Troposphere delays from space geodetic techniques, water vapor radiometers, and numerical weather models over a series of continuous VLBI campaigns

Continuous, very long baseline interferometry (VLBI) campaigns over 2 weeks have been carried out repeatedly, i.e., CONT02 in October 2002, CONT05 in September 2005, CONT08 in August 2008, and CONT11 in September 2011, to demonstrate the highest accuracy the current VLBI was capable at that time. In this study, we have compared zenith total delays (ZTD) and troposphere gradients as consistently estimated from the observations of VLBI, Global Navigation Satellite Systems (GNSS), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) at VLBI sites participating in the CONT campaigns. We analyzed the CONT campaigns using the state-of-the-art software following common processing strategies as closely as possible. In parallel, ZTD and gradients were derived from numerical weather models, i.e., from the global European Centre for Medium-Range Weather Forecasts (ECMWF) analysis fields, the High Resolution Limited Area Model (European sites), the Japan Meteorological Agency-Operational Meso-Analysis Field (MANAL, over Japan), and the Cloud Resolving Storm Simulator (Tsukuba, Japan). Finally, zenith wet delays were estimated from the observations of water vapor radiometers (WVR) at sites where the WVR observables are available during the CONT sessions. The best ZTD agreement, interpreted as the smallest standard deviation, was found between GNSS and VLBI techniques to be about 5–6 mm at most of the co-located sites and CONT campaigns. We did not detect any significant improvement in the ZTD agreement between various techniques over time, except for DORIS and MANAL. On the other hand, the agreement and thus the accuracy of the troposphere parameters mainly depend on the amount of humidity in the atmosphere.     

用于GNSS的SpT星载原子钟及时间系统介绍

研究了空间铷钟和被动型氢钟的地面批量和寿命试验测试结果,以及卫星在轨试验所达到的最新性能结果。基于这些星载钟的试验结果,对全球卫星导航系统的地面时间站的关键设备及其相关算法作了简要描述,并介绍了一种新颖的在轨技术,即从星载原子钟组（ONCLE）直接产生高度稳健的时间频率信号。     

A GIS‐based Approach in Support of an Assessment of Food Safety Risks

A Geographical Information System (GIS)‐based approach was developed for the identification of vulnerabilities and the measurement of risks associated with contamination of food systems with biological agents. In this research work, a tight integration of ArcGIS with the Arena simulation tool has been implemented. Arena was used to simulate and track contamination in a food distribution network and transmit the time dependent information to GIS. ArcGIS was employed to provide the primary user interface, process network data, and visualize the results. In addition, the GIS, through its powerful capabilities to process spatial data, could allow decision‐ makers to quickly determine the potential impact of a contamination event, at any stage, as a function of both time and geography. Two contamination scenarios along the farm‐to‐fork chain were examined to show the geographic zone and the proportion of the population affected by the contamination. A constraint Voronoi data structure was developed to define influence zones (these were color coded according to a dynamic risk index), to identify those areas that are at greatest immediate risk as time progresses, and to estimate the population affected by these contamination events. This approach thus appears to have general application to many GIS‐based risk assessment problems.     

Water and the density of silicate glasses

A review of published and newly measured densities for 40 hydrous silicate glasses indicates that the room-temperature partial molar volume of water is 12.0 ± 0.5 cm³/mol. This value holds for simple or mineral compositions as well as for complex natural glasses, from rhyolite to tephrite compositions, prepared up to 10–20 kbar pressures and containing up to 7 wt% H₂O. This volume does not vary either with the molar volume of the water-free silicate phase, with its degree of polymerization or with water speciation. Over a wide range of compositions, this constant value implies that the volume change for the reaction between hydroxyl ions and molecular water is zero and that, at least in glasses, speciation does not depend on pressure. Consistent with data from Ochs and Lange (1997, 1999), systematics in volume expansion for SiO₂–M₂O systems (M=H, Li, Na, K) suggests that the partial molar thermal expansion coefficient of H₂O is about 4 × 10⁻⁵ K⁻¹ in silicate glasses. Received: 30 June 1999 / Accepted: 5 November 1999     

Analytical Procedures for the Measurement of Boron Isotope Compositions by Ion Microprobe in Meteorites and Mantle Rocks

An analytical procedure has been developed to measure in situ the ¹¹ B/¹⁰ B ratio in terrestrial basaltic rocks and meteoritic chondrules having B concentration of less than 1 μg g^?1 using a small radius ims3f ion microprobe. The central difficulties for these measurements are (i) the removal of the trace amount of B contamination introduced in the sample during polishing, (ii) the precise calibration of instrumental mass fractionation of B isotopes and (iii) the low count rates of ¹⁰ B and ¹¹ B. Contamination experiments conducted with isotopically labelled B enriched in ¹⁰ B showed that ultrasonic cleaning in bi-distilled water (< 1 ng g^?1 B) and pre-sputtering of the analysed area decrease B contamination to the level of 0.01 μg g^?1. Analyses of isotope standards spanning a range of ¹¹ B/¹⁰ B between 3.93 and 4.20 showed that instrumental mass fractionation was constant within? during one session of analyses. Repeated analyses of a standard glass showed a reproducibility of instrumental mass fractionation between February 1991 and October 1996 of 1.3. Taking into account all sources of error, boron isotope measurements are accurate to within 5 for meteoritic samples having B contents in the range 0.1 to 1 μg g^?1. A slightly better accuracy of 1.5 can be achieved for basaltic glasses which can be sputtered with very intense primary beams.     

Paléoclimat des Petites Antilles depuis 4000 ans BP : l'enregistrement de la lagune de Grand-Case à Saint-Martin

A 2.5-m-thick sequence of lake sediments at the Étang de Grand-Case, Saint Martin (French West Indies) is studied here. Significant hydrological budget fluctuations allows distinction of three main climatic periods: (1) an overall dry period (4200 BP–2300 BP), characterised by carbonated mud, gypsum and storm sand layers; (2) a wet phase (2300 BP–1150 BP) dominated by organic mud; and (3) a more complex phase (1150 BP to present), with detrital inputs due to human activities. Comparison with other regional high-resolution records shows that similar climate modifications typify the whole Mesoamerican and Caribbean area. The climatic phenomena that are implicated in the variations of both precipitation and hurricane frequency over the Lesser Antilles are due to the latitudinal displacement of the inter-tropical convergence zone. These data give new support to the hypothesis of the existence of a correlation between peopling phases and climate variations in the Caribbean as previously proposed by some archaeologists. To cite this article: P. Bertran et al., C. R. Geoscience 336 (2004).