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511.
G. Palladino A. Basili G. Di Cocco T. Franceschini G. Landini S. Silvestri A. Barbini M. Galimberti L. A. Gizzi 《Experimental Astronomy》2006,21(3):169-187
This paper describes the design of a star sensor based upon a high dynamic range CCD in order to reach an arcsec-level attitude
determination in balloon-borne missions. A custom star identification software was developed and laboratory-tested on a prototype
assembled using commercial components. A set of numerical simulations have been carried out to study the dependence on the
pointing precision of the centroid position accuracy, the number of detected stars and the effect of the image focusing. Moreover,
the role of the electronic noise and the discrete pixel structure on the light signals is identified by the analysis of numerical
simulations. Laboratory tests confirm that the arcsec pointing accuracy with a 1 Hz update rate can be achieved with our combination
of custom-developed software and selected hardware components. 相似文献
512.
513.
Erosion of man-made, forestry drainage channels occurs when the plough cuts through the surface peat layer into the underlying erodible mineral soil. A procedure is developed, based on hydraulic considerations, which will allow the drainage engineer to design stable drainage networks in upland forestry plantations. An example design chart is given for an erodible sandy loam type soil. 相似文献
514.
Based on our 1997 observations with a CCD camera and narrow-band filters attached to the 1-m Assy Obsev vatory telescope, we extensively study the latitudinal variations in limb darkening and normal albedo on Jupiter's disk at wavelengths of 387, 445, 502, 619, and 702 nm. In addition, we carried out observations in 1998 with broad-band red, green, and blue filters. Apart from a general regularity—the increase in darkening coefficient with normal albedo of Jupiter's cloud cover—there is an appreciable scatter of darkening coefficients for the same albedo, which is most pronounced in the ultraviolet. This scatter may result from differences in the optical depth of the aerosol haze above the clouds. The lack of any wavelength dependence of the darkening coefficients is confirmed for Jupiter's polar regions, while at other latitudes, they decrease with decreasing wavelength. 相似文献
515.
A. El-Bassuny Alawy 《Astrophysics and Space Science》2001,277(3):473-495
A new approach is proposed and developed to handle pre-processed CCD frames in order to identify stellar images and derive
their relevant parameters. The present method relies on: 1) identifying stellar images and assigning approximate positions
of their centres using artificial intelligence techniques, 2) accurate determination of the centre coordinates applying an
elementary statistical concept and 3) estimating the image peak intensity as a stellar magnitude measure employing a simple
numerical analysis approach. The method has been coded for personal computer users. A CCD frame of the star cluster M67 was
adopted as a test case. The results obtained are discussed in comparison with the DAOPHOTII ones and the corresponding published
data. Exact coincidence has been found between both results except very few cases. These exceptions have been discussed in
view of both methods' bases and the cluster plates. It has been realized that the method suggested exhibits very simple, extremely
fast and high precise approach in stellar CCD photometry domain. Moreover, it is more capable for handling blended and distorted
stellar images than the DAOPHOTII. These characteristics show the usefulness of the present method in some astronomical applications
such as auto-focusing and auto-guiding sensing approaches beside the main purpose, viz. stellar photometry.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
516.
Bjerrum-Niese C. Bjorno L. Pinto M.A. Quellec B. 《Oceanic Engineering, IEEE Journal of》1996,21(2):143-149
The paper discusses the development of a simulation tool to model high data-rate acoustic communication in shallow water. The simulation tool is able to generate synthetic time series of signals received at a transducer array after transmission across a shallow-water communication channel. The simulation tool is suitable for testing advanced signal processing techniques for message recovery. A channel model has been developed based on the physical aspects of the acoustic channel. Special emphasis has been given to fluctuations of the signal transmission caused by time-varying multipath effects. At shorter ranges, the temporal variations are dominated by acoustic scattering from the moving sea surface. Therefore, the channel model produces a coherence function which may be interpreted as a time-varying reflection coefficient for the surface scattered acoustical path. A static, range-independent ray model identifies the significant multipaths, and the surface path is modulated with the time-varying reflection coefficient. The advantages and limitations of the channel model are discussed and assumptions necessary to overcome the limitations are emphasised. Based on the assumptions, an algorithm has been developed and implemented to model how a binary message will be modulated when transmitted by a transducer, is distorted in the channel and finally is received by a transducer array 相似文献
517.
OSCR is an HF radar system that has been developed for high spatial resolution coastal surface current measurement. This paper describes preliminary results that demonstrate that wave measurement can be successfully obtained from suitably processed OSCR data. Comparisons with data from a WAVEC directional buoy are presented and show encouraging agreement. Some of the limitations to the measurement process are discussed and indicate a maximum range of about 20 km. Surface current variability on short time scales presents the most serious obstacle to wave measurement. This appears to be more of a problem when the mean currents are large, in that in these circumstances the data fail initial quality control criteria. However, in lower mean currents, the effect is often still present and leads to errors in long wave measurement 相似文献
518.
519.
Mercury's close orbit around the Sun, its weak intrinsic magnetic field and the absence of an atmosphere (Psurface<1×10−8 Pa) results in a strong direct exposure of the surface to energetic ions, electrons and UV radiation. Thermal processes and particle-surface-collisions dominate the surface interaction processes leading to surface chemistry and physics, including the formation of an exosphere (N?1014 cm−2) in which gravity is the dominant force affecting the trajectories of exospheric atoms. NASA's Mariner 10 spacecraft observed the existence of H, He, and O in Mercury's exosphere. In addition, the volatile components Na, K, and Ca have been observed by ground based instrumentation in the exosphere. We study the efficiency of several particle surface release processes by calculating stopping cross-sections, sputter yields and exospheric source rates. Our study indicates surface sputter yields for Na between values of about 0.27 and 0.35 in an energy range from 500 eV up to 2 keV if Na+ ions are the sputter agents, and about 0.037 and 0.082 at an energy range between 500 eV up to 2 keV when H+ are the sputter agents and a surface binding energy of about 2 eV to 2.65 eV. The sputter yields for Ca are about 0.032 to 0.06 and for K atoms between 0.054 to 0.1 in the same energy range. We found a sputter yield for O atoms between 0.025 and 0.04 for a particle energy range between 500 eV up to 2 keV protons. By taking the average solar wind proton surface flux at the open magnetic field line area of about 4×108 cm−2 s−1 calculated by Massetti et al. (2003, Icarus, in press) the resulting average sputtering flux for O is about 0.8-1.0×107 cm−2 s−1 and for Na approximately 1.3-1.6×105 cm−2 s−1 depending on the assumed Na binding energies, regolith content, sputtering agents and solar activity. By using lunar regolith values for K we obtain a sputtering flux of about 1.0-1.4×104 cm−2 s−1. By taking an average open magnetic field line area of about 2.8×1016 cm2 modelled by Massetti et al. (2003, Icarus, in press) we derive an average surface sputter rate for Na of about 4.2×1021 s−1 and for O of about 2.5×1023 s−1. The particle sputter rate for K atoms is about 3.0×1020 s−1 assuming lunar regolith composition for K. The sputter rates depend on the particle content in the regolith and the open magnetic field line area on Mercury's surface. Further, the surface layer could be depleted in alkali. A UV model has been developed to yield the surface UV irradiance at any time and latitude over a Mercury year. Seasonal and diurnal variations are calculated, and Photon Stimulated Desorption (PSD) fluxes along Mercury's orbit are evaluated. A solar UV hotspot is created towards perihelion, with significant average PSD particle release rates and Na fluxes of about 3.0×106 cm−2 s−1. The average source rates for Na particles released by PSD are about 1×1024 s−1. By using the laboratory obtained data of Madey et al. (1998, J. Geophys. Res. 103, 5873-5887) for the calculation of the PSD flux of K atoms we get fluxes in the order of about 104 cm−2 s−1 along Mercury's orbit. However, these values may be to high since they are based on idealized smooth surface conditions in the laboratory and do not include the roughness and porosity of Mercury's regolith. Further, the lack of an ionosphere and Mercury's small, temporally and spatially highly variable magnetosphere can result in a large and rapid increase of exospheric particles, especially Na in Mercury's exosphere. Our study suggests that the average total source rates for the exosphere from solar particle and radiation induced surface processes during quiet solar conditions may be of the same order as particles produced by micrometeoroid vaporization. We also discuss the capability of in situ measurements of Mercury's highly variable particle environment by the proposed NPA-SERENA instrument package on board ESA's BepiColombo Mercury Planetary Orbiter (MPO). 相似文献
520.