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Koleksiyon [3]
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Yazar [20]
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A Tikhonov regularization method to estimate Earth's oblateness variations from global GPS observations

Jin S. | Zhang X.

Article | 2014 | Journal of Geodynamics79 , pp.23 - 29

Earth's oblateness is varying due to the redistribution of Earth's fluid mass and the interaction of various components in the Earth system. Nowadays, continuous Global Positioning System (GPS) observations can estimate Earth's oblateness (J2) variations with the least squares method, but are subject to ill-conditioned equations with limited GPS observations and aliasing errors from truncated degrees. In this paper, a Tikhonov regularization method is used to estimate J2 variations from global continuous GPS observations. Results show that the J2 has been better estimated from GPS observations based on a Tikhonov regularization meth . . .od than the usual least squares method when compared to SLR solutions. Furthermore, the amplitudes and phases of the annual and semi-annual J2 variations are closer to the SLR results with truncated degrees from 2 to 5. Higher truncated degrees will degrade the J2 estimate. Annual J2 variations are best estimated from GPS observations with truncated degree 4 and semi-annual J2 variations are best estimated with truncated degree 2. © 2014 Elsevier Ltd Daha fazlası Daha az

A new bound constraints method for 3-D potential field data inversion using Lagrangian multipliers

Zhang Y. | Yan J. | Li F. | Chen C. | Mei B. | Jin S. | Dohm J.H.

Article | 2015 | Geophysical Journal International201 ( 1 ) , pp.267 - 275

In this paper, we present a method for incorporating prior geological information into potential field data inversion problem. As opposed to the traditional inverse algorithm, our proposed method takes full advantage of prior geological information as a constraint and thus obtains a new objective function for inversion by adding Lagrangian multipliers and slack variables to the traditional inversion method. These additional parameters can be easily solved during iterations.We used both synthetic and observed data sets to test the stability and validity of the proposed method. Our results using synthetic gravity data show that our ne . . .w method predicts depth and density anomalies more efficiently and accurately than the traditional inversion method that does not include prior geological constraints. Then using observed gravity data in the Three Gorges area and geological constraint information, we obtained the density distribution of the upper and middle crust in this area thus revealing its geological structure. These results confirm the proposed method's validity and indicate its potential application for magnetism data inversion and exploration of geological structures. © The Authors 2015 Daha fazlası Daha az

Accelerated ice-sheet mass loss in antarctica from 18-year satellite laser ranging measurements

Jin S. | Abd-Elbaky M. | Feng G.

Article | 2016 | Annals of Geophysics59 ( 1 ) , pp.267 - 275

Accurate estimation of the ice-sheet mass balance in Antarctic is very difficult due to complex ice sheet condition and sparse in-situ measurements. In this paper, the low-degree gravity field coefficients of up to degree and order 5 derived from Satellite Laser Ranging (SLR) measurements are the first used to determine the ice mass variations in Antarctica for the period 1993-2011, which are compared to Gravity Recovery and Climate Experiment (GRACE). Results show that the ice mass is is decreasing at the rate of -36±13 Gt/y in Antarctica, -42±11 Gt/y in West Antarctica and increasing at 6±10 Gt/y in East Antarctica from 1993 to 20 . . .11. The ice mass variations from the SLR 5×5 have a good agreement with the GRACE 5×5, GRACE 5×5 (1&2) and GRACE (60×60) for the entire continent since 2003, but SLR solution of 5×5 is not sufficient to quantify ice losses in West and East Antarctica, respectively. The rate of ice loss in Antarctica is -28±17 Gt/y for 1993-2002 and -55±17 Gt/y for 2003-2011 within one sigma, indicating significant accelerated ice mass losses since 2003. Furthermore, the results from SLR are comparable with GRACE measurements. © 2016 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved Daha fazlası Daha az

High-order ionospheric effects on electron density estimation from Fengyun-3C GPS radio occultation

Li J. | Jin S.

Article | 2017 | Annales Geophysicae35 ( 3 ) , pp.403 - 411

GPS radio occultation can estimate ionospheric electron density and total electron content (TEC) with high spatial resolution, e.g., China's recent Fengyun-3C GPS radio occultation. However, high-order ionospheric delays are normally ignored. In this paper, the high-order ionospheric effects on electron density estimation from the Fengyun-3C GPS radio occultation data are estimated and investigated using the NeQuick2 ionosphere model and the IGRF12 (International Geomagnetic Reference Field, 12th generation) geomagnetic model. Results show that the high-order ionospheric delays have large effects on electron density estimation with . . .up to 800 el cm-3, which should be corrected in high-precision ionospheric density estimation and applications. The second-order ionospheric effects are more significant, particularly at 250-300 km, while third-order ionospheric effects are much smaller. Furthermore, the high-order ionospheric effects are related to the location, the local time, the radio occultation azimuth and the solar activity. The large high-order ionospheric effects are found in the low-latitude area and in the daytime as well as during strong solar activities. The second-order ionospheric effects have a maximum positive value when the radio occultation azimuth is around 0-20°, and a maximum negative value when the radio occultation azimuth is around -180 to -160 °. Moreover, the geomagnetic storm also affects the high-order ionospheric delay, which should be carefully corrected. © Author(s) 2017. CC Attribution 3.0 License Daha fazlası Daha az

New tracking modes and performance for Mars spacecraft orbit determination and lander positioning

Yan J. | Yang X. | Ye M. | Li F. | Jin S. | Jin W. | Barriot J.-P.

Article | 2018 | Planetary and Space Science163 , pp.5 - 13

Precise orbit determination and lander positioning are still a challenge for Mars exploration. In this paper, three positioning methods are proposed, including the same-beam differential Very Long Baseline Interferometry (VLBI), four-way orbiter-lander Doppler and three-way loop Doppler for Mars orbiter and lander. Measurement models and these three positioning methods are further analyzed and evaluated using simulation experiments. The results show that about 60-m level accuracy for the Mars lander can be achieved by a combination same-beam differential VLBI data at 10 ps noise level with two-way Doppler data at the 0.1 mm/s noise . . .level. Since VLBI data are sensitive to the angle between two radio sources, the accuracy of the Mars orbiter was not improved. The geometry of four-way orbiter-lander Doppler is more sensitive than the geometry of ordinary two-way Doppler, and therefore, after adding the four-way orbiter-lander Doppler data, the accuracy of the orbiter and lander positioning can reach the meter and decimeter level. Furthermore, the three-way loop Doppler reduces the number of orbital repeater signals and improves the POD accuracy at the meter level as well as the lander positioning accuracy at the decimeter level. Therefore, the three-way loop tracking mode provides an important reference for future Mars missions. © 2018 Elsevier Lt Daha fazlası Daha az

Absolute Navigation and Positioning of Mars Rover Using Gravity-Aided Odometry

Liu J. | Wei E. | Jin S. | Liu J.

Article | 2018 | Journal of Navigation71 ( 3 ) , pp.530 - 546

Positioning and Navigation (PN) of Martian rovers still faces challenges due to limited observations. In this paper, the PN feasibilities of Mars rovers based on a Gravity-aided Odometry (GO) system are proposed and investigated in terms of numeric simulations and a case study. Statistical features of the Mars gravity field are studied to evaluate the feature diversity of the background map. The Iterative Closest Point (ICP) algorithm is introduced to match gravity measurements with the gravitational map. The trajectories of Mars Exploration Rovers (MER) and Mars Gravity Map 2011 (MGM2011) are used to complete the experiments. Sever . . .al key factors of GO including odometry errors, measurement uncertainties, and grid resolution of the map are investigated to evaluate their influences on the positioning ability of the system. Simulated experiments indicate that the GO method could provide an alternative positioning solution for Martian surface rovers. © 2017 The Royal Institute of Navigation Daha fazlası Daha az

Cassini's motions of the Moon and Mercury and possible excitations of free librations

Barkin Y.V. | Ferrandiz J. | Jin S. | Barkin M.Y.

Article | 2018 | Geodesy and Geodynamics9 ( 6 ) , pp.474 - 484

On the basis of conditionally-periodic solutions of Hamiltonian systems at resonance of main frequencies Cassini's motions, their stability, Cassini's angle and periods of free librations of the Moon and Mercury have been recently studied and determined. The generalized formulations of Cassini's laws for the motion of the Moon and Mercury, that are considered as absolutely rigid non-spherical bodies, have been determined. The study of the second approximation equations of the desired quasi-periodic solutions in the case of the Moon allows us to determine the constant components of the first order for six Andoyer variables and the co . . .nstant component of the second order for the angular velocity of the Moon. These effects are caused by the influence of the third harmonic of selenopotential. In this paper, these effects are described by analytical formulas, the dynamic and geometric interpretations are given, and a new interpretation of Mercury's motion under the generalized Cassini's laws has been proposed. Predictions of the existence of free librations of significant amplitude in the Mercury longitude, that are confirmed by the radar measurements data of the Mercury angular velocity, and in its pole motion in the body and in space have been made. The mechanism describing free librations of celestial bodies and their pole oscillations has been proposed due to the forced relative oscillations and wobble of the core-mantle system of celestial bodies (Moon, Mercury, Earth and other bodies in the solar system) under gravitational action of the external celestial bodies. The paper shows that the ascending node of equator of Mercury (and the intermediate plane orthogonal to the angular momentum) of epoch 2000.0 on the ecliptic does not coincide with the ascending node of orbital plane of Mercury on the same plane, and is ahead of it at an angle 23º4’. Angular momentum vector of the rotational motion of Mercury forms a constant angle ?G=4’1±1’1 with normal to the moveable plane of its orbit. The observed inclination of the angular velocity??=2’1±0’1, can be considered as a possible evidence of a significant amplitude of the poles free motion of the Mercury rotation axis (c amplitude of about 2'- 4'). © 2018 The Author Daha fazlası Daha az

Estimation of Snow Depth from GLONASS SNR and Phase-Based Multipath Reflectometry

Qian X. | Jin S.

Article | 2016 | IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing9 ( 10 ) , pp.4817 - 4823

Monitoring snow variations can know how fast the snow is melted and how much water is frozen on the Earth. However, traditional ground techniques have some limitations to monitor snow variations due to high cost. Recently, global navigation satellite systems-reflectometry (GNSS-R) has been developed and used to sense snow variations, while previous studies mostly used global positioning system (GPS) observations to estimate snow depth. In this paper, snow depth is estimated from GLONASS signal-to-noise ratio (SNR) and phase-based multipath reflectometry at one IGS GANP station in Slovakia, which is compared with in situ observations . . .. For two snow seasons, snow depth estimations from GLONASS SNR1 and SNR2 data have a good agreement with in situ results with correlations of 0.94 and 0.94 in 2012 and 0.92 and 0.89 in 2013, respectively. Compared to GPS results, snow depth results from GLONASS are almost similar, but have some differences due to low coverage of less GLONASS satellites. However, results from GLONASS geometry-free linear combination (L4) are not pretty much, which needs more works to improve it in the future. Combined GPS and GLONASS observations have no significant improvement on the precision, but improve the spatial resolution because of more satellites. © 2016 IEEE Daha fazlası Daha az

A new GPS-based calibration of GRACE accelerometers using the arc-to-chord threshold uncovered sinusoidal disturbing signal

Calabia A. | Jin S. | Tenzer R.

Article | 2015 | Aerospace Science and Technology45 , pp.265 - 271

Abstract In order to guarantee an unbiased solution in accelerometer measurements, calibration parameters have been finally calculated without using any kind of regularization or constraint. In this paper, a better calibration of the GRACE accelerometers is achieved from the instantaneous GPS-based non-gravitational accelerations. The first derivatives of the precise-orbit velocity are computed under an a priori arc-to-chord threshold, while the modelled time-varying forces of gravitational origin and reference-system rotations are computed according to current conventions (including sub-daily tide variations). After subtracting the . . . modelled time-varying gravity from the GPS-based accelerations, cross-track axes of both GRACE satellites seem to be affected by a periodic error of unknown source. With the purpose of extracting the underlying information from the resulting data, the systematic error is modelled and subtracted successfully. According to this approach, the resulting accelerations serve as a reliable reference for accelerometer calibration. © 2015 Elsevier Masson SAS Daha fazlası Daha az

Martian sub-crustal stress from gravity and topographic models

Tenzer R. | Eshagh M. | Jin S.

Article | 2015 | Earth and Planetary Science Letters425 , pp.84 - 92

The latest Martian gravity and topographic models derived from the Mars Orbiter Laser Altimeter and the Mars Global Surveyor spacecraft tracking data are used to compute the sub-crustal stress field on Mars. For this purpose, we apply the method for a simultaneous determination of the horizontal sub-crustal stress component and the crustal thickness based on solving the Navier-Stokes problem and incorporating the Vening Meinesz-Moritz inverse problem of isostasy. Results reveal that most of the Martian sub-crustal stress is concentrated in the Tharsis region, with the most prominent signatures attributed to a formation of Tharsis ma . . .jor volcanoes followed by crustal loading. The stress distribution across the Valles Marineris rift valleys indicates extensional tectonism. This finding agrees with more recent theories of a tectonic origin of Valles Marineris caused, for instance, by a crustal loading of the Tharsis bulge that resulted in a regional trusting and folding. Aside from these features, the Martian stress field is relatively smooth with only a slightly enhanced pattern of major impact basins. The signatures of active global tectonics and polar ice load are absent. Whereas the signature of the hemispheric dichotomy is also missing, the long-wavelength spectrum of the stress field comprises the signature of additional dichotomy attributed to the isostatically uncompensated crustal load of Tharsis volcanic accumulations. These results suggest a different origin of the Earth's and Martian sub-crustal stress. Whereas the former is mainly related to active global tectonics, the latter is generated by a crustal loading and regional tectonism associated with a volcanic evolution on Mars. The additional sub-crustal stress around major impact basins is likely explained by a crustal extrusion after impact followed by a Moho uplift. © 2015 Elsevier B.V Daha fazlası Daha az

Thermospheric density estimation and responses to the March 2013 geomagnetic storm from GRACE GPS-determined precise orbits

Calabia A. | Jin S.

Article | 2017 | Journal of Atmospheric and Solar-Terrestrial Physics154 , pp.167 - 179

The thermospheric mass density variations and the thermosphere-ionosphere coupling during geomagnetic storms are not clear due to lack of observables and large uncertainty in the models. Although accelerometers on-board Low-Orbit-Earth (LEO) satellites can measure non-gravitational accelerations and derive thermospheric mass density variations with unprecedented details, their measurements are not always available (e.g., for the March 2013 geomagnetic storm). In order to cover accelerometer data gaps of Gravity Recovery and Climate Experiment (GRACE), we estimate thermospheric mass densities from numerical derivation of GRACE determ . . .ined precise orbit ephemeris (POE) for the period 2011–2016. Our results show good correlation with accelerometer-based mass densities, and a better estimation than the NRLMSISE00 empirical model. Furthermore, we statistically analyze the differences to accelerometer-based densities, and study the March 2013 geomagnetic storm response. The thermospheric density enhancements at the polar regions on 17 March 2013 are clearly represented by POE-based measurements. Although our results show density variations better correlate with Dst and k-derived geomagnetic indices, the auroral electroject activity index AE as well as the merging electric field Em picture better agreement at high latitude for the March 2013 geomagnetic storm. On the other side, low-latitude variations are better represented with the Dst index. With the increasing resolution and accuracy of Precise Orbit Determination (POD) products and LEO satellites, the straightforward technique of determining non-gravitational accelerations and thermospheric mass densities through numerical differentiation of POE promises potentially good applications for the upper atmosphere research community. © 2017 Elsevier Lt Daha fazlası Daha az

Effects of ocean tide models on GNSS-estimated ZTD and PWV in Turkey

Gurbuz G. | Jin S. | Mekik C.

Conference Object | 2015 | International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences - ISPRS Archives40 ( 1W5 ) , pp.255 - 258

Global Navigation Satellite System (GNSS) observations can precisely estimate the total zenith tropospheric delay (ZTD) and precipitable water vapour (PWV) for weather prediction and atmospheric research as a continuous and all-weather technique. However, apart from GNSS technique itself, estimations of ZTD and PWV are subject to effects of geophysical models with large uncertainties, particularly imprecise ocean tide models in Turkey. In this paper, GNSS data from Jan. 1st to Dec. 31st of 2014 are processed at 4 co-located GNSS stations (GISM, DIYB, GANM, and ADAN) with radiosonde from Turkish Met-Office along with several nearby I . . .GS stations. The GAMIT/GLOBK software has been used to process GNSS data of 30-second sample using the Vienna Mapping Function and 10° elevation cut-off angle. Also tidal and non-tidal atmospheric pressure loadings (ATML) at the observation level are also applied in GAMIT/GLOBK. Several widely used ocean tide models are used to evaluate their effects on GNSS-estimated ZTD and PWV estimation, such as IERS recommended FES2004, NAO99b from a barotropic hydrodynamic model, CSR4.0 obtained from TOPEX/Poseidon altimetry with the model FES94.1 as the reference model and GOT00 which is again long wavelength adjustments of FES94.1 using TOPEX/Poseidon data at 0.5 by 0.5 degree grid. The ZTD and PWV computed from radiosonde profile observations are regarded as reference values for the comparison and validation. In the processing phase, five different strategies are taken without ocean tide model and with four aforementioned ocean tide models, respectively, which are used to evaluate ocean tide models effects on GNSS-estimated ZTD and PWV estimation through comparing with co-located Radiosonde. Results showed that ocean tide models have greatly affected the estimation of the ZTD in centimeter level and thus the precipitable water vapour in millimeter level, respectively at stations near coasts. The ocean tide model FES2004 that is the product of assimilation of the altimetric data of ERS2, TOPEX/POSEIDON and the data of a global tide gauge network, gave the most accurate results when compared to radiosonde with ±1.99 mm in PWV at stations near coastline. While other ocean tides models agree each other at millimeter level in PWV. However, at inland GNSS stations, ocean tide models have less effects on GNSS-estimated ZTD and PWV, e.g., with ±1.0 mm in ZTD and ±0.1 mm in PWV Daha fazlası Daha az

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