–46 mm in amplitude, we show that our estimates are stable for the principal OTL components when data from at least 90 days are stacked. Our final observations are compared to model predictions (using the FES95.2, FES99, GOT00.2, TPXO.2, TPXO.6, NAO.99b and CSR3 models) and are in agreement with OTL predicted by FES99, but differ significantly at some locations from those predicted by the CSR3 and TPXO.2 tidal models.
Richard D. Ray - One of the best experts on this subject based on the ideXlab platform.
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tests of Ocean Tide models by analysis of satellite to satellite range measurements an update
Geophysical Journal International, 2019Co-Authors: Richard D. Ray, Bryant D Loomis, S B Luthcke, K E RachlinAbstract:Seven years of GRACE intersatellite range-rate measurements are used to test the new Ocean Tide model FES2014 and to compare against similar results obtained with earlier models. These qualitative assessments show that FES2014 represents a marked improvement in accuracy over its earlier incarnation, FES2012, with especially notable improvements in the Arctic Ocean for constituents K1 and S2. Degradation appears to have occurred in two anomalous regions: the Ross Sea for the O1 constituent and the Weddell Sea for M2.
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investigating the 59 day error signal in the mean sea level derived from topex poseidon jason 1 and jason 2 data with fes and got Ocean Tide models
IEEE Transactions on Geoscience and Remote Sensing, 2018Co-Authors: Lionel Zawadzki, Richard D. Ray, M Ablain, Loren Carrere, Nikita P Zelensky, Florent Lyard, Amandine Guillot, Nicolas PicotAbstract:Since the beginning of the altimeter mission TOPEX/Poseidon (T/P), followed by Jason-1 and Jason-2 on similar orbits, and many other missions on different orbits (ERS, EnviSat, etc.), mean sea level (MSL) products became essential for the comprehension of global Ocean circulation. Since early in the T/P mission, a suspicious signal, having a period of near 59 days and amplitude of roughly 5 mm, was apparent in the Global MSL record. Compared with the 4–5-mm amplitude of the annual signal, the 59-day signal has understandably attracted attention. Moreover, the same signal has been subsequently detected in Jason-1 and later in Jason-2 MSLs. In 2010, the Ocean Surface Topography Science Team (OSTST) concluded this signal as the aliasing of a higher frequency error inherited from the Tide model correction: the semi-diurnal wave S2. The source of this error was mainly attributed to T/P measurements, which were assimilated in Ocean Tide models. When these models are used in the computation of T/P MSL, most of the error cancels. However, this error is communicated to Jason-1 and Jason-2 MSLs. In order to gather and publish the OSTST analyses on this matter, this paper first attempts to list the myriad possibilities for the puzzling 59-day error in MSL. Then, this paper goes deeper into the description of the main contributor to this list: the Tide models error. Indeed, since 2010, considerable efforts have been undertaken within the Ocean Tide community in order to correct Ocean Tide S2-waves from this error, particularly in the Goddard Ocean Tide (GOT) and finite element solution (FES) latest versions. Comparing several GOT and FES versions and a pure hydrodynamic Tide model, this paper assesses, quantifies, and describes a reduction of the MSL 59-day error thanks to the latest releases. These analyses also confirm that a large part of this error has its origins in the T/P mission and has contaminated Ocean Tide solutions and Jason-1 and Jason-2 MSLs. They also suggest that Ocean Tide is not the only possible vector. Jason-1 and Jason-2 MSLs contain additional 59-day error—though to a lesser extent—that may either come from the measurements themselves or from another vector.
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qualitative comparisons of global Ocean Tide models by analysis of intersatellite ranging data
Journal of Geophysical Research, 2009Co-Authors: Richard D. Ray, S B Luthcke, Jeanpaul BoyAbstract:[1] Four global Ocean Tide models are compared in terms of their contribution to Gravity Recovery and Climate Experiment (GRACE) satellite-to-satellite tracking residuals. The residuals are computed relative to a comprehensive model of Earth's time-varying gravity, including allowance for mass motions in the atmosphere, Ocean, terrestrial hydrology, and mantle, in addition to Tides. For each analyzed Tide model, 4 years of GRACE range rate data are processed. Range and range acceleration residuals are tidally analyzed by geographic location. All four global Tide models are shown to be error prone in various ways, leaving tidally coherent residuals especially in polar regions but also in some lower-latitude regions. Considerable power in the solar semidiurnal S2 Tide in low latitudes suggests errors in our adopted model of atmospheric Tides, which is based on 3 hourly European Centre for Medium-Range Weather Forecasts operational analyses. Anomalies in the μ2 tidal constituent over some shallow seas suggest the presence of unmodeled nonlinear compound Tides, in this case 2MS2. Similarly, anomalies in the nonlinear M4 Tide are seen if this constituent is omitted from the models. Errors in assumed seawater density may be contributing to some residuals.
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A Global Ocean Tide Model From TOPEX/POSEIDON Altimetry: GOT99.2
1999Co-Authors: Richard D. RayAbstract:Goddard Ocean Tide model GOT99.2 is a new solution for the amplitudes and phases of the global Oceanic Tides, based on over six years of sea-surface height measurements by the TOPEX/POSEIDON satellite altimeter. Comparison with deep-Ocean Tide-gauge measurements show that this new tidal solution is an improvement over previous global models, with accuracies for the main semidiurnal lunar constituent M2 now below 1.5 cm (deep water only). The new solution benefits from use of prior hydrodynamic models, several in shallow and inland seas as well as the global finite-element model FES94.1. This report describes some of the data processing details involved in handling the altimetry, and it provides a comprehensive set of global cotidal charts of the resulting solutions. Various derived tidal charts are also provided, including tidal loading deformation charts, tidal gravimetric charts, and tidal current velocity (or transport) charts. Finally, low-degree spherical harmonic coefficients are computed by numerical quadrature and are tabulated for the major short-period Tides; these are useful for a variety of geodetic and geophysical purposes, especially in combination with similar estimates from satellite laser ranging.
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a global Ocean Tide model from topex poseidon altimetry got99 2
1999Co-Authors: Richard D. RayAbstract:Goddard Ocean Tide model GOT99.2 is a new solution for the amplitudes and phases of the global Oceanic Tides, based on over six years of sea-surface height measurements by the TOPEX/POSEIDON satellite altimeter. Comparison with deep-Ocean Tide-gauge measurements show that this new tidal solution is an improvement over previous global models, with accuracies for the main semidiurnal lunar constituent M2 now below 1.5 cm (deep water only). The new solution benefits from use of prior hydrodynamic models, several in shallow and inland seas as well as the global finite-element model FES94.1. This report describes some of the data processing details involved in handling the altimetry, and it provides a comprehensive set of global cotidal charts of the resulting solutions. Various derived tidal charts are also provided, including tidal loading deformation charts, tidal gravimetric charts, and tidal current velocity (or transport) charts. Finally, low-degree spherical harmonic coefficients are computed by numerical quadrature and are tabulated for the major short-period Tides; these are useful for a variety of geodetic and geophysical purposes, especially in combination with similar estimates from satellite laser ranging.
S D Desai - One of the best experts on this subject based on the ideXlab platform.
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Application of the convolution formalism to the Ocean Tide potential: Results from the Gravity Recovery and Climate Experiment (GRACE)
Journal of Geophysical Research, 2006Co-Authors: S D Desai, Dah-ning YuanAbstract:[1] A computationally efficient approach to reducing omission errors in Ocean Tide potential models is derived and evaluated using data from the Gravity Recovery and Climate Experiment (GRACE) mission. Ocean Tide height models are usually explicitly available at a few frequencies, and a smooth unit response is assumed to infer the response across the tidal spectrum. The convolution formalism of Munk and Cartwright (1966) models this response function with a Fourier series. This allows the total Ocean Tide height, and therefore the total Ocean Tide potential, to be modeled as a weighted sum of past, present, and future values of the Tide-generating potential. Previous applications of the convolution formalism have usually been limited to Tide height models, but we extend it to Ocean Tide potential models. We use luni-solar ephemerides to derive the required Tide-generating potential so that the complete spectrum of the Ocean Tide potential is efficiently represented. In contrast, the traditionally adopted harmonic model of the Ocean Tide potential requires the explicit sum of the contributions from individual tidal frequencies. It is therefore subject to omission errors from neglected frequencies and is computationally more intensive. Intersatellite range rate data from the GRACE mission are used to compare convolution and harmonic models of the Ocean Tide potential. The monthly range rate residual variance is smaller by 4–5%, and the daily residual variance is smaller by as much as 15% when using the convolution model than when using a harmonic model that is defined by twice the number of parameters.
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Error analysis of empirical Ocean Tide models estimated from TOPEX/POSEIDON altimetry
Journal of Geophysical Research: Oceans, 1997Co-Authors: S D Desai, J M Wahr, Yi ChaoAbstract:An error budget is proposed for the TOPEX/POSEIDON (T/P) empirical Ocean Tide models estimated during the primary mission. The error budget evaluates the individual contribution of errors in each of the altimetric range corrections, orbit errors caused by errors in the background Ocean Tide potential, and errors caused by the general circulation of the Oceans, to errors in the Ocean Tide models of the eight principal diurnal and semidiurnal tidal components, and the two principal long-period tidal components. The effect of continually updating the T/P empirical Ocean Tide models during the primary T/P mission is illustrated through Tide gauge comparisons and then used to predict the impact of further updates during the extended mission. Both the Tide gauge comparisons and the error analysis predict errors in the Tide models for the eight principal diurnal and semidiurnal constituents to be of the order of 2–3 cm root-sum-square. The dominant source of errors in the T/P Ocean Tide models appears to be caused by the general circulation of the Oceans observed by the T/P altimeter. Further updates of the T/P empirical Ocean Tide models during the extended mission should not provide significant improvements in the diurnal and semidiurnal Ocean Tide models but should provide significant improvements in the long-period Ocean Tide models, particularly in the monthly (Mm) tidal component.
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empirical Ocean Tide models estimated from topex poseidon altimetry
Journal of Geophysical Research, 1995Co-Authors: S D Desai, J M WahrAbstract:Three empirical Ocean Tide models are determined from repeat cycles 10 to 78 of the TOPEX/POSEIDON (T/P) altimeter mission. The three models investigate the effects of the satellite orbit ephemeris on the Ocean Tides determined from T/P altimetry and the effect of extracting the free core nutation resonance in the definition of the diurnal Ocean Tide admittance. The altimetric data series use the Joint Gravity Model JGM-2 geopotential orbit ephemeris and the preliminary JGM-3 orbit ephemeris computed at the University of Texas, Center for Space Research. The altimetric data from the T/P mission are shown to have the precision necessary to estimate the monthly and fortnightly Ocean Tides in each bin. Inclusion of existing models of the Ocean Tides in the polar latitudes not sampled by the altimeter demonstrates the importance of these latitudes on spherical harmonic representations of the Ocean Tides. The Ocean Tides are first estimated in bins of size 2.834° in longitude by 1° in latitude and then smoothed to 1° by 1° grids within ±66° latitude. The orthoTide response formalism of Groves and Reynolds (1975) is used to represent the diurnal and semidiurnal Ocean Tides, while a constant admittance is assumed across narrow bandwidths around each of the monthly (Mm), fortnightly (Mf), and termensual (Mt) tidal components. Comparisons of the T/P Ocean Tide models to Tide gauge observations indicate their accuracies to be of the order of 2–3 cm. The T/P-derived Ocean Tide models remove approximately 20 cm2 more of the T/P measured sea surface variance than the Cartwright and Ray (1991) Tide model and show a 19 cm2 and 15 cm2 improvement over the Schwiderski (1980a, b) and Cartwright and Ray (1991) Tide models, respectively, when compared to Tide gauge estimates of the Ocean Tides.
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Empirical Ocean Tide models estimated from TOPEX/POSEIDON altimetry
Journal of Geophysical Research, 1995Co-Authors: S D Desai, J M WahrAbstract:Three empirical Ocean Tide models are determined from repeat cycles 10 to 78 of the TOPEX/POSEIDON (T/P) altimeter mission. The three models investigate the effects of the satellite orbit ephemeris on the Ocean Tides determined from T/P altimetry and the effect of extracting the free core nutation resonance in the definition of the diurnal Ocean Tide admittance. The altimetric data series use the Joint Gravity Model JGM-2 geopotential orbit ephemeris and the preliminary JGM-3 orbit ephemeris computed at the University of Texas, Center for Space Research. The altimetric data from the T/P mission are shown to have the precision necessary to estimate the monthly and fortnightly Ocean Tides in each bin. Inclusion of existing models of the Ocean Tides in the polar latitudes not sampled by the altimeter demonstrates the importance of these latitudes on spherical harmonic representations of the Ocean Tides. The Ocean Tides are first estimated in bins of size 2.834° in longitude by 1° in latitude and then smoothed to 1° by 1° grids within +/-66° latitude. The orthoTide response formalism of Groves and Reynolds (1975) is used to represent the diurnal and semidiurnal Ocean Tides, while a constant admittance is assumed across narrow bandwidths around each of the monthly (Mm), fortnightly (Mf), and termensual (Mt) tidal components. Comparisons of the T/P Ocean Tide models to Tide gauge observations indicate their accuracies to be of the order of 2-3 cm. The T/P-derived Ocean Tide models remove approximately 20 cm2 more of the T/P measured sea surface variance than the Cartwright and Ray (1991) Tide model and show a 19 cm2 and 15 cm2 improvement over the Schwiderski (1980a, b) and Cartwright and Ray (1991) Tide models, respectively, when compared to Tide gauge estimates of the Ocean Tides.
Hans-georg Scherneck - One of the best experts on this subject based on the ideXlab platform.
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Computation of green's functions for Ocean Tide loading
Sciences of Geodesy - II, 2012Co-Authors: Machiel Bos, Hans-georg ScherneckAbstract:The Earth deforms periodically due to the varying weight of the Ocean Tides. Classical terrestrial geodetic techniques such as gravimetry, strain and tilt observe this deformation clearly. Also space geodetic techniques have reached an accuracy level where this loading signal can no longer be ignored. We present here the basic physical assumptions that underlie, and the derivation of, the mathematical framework that is used nowadays to compute these deformations. Special attention has been paid to the definition of the boundary conditions, how to treat the fluid core and the peculiarities that surround the deformation of the solid Earth at degree one. Also a short explanation of the Longman-Paradox is given. Finally, we discuss numerical methods to solve the differential equations and how to form the Green's functions that describe the Ocean Tide loading due to a point load.
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Assessing the accuracy of predicted Ocean Tide loading displacement values
Journal of Geodesy, 2008Co-Authors: Nigel T. Penna, T F Baker, Machiel Bos, Hans-georg ScherneckAbstract:The accuracy of Ocean Tide loading (OTL) displacement values has long been assumed to be dominated by errors in the Ocean Tide models used, with errors due to the convolution scheme used considered very small (2–5%). However, this paper shows that much larger convolution errors can arise at sites within approximately 150km of the coastline, depending on the method used to refine the discrete regularly spaced grid cells of the Ocean Tide model to better fit the coastline closest to the site of interest. If the local water mass redistribution approach is implemented, as used in the OLFG/OLMPP software recommended in the IERS 2003 conventions, OTL height displacement errors of up to around 20% can arise, depending on the Ocean Tide model used. Bilinear interpolation only, as used in theSPOTL and CARGA softwares for example, is shown from extensive global and regional comparisons of OTL displacement values derived from the different methods and softwares to be more appropriate. This is verified using GPS observations. The coastal refinement approach used in the OLFG/OLMPP software was therefore changed in August 2007 to use bilinear interpolation only. It is shown that with this change, OTL displacement values computed using OLFG/OLMPP, SPOTL and CARGA invariably agree to the millimetre level for coastal sites, and better than 0.2mm for sites more than about 150km inland.
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The M _2 Ocean Tide loading wave in Alaska: vertical and horizontal displacements, modelled and observed
Journal of Geodesy, 2003Co-Authors: Shfaqat Abbas Khan, Hans-georg ScherneckAbstract:Crustal deformations caused by surface load due to Ocean Tides are strongly dependent on the surface load closest to the observation site. In order to correctly model this Ocean loading effect near irregular coastal areas, a high-resolution coastline is required. A test is carried out using two GPS sites located in Alaska, where the Ocean Tide loading effect is large and consequently observed easily by relative positioning with GPS. The selected sites are Fair (Fairbanks) and Chi3 (located on an island that separates Prince William Sound from the Gulf of Alaska). Processing of hourly baseline solutions between Fair and Chi3 over a period of 49 days yields a significant Ocean Tide loading effect. The data are processed using different strategies for the tropospheric delay correction. However, the best results are obtained when 1-h ZTD (Zenith Tropospheric Delay) parameters for hourly solutions are used. In this case Ocean Tide loading is not absorbed into the ZTD parameters. Hence, Ocean Tide loading can be well resolved in the GPS data analysis. In addition, the M _2 Ocean Tide wave in the Gulf of Alaska has a very large amplitude. Although the horizontal M _2 Ocean Tide loading amplitude in general is only about 1/4 of the vertical M _2 Ocean Tide loading amplitude, the differential horizontal M _2 Ocean Tide loading displacements are nevertheless measurable using differential GPS (DGPS). When using the GOT99.2 Ocean Tide model and taking the coastal structure into account, the predicted differential vertical M _2 amplitude and Greenwich phase lag due to Ocean Tide loading are 19.3 mm and 110.2 degrees respectively, while GPS measurements yield 21.3 ± 1.0 mm and 99.7±2.8 degrees. Similarly, the predicted differential horizontal M _2 amplitude and Greenwich phase lag (in the north–south direction) are 4.5 mm and −77.0 degrees, while GPS yields 5.4 ± 0.3 mm and −106.3±3.3 degrees. Only the north-south component of the differential horizontal M _2 Ocean Tide loading wave is considered, because the east–west component is too small for the processed baseline and not detectable using DGPS.
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Ocean Tide and Atmospheric Loading
2002Co-Authors: Hans-georg Scherneck, Machiel BosAbstract:We describe the ingredients of an automatic service for the dissemination of Ocean Tide loading coefficients and the considerations that led to the solution employed. The paper reviews the surface loading problem, methods for computation and especially the improvement of coastline resolution for accurate representation of coastal loads. We finally compare the loading results based on several Ocean Tide models with coefficients estimated from VLBI observations.
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Effect of horizontal displacements due to Ocean Tide loading on the determination of polar motion and UT1
Geophysical Research Letters, 1999Co-Authors: Hans-georg Scherneck, Rüdiger HaasAbstract:We show the influence of horizontal displacements due to Ocean Tide loading on the determination of polar motion and UT1 (PMU) on the daily and subdaily timescale. So called ‘virtual PMU variations’ due to modelling errors of Ocean Tide loading are predicted for geodetic Very Long Baseline Interferometry (VLBI) networks. This leads to errors of subdaily determination of PMU. The predicted effects are confirmed by the analysis of geodetic VLBI observations.
Nigel T. Penna - One of the best experts on this subject based on the ideXlab platform.
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Benefits of combining GPS and GLONASS for measuring Ocean Tide loading displacement
Journal of Geodesy, 2020Co-Authors: Majid Abbaszadeh, P J Clarke, Nigel T. PennaAbstract:GPS has been used to estimate Ocean Tide loading (OTL) height displacement amplitudes to accuracies of within 0.5 mm at the M2 frequency, but such estimation has been problematic at luni-solar K2 and K1 frequencies because they coincide with the GPS orbital period and revisit period, leading to repeating multipath and satellite orbit errors. We therefore investigate the potential of using the GLONASS constellation (with orbital period 11.26 h and true site revisit period of 8 sidereal days distinct from K2 and K1) for OTL displacement estimation, analysing 3–7 years of GPS and GLONASS data from 49 globally distributed stations. Using the PANDA software in kinematic precise point positioning mode with float ambiguities, we demonstrate that GLONASS can estimate OTL height displacement at the M2, N2, O1 and Q1 lunar frequencies with similar accuracy to GPS: 95th percentile agreements of 0.6–1.3 mm between estimated and FES2014b Ocean Tide model displacements. At the K2 and K1 luni-solar frequencies, 95th percentile agreements between GPS estimates and model values of 3.9–4.4 mm improved to 2.0–2.8 mm using GLONASS-only solutions. A combined GPS+GLONASS float solution improves accuracy of the lunar OTL constituents and P1 (but not significantly for K1 or K2) compared with a single-constellation solution and results in hourly-to-weekly spectral noise very similar to a GPS ambiguity-fixed solution, but without needing uncalibrated phase delay information. GLONASS estimates are more accurate at higher compared with lower latitudes because of improved satellite visibility, although this can be countered by using a lower elevation cut-off angle.
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asthenospheric anelasticity effects on Ocean Tide loading around the east china sea observed with gps
Solid Earth, 2020Co-Authors: Junjie Wang, Nigel T. Penna, P J Clarke, M S BosAbstract:Abstract. Anelasticity may decrease the shear modulus of the asthenosphere by 8 %–10 % at semidiurnal tidal periods compared with the reference 1 s period of seismological Earth models. We show that such anelastic effects are likely to be significant for Ocean Tide loading displacement at the M2 tidal period around the East China Sea. By comparison with Tide gauge observations, we establish that from nine selected Ocean Tide models (DTU10, EOT11a, FES2014b, GOT4.10c, HAMTide11a, NAO99b, NAO99Jb, OSU12, and TPXO9-Atlas), the regional model NAO99Jb is the most accurate in this region and that related errors in the predicted M2 vertical Ocean Tide loading displacements will be 0.2–0.5 mm. In contrast, GPS observations on the Ryukyu Islands (Japan), with an uncertainty of 0.2–0.3 mm, show 90th-percentile discrepancies of 1.3 mm with respect to Ocean Tide loading displacements predicted using the purely elastic radial Preliminary Reference Earth Model (PREM). We show that the use of an anelastic PREM-based Earth model reduces these 90th-percentile discrepancies to 0.9 mm. Use of an anelastic radial Earth model consisting of a regional average of the laterally varying S362ANI model reduces the 90th-percentile to 0.7 mm, which is of the same order as the sum of the remaining errors due to uncertainties in the Ocean Tide model and the GPS observations.
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asthenospheric anelasticity effects on Ocean Tide loading in the east china sea region observed with gps
Solid Earth Discussions, 2019Co-Authors: Junjie Wang, Nigel T. Penna, P J Clarke, M S BosAbstract:Abstract. Anelasticity may decrease the shear modulus of the asthenosphere by 8–10 % at semi-diurnal tidal periods compared with the reference 1 s period of seismological Earth models. We show that such anelastic effects are likely to be significant for Ocean Tide loading displacement at the M2 tidal period around the East China Sea. By comparison with Tide gauge observations, we establish that NAO99Jb is the most accurate numerical Ocean Tide model in this region, and that related errors in the predicted M2 vertical Ocean Tide loading displacements will be 0.2–0.5 mm. In contrast, GPS observations on the Ryukyu Islands (Japan), with uncertainty 0.2–0.3 mm, show discrepancies of over 1.5 mm with respect to Ocean Tide loading displacements predicted using the purely elastic radial Preliminary Reference Earth Model. We show that the use of an anelastic PREM-based Earth model reduces these discrepancies to no more than 0.8 mm, which is of the same order as the sum of the remaining errors due to uncertainties in the Ocean Tide model and the GPS observations. Use of a regional Earth model based on the laterally-varying S362ANI, with or without further empirical tuning, results in minor additional improvements in fit.
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Ocean Tide loading displacements in western europe 2 gps observed anelastic dispersion in the asthenosphere
Journal of Geophysical Research, 2015Co-Authors: M S Bos, T F Baker, Nigel T. Penna, P J ClarkeAbstract:GPS-observed vertical Ocean Tide loading displacements show in Cornwall, southwest England, and in Brittany, northwest France, discrepancies of 2–3 mm with predicted values based on the isotropic Preliminary Reference Earth Model for the main tidal harmonic M2, yet in central Europe the agreement is better than 0.5 mm. By comparison of Ocean Tide models and validation with Tide gauge observations, we demonstrate that the uncertainties in the former are too small to cause this disagreement. Furthermore, we find that different local models of the crust and different global elastic reference models derived from seismological observations can only reduce the observed discrepancies to 1–2 mm, which still exceeds the GPS observational uncertainty of 0.2–0.4 mm. It is customary to use the elastic properties of the Earth as given by seismic models. Previously, there has been insufficient evidence to determine how to modify these properties during the transformation from seismic to tidal frequencies to account for possible anelastic dispersion in the asthenosphere, and so this effect has been ignored. If we include this effect, then our discrepancies reduce further to 0.2–0.4 mm. This value is of the same order as the sum of the remaining errors due to uncertainties in the Ocean Tide models and in the GPS observations themselves. This research provides evidence in western Europe of a reduction of around 8–10% of the seismic shear modulus in the asthenosphere at tidal frequencies. In addition, we find that the asthenosphere absorption band frequencies can be represented by a constant quality factor Q.
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Ocean Tide loading displacements in western europe 1 validation of kinematic gps estimates
Journal of Geophysical Research, 2015Co-Authors: Nigel T. Penna, M S Bos, P J Clarke, T F BakerAbstract:GPS has been extensively used to estimate tidal ground displacements, but the accuracy of this has not been systematically verified. Using more than 20 sites distributed across western Europe, we show that postprocessed kinematic precise point positioning GPS with appropriately tuned process noise constraints is capable of recovering synthetic tidal displacements inserted into real data, with a typical accuracy of 0.2 mm depending on the time series noise. The kinematic method does not result in erroneous propagation of signals from one coordinate component to another or to the simultaneously estimated tropospheric delay parameters. It is robust to the likely effects of day-to-day equipment and reference frame changes, and to outages in the data. A minimum data span of 4 years with at least 70% availability is recommended. Finally, we show that the method of reducing apparent coordinate time series noise by constraining the tropospheric delay to values previously estimated in static batch GPS analysis, in fact, results in the suppression of true tidal signals. Using our kinematic GPS analysis approach, periodic displacements can be reliably observed at the 0.2 mm level, which is suitable for the testing and refinement of Ocean Tide and solid Earth response models.