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31.
The realization of a rectangular pulse-compression waveform having low time sidelobes and zero mismatch loss due to spectral weighting is discussed. The theoretical aspects of the design of such a waveform are presented with particular reference to frequency modulated, rectangular pulses. The design and performance of a matched-filter pulse-compression system having essentially zero mismatch loss are presented. The system discussed has a time-bandwidth product of 22 and time sidelobes suppressed at least 27 dB; the measured mismatch loss is 0.1 dB. The difficulty of achieving the required nonlinear time delay dispersion is overcome by synthesizing the dispersive network as a cascade of all-pass networks. 相似文献
32.
In order to stabilize the altitude calculation in an inertial navigation system, an altimeter is commonly used. In a conventional local-level mechanization, this is generally accomplished by correcting the vertical channel integrators with the difference between the inertial system and altimeter indication of vertical position. However, in a space-stable system the procedure is not as clear since a vertical channel is not physically present. Three altitude damping mechanizations for a space-stable inertial navigation system are proposed. The equivalent local-level mechanizations are then found by comparing error propagation equations in a common coordinate frame. 相似文献
33.
Three radar Doppler shift measurements are sufficient to determine the range and velocity, and the angle between them, of a target in uniform rectilinear motion. A solution using derivative approximations has recently appeared. An exact solution in terms of the original measurements is presented here. 相似文献
34.
Problems involving kinematics of a counterweapon intercepting an attacking missile are well known. However, when the attacker's target, the counterweapon's launch platform, is moving, the problem generally becomes amenable only to trajectory simulation. Such may be the case in defense against torpedo attack against a ship and other situations involving the use of antimissile missiles. This paper derives the kinematic relations among the three bodies (missile, target, and counterweapon) in closed form under the asymptotic approximation of constant-bearing trajectories (collision courses). It is shown that the defending target, even if moving, can still determine the optimum course for its counterweapon when range and speed of the attacking vehicle are unknown. A simple relation is derived for the ratio between the attacker-target range (and time to impact) at counterweapon launch and the range at interception of the attacker by the counterweapon. Normalized curves are presented for some representative cases of defense by a counterweapon of known speed against a torpedo or missile attack on a moving target. The equations are shown to reduce to the familiar form for a stationary target. 相似文献
35.
36.
M R Sims C T Pillinger I P Wright J Dowson S Whitehead A Wells J E Spragg G Fraser L Richter H Hamacher A Johnstone N P Meredith C de la Nougerede B Hancock R Turner S Peskett A Brack J Hobbs M Newns A Senior M Humphries H U Keller N Thomas J S Lingard T C Ng 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,23(11):1925-1928
The aim of the proposed Beagle 2 small lander for ESA's 2003 Mars Express mission is to search for organic material on and below the surface of Mars and to study the inorganic chemistry and mineralogy of the landing site. The lander will have a total mass of 60kg including entry, descent, and landing system. Experiments will be deployed on the surface using a robotic arm. It will use a mechanical mole and grinder to obtain samples from below the surface, under rocks, and inside rocks. Sample analysis by a mass spectrometer will include isotopic analysis. An optical microscope, an X-ray spectrometer and a Mossbauer spectrometer will conduct in-situ rock studies. 相似文献
37.
L.M. Mukhin D.F. Nenarokov N.V. Porschnev V.B. Bondarev B.G. Gelman G. Israe&#x;l F. Raulin J. Runavot R. Thomas 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1987,7(12):329-335
SIGMA - 3 gas chromatograph on board VEGA 1 and 2 landing probes has been operated successfully in the 60 - 50 km altitude range, providing several in - situ chemical analysis of the gas and the aerosols of Venus cloud layers. Post flight calibration required to derive atmospheric abundancies from gas chromatograms were carried out using the SIGMA - 3 spare model. A Venus atmospheric aerosol simulation chamber was used in which sulfuric acid droplets were generated. Preliminary results of these calibration experiments indicate that the concentration of sulfuric acid in the upper part of the clouds ( 60 to 55 km) is about 1 mg/m3 and suggest that an additional constituant must be present in noticeable amount in the aerosols. From these experiments the mixing ratio upper limits of SO2 is 100 ppmV and of H2S and COS is few 10 ppmV. 相似文献
38.
Bellegarda J.R. Titlebaum E.L. 《IEEE transactions on aerospace and electronic systems》1988,24(6):726-742
Time-frequency hop codes are developed that can be used for coherent multiuser echolocation and asynchronous spread spectrum communication systems. They represent a compromise between Costas codes, which have nearly ideal autoambiguity but not so good cross-ambiguity properties, and linear congruential codes, which have nearly ideal cross-ambiguity but unattractive autoambiguity properties. Extended quadratic congruential code words are shown to have reasonably good autoambiguity and cross-ambiguity properties across the whole class of code sets considered. A uniform upper bound is placed on the entire cross-ambiguity function surface, and bounds are placed on the position and amplitude of spurious peaks in the autoambiguity function. These bounds depend on the time/bandwidth product and code length exclusively and lead naturally to a discussion of the design tradeoffs for these two parameters. Examples of typical autoambiguity and cross-ambiguity functions are given to illustrate the performance of the new codes 相似文献
39.
R. Von Steiger 《Space Science Reviews》1998,85(1-2):407-418
This rapporteur paper discusses the solar corona and the solar wind in the context of their chemical composition. The abundances of elements, both obtained by optical and by in situ observations, are used to infer the sources of the slow solar wind and of the fast streams. The first ionisation potential (FIP) fractionation effect is also discussed, in particular the agreed basics and the open questions. This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
40.
K. C. Hansen T. Bagdonat U. Motschmann C. Alexander M. R. Combi T. E. Cravens T. I. Gombosi Y.-D. Jia I. P. Robertson 《Space Science Reviews》2007,128(1-4):133-166
The plasma environment of comet 67P/Churyumov-Gerasimenko, the Rosetta mission target comet, is explored over a range of heliocentric distances throughout the mission: 3.25 AU (Rosetta instruments on), 2.7 AU (Lander down), 2.0 AU, and 1.3 AU (perihelion). Because of the large range of gas production rates, we have used both a fluid-based magnetohydrodynamic (MHD) model as well as a semi-kinetic hybrid particle model to study the plasma distribution. We describe the variation in plasma environs over the mission as well as the differences between the two modeling approaches under different conditions. In addition, we present results from a field aligned, two-stream transport electron model of the suprathermal electron flux when the comet is near perihelion. 相似文献