The International DORIS Service (IDS): Toward maturity

DORIS is one of the four space-geodetic techniques participating in the Global Geodetic Observing System (GGOS), particularly to maintain and disseminate the Terrestrial Reference Frame as determined by International Earth rotation and Reference frame Service (IERS). A few years ago, under the umbrella of the International Association of Geodesy, a DORIS International Service (IDS) was created in order to foster international cooperation and to provide new scientific products. This paper addresses the organizational aspects of the IDS and presents some recent DORIS scientific results. It is for the first time that, in preparation of the ITRF2008, seven Analysis Centers (AC’s) contributed to derive long-term time series of DORIS stations positions. These solutions were then combined into a homogeneous time series IDS-2 for which a precision of less than 10 mm was obtained. Orbit comparisons between the various AC’s showed an excellent agreement in the radial component, both for the SPOT satellites (e.g. 0.5–2.1 cm RMS for SPOT-2) and Envisat (0.9–2.1 cm RMS), using different software packages, models, corrections and analysis strategies. There is now a wide international participation within IDS that should lead to future improvements in DORIS analysis strategies and DORIS-derived geodetic products.     

On the convergence of iterative methods for bearings-only tracking

This paper deals with the analysis of the convergence of iterative methods for bearings only tracking (BOT). A geometric and unified framework is developed. Explicit sufficient conditions ensuring convergence of iterative methods for maximization of the likelihood functional are then derived     

Geochemical Reservoirs and Timing of Sulfur Cycling on Mars

Sulfate-dominated sedimentary deposits are widespread on the surface of Mars, which contrasts with the rarity of carbonate deposits, and indicates surface waters with chemical features drastically different from those on Earth. While the Earth’s surface chemistry and climate are intimately tied to the carbon cycle, it is the sulfur cycle that most strongly influences the Martian geosystems. The presence of sulfate minerals observed from orbit and in-situ via surface exploration within sedimentary rocks and unconsolidated regolith traces a history of post-Noachian aqueous processes mediated by sulfur. These materials likely formed in water-limited aqueous conditions compared to environments indicated by clay minerals and localized carbonates that formed in surface and subsurface settings on early Mars. Constraining the timing of sulfur delivery to the Martian exosphere, as well as volcanogenic H₂O is therefore central, as it combines with volcanogenic sulfur to produce acidic fluids and ice. Here, we reassess and review the Martian geochemical reservoirs of sulfur from the innermost core, to the mantle, crust, and surficial sediments. The recognized occurrences and the mineralogical features of sedimentary sulfate deposits are synthesized and summarized. Existing models of formation of sedimentary sulfate are discussed and related to weathering processes and chemical conditions of surface waters. We also review existing models of sulfur content in the Martian mantle and analyze how volcanic activities may have transferred igneous sulfur into the exosphere and evaluate the mass transfers and speciation relationships between volcanic sulfur and sedimentary sulfates. The sedimentary clay-sulfate succession can be reconciled with a continuous volcanic eruption rate throughout the Noachian-Hesperian, but a process occurring around the mid-Noachian must have profoundly changed the composition of volcanic degassing. A hypothetical increase in the oxidation state or in water content of Martian lavas or a decrease in atmospheric pressure is necessary to account for such a change in composition of volcanic gases. This would allow the pre mid-Noachian volcanic gases to be dominated by water and carbon-species but late Noachian and Hesperian volcanic gases to be sulfur-rich and characterized by high SO₂ content. Interruption of early dynamo and impact ejection of the atmosphere may have decreased the atmospheric pressure during the early Noachian whereas it remains unclear how the redox state or water content of lavas could have changed. Nevertheless, volcanic emission of SO₂ rich gases since the late Noachian can explain many features of Martian sulfate-rich regolith, including the mass of sulfate and the particular chemical features (i.e. acidity) of surface waters accompanying these deposits. How SO₂ impacted on Mars’s climate, with possible short time scale global warming and long time scale cooling effects, remains controversial. However, the ancient wet and warm era on Mars seems incompatible with elevated atmospheric sulfur dioxide because conditions favorable to volcanic SO₂ degassing were most likely not in place at this time.     

Bearings-only tracking for maneuvering sources

Classical bearings-only target-motion analysis (TMA) is restricted to sources with constant motion parameters (usually position and velocity). However, most interesting sources have maneuvering abilities, thus degrading the performance of classical TMA. In the passive sonar context a long-time source-observer encounter is realistic, so the source maneuver possibilities may be important in regard to the source and array baseline. This advocates for the consideration and modeling of the whole source trajectory including source maneuver uncertainty. With that aim, a convenient framework is the hidden Markov model (HMM). A basic idea consists of a two-levels discretization of the state-space. The probabilities of position transition are deduced from the probabilities of velocity transitions which, themselves, are directly related to the source maneuvering capability. The source state sequence estimation is achieved by means of classical dynamic programming (DP). This approach does not require any prior information relative to the source maneuvers. However, the probabilistic nature of the source trajectory confers a major role to the optimization of the observer maneuvers. This problem is then solved by using the general framework of the Markov decision process (MDP)     

The Solar Sail Materials (SSM) project – Status of activities

SSM (Solar Sail Materials) is an on-going project for the European Space Agency (ESA) relying on past and recent European solar sail design projects. It aims at developing and testing future technologies suitable for large, operational solar sailcrafts.     

Target motion analysis with multiple arrays: performance analysis

The performance analysis of source trajectory estimation using measurements provided by multiple platforms (or arrays) is studied. In numerous practical situations, the maneuvering ability of the receiver (e.g., a ship towing linear arrays) is limited, leading to the assumption that the observer motion is rectilinear and uniform. Even if this hypothesis appears quite restrictive, practical and tactical considerations fully justify its interest. This leads to consider multiple (platform) target motion analysis (denoted MTMA) and to analyze the performance of such trajectory estimation methods     

PR状态方程在超临界喷射模型中的应用

针对超临界流体物性的特殊性,对超临界喷射数值模拟方法进行研究。基于PR状态方程建立了考虑超临界流体特点的超临界喷射数值模型,并采用该模型对超临界C10H22喷射到超临界N2环境中的喷射进行了数值模拟。对比研究了采用PR状态方程的真实气体模型和理想气体模型得出的密度、温度、质量分数分布以及超临界喷射长度和喷射扩张角的变化规律和差异性,并与试验数据进行了对比。结果表明:2种模型在物性预测上的差异会造成以上喷射特性模拟结果的巨大差异,理想气体模型模拟结果与试验数据误差很大,利用真实气体模型能够得到与试验数据较为吻合的结果。基于PR状态方程的超临界喷射数值模型准确可靠,可为碳氢燃料的超临界喷射现象提供参考。     

波音777飞机ACARS系统及马航MH370航班失联事件分析

马来西亚航空公司MH370客机失联事件一度引发媒体与公众的诸多猜测,猜测的焦点之一就是执飞的波音777飞机的飞机通信寻址与报告系统(Aircraft Communication Addressing and Reporting System,简称ACARS)功能是如何被关闭的。从专业角度出发,首先采用Sys ML建模的方式描述了波音777飞机ACARS系统的详细架构和人机接口,并据此深入分析了马航MH370航班失联事件中ACARS被关闭的情景。最后,提出了此次事件暴露出的现代民用飞机设计中存在的问题以及改进方法。     

Influence of the gravitational vertical on geometric visual illusions

The occurrence of geometric orientation illusions and the perception of ambiguous figures were analyzed in 24 subjects during static body tilt relative to gravity on Earth. Results showed that illusions such as the Rock's diamond/square, the Ponzo illusion, and orientation contrast illusions occurred less frequently, and that depth reversal of ambiguous figures took more time when subjects were lying on their side or supine compared to upright, thus suggesting that the gravitational reference plays a significant role in these “visual” illusions. The structure of images, our representation of the environment, and orientation relative to gravity are all integral parts in interpreting visual images. In a weightless environment where no gravitational reference can be used, it is expected that similar alterations in visual perception will occur.     

纳米碳粉掺杂对激光烧蚀GAP的推进性能影响

实验以含能聚合物聚叠氮缩水甘油醚(Glycidyl azide polymer,GAP）作为激光烧蚀微推力器的靶材。通过对不同浓度纳米碳粉掺杂和靶材厚度下激光烧蚀GAP的比冲、冲量耦合系数和能量转化效率测量，结合靶材喷射羽流图像，分析了纳米碳粉掺杂提高激光烧蚀聚合物靶材推进性能的机理，给出纳米碳粉掺杂的适用方式。实验结果表明：透射式下，掺杂纳米碳粉之后，聚合物对激光的吸收大幅增强，但激光烧蚀推进性能不随掺杂浓度增加而显著提升；纳米碳粉吸收激光能量形成温度极高的局部热区促进聚合物中化学能的释放，是推进性能提升的主要原因；掺杂纳米碳粉之后的GAP烧蚀深度降低，表现出面吸收特性；随着靶材厚度的增加，未完全烧蚀的工质质量增加，使得靶材的利用率大大降低，导致聚合物推进性能下降。实验中掺杂3%纳米碳粉、厚度为54 μm的GAP靶材最优能量转化效率超过250%，适合作为透射式激光烧蚀微推力器的靶材。