The future of artificial satellite theories Hybrid ephemeris compression model

Since the time of Newton, astrodynamics has focused on the analytical solution of orbital problems. This was driven by the desire to obtain a theoretical understanding of the motion and the practical desire to be able to produce a computational result, Only with the advent of the computer did numerical integration become a practical consideration for solving dynamical problems. Although computer technology is not yet to the point of being able to provide numerical integration support for all satellite orbits, we are in a transition period which is being driven by the unprecedented increase in computational power, This transition will affect the future of analytical, semi-analytical and numerical artificial satellite theories in a dramatic way, In fact, the role for semi-analytical theories may disappear. During the time of transition, a central site may have the capacity to maintain the orbits using numerical integration, but the user may not have such a capacity or may need results in a more timely manner, One way to provide for this transition need is through the use of some type of satellite ephemeris compression. Through the combined use of a power series and a Fourier series, good quality ephemeris compression has been achieved for 7 day periods, The ephemeris compression requires less than 40 terms and is valid for all eccentricities and inclinations.     

Cyclicities in Triassic platform carbonates: synchronizing radio-isotopic and orbital clocks

ABSTRACT Sedimentary cycles recorded in young sediments are often attributed to fluctuations of the Earth's climate on a 10⁴−10⁶-year scale which in turn is governed by periodic variations in solar insolation linked to orbital (Milankovitch) parameters. A spectacular example of cyclic stratal patterns in ancient deposits is the Middle Triassic Latemar carbonate platform (W Dolomites, N Italy). Based on spectral analyses from previous studies, a superimposition of precession (∼20 ka) and eccentricity (∼100 ka) controlled sea-level fluctuations has been suggested to account for the stacking hierarchy at Latemar, with ∼20 ka being assigned to each highest-order depositional cycle. Zircon U–Pb isotopic ages from volcanic-ash layers within the cyclic succession, corroborated by biostratigraphic constraints, suggest that the average time interval for every individual cycle is significantly smaller than the shortest Milankovitch period and therefore challenge previously published interpretations relating distinct spectral peaks to the above mentioned hierarchy. However, our new spectral data indicate that cyclicities resembling Milankovitch characteristics might exist, but on an entirely different scale. Our findings show that frequency spectra should only be interpreted in combination with robust age control. They also encourage the search for complementary mechanisms controlling carbonate deposition.     

Time-domain sparsity promoting least-squares reverse time migration with source estimation

Least-squares reverse-time migration is well known for its capability to generate artefact-free true-amplitude subsurface images through fitting observed data in the least-squares sense. However, when applied to realistic imaging problems, this approach is faced with issues related to overfitting and excessive computational costs induced by many wave-equation solves. The fact that the source function is unknown complicates this situation even further. Motivated by recent results in stochastic optimization and transform-domain sparsity promotion, we demonstrate that the computational costs of inversion can be reduced significantly while avoiding imaging artefacts and restoring amplitudes. While powerful, these new approaches do require accurate information on the source-time function, which is often lacking. Without this information, the imaging quality deteriorates rapidly. We address this issue by presenting an approach where the source-time function is estimated on the fly through a technique known as variable projection. Aside from introducing negligible computational overhead, the proposed method is shown to perform well on imaging problems with noisy data and problems that involve complex settings such as salt. In either case, the presented method produces high-resolution high-amplitude fidelity images including an estimate for the source-time function. In addition, due to its use of stochastic optimization, we arrive at these images at roughly one to two times the cost of conventional reverse-time migration involving all data.     

Reconstructing ice-flow fields from streamlined subglacial bedforms: A kriging approach

The orientation of several landforms, e.g. drumlins, flutes, crag-and-tails, and mega-scale glacial lineations, records the direction of the overlying ice flow that created them. Populations of such features are used routinely to infer former ice-flow patterns, which serve as the building blocks of reconstructions of palaeo ice-sheet evolution. Currently, the conceptualisation of flow patterns from these flow-direction records is done manually and qualitatively, so the extractable glaciological information is limited. We describe a kriging method (with Matlab code implementation) that calculates continuous fields of ice-flow direction, convergence, and curvature from the flow-direction records, and which yields quantitative results with uncertainty estimates. We test the method by application to the subglacial bedforms of the Tweed Valley Basin, UK. The results quantify the convergent flow pattern of the Tweed Palaeo-Ice Stream in detail and pinpoint its former lateral shear margins and where ice flowed around basal bumps. Ice-flow parameters retrieved by this method can enrich ice-sheet reconstructions and investigations of subglacial till processes and bedform genesis. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

The mechanism of formation of the Baikal basin

The Baikal is a deep long and narrow basin in East Siberia which follows a huge fault zone adjoining the Siberian Platform. The basin was formed by rapid subsidence of continental crust during the pas 3–4 Ma. It is bounded by normal faults which indicate extension of the crust during the subsidence. According to seismic reflection profiling data, the intensity of extension is not large (3–7%). It is much smaller than the thinning of the crystalline crust under the basin (up to 38%). The thinning and crustal subsidence can be explained by the transformation of gabbro in the lower crust into dense garnet granulites. The latter rocks (with V_p 7.7−7.8 km/sec) are still located under the remnant part of the crust. Rapid transformation took place due to an inflow of catalyzing fluid along the fault zone from the asthenospheric upwelling. This upwelling, which is at a depth of 80–90 km, caused a general uplift of a broad area in the south of East Siberia.     

A generic approach to modelling flexible confined boundary conditions in SPH and its application

In this paper, a new approach to applying confining stress to flexible boundaries in the smoothed particle hydrodynamics (SPH) method is developed to facilitate its applications in geomechanics. Unlike the conventional SPH methods that impose confining boundary conditions by creating extra boundary particles, the proposed approach makes use of kernel truncation properties of SPH approximations that occur naturally at free-surface boundaries. Therefore, it does not require extra boundary particles and, as a consequence, can be utilised to apply confining stresses onto any boundary with arbitrary geometry without the need for tracking the curvature change during the computation. This enables more complicated problems that involve moving confining boundaries, such as confining triaxial tests, to be simulated in SPH without difficulties. To further enhance SPH applications in elasto-plastic computations of geomaterials, a robust numerical procedure to implement Mohr-Coulomb plasticity model in SPH is presented for the first time to avoid difficulties associated with corner singularities in Mohr-Coulomb model. The proposed approach was first validated against two-dimensional finite element (FE) solutions for confining biaxial compression tests to demonstrate its predictive capability at small deformation range when FE solutions are still valid. It is then further extended to three-dimensional conditions and utilised to simulate triaxial compression experiments. Simulation results predicted by SPH show good agreement with experiments, FE solutions, and other numerical results available in the literature. This suggests that the proposed approach of imposing confining stress boundaries is promising and can handle complex problems that involve moving confining boundary conditions.     

Characterization of indoor and outdoor atmospheric pollutants impacting architectural monuments: the case of San Jerónimo Monastery (Granada, Spain)

Indoor and outdoor concentrations of atmospheric gaseous pollutants as well as composition, size, and morphology of particulate matter have been investigated at the monastery of San Jerónimo in Granada (Southern Spain). Complementary micro- and nano-analytical techniques were applied; elemental and mineralogical composition and morphological characteristics of particulate matter were investigated combining electron probe microanalysis at the single particle level, and bulk aerosol samples were analyzed using energy-dispersive X-ray fluorescence, X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray analyzer and transmission electron microscopy (TEM). Microclimatic conditions at the monastery were monitored, and gas concentrations were assessed by means of diffusion tubes subsequently analyzed with ion chromatography. Results revealed high abundances of soil dust particles (aluminosilicates, calcite, dolomite, quartz), salt aerosols (chlorides, sulfates and ammonium-rich salts), and NO₂ and SO₂ both outdoors and indoors. Amorphous black carbon particles had surprisingly high abundances for Granada, a non-industrialized city. The composition of indoor particles corresponds to severe weathering affecting the construction materials and artworks inside the church; moreover their composition promotes a feedback process that intensifies the deterioration. Chemical reactions between chloride-rich salts and pigments from paintings were confirmed by TEM analyses. Indoors, blackening of surface decorative materials is fostered by particle re-suspension due to cleaning habits in the monastery (i.e. dusting). This is the first air quality study performed in a monument in the city of Granada with the aim of developing a strategy for preventive conservation.