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781.
Published comments on a recent paper criticized the use of inverse filtering as applied to resolution of overlapping radar signal returns. It is shown that an inadequate model of the inverse filter was assumed by the critic, which lead him to predict excessive time sidelobes at the filter output. It is demonstrated, by computer simulation, that the time sidelobes at the output of the true inverse filter are down 30 dB or more. 相似文献
782.
Bethany L. Ehlmann Gilles Berger Nicolas Mangold Joseph R. Michalski David C. Catling Steven W. Ruff Eric Chassefière Paul B. Niles Vincent Chevrier Francois Poulet 《Space Science Reviews》2013,174(1-4):329-364
Clays form on Earth by near-surface weathering, precipitation in water bodies within basins, hydrothermal alteration (volcanic- or impact-induced), diagenesis, metamorphism, and magmatic precipitation. Diverse clay minerals have been detected from orbital investigation of terrains on Mars and are globally distributed, indicating geographically widespread aqueous alteration. Clay assemblages within deep stratigraphic units in the Martian crust include Fe/Mg smectites, chlorites and higher temperature hydrated silicates. Sedimentary clay mineral assemblages include Fe/Mg smectites, kaolinite, and sulfate, carbonate, and chloride salts. Stratigraphic sequences with multiple clay-bearing units have an upper unit with Al-clays and a lower unit with Fe/Mg-clays. The typical restriction of clay minerals to the oldest, Noachian terrains indicates a distinctive set of processes involving water-rock interaction that was prevalent early in Mars history and may have profoundly influenced the evolution of Martian geochemical systems. Current analyses of orbital data have led to the proposition of multiple clay-formation mechanisms, varying in space and time in their relative importance. These include near-surface weathering, formation in ice-dominated near-surface groundwaters, and formation by subsurface hydrothermal fluids. Near-surface, open system formation of clays would lead to fractionation of Mars’ crustal reservoir into an altered crustal reservoir and a sedimentary reservoir, potentially involving changes in the composition of Mars’ atmosphere. In contrast, formation of clays in the subsurface by either aqueous alteration or magmatic cooling would result in comparatively little geochemical fractionation or interaction of Mars’ atmospheric, crustal, and magmatic reservoirs, with the exception of long-term sequestration of water. Formation of clays within ice would have geochemical consequences intermediate between these endmembers. We outline the future analyses of orbital data, in situ measurements acquired within clay-bearing terrains, and analyses of Mars samples that are needed to more fully elucidate the mechanisms of martian clay formation and to determine the consequences for the geochemical evolution of the planet. 相似文献
783.
W Trettnak C Kolle F Reininger C Dolezal P O'Leary R A Binot 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1998,22(10):1465-1474
Optical oxygen sensors are mainly based on the principle of luminescence quenching. In contrast to arready existing intensity-based systems, the measurement of the luminescence lifetime provides certain advantages, such as insensitivity to photobleaching or leaching of the dye, or changes in the intensity of excitation light. This facilitates the use of simple optical systems or optical fibres. A new family of oxygen-sensitive dyes, the porphyrin-ketones, has been introduced, which exhibits favorable spectral properties and decay times in the order of tens and hundreds of microseconds. This allows the use of simple optoelectronic circuitry and low-cost processing electronics. An optical oxygen sensor module has been developed with the dimensions of only 120 x 60 x 30 mm. The prototype is based on the measurement of the decay time of the luminophore by measuring the phase shift between the square-wave excitation and the detected square-wave of the emission coming from the sensor. The instrument is based on semiconductor devices (light-emitting diodes, photodiodes) and may be used for the detection of oxygen in gaseous or liquid samples. The measurement range of the device is from 0 to 200 hPa oxygen partial pressure with a resolution of < 1 hPa over the whole measurement range. The overall measurement accuracy of < +/- 1 hPa has been obtained for periods of 24 h of continuous measurement in a thermostatted environment. The sensor response times t90 are typically < 1 s for gases and 0.5 to 5 min for liquid samples. 相似文献
784.
Bergland G.D. Hunnicutt C.F. 《IEEE transactions on aerospace and electronic systems》1972,(2):161-167
The application of a highly parallel computer known as PEPE (Parallel Element Processing Ensemble) to radar data processing is described. The PEPE computer consists of a large number of identical processing elements which operate in parallel and are controlled by a common control unit. Each of the processing elements is assigned to a particular radar target such that many targets can be tracked in parallel. PEPE is designed to augment a conventional computer rather than to stand alone. The total data processing load associated with a radar tracking system is distributed between PEPE and the sequential machine in a manner than maximizes the overall system efficiency and desensitizes the system performance to fluctuations in traffic levels. The use of PEPE provides very high data processing throughput potential to a radar data processing system. 相似文献
785.
C Schimek P Eibel T Horie P Galland T Ootaki 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,24(6):687-696
The sporangiophores of the zygomycete fungus Phycomyces blakesleeanus contain octahedral crystals with diameters of up to 5 micrometers in their vacuole. The crystals are associated with the intracellular membrane system. In tilted or horizontally placed sporangiophores, the crystals sediment to the respective lower face of the vacuole with a velocity of up to 100 micrometers per minute. The sedimentation is completed within about 2 minutes, well within the latency period for the negative gravitropic response of Phycomyces. Crystal-lacking mutant strains display a smaller maximal bending angle and a reduced gravitropic bending rate in comparison to the wild type. We therefore conclude that the crystals serve as statoliths for gravitropism in Phycomyces. 相似文献
786.
Glisson T.H. Black C.I. Sage A.P. 《IEEE transactions on aerospace and electronic systems》1970,(1):37-50
Systems which utilize acoustic energy to explore an undersea environment are called sonars. This introductory and tutorial paper presents a discussion of active sonar signal analysis concentrating upon sinusoidal, linear FM, and pseudorandom echolocation pulses. Many previously published results concerning sonar signals and their haracteristics are integrated, collected, and presented in a unified form such as to portray typical signal design considerations. 相似文献
787.
A typical function of an angle tracking loop is to keep a radar antenna pointed at a target. The error in pointing is directly related to successful operation of the tracking device; therefore, its behavior is of interest. For a tracker with a general polynomial nonlinearity, an arbitrary initial pointing error, and a bounded deterministic input, a method is developed for finding upper bounds on the magnitude of the tracking error using Volterra series techniques. Convergence regions of the Volterra series are also obtained. Applications of these results are made to a second-order tracking device. 相似文献
788.
Although weaknesses have been demonstrated in some security techniques (encryption, protocols, mobile code such as Java, etc.), current security technology is quite strong in many areas. Despite this, information security has proved difficult to achieve in large modern software systems. Many problems have been reported in which supposedly secure systems have been penetrated and, in some cases, significant damage done. One problem considered is a buffer-overrun attack. The idea called a 〈wrapper〉 which is a layer of software that logically surrounds a software artifact and enhances the functionality of the artifact in some way, is then discussed. Most proposals for the use of wrappers assume that their presence is transparent to the artifact being wrapped. In other words, the artifact sees its operating environment as unchanged and the artifact does not have to be modified in i order to permit it to be wrapped. Wrappers have been proposed as an approach to dealing with deficiencies in existing systems, deficiencies in security for example. The use of shells as a feasible solution to the problem of security is also considered 相似文献
789.
F. Sabri N. Leventis J. Hoskins A.C. Schuerger M. Sinden-Redding D. Britt R.A. Duran 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
Room temperature vulcanizing (RTV)-based components have been used on Mars Pathfinder, the Mars rovers, Spirit and Opportunity, as well as the Phoenix Lander as a support matrix for pigmented panoramic camera calibration targets. RTV 655 has demonstrated superiority to other polymers due to its unique range of material properties namely mechanical stability between −115 and 204 °C and UV radiation tolerance. As a result, it has been the number one choice for many space-related missions. However, due to the high mass density and the natural tendency for electrostatic charging RTV materials have caused complications by attracting and retaining dust particles (Sabri et al., 2008). In the current project we have investigated the relevant properties of polymer-reinforced (crosslinked) silica aerogels with the objective of substituting RTV-based calibration targets with an aerogel based design. The lightweight, mechanical strength, ability to accept color pigments, and extremely low dust capture makes polyurea crosslinked aerogels a strong candidate as a chromatic standard for extraterrestrial missions. For this purpose, the reflection spectra, gravimetric analysis, and low temperature response of metal oxide pigmented, polyurea crosslinked silica aerogels have been investigated and reported here. 相似文献
790.
O.J. Olwendo P. Baki P.J. Cilliers C. Mito P. Doherty 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
This work presents an analysis of the Total Electron Content (TEC) derived from the International GNSS Service (IGS) receivers at Malindi (mal2: 2.9oS, 40.1oE, dip −26.813o), Kasarani (rcmn: 36.89oE, 1.2oS, dip −23.970o), Eldoret (moiu: 35.3oE, 0.3oN, dip −21.037o) and GPS-SCINDA (36.8oE, 1.3oS, dip −24.117o) receiver located in Nairobi for the period 2009–2011. The diurnal, monthly and seasonal variations of the GPS derived TEC (GPS-TEC) and effects of space weather on TEC are compared with TEC from the 2007 International Reference Ionosphere model (IRI-TEC) using the NeQuick option for the topside electron density. The diurnal peaks in GPS-TEC is maximum during equinoctial months (March, April, October) and in December and minimum in June solstice months (May, June, July). The variability in GPS-TEC is minimal in all seasons between 0:00 and 04:00 UT and maximum near noon between 10:00 and 14:00 UT. Significant variability in TEC at post sunset hours after 16:00 UT (19:00 LT) has been noted in all the seasons except in June solstice. The TEC variability of the post sunset hours is associated with the occurrence of the ionization anomaly crest which enhances nighttime TEC over this region. A comparison between the GPS-TEC and IRI-TEC indicates that both the model and observation depicts a similar trend in the monthly and seasonal variations. However seasonal averages show that IRI-TEC values are higher than the GPS-TEC. The IRI-TEC also depicts a double peak in diurnal values unlike the GPS-TEC. This overestimation which is primarily during daytime hours could be due to the model overestimation of the equatorial anomaly effect on levels of ionospheric ionization over the low latitude regions. The IRI-TEC also does not show any response to geomagnetic activity, despite the STORM option being selected in the model; the IRI model generally remains smooth and underestimates TEC during a storm. The GPS-TEC variability indicated by standard deviation seasonal averages has been presented as a basis for extending the IRI-model to accommodate TEC-variability. 相似文献