欧洲通过内外的合作推动无人机的发展

欧洲正在通过欧洲各国之间及其与非欧洲国家之间的合作,加速推动欧洲无人机的发展。在过去的两年间,欧洲为了缩短与美国及以色列在无人机领域的差距,已经作了艰苦努力,开始自主开展无人作战飞机(UCAV)以及中空长航时无人侦察系统的研制工作,并在美国及以色列的经验基础上,重新评估对战术无人机的需求。而海军对垂直起降无人机的需求也逐渐明朗化了。     

Estimating low resolution gravity fields at short time intervals to reduce temporal aliasing errors

The Gravity Recovery and Climate Experiment (GRACE) satellite mission has been estimating temporal changes in the Earth’s gravitational field since its launch in 2002. While it is not yet fully resolved what the limiting source of error is for GRACE, studies on future missions have shown that temporal aliasing errors due to undersampling signals of interest (such as hydrological variations) and errors in atmospheric, ocean, and tide models will be a limiting source of error for missions taking advantage of improved technologies (flying drag-free with a laser interferometer). This paper explores the option of reducing the effects of temporal aliasing errors by directly estimating low degree and order gravity fields at short time intervals, ultimately resulting in data products with improved spatial resolution. Three potential architectures are considered: a single pair of polar orbiting satellites, two pairs of polar orbiting satellites, and a polar orbiting pair of satellites coupled with a lower inclined pair of satellites. Results show that improvements in spatial resolution are obtained when one estimates a low resolution gravity field every two days for the case of a single pair of satellites, and every day for the case of two polar pairs of satellites. However, the spatial resolution for these cases is still lower than that provided by simply destriping and smoothing the solutions via standard GRACE post-processing techniques. Alternately, estimating daily gravity fields for the case of a polar pair of satellites coupled with a lower inclined pair results in solutions with superior spatial resolution than that offered by simply destriping and smoothing the solutions.     

A Review of Hypervelocity Debris Testing at the Air Force Research Laboratory

In this paper, we describe a compact, low cost, fast turn-around-time technique used at the Air Force Research Laboratory to study hypervelocity debris impact effects on spacecraft structures and components. The technique described was used to study debris effects in the areas of: shock physics, debris-produced contamination, chemical analyses of the impact ejecta and debris initiated spacecraft discharge. Examples of research results obtained with the technique are presented and illustrate problems encountered in the field of space debris effects on spacecraft.     

The Cassini Ultraviolet Imaging Spectrograph Investigation

The Cassini Ultraviolet Imaging Spectrograph (UVIS) is part of the remote sensing payload of the Cassini orbiter spacecraft. UVIS has two spectrographic channels that provide images and spectra covering the ranges from 56 to 118 nm and 110 to 190 nm. A third optical path with a solar blind CsI photocathode is used for high signal-to-noise-ratio stellar occultations by rings and atmospheres. A separate Hydrogen Deuterium Absorption Cell measures the relative abundance of deuterium and hydrogen from their Lyman-α emission. The UVIS science objectives include investigation of the chemistry, aerosols, clouds, and energy balance of the Titan and Saturn atmospheres; neutrals in the Saturn magnetosphere; the deuterium-to-hydrogen (D/H) ratio for Titan and Saturn; icy satellite surface properties; and the structure and evolution of Saturn’s rings.This revised version was published online in July 2005 with a corrected cover date.     

The Cluster Spatio-Temporal Analysis of Field Fluctuations (STAFF) Experiment

The Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment is one of five experiments which together comprise the Wave Experiment Consortium (WEC). STAFF consists of a three-axis search coil magnetometer to measure magnetic fluctuations at frequencies up to 4 kHz, and a spectrum analyser to calculate in near-real time aboard the spacecraft, the complete auto- and cross-spectral matrices using the three magnetic and two electric components of the electromagnetic field. The magnetic waveform at frequencies below either 10 Hz or 180 Hz is also transmitted. The sensitivity of the search coil is adapted to the phenomena theo be studied: the values 3 × 10^-3 nT Hz^-1/2 and 3 × 10^-5 nT Hz^-1/2 are achieved respectively at 1 Hz and 100 Hz. The dynamic range of the STAFF instruments is about 96 dB in both waveform and spectral power, so as to allow the study of waves near plasma boundaries. Scientific objectives of the STAFF investigations, particularly those requiring four point measurements, are discussed. Methods by which the wave data will be characterised are described with emphasis on those specific to four-point measurements, including the use of the Field Energy Distribution function.     

Book reviews

Space Science Reviews -     

Viscoelastic characterization of polymers for deployable composite booms

Deployable space structures are being built from thin-walled fiber-reinforced polymer composite materials due to their high specific strength, high specific stiffness, and designed bistability. However, the inherent viscoelastic behavior of the resin matrix can cause dimensional instability when the composite is stored under strain. The extended time of stowage between assembly and deployment in space can result in performance degradation and in the worst case, mission failure. In this study, the viscoelastic properties of candidate commercial polymers consisting of difunctional and tetrafunctional epoxies and thermoplastic and thermosetting polyimides were evaluated for deployable boom structures of solar sails. Stress relaxation master curves of the candidate polymers were used to predict the relaxation that would occur in 1 year at room temperature under relatively low strains of about 0.1%. A bismaleimide (BMI) showed less stress relaxation (about 20%) than the baseline novolac epoxy (about 50%). Carbon fiber composites fabricated with the BMI resin showed a 44% improvement in resistance to relaxation compared to the baseline epoxy composite.     

Searching for life in extreme environments relevant to Jovian’s Europa: Lessons from subglacial ice studies at Lake Vostok (East Antarctica)

The objective was to estimate the genuine microbial content of ice samples from refrozen water (accretion ice) from the subglacial Lake Vostok (Antarctica) buried beneath the 4-km thick East Antarctic ice sheet. The samples were extracted by heavy deep ice drilling from 3659 m below the surface. High pressure, a low carbon and chemical content, isolation, complete darkness and the probable excess of oxygen in water for millions of years characterize this extreme environment. A decontamination protocol was first applied to samples selected for the absence of cracks to remove the outer part contaminated by handling and drilling fluid. Preliminary indications showed the accretion ice samples to be almost gas free with a low impurity content. Flow cytometry showed the very low unevenly distributed biomass while repeated microscopic observations were unsuccessful.