Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy最新文献

To deliver tropospheric water vapor contents derived from GPS ground networks for the assimilation into numerical weather prediction models the GPS data have to be analyzed within near real-time with a delay of about one hour. Two near real-time processors for the trophospheric water vapor monitoring developed at IEEC Barcelona and GFZ Potsdam are compared using one week of data acquired in the German near real-time network. The resulting small differences in the integrated water vapor being on the level of 1–2 mm precipitable water vapor reflect that both processors fulfill the requirements for numerical weather predictions. Therefore the standardization for processors should not be too strict not to obstruct a fruitful competition in the software developments and improvements.

The Crustal Dynamics Data Information System (CDDIS) has served as a global data center for the International GPS Service (IGS) since its start in June 1992, providing on-line access to data from over 175 sites on a daily basis. This paper will present information about the GPS and GLONASS data and products archive at the CDDIS. General information about the system and its support of other international space geodesy services (the ILRS, IVS, IGLOS-PP, and DPE) will also be discussed.

In this paper we review the atmospheric modeling methods used in GPS data analysis. Due to the strong spatial inhomogeneity and temporal variability of atmospheric constituents, especially water vapor, accurate modeling of path delay in GPS signals is necessary for high-accuracy positioning (e.g., tectonics and sea-level change) and meteorological applications (climatology and weather forecasting). State-of-the-art path delay modeling consists primarily in parameter estimation. In this strategy, zenith path delays are estimated during the GPS data reduction. External correction is another common strategy, in which the wet path delay is measured by a remote sensing instrument (usually a microwave radiometer). However, the latter is not as generalized, and is rather used for specific field campaigns or local long term observations. Both strategies have led to quite similar coordinate accuracies (using daily GPS observations), at the level of 1–2 mm in the horizontal component and 5–10 mm in the vertical component. The external correction strategy is capable of achieving even higher accuracy under specific conditions. Recent models, including gradients in the parameter estimation strategy have only led to marginal improvement. A major limitation of both strategies seems to be the use of mapping functions for the hydrostatic path delay correction. With the parameter estimation strategy, this limitation applies also to wet path delay correction. The use of numerical weather prediction and analysis models, and/or spaceborne sounding instruments, is suggested for replacing mapping functions and possibly for performing directly the hydrostatic correction. New instruments, such as Raman lidars, might also be used for a more accurate external wet path delay correction in the presence of strong atmospheric inhomogeneity. Further work is still needed for achieving measurements of absolute water vapor distribution in the atmosphere for this purpose.

The oxygen isotope composition of whole-rock and mineral separates was measured for granulite rocks from Dabieshan. According to their whole-rock δ¹⁸O values relative to the normal mantle δ¹⁸O values of 5.7±0.5‰, two groups are classified: (1) mafic granulite which exhibits lower δ¹⁸O values of 3.5 to 4.7‰, and (2) felsic granulite which shows higher δ¹⁸O values of 7.6 to 7.8‰. Consistent isotope temperatures of 800 to 900 °C are obtained for mineral pairs containing such refractory minerals as pyroxene, garnet, hornblende and iron oxides, suggesting the achievement and preservation of oxygen isotope equilibrium at the conditions of the peak granulite-facies metamorphism. This not only points to a rapid cooling and ascent for the granulite rocks in the early stage of exhumation, but also precludes the infiltration of external fluids during exhumation as the cause for the ¹⁸O-depletion in the mafic granulite. It is evident that the granulite rocks acquired the low δ¹⁸O values before the granulite-facies metamorphism by interaction with a certain ¹⁸O-depleted surface fluid. The surface fluid is assumed to exchange oxygen isotopes with the granulite protoliths prior to plate subduction. Fluid-absent metamorphism is suggested for the formation of the granulites on local scales. It is likely that the granulites together with the ultrahigh pressure eclogites and gneisses in Dabieshan were part of a single tectonic entity in the processes of subduction and Triassic metamorphism but experienced differential two-stage uplifts prior to amphibolite-facies retrogression.

The possibilities of reconstruction of various atmospheric structures and influence of localized atmospheric and ionospheric irregularities on the reconstructed atmospheric profiles are discussed. In the case of vast atmospheric structures (above 400–600 km long), the radiooccultation method of atmospheric studies makes it possible to reconstruct the vertical profiles of meteorological parameters with the accuracy acceptable for applications (the errors in the refractive index profile are 0.5–1% for the altitude region 8–40 km). However, the presence of the atmospheric irregularities with horizontal dimensions up to 300–400 km leads to significant reconstruction errors, the fact being related to the ‘spreading’ of the disturbance to the entire region of the sounding ray intersection of the troposphere. Therefore, near such structures as secondary typo atmospheric fronts and developing cyclones, the profiles are reconstructed with strong distortions reaching 1–10%. The impact of the irregularities in the radio signal propagation media (atmosphere, ionosphere, and protonosphere) on the accuracy of the profiles reconstructed is studied. Possibilities to study various atmospheric irregularities by the tomographic method, using a network of ground-based receivers, are considered.

We have made Monte Carlo simulations in order to quantify the influence of several error sources on the atmospheric delay derived in our regular “Precise Point Positioning” analysis of GPS data. Satellite position and clock errors were found to be the greatest sources of error. The RMS size of the atmospheric delay error due to these sources is suggested to be 4–8 mm, and the correlation of the delay error over 1000 km is 0.5-0.7. In our real-time analysis of the atmospheric delay satellite orbits and clock offsets are modelled in the Kalman filter. The modelled errors suggest an uncertainty in the atmospheric delay of 10–13 mm, and a decrease in the correlation coefficients to about 0.65 over a distance of 1000 km. For both the Precise Point Positioning and the real-time analysis the influence from antenna signal scatter will give additional error contributions.

To estimate total absolute tropospheric zenith delays in a global network of GPS stations, residual delays are estimated as corrections to an a priori delay model using a zenith angle dependent mapping function. Because the a priori model relies on a standard atmosphere and the wet and hydrostatic delays have different zenith angle dependence, the total zenith delay estimates are biased. A simulation has shown that biases of several mm are possible, especially when a crude mapping function, large zenith cut-off angles and no zenith angle dependent weighting is used.

Recent comparisons have shown that zenith total delays (ZTD's) derived from Global Positioning System (GPS) observations and ZTD's calculated from analysis fields of numerical weather prediction (NWP) models agree within 1–2 cm (rms). Thus NWP models describe the tropospheric delay quite well, even though no GPS data are assimilated into the model. In this work we investigate how a NWP model can be used to calculate and eliminate the influence of the troposphere on GPS measurements. Using the European Centre for Medium-Range Weather Forecasts (ECMWF) global weather model we calculate the GPS path delays for each single GPS observation using a high precision 3D ray tracing procedure and subtract it from the GPS observations.

We discuss two methods to process the corrected GPS observations. We find a reduction of the rms value of the phase residuals at low elevations, if the dry delay is eliminated and the wet delay is modeled in the adjustment. If the total delay is eliminated, tropospheric modeling of the remaining delay, whose correlation matrix will be determined, is still necessary. The analysis of the eigenvectors of the correlation matrix gives a set of mapping functions and weights which can be used to process the corrected GPS observations.

Peat contains mostly diamagnetic organic matter, which is a good collector of all kinds of atmospheric dusts and industrial pollution. Ombrotrophic peat bogs are built up above the ground water table, so deposition and accumulation of magnetic particles are not influenced by ground water and the deposited particles remain largely “in situ”. During this study, 6 Polish ombrotrophic peat bogs from different locations have been investigated. Peat cores of about 30–50 cm in depth were taken from every bog. The specific low-frequency magnetic susceptibility was measured along the peat profile in fresh, not dried sample. In most profiles the susceptibility below 10 cm was slightly negative, which is characteristic for clean organic material. Above a depth 8–10 cm the susceptibility starts to increase. Independently of the profile location, the increase is observed in all bogs and according to isotopic (C-14) dating it is connected with the post war industrialisation (1945–1955). The maximum of the magnetic deposition noticed as maximum susceptibility enhancement is observed in a depth of about 5 cm below the surface. In the south-western part of Poland the peat profiles show the maximum susceptibility above 350 ×10⁻⁸m³kg⁻¹. The susceptibility decreases in profiles from the central part of Poland to 30–60. In the northern and southeastern part of the country the maximum observed susceptibility is about 10 ×10⁻⁸m³kg⁻¹. Hysteresis parameters pointed at anthropogenic ferromagnetic minerals as a carrier of magnetic signal.