Dielectric-Covered Ground Reflectors in GPS Multipath Reception—Theory and Measurement

Global positioning system receivers sometimes operate under severe multipath conditions, including those which include a ground specularly reflected signal along with a direct signal. Here, such a case is discussed which involves a dielectric-covered flat ground reflector, with incident energy at 1.57542-GHz right-hand circularly polarized at elevation angles between 5$^{circ}$ and 50 $^{circ}$. The relative received powers produced by the sum of the specularly reflected and direct signals are computed from conventional theory and are measured for various thicknesses of dielectrics. Both theory and measurement show that the received powers produced by the sum of the specularly reflected and direct signals have the same behavior throughout a range of elevation angles. A snow case study shows potential for inferring snow depth by fitting the theory to the measurements.      

Analysis of nonintrinsic spatial variability by residual kriging with application to regional groundwater levels

A method for obtaining pointwise or spatially averaged estimates of a nonintrinsic function is introduced based on residual kriging. The method relies on a stepwise iterative regression process for simultaneously estimating the global drift and residual semivariogram. Estimates of the function are then obtained by solving a modified set of simple kriging equations written for the residuals. The modification consists of replacing the true variogram in the kriging equations by the variogram of the residual estimates as obtained from the iterative regression process. The method is illustrated by considering groundwater levels in an Arizona aquifer. The results are compared with those obtained for the aquifer by the generalized covariance package BLUEPACK-3D.     

Climatic and tectonic controls on chemical weathering in the New Zealand Southern Alps

Climatic and tectonic controls on the relative abundance of solutes in streams draining the New Zealand Southern Alps were investigated by analyzing the elemental and Sr isotope geochemistry of stream waters, bedload sediment, and hydrothermal calcite veins. The average relative molar abundance of major cations and Si in all stream waters follows the order Ca²⁺ (50%) > Si (22%) > Na⁺ (17%) > Mg²⁺ (6%) > K⁺ (5%). For major anions, the relative molar abundance is HCO₃⁻ (89%) > SO₄²⁻ (7%) > Cl⁻ (4%). Weathering reactions involving plagioclase and volumetrically small amounts of hydrothermal calcite define the ionic chemistry of stream waters, but nearly all streams have a carbonate-dominated Ca²⁺ and HCO₃⁻ mass-balance. Stream water Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios vary from 0.173 to 0.439 μmol/nmol and from 0.7078 to 0.7114, respectively. Consistent with the ionic budget, these ratios lie solely within the range of values measured for bedload carbonate (Ca/Sr = 0.178 to 0.886 μmol/nmol; ⁸⁷Sr/⁸⁶Sr = 0.7081 to 0.7118) and hydrothermal calcite veins (Ca/Sr = 0.491 to 3.33 μmol/nmol; ⁸⁷Sr/⁸⁶Sr = 0.7076 to 0.7097).Streams draining regions in the Southern Alps with high rates of physical erosion induced by rapid tectonic uplift and an extremely wet climate contain ∼10% more Ca²⁺ and ∼30% more Sr²⁺ from carbonate weathering compared to streams draining regions in drier, more stable landscapes. Similarly, streams draining glaciated watersheds contain ∼25% more Sr²⁺ from carbonate weathering compared to streams draining non-glaciated watersheds. The highest abundance of carbonate-derived solutes in the most physically active regions of the Southern Alps is attributed to the tectonic exhumation and mechanical denudation of metamorphic bedrock, which contains trace amounts of calcite estimated to weather ∼350 times faster than plagioclase in this environment. In contrast, regions in the Southern Alps experiencing lower rates of uplift and erosion have a greater abundance of silicate- versus carbonate-derived cations. These findings highlight a strong coupling between physical controls on landscape development and sources of solutes to stream waters. Using the Southern Alps as a model for assessing the role of active tectonics in geochemical cycles, this study suggests that rapid mountain uplift results in an enhanced influence of carbonate weathering on the dissolved ion composition delivered to seawater.     

Propagation of the Bastille Day 2000 CME Shock in the Outer Heliosphere

The Bastille Day (14 July) 2000 CME is a fast, halo coronal mass ejection event headed earthward. The ejection reached Earth on 15 July 2000 and produced a very significant magnetic storm and widespread aurora. At 1 AU the Wind spacecraft recorded a strong forward shock with a speed jump from ∼ 600 to over 1000 km s⁻¹. About 6 months later, this CME-driven shock arrived at Voyager 2 (∼ 63 AU) on 12 January 2001 with a speed jump of ∼ 60 km s⁻¹. This provides a good opportunity to study the shock propagation in the outer heliosphere. In this study, we employ a 2.5-D MHD numerical model, which takes the interaction of solar wind protons and interstellar neutrals into account, to investigate the shock propagation in detail and compare the model predictions with the Voyager 2 observations. The Bastille Day CME shock undergoes a dramatic change in character from the inner to outer heliosphere. Its strength and propagation speed decay significantly with distance. The model results at the location of Voyager 2 are in good agreement with in-situ observations. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014293527951     

Orbits and masses of Saturn's coorbital and F-ring shepherding satellites

We have obtained numerically integrated orbits for Saturn's coorbital satellites, Janus and Epimetheus, together with Saturn's F-ring shepherding satellites, Prometheus and Pandora. The orbits are fit to astrometric observations acquired with the Hubble Space Telescope and from Earth-based observatories and to imaging data acquired from the Voyager spacecraft. The observations cover the 38 year period from the 1966 Saturn ring plane crossing to the spring of 2004. In the process of determining the orbits we have found masses for all four satellites. The densities derived from the masses for Janus, Epimetheus, Prometheus, and Pandora in units of g cm⁻³ are , , , and , respectively.     

A shallow subsurface controlled release facility in Bozeman, Montana, USA, for testing near surface CO2 detection techniques and transport models

A controlled field pilot has been developed in Bozeman, Montana, USA, to study near surface CO₂ transport and detection technologies. A slotted horizontal well divided into six zones was installed in the shallow subsurface. The scale and CO₂ release rates were chosen to be relevant to developing monitoring strategies for geological carbon storage. The field site was characterized before injection, and CO₂ transport and concentrations in saturated soil and the vadose zone were modeled. Controlled releases of CO₂ from the horizontal well were performed in the summers of 2007 and 2008, and collaborators from six national labs, three universities, and the U.S. Geological Survey investigated movement of CO₂ through the soil, water, plants, and air with a wide range of near surface detection techniques. An overview of these results will be presented.     

Dissociation constants of calcite and CaHC03+ from 0 to 50°C

From conductance measurements, the negative logarithm of the dissociation constant of the CaHCO₃₊ ion pair, pK(CaHCO₃⁺), is 0.7, 1.0 and 1.35 within ±0.05 units at 0, 25 and 60°C, respectively. A revaluation of published and unpublished data yields pK(CaCO₃⁰) ≈ 3.2 at 25°C. Use of these pK's to compute the dissociation constant of calcite (Kc) from published calcite solubility measurements in pure water gives pKc values which increase markedly with ionic strength. However, if the ion pairs are ignored, computed pKc values are nearly constant with ionic strength. All reasonable attempts to eliminate the trend in pKc by adjusting ion activity coefficients, and/or values of K(CaCO₃⁰) failed, so the dilemma remains. Kc values computed from the most reliable published calcite solubility data are in good agreement with such values based on solubility data measured in this study at 5, 15, 35 and 50°C. Study results ignoring ion pairs are accurately represented by the equation log Kc = 13.870 — (3059/T) ?0.04035T, and correspond to ?8.35, ?8.42, and ?8.635 at 0, 25 and 50°C, respectively. The logarithmic expression leads to ΔH_r^o = ?2420 ± 300 cal/mol, ΔCp = ?110 ± 2 cal/deg mol, and ΔS_r^o = ?46.6 ± 1.0 cal/deg mol for the calcite dissociation reaction at 25°C. The dependence of Kc on temperature when CaCO₃⁰ and CaHCO₃⁺ are assumed, is described by log Kc = 13.543 ? (3000/T) ? 0.0401T which yields ?8.39, ?8.47, and -8.70 at 0, 25 and 50°C. This gives ΔH_r^o = ?2585 ± 300 cal/mol, ΔCp = ?109 ± 2 cal/deg mol, and ΔS_r⁰ = ?47.4 ± 1.0 cal/deg mol at 25°C.     

Estimation of semivariogram parameters and evaluation of the effects of data sparsity

Semivariogram parameters are estimated by a weighted least-squares method and a jackknife kriging method. The weighted least-squares method is investigated by differing the lag increment and maximum lag used in the fit. The jackknife kriging method minimizes the variance of the jackknifing error as a function of semivariogram parameters. The effects of data sparsity and the presence of a trend are investigated by using 400-, 200-, and 100-point synthetic data sets. When the two methods yield significantly different results, more data may be needed to determine reliably the semivariogram parameters, or a trend may be present in the data.