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61.
James R. Drummond Jiansheng Zou Florian Nichitiu Jayanta Kar Robert Deschambaut John Hackett 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
The MOPITT (Measurements of Pollution in the Troposphere) instrument has provided more than nine years of global carbon monoxide (CO) measurements on a continuous basis since its launch aboard the Terra Spacecraft on December 18th, 1999. This paper gives an overview of the core sub-system performance and major issues of the in-flight instrument over the mission period. Some of the instrument anomalies are also discussed. The major successes are: (1) the concept of using a combination of correlation systems such as Length Modulated Cells (LMCs) and Pressure Modulated Cells (PMCs) to retrieve CO profiles in the troposphere; (2) the redundant design in the instrumentation which was crucial for coping with unexpected in-flight anomalies and for continuing the mission in the case of component failure; (3) the thermal environment on orbit that is so stable that some calibration procedures are not necessary; and (4) the recent production of CO total column retrieved from the MOPITT 2.3 μm channel. 相似文献
62.
The performance of FM multivibrator systems for high quality communications applications is limited by the presence of unwanted spectral components. A theory is presented for an improved system in which unwanted spectral components are cancelled, achieved by generating a cancelling component in a square-law device. The cancelling component tracks the unwanted component in both frequency and amplitude over a wide bandwidth of signal frequencies. Reductions in magnitude of the unwanted components by at least 16 dB are predicted; reductions of better than 14 dB have been achieved in practice. 相似文献
63.
64.
Xu J Ramian GJ Galan JF Savvidis PG Scopatz AM Birge RR Allen SJ Plaxco KW 《Astrobiology》2003,3(3):489-504
We propose a terahertz (far-infrared) circular dichroism-based life-detection technology that may provide a universal and unequivocal spectroscopic signature of living systems regardless of their genesis. We argue that, irrespective of the specifics of their chemistry, all life forms will employ well-structured, chiral, stereochemically pure macromolecules (>500 atoms) as the catalysts with which they perform their metabolic and replicative functions. We also argue that nearly all such macromolecules will absorb strongly at terahertz frequencies and exhibit significant circular dichroism, and that this circular dichroism unambiguously distinguishes biological from abiological materials. Lastly, we describe several approaches to the fabrication of a terahertz circular dichroism spectrometer and provide preliminary experimental indications of their feasibility. Because terahertz circular dichroism signals arise from the molecular machinery necessary to carry out life's metabolic and genetic processes, this life-detection method differs fundamentally from more well-established approaches based on the detection of isotopic fractionation, "signature" carbon compounds, disequilibria, or other by-products of metabolism. Moreover, terahertz circular dichroism spectroscopy detects this machinery in a manner that makes few, if any, assumptions as to its chemical nature or the processes that it performs. 相似文献
65.
Cleaves HJ 《Astrobiology》2002,2(4):403-415
It has been suggested that life began with a self-replicating RNA molecule. However, after much research into the prebiotic synthesis of RNA, the difficulties encountered have lead some to hypothesize that RNA was preceded by a simpler molecule, one more easily synthesized prebiotically. Many of the proposed alternative molecules are based on acrolein, since it reacts readily with nucleophiles, such as the nucleobases, via Michael addition and is readily synthesized from formaldehyde and acetaldehyde. Reports regarding the reactions of nucleobases with concentrated acrolein solutions suggest that this is a plausible reaction mechanism, though there are also reports that the "incorrect" isomers are obtained. The scope and kinetics of the reaction of acrolein with various nitrogen heterocycles are reported here. Reactions of pyrimidines often give N(1) adducts as the major products. Reactions of purines often give N(9) adducts in good yield. The reactions are rapid under neutral to slightly alkaline conditions, and proceed at low temperatures and dilutions. The implications of these findings for the origin of life are discussed. 相似文献
66.
The Earth's auroral electrons produce copious non-thermal radio emissions of various types, including auroral kilometric radiation (AKR), whistler mode auroral hiss, mode conversion radiation such as auroral roar and MF-burst, and possibly HF/VHF emissions. In some cases, mechanisms have been identified and quantitatively described, whereby the energy of the auroral electrons is converted into electromagnetic radiation. In many other cases, the radiation mechanism, or the relative significance of several possible mechanisms, remains uncertain. This review covers fairly comprehensively experimental and theoretical research on types of auroral radiation other than AKR, concentrating on emissions with frequency higher than about 1kHz and treating only emissions which are unique to the auroral zone. The review covers both ground-based and in-situ observations. It covers a wide range of theoretical approaches, emphasizing those which at present appear most important for producing non-AKR auroral radiations. 相似文献
67.
High accuracy satellite drag model (HASDM) 总被引:2,自引:0,他引:2
Mark F. Storz Bruce R. Bowman Major James I. Branson Stephen J. Casali W. Kent Tobiska 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2005,36(12):2497-2505
The dominant error source in force models used to predict low-perigee satellite trajectories is atmospheric drag. Errors in operational thermospheric density models cause significant errors in predicted satellite positions, since these models do not account for dynamic changes in atmospheric drag for orbit predictions. The Air Force Space Battlelab’s High Accuracy Satellite Drag Model (HASDM) estimates and predicts (out three days) a dynamically varying global density field. HASDM includes the Dynamic Calibration Atmosphere (DCA) algorithm that solves for the phases and amplitudes of the diurnal and semidiurnal variations of thermospheric density near real-time from the observed drag effects on a set of Low Earth Orbit (LEO) calibration satellites. The density correction is expressed as a function of latitude, local solar time and altitude. In HASDM, a time series prediction filter relates the extreme ultraviolet (EUV) energy index E10.7 and the geomagnetic storm index ap, to the DCA density correction parameters. The E10.7 index is generated by the SOLAR2000 model, the first full spectrum model of solar irradiance. The estimated and predicted density fields will be used operationally to significantly improve the accuracy of predicted trajectories for all low-perigee satellites. 相似文献
68.
G. Randall Gladstone Steven C. Persyn John S. Eterno Brandon C. Walther David C. Slater Michael W. Davis Maarten H. Versteeg Kristian B. Persson Michael K. Young Gregory J. Dirks Anthony O. Sawka Jessica Tumlinson Henry Sykes John Beshears Cherie L. Rhoad James P. Cravens Gregory S. Winters Robert A. Klar Walter Lockhart Benjamin M. Piepgrass Thomas K. Greathouse Bradley J. Trantham Philip M. Wilcox Matthew W. Jackson Oswald H. W. Siegmund John V. Vallerga Rick Raffanti Adrian Martin J.-C. Gérard Denis C. Grodent Bertrand Bonfond Benoit Marquet François Denis 《Space Science Reviews》2017,213(1-4):447-473
The ultraviolet spectrograph instrument on the Juno mission (Juno-UVS) is a long-slit imaging spectrograph designed to observe and characterize Jupiter’s far-ultraviolet (FUV) auroral emissions. These observations will be coordinated and correlated with those from Juno’s other remote sensing instruments and used to place in situ measurements made by Juno’s particles and fields instruments into a global context, relating the local data with events occurring in more distant regions of Jupiter’s magnetosphere. Juno-UVS is based on a series of imaging FUV spectrographs currently in flight—the two Alice instruments on the Rosetta and New Horizons missions, and the Lyman Alpha Mapping Project on the Lunar Reconnaissance Orbiter mission. However, Juno-UVS has several important modifications, including (1) a scan mirror (for targeting specific auroral features), (2) extensive shielding (for mitigation of electronics and data quality degradation by energetic particles), and (3) a cross delay line microchannel plate detector (for both faster photon counting and improved spatial resolution). This paper describes the science objectives, design, and initial performance of the Juno-UVS. 相似文献
69.
JamesM.Dahl 《航空科学技术》2004,(6):43-45
进入新世纪,民用与军用飞机合金的选择设计标准发生了巨大变化。过去的设计标准主张追求更高、更快和更远的目标而不计成本,已不适应时代的发展。新的设计要求包括:一是减少瑕疵、增加使用寿命和降低维修和飞行成本;二是寻求更廉价的零件与部件,以降低产品生命周期成本;三是对更环保的“绿色”部件的需求,如淘汰需要以铬和镉进行表面处理的部件。这些要求使得H-11和300M等一些标准合金失宠,尽管这些合金异常坚硬,但却易产生应力腐蚀断裂。本文介绍了五种新型合金,这五种合金具有延长使用寿命和降低寿命周期成本的优点。它们可用作曾用于起… 相似文献
70.
Over the past several years Satellites International has developed an integrated suite of satellite sub-systems and small satellite buses. The sub-systems include S-band communications, attitude sensing and control, power conversion and distribution, and on-board data handling. They are inherently modular and readily adaptable to different satellite configurations, a concept known as semi-standardisation. This concept has been adopted by two generic low-cost buses: MicroSIL for satellites in the mass range 40–80kg; and MiniSIL for satellites in the range 100–500kg. Their architecture is based on the semi-standard sub-systems, but easily modified to utilise sub-systems from other manufacturers. They can support all stabilisation methods including spinning, 3-axis control and gravity gradient and are adaptable to a wide variety of missions including Earth resources, scientific, communications and technology demonstration. The Company also manufactures a range of low cost ground support equipment and complete ground stations to complement the space-borne systems. 相似文献