Is there utility in rigorous combinations of VLBI and GPS Earth orientation parameters?

Combinations of station coordinates and velocities from independent space-geodetic techniques have long been the standard method to realize robust global terrestrial reference frames (TRFs). In principle, the particular strengths of one observing method can compensate for weaknesses in others if the combination is properly constructed, suitable weights are found, and accurate co-location ties are available. More recently, the methodology has been extended to combine time-series of results at the normal equation level. This allows Earth orientation parameters (EOPs) to be included and aligned in a fully consistent way with the TRF. While the utility of such multi-technique combinations is generally recognized for the reference frame, the benefits for the EOPs are yet to be quantitatively assessed. In this contribution, which is a sequel to a recent paper on co-location ties (Ray and Altamimi in J Geod 79(4–5): 189–195, 2005), we have studied test combinations of very long baseline interferometry (VLBI) and Global Positioning System (GPS) time-series solutions to evaluate the effects on combined EOP measurements compared with geophysical excitations. One expects any effect to be small, considering that GPS dominates the polar motion estimates due to its relatively dense and uniform global network coverage, high precision, continuous daily sampling, and homogeneity, while VLBI alone observes UT1-UTC. Presently, although clearly desirable, we see no practical method to rigorously include the GPS estimates of length-of-day variations due to significant time-varying biases. Nevertheless, our results, which are the first of this type, indicate that more accurate polar motion from GPS contributes to improved UT1-UTC results from VLBI. The situation with combined polar motion is more complex. The VLBI data contribute directly only very slightly, if at all, with an impact that is probably affected by the weakness of the current VLBI networks (small size and sparseness) and the quality of local ties relating the VLBI and GPS frames. Instead, the VLBI polar motion information is used primarily in rotationally aligning the VLBI and GPS frames, thereby reducing the dependence on co-location tie information. Further research is needed to determine an optimal VLBI-GPS combination strategy that yields the highest quality EOP estimates. Improved local ties (including internal systematic effects within the techniques) will be critically important in such an effort.     

Terrestrial reference frame requirements within GGOS perspective

One of the main objectives of the promising and challenging IAG project GGOS (Global Geodetic Observing System) is the availability of a global and accurate Terrestrial Reference Frame for Earth Science applications, particularly Earth Rotation, Gravity Field and geophysics. With the experience gained within the activities related to the International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF), the combination method proved its efficiency to establish a global frame benefiting from the strengths of the various space geodetic techniques and, in the same time, underlining their biases and weaknesses. In this paper we focus on the limitation factors inherent to each individual technique and to the combination, such as the current status of the observing networks, distribution of the co-location sites and their quality and accuracy of the combined frame parameters. Results of some TRF and EOP simultaneous combinations using CATREF software will be used to illustrate the current achievement and to help drawing up future goals and improvements in the GGOS framework. Beyond these technical aspects, the overall visibility and acceptance of ITRS/ITRF as international standard for science and applications is also discussed.     

Effect of different tropospheric mapping functions on the TRF, CRF and position time-series estimated from VLBI

This paper compares estimated terrestrial reference frames (TRF) and celestial reference frames (CRF) as well as position time-series in terms of systematic differences, scale, annual signals and station position repeatabilities using four different tropospheric mapping functions (MF): The NMF (Niell Mapping Function) and the recently developed GMF (Global Mapping Function) consist of easy-to-handle stand-alone formulae, whereas the IMF (Isobaric Mapping Function) and the VMF1 (Vienna Mapping Function 1) are determined from numerical weather models. All computations were performed at the Deutsches Geodätisches Forschungsinstitut (DGFI) using the OCCAM 6.1 and DOGS-CS software packages for Very Long Baseline Interferometry (VLBI) data from 1984 until 2005. While it turned out that CRF estimates only slightly depend on the MF used, showing small systematic effects up to 0.025 mas, some station heights of the computed TRF change by up to 13 mm. The best agreement was achieved for the VMF1 and GMF results concerning the TRFs, and for the VMF1 and IMF results concerning scale variations and position time-series. The amplitudes of the annual periodical signals in the time-series of estimated heights differ by up to 5 mm. The best precision in terms of station height repeatability is found for the VMF1, which is 5–7% better than for the other MFs.     

On the Equation of Position Measurement of XNAV

The equation of position measurement of XNAV (X-ray pulsar-based autonomous navigation) reveals the relation between the time of arrival (TOA) of an X-ray photon and the spacecraft position at this moment. In specific dis- cussions of navigation, the TOA is generally replaced by the difference between the TOA and a preset “time reference”, and the spacecraft position is expressed by the position vector in the barycentric system of the solar system. The “time reference” may be the true TOA at the solar-system barycenter (SSB), or an “equivalent TOA” at SSB. As the true TOA at SSB is difficult to obtain, the “equivalent TOA” is more convenient for navigation. The equations of position measurement based on the two kinds of “time reference” are discussed, the equa- tion based on the “equivalent TOA”, which has the time accuracy of 0.1 ns, is given, and the physical meanings of the every term in which are analyzed.     

A novel approach for modelling soil–water characteristic curves with hysteresis

Soil–water characteristic curves can be defined as the relationship between the degree of saturation and suction of an unsaturated soil. Geomaterials, such as clays, sands, and geotextiles, usually exhibit hysteresis between drying and wetting curves. In addition, each drying and wetting curve is nonlinear in shape, which may be approximated by sigmoid curves. In geotechnical engineering, it is common to adopt analytical expressions for these curves that must be calibrated iteratively by trying different values for the constitutive parameters. In this paper, a novel approach for modelling the nonlinear saturation–suction response with hysteresis is presented, where a simple differential equation is introduced to describe the shapes of the curves. The great advantage of this new technique is the ease with which the parameters can be determined. In addition, the implementation of the resulting equations into fully hydro-mechanical models for numerical analyses is straightforward. Some features of the behaviour predicted with the new representation are studied and validations against real laboratory curves for soils are presented. The technique is simple, yet versatile due to the rational basis used in the deduction of the equations, which allows for future extensions to soils displaying more complex unsaturated behaviour.     

Selenium and heavy metals content in some Mediterranean soils

The study of metal contents in industrial, agricultural or/and polluted soils compared with natural or unpolluted soils is currently necessary to obtain reference values and to assess soil contamination. Nonetheless, very few works published appear in international journals on elements like Se, Li and Sr in Spanish soils. This study determines the total levels of Se, Li, Sr, As, Cd, Co, Cr, Cu, Ni, Pb, V, Zn, Fe, Mn and Ba in 14 natural (unpolluted) soils (Gypsisols, Leptosols, Arenosols and Acrisols), 14 agricultural soils (Anthrosols, Fluvisols and Luvisols), and 4 industrial–urban affected-surface soil horizons (Anthrosols and Fluvisols) of Eastern Spain. The geochemical baseline concentrations (GBC) and reference values (RV) have been established, and the relationships among elements and also between soil properties and elemental concentrations have been analysed. The RV obtained in this study were (mg kg⁻¹): Se 2.68, Li 115, Sr 298, Cd 0.97, Co 35, Cr 217, Cu 46, Ni 50, Pb 137, V 120, Zn 246, Fe 124,472, Mn 2691, and Ba 743. The RV for Se and Li were used as a preliminary approach to assess soil contamination in Spanish soils. The results confirm human impact on Sr, As, Cd, Cr, Cu, Ni, Pb and Zn soil concentrations, but evidence no deviation from natural Se, Li, Co, V, Fe, Mn and Ba concentrations. The results obtained from the statistical analysis reveal significant correlations between some elements and clay and soil organic matter (SOM) contents, indicating that metal concentrations are controlled by soil composition. One particularly interesting finding is the high correlation coefficients obtained between SOM and Se, Cd, Cr, V, Fe, and Mn, and between clay and Cd, Zn, V, Fe and Mn. Once again, these facts confirm the role of SOM and clay minerals in soil functions and that soil is an ecosystem element responsible for maintaining environmental quality.     

氩-氩定年法国际标准物质BSP-1角闪石的研制

BSP-1角闪石选自中国吉林省桦甸县老牛沟的角闪石岩。~(40)Ar-~(39)Ar分析和均匀性检查表明:此角闪石结晶以后未受过热扰动,~(40)Ar~*保存性好,谱线平坦,K/Ca和Cl/K比值稳定。定值结果显示,在置信概率为0.95时,~(40)Ar~*和K含量的相对偏差均<5‰,是目前最理想的Ar-Ar和K-Ar法国际标准物质,可供全世界Ar-Ar实验室使用80年以上。     

辽宁参考作物腾发量变化特征及成因分析

利用1961-2020年辽宁地区62个国家级气象观测站生长季的逐日气象观测数据,采用Penman-Monteith公式计算了辽宁地区参考作物腾发量（ET0）,利用ArcGIS的克里格插值法、M-K检验分析了辽宁地区生长季ET0的时空分布特征,对影响ET0变化的成因进行了分析。结果表明,近60年辽宁地区生长季参考作物腾发量呈现由西北向东南递减的变化趋势;ET0在1961-2010年呈下降趋势,2011-2020年呈升高趋势,生长季多年平均ET0变化趋势表现为波动下降趋势;生长季内ET0对相对湿度的响应最为敏感,为负效应。ET0对风速和温度变化的响应敏感性相对较小,为正效应。湿度的敏感系数绝对值明显高于风速和温度,7月份达到峰值;多年相对变化率绝对值最大的是风速,其次是温度和相对湿度;三个气象要素对ET0贡献最大的是风速,温度和湿度对ET0的正贡献不及风速的负贡献,综合敏感性和贡献两方面因素分析,风速的变化趋势为ET0呈下降趋势的主导因子。     

国际前寒武纪地质年代表划分方案研究进展

传统的前寒武纪地质年代表划分方案以全球标准地层年龄（GSSA）为基础,不代表任何特殊的岩石实体,仅以推测的绝对测年值为界线进行单元划分,脱离了客观的岩石记录和地球演化系统,不利于对前寒武纪地球系统的研究。2004年—2008年前寒武纪划分参考方案,以反映地球历史阶段特征的“关键事件”为界线,创建前寒武纪地层划分的“金钉子”,建立客观的、“自然的” 前寒武纪地质年代表,并且通过全球一级事件群把前寒武纪划分为5个宙,即创世宙、冥古宙、太古宙、过渡宙和元古宙。另外,经过综合分析建议将埃迪卡拉纪归到显生宙。因此,对前寒武纪的研究实际上变为对“前埃迪卡拉纪”的研究,使术语“显生宙”在内涵和应用上更加一致。虽然“参考方案”在一定程度上还仅仅是一个理论框架,需要大量的研究去充实和细化,但是对这两种划分方案的系统研究和对比,可以给我国前寒武纪工作者提供重要的研究思路和方向。