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51.
Piyush M. Mehta Andrew Walker Earl Lawrence Richard Linares David Higdon Josef Koller 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
Satellite drag coefficients are a major source of uncertainty in predicting the drag force on satellites in low Earth orbit. Among other things, accurately predicting the orbit requires detailed knowledge of the satellite drag coefficient. Computational methods are an important tool in computing the drag coefficient but are too intensive for real-time and predictive applications. Therefore, analytic or empirical models that can accurately predict drag coefficients are desired. This work uses response surfaces to model drag coefficients. The response surface methodology is validated by developing a response surface model for the drag coefficient of a sphere where the closed-form solution is known. The response surface model performs well in predicting the drag coefficient of a sphere with a root mean square percentage error less than 0.3% over the entire parameter space. For more complex geometries, such as the GRACE satellite, the Hubble Space Telescope, and the International Space Station, the model errors are only slightly larger at about 0.9%, 0.6%, and 1.0%, respectively. 相似文献
52.
John H. Heinbockel Tony C. Slaba Steve R. Blattnig Ram K. Tripathi Lawrence W. Townsend Thomas Handler Tony A. Gabriel Lawrence S. Pinsky Brandon Reddell Martha S. Clowdsley Robert C. Singleterry John W. Norbury Francis F. Badavi Sukesh K. Aghara 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
The protection of astronauts and instrumentation from galactic cosmic rays and solar particle events is one of the primary constraints associated with mission planning in low earth orbit or deep space. To help satisfy this constraint, several computational tools have been developed to analyze the effectiveness of various shielding materials and structures exposed to space radiation. These tools are now being carefully scrutinized through a systematic effort of verification, validation, and uncertainty quantification. In this benchmark study, the deterministic transport code HZETRN is compared to the Monte Carlo transport codes HETC-HEDS and FLUKA for a 30 g/cm2 water target protected by a 20 g/cm2 aluminum shield exposed to a parameterization of the February 1956 solar particle event. Neutron and proton fluences as well as dose and dose equivalent are compared at various depths in the water target. The regions of agreement and disagreement between the three codes are quantified and discussed, and recommendations for future work are given. 相似文献
53.
The Lunar Orbiter Laser Altimeter Investigation on the Lunar Reconnaissance Orbiter Mission 总被引:3,自引:0,他引:3
David E. Smith Maria T. Zuber Glenn B. Jackson John F. Cavanaugh Gregory A. Neumann Haris Riris Xiaoli Sun Ronald S. Zellar Craig Coltharp Joseph Connelly Richard B. Katz Igor Kleyner Peter Liiva Adam Matuszeski Erwan M. Mazarico Jan F. McGarry Anne-Marie Novo-Gradac Melanie N. Ott Carlton Peters Luis A. Ramos-Izquierdo Lawrence Ramsey David D. Rowlands Stephen Schmidt V. Stanley Scott III George B. Shaw James C. Smith Joseph-Paul Swinski Mark H. Torrence Glenn Unger Anthony W. Yu Thomas W. Zagwodzki 《Space Science Reviews》2010,150(1-4):209-241
The Lunar Orbiter Laser Altimeter (LOLA) is an instrument on the payload of NASA’s Lunar Reconnaissance Orbiter spacecraft (LRO) (Chin et al., in Space Sci. Rev. 129:391–419, 2007). The instrument is designed to measure the shape of the Moon by measuring precisely the range from the spacecraft to the lunar surface, and incorporating precision orbit determination of LRO, referencing surface ranges to the Moon’s center of mass. LOLA has 5 beams and operates at 28 Hz, with a nominal accuracy of 10 cm. Its primary objective is to produce a global geodetic grid for the Moon to which all other observations can be precisely referenced. 相似文献
54.
Dynamic stall delay of flow over airfoils rapidly pitching past the static stall angle has been studied by many scientists. However, the effect of compressibility on this dynamic stall behavior has been less comprehensively studied. This review presents a detailed assessment of research performed on this subject, including a historical review of work performed on both aircraft and helicopters, and offers insight into the impact of compressibility on the complex aerodynamic phenomenon known as dynamic stall. It also documents the major effect that compressibility can have on dynamic stall events, and the complete change of physics of the stall process that can occur as free-stream Mach number is increased. 相似文献
55.
We have explored the direct and indirect radiative effects on climate of organic particles likely to have been present on early Earth by measuring their hygroscopicity and cloud nucleating ability. The early Earth analog aerosol particles were generated via ultraviolet photolysis of an early Earth analog gas mixture, which was designed to mimic possible atmospheric conditions before the rise of oxygen. An analog aerosol for the present-day atmosphere of Saturn's moon Titan was tested for comparison. We exposed the early Earth aerosol to a range of relative humidities (RHs). Water uptake onto the aerosol was observed to occur over the entire RH range tested (RH=80-87%). To translate our measurements of hygroscopicity over a specific range of RHs into their water uptake ability at any RH < 100% and into their ability to act as cloud condensation nuclei (CCN) at RH > 100%, we relied on the hygroscopicity parameter κ, developed by Petters and Kreidenweis. We retrieved κ=0.22?±0.12 for the early Earth aerosol, which indicates that the humidified aerosol (RH < 100 %) could have contributed to a larger antigreenhouse effect on the early Earth atmosphere than previously modeled with dry aerosol. Such effects would have been of significance in regions where the humidity was larger than 50%, because such high humidities are needed for significant amounts of water to be on the aerosol. Additionally, Earth organic aerosol particles could have activated into CCN at reasonable-and even low-water-vapor supersaturations (RH > 100%). In regions where the haze was dominant, it is expected that low particle concentrations, once activated into cloud droplets, would have created short-lived, optically thin clouds. Such clouds, if predominant on early Earth, would have had a lower albedo than clouds today, thereby warming the planet relative to current-day clouds. 相似文献
56.
Brennan Lawrence E. Reed Irving S. 《IEEE transactions on aerospace and electronic systems》1966,(6):655-658
This paper deals precisely with the effect of quantum or step size in an analog-to-digital converter on the quantization noise in the output of MTI (moving target indication) radar processors. The correlation between quantization errors on successive pulses is considered in the analysis and is shown to have a small quieting effect on the output quantization noise. 相似文献
57.
Nitrous oxide as a rocket propellant 总被引:1,自引:0,他引:1
Nitrous oxide is introduced as a multi-purpose propellant for spacecraft. Potential space applications of this propellant are given. Based on comparison to conventional systems, a multi-mode nitrous oxide propulsion concept is expected to deliver higher performance. Main features of a self-pressurising, nitrous oxide storage system are described. A nitrous oxide catalytic decomposition technique is suggested for restartable spacecraft propulsion. Up-to-date experimental results are presented. A conclusion describes the long-term feasibility of novel nitrous oxide propulsion option concepts. 相似文献
58.
Over the past two decades, hypothetical models of “worst-case” solar particle event (SPE) spectra have been proposed in order to place an upper bound on radiation doses to critical body organs of interplanetary crews on deep space missions. These event spectra are usually formulated using hypothetical extrapolations of space measurements for previous large events. Here we take a different approach. Recently reported analyses of ice core samples indicate that the Carrington flare of 1859 is the largest event observed in the past 500 years. These ice core data yield estimates of the proton fluence for energies greater than 30 MeV, but provide no other spectrum information. Assuming that the proton energy distribution for such an event is similar to that measured for other recent, large events, interplanetary crew doses are estimated for these hypothetical worst case SPE spectra. These estimated doses are life threatening unless substantial shielding is provided. 相似文献
59.
Intruder alarm systems, IAS, are seen by many as an essential part of the United Kingdom's defence against the crime of burglary. IAS are intended either to deter the potential intruder or detect and give warning of an intruder directly or indirectly to the police. Unfortunately the false alarm rate, at over 90%, is so high that it leads to ineffectual use of police resources and calls into question the ability of IAS to perform effectively either as a deterrent or detector. To try and improve the situation the Police Forces in the UK have in conjunction with the alarm industry, over the past few years, implemented special measures in an attempt to reduce the number of false alarms and improve the effectiveness of the systems. However, from a study of available data on the deterrent effect of alarm systems, the national statistics for all alarm call types, the numbers of arrests arising from the calls, etc., this paper concludes that IAS on their own are not an effective police crime prevention measure. Thus a radical rethink of their use is required. The paper also suggests an alternative strategy which may improve the situation 相似文献
60.
Joan Johnson-Freese Professor 《Space Policy》1994,10(1)
The development of a global environmental and disaster satellite observation system (EDOS) has recently received attention, particularly as a potential cooperative project with the USA and Japan as the initiators. Such a system, proponents contend, would provide valuable and potentially lifesaving information to developed and particularly developing countries which might not otherwise have such information available, as well as provide a vehicle to strengthen ties between the USA and Japan at a time when strife over trade issues is too often emphasized. The initiation of such a project, however, has not been without difficulty. Beyond the technical issues, there has been hesitation and scepticism on the part of some national space players concerning the project, often motivated by parochial political concerns rather than conceptual or technical issues. The focus of this article is an examination of those political factors which have acted as impetus and barriers for the initiative, using interviews with programme participants, direct author observation at related meetings and internal documents and correspondence. 相似文献