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881.
Generating chirp waveforms by means of phase coding yields a simple, cost-effective mechanization. The coding process, however, introduces phase errors whose effect must be included in the design. An approximate analysis is presented, valid for moderate to high compression ratios, which allows error effects on compressed pulse amplitude and sidelobes to be calculated in a simple manner. The anaylsis provides criteria for selecting the coding bit width (sample rate), weighting network bandwidth, and phase-coder quantization interval and transition times. Weighting functions for maximizing the signal-to-noise ratio (SNR) or for producing desired close-in sidelobe performance are derived, as is an exact expression for the transmitted spectrum. Numerical results are presented for Gaussian and the maximum-SNR weighting. The results indicate that performance will be satisfactory for many applications. 相似文献
882.
M.C. Flemings J. Szekely N. El-Kaddah Y. Shiohara R.T. Frost 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(5):99-103
An electromagnetic furnace is being prepared for flights aboard the space shuttle. This apparatus is capable of melting metals and alloys up to 1400°C melting point by induction heating with subsequent solidification of the freely levitated melt without contact with any container. The solidification can be carried out with greatly reduced fields resulting in minimal heating and stirring of the free melt. Sequential specimens can be processed during flight. Several experiments are planned for a series of flights, beginning in 1985 with an undercooling experiment on NiSn alloys. These will be interspersed with detailed studies of fluid flow caused by low and high field levels in order to quantify the corresponding effect upon the solidification process. 相似文献
883.
Closed-form expressions are given for the standard deviation of the error in estimating angle (usually azimuth) in a scanning radar. The formulas apply to Swerling's lower bound and to the error using a pulse-to-pulse two-pole filter. They apply to non-fluctuating and Swerling II targets and hold for all signal-to-noise ratios. Comparison with graphical results in the literature shows that the average deviation obtained using the formulas is less than 4 percent. 相似文献
884.
K Baltschukat G Horneck H Bucker R Facius M Schafer 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1986,6(11):109-115
Using spores of two Bacillus subtilis strains differing in repair capacity, we have studied repair and mutation induction in the spores after irradiation with very heavy ions up to uranium with specific particle energies up to 18.6 MeV/u. The results indicate that repair and mutation induction after heavy ion irradiation are closely related to each other and that both phenomena strongly depend on the atomic number and specific energy of the ions. The effects are discussed in comparison with results obtained after X-irradiation. 相似文献
885.
H.B. Niemann W.T. Kasprzak 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(10):261-270
Composition and gas density measurement at all altitudes in the atmospheres of earth and other planets are made by mass spectrometers. Because of the impartiality and large dynamic range they are particularly favored for exploratory missions. Measurements of trace constituents, inert gases and height profiles, especially below clouds, are objectives where mass spectrometry is most useful. Significant advances have been made in recent years in development of light weight automated instruments. Experiments conducted in rarified atmospheres have been more successful or results were less controversial than in attempts to analyze high pressure atmospheres. Gas sampling and conditioning techniques are highly specific because of measurement environments and engineering constraints on the mission, and are usually the most critical elements in the experiment. Chemical sample enrichment and scrubbing for noble gas enhancement are additional sample conditioning techniques now employed. Dynamic range of more than 108 is achievable. Reliable measurements of complex organic or chemically active trace constituents with mixing ratios of less than 10?9 still require significant instrument development particularly where weight, power and sampling time are severely restricted. Adaptation of familiar and proven laboratory techniques for flight instruments is usually not straightforward and practical. 相似文献
886.
Arthur C. Clarke 《Space Policy》1986,2(2)
Soviet General Secretary Gorbachev, in his January 1986 speech, expressed a hope that the level of nuclear weapons would before long be greatly reduced. The Soviet Embassy in Sri Lanka invited Arthur C. Clarke to comment on the General Secretary's speech. This article is his response. 相似文献
887.
The performance of electronically scanned radar systems is evaluated for clear and barrage jamming environments. Radar figures of merit in jamming are derived for search and track modes and are directly related to antenna pattern relative sidelobe levels. A random sidelobe model is analyzed with various numbers of independent jammers. Probability distributions of detection probability are derived to determine detection performance versus average jamming level and number of jammers. 相似文献
888.
We describe issues related to the design and performance of a multimedia information server for an office environment. Multimedia documents are composed of text, image, voice, and attribute information. We describe the multimedia document structure, their internal representation and their presentation form. Content addressibility in this environment is achieved by specifying conditions on attributes, text, images, and document presentation form. Query reformulation, multimedia document formation in this environment and communication with the outside world are also discussed. Archived documents are stored in a centralized server. Users submit their queries in their local workstations using a local area network. The server retrieves the qualifying documents and returns them to the users. We analyze the performance of various access methods for multimedia documents that reside in the server. 相似文献
889.
C. Sawyer 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(11):265-270
A multidisciplinary study of this solar-interplanetary event is summarized by two main points: this flare was an incident in a process that began days before the flare, and continued after the flare; and the chain of events can be interpreted most simply in terms of energy input over scales of time and space that are large compared to the flare seen in the light of Hα. In support of these points, 5 aspects of the flare are described here: (1) hours before the flare, slow changes in coronal structure were associated with radio continuum emission, suggesting large-scale magnetic-field changes and the presence of energetic electrons; (2) long-lived X-ray loops require sustained energy input for at least an hour after the flare start; (3) interplanetary disturbance near earth is probably related to this limb flare, although the (expected) absence of a shock makes identification uncertain; (4) the coronal mass ejection overlay decaying magnetic field; (5) speed derived from frequency drift of the type II radio burst in the low corona, and from the travel time of the disturbance to 1 a.u., are about twice as great as the observed speed of the coronal mass ejection and of the disturbed solar-wind speed. 相似文献
890.