A composite focal plane assembly for the GERB instrument

In this paper, we present the design and construction of a composite focal plane assembly (FPA) for the Geostationary Earth Radiation Budget (GERB) Earth observing, visible–infrared imaging radiometer. The FPA is a complex structure that incorporates a number of features aimed at reducing mass, power and volume. These include the use of silicon and ceramic substrates for high density tracking and temperature stabilisation, removable test structures for component verification prior to final assembly and high levels of integration.     

Proposal for ionospheric tomography technique using a new HF radio source

Measurements of the line integral of the electron density along satellite-to-ground ray paths (i.e. TEC) using differential phase or Doppler of two coherent VHF/UHF signals transmitted from NNSS or GPS satellite networks have been used in the ionospheric tomography for mapping large-scale ionospheric images over region of interest. In this paper, we present theoretical studies of using a new signal source HF frequency in tomographic imaging. The initial phase problem inherent in the phase measurement can be eliminated by measuring Faraday rotations. Relative rotation on two adjacent HF frequencies is used to solve the ambiguity problem. A second-order approximation of the Faraday rotation incorporated with ray-tracing technique improves the reconstruction degradation due to ionospheric refractions. CASSIOPE is a multi-purpose small satellite that receives HF signals from ground radar facilities and it is scheduled for launch in early 2008. Simulations have demonstrated the potential applications of the ionospheric tomography in CASSIOPE/e-POP satellite experiment.     

FIBERSIM(R)在无人机复合材料部件开发中的优点

FiberSIM能辅助无人机复合材料结构在更低的成本下取得更轻的重量,同时能标准化和自动化完成开发过程。高空无人机的机身约占起飞总重的三分之一。任何对机身的减重都直接转化成增加传感器或有效载荷、飞行持续时间。因此制造者专注于采用工业定义的工程过程来设计     

Gas exchange rates of potato stands for bioregenerative life support

Plants can provide a means for removing carbon dioxide (CO₂) while generating oxygen (O₂) and clean water for life support systems in space. To study this, 20 m² stands of potato (Solanum tuberosum L.) plants were grown in a large (113 m³ vol.), atmospherically closed chamber. Photosynthetic uptake of CO₂ by the stands was detected about 10 DAP (days after planting), after which photosynthetic rates rose rapidly as stand ground cover and total light interception increased. Photosynthetic rates peaked ca. 50 DAP near 45 μmol CO₂ m⁻² s⁻¹ under 865 μmol m⁻² s⁻¹ PPF (average photosynthetic photon flux), and near 35 μmol CO₂ m⁻² s⁻¹ under 655 μmol m⁻² s⁻¹ PPF. Short term changes in PPF caused a linear response in stand photosynthetic rates up to 1100 μmol m⁻² s⁻¹ PPF, with a light compensation point of 185 μmol m⁻² s⁻¹ PPF. Comparisons of stand photosynthetic rates at different CO₂ concentrations showed a classic C₃ response, with saturation occurring near 1200 μmol mol⁻¹ CO₂ and compensation near 100 μmol mol⁻¹ CO₂. In one study, the photoperiod was changed from 12 h light/12 h dark to continuous light at 58 DAP. This caused a decrease in net photosynthetic rates within 48 h and eventual damage (scorching) of upper canopy leaves, suggesting the abrupt change stressed the plants and/or caused feedback effects on photosynthesis. Dark period (night) respiration rates increased during early growth as standing biomass increased and peaked near 9 μmol CO₂ m⁻² s⁻¹ ca. 50 DAP, after which rates declined gradually with age. Stand transpiration showed a rapid rise with canopy ground cover and peaked ca. 50 DAP near 8.9 L m⁻² d⁻¹ under 860 μmol m⁻² s⁻¹ PPF and near 6.3 L m⁻² d⁻¹ under 650 μmol m⁻² s⁻¹ PPF. Based on the best photosynthetic rates from these studies, approximately 25 m² of potato plants under continuous cultivation would be required to support the CO₂ removal and O₂ requirements for one person.     

The Dawn Spacecraft

The Dawn spacecraft is designed to travel to and operate in orbit around the two largest main belt asteroids, Vesta and Ceres. Developed to meet a ten-year life and fully redundant, the spacecraft accommodates an ion propulsion system, including three ion engines and xenon propellant tank, utilizes large solar arrays to power the engines, carries the science instrument payload, and hosts the hardware and software required to successfully collect and transmit the scientific data back to Earth. The launch of the Dawn spacecraft in September 2007 from Cape Canaveral Air Force Station was the culmination of nearly five years of design, development, integration and testing of this unique system, one of the very few scientific spacecraft to rely on ion propulsion. The Dawn spacecraft arrived at its first destination, Vesta, in July 2011, where it will conduct science operations for twelve months before departing for Ceres.     

Microwave policy issues for solar space power

This article reviews the three major policy issues likely to arise from an SSP system: environmental safety, frequency allocation and prevention of interference with other frequency-using activities. Supporters of SSP must make sure that their case is heard clearly at the ITU, but they must also do more to promote public awareness of the technology's potential benefits in order to counter inappropriate use of the Precautionary Principle by anti-technology groups. The strengthening of standard-setting groups world-wide will also assist this process.     

Newly developed techniques for the analysis of micrometer-sized interplanetary dust particles and comet grains

Electron transparent thin sections (30–100 nm thick) of interplanetary dust particles and other fine-grained meteoritic materials are produced using an ultramicrotome equipped with a diamond knife. An analytical electron microscope (AEM) is imployed to examine indigenous physical properties (e.g. porosity), mineralogy, and petrography. Large data sets of quantitative point count analyses obtained from thin sections enable direct mineralogical comparison of IDPs and Halley.     

Impact craters as biospheric microenvironments, Lawn Hill Structure, Northern Australia

Impact craters on Mars act as traps for eolian sediment and in the past may have provided suitable microenvironments that could have supported and preserved a stressed biosphere. If this is so, terrestrial impact structures such as the 18-km-diameter Lawn Hill Structure, in northern Australia, may prove useful as martian analogs. We sampled outcrop and drill core from the carbonate fill of the Lawn Hill Structure and recorded its gamma-log signature. Facies data along with whole rock geochemistry and stable isotope signatures show that the crater fill is an outlier of the Georgina Basin and was formed by impact at, or shortly before, approximately 509-506 million years ago. Subsequently, it was rapidly engulfed by the Middle Cambrian marine transgression, which filled it with shallow marine carbonates and evaporites. The crater formed a protected but restricted microenvironment in which sediments four times the thickness of the nearby basinal succession accumulated. Similar structures, common on the martian surface, may well have acted as biospheric refuges as the planet's water resources declined. Low-pH aqueous environments on Earth similar to those on Mars, while extreme, support diverse ecologies. The architecture of the eolian crater fill would have been defined by long-term ground water cycles resulting from intermittent precipitation in an extremely arid climate. Nutrient recycling, critical to a closed lacustrine sub-ice biosphere, could be provided by eolian transport onto the frozen water surface.     

Application of neural networks to South African GPS TEC modelling

The propagation of radio signals in the Earth’s atmosphere is dominantly affected by the ionosphere due to its dispersive nature. Global Positioning System (GPS) data provides relevant information that leads to the derivation of total electron content (TEC) which can be considered as the ionosphere’s measure of ionisation. This paper presents part of a feasibility study for the development of a Neural Network (NN) based model for the prediction of South African GPS derived TEC. The South African GPS receiver network is operated and maintained by the Chief Directorate Surveys and Mapping (CDSM) in Cape Town, South Africa. Vertical total electron content (VTEC) was calculated for four GPS receiver stations using the Adjusted Spherical Harmonic (ASHA) model. Factors that influence TEC were then identified and used to derive input parameters for the NN. The well established factors used are seasonal variation, diurnal variation, solar activity and magnetic activity. Comparison of diurnal predicted TEC values from both the NN model and the International Reference Ionosphere (IRI-2001) with GPS TEC revealed that the IRI provides more accurate predictions than the NN model during the spring equinoxes. However, on average the NN model predicts GPS TEC more accurately than the IRI model over the GPS locations considered within South Africa.