Levelling of helicopter-borne frequency-domain electromagnetic data     

Bernhard Siemon 《Journal of Applied Geophysics》2009,67(3):206-218

For about three decades helicopter-borne electromagnetic (HEM) measurements have been used to reveal the resistivity distribution of the upper one hundred metres of the earth's subsurface. HEM systems record secondary fields, which are 3–6 orders of magnitude smaller than the transmitted primary fields. As both the primary fields and the secondary fields are present at the receivers, well-designed bucking coils are often used to reduce the primary fields at the receivers to a minimum. Remaining parts of the primary fields, the zero levels, are generally corrected by subtracting field values recorded at high altitudes (standard zero levelling) or estimated from resistivities of neighbouring lines or from resistivity maps (advanced zero levelling). These zero-levelling techniques enable the correction for long-term, quasi-linear instrumental drift. Short-term variations caused by temperature changes due to altitude variations, however, cannot be completely corrected by this procedure resulting in stripe patterns on thematic maps.Statistical methods and/or 2-D filter techniques called statistical levelling (tie-line levelling) and empirical levelling (microlevelling), respectively, used to correct stripe patterns in airborne geophysical data sets are, in general, not directly applicable to HEM data. Because HEM data levelling faces the problem that the parameter affected by zero-level errors, the secondary field, differs from the parameter generally levelled, the apparent resistivity. Furthermore, the dependency of the secondary field on both the resistivity of the subsurface and the sensor altitude is strongly nonlinear.A reasonable compromise is to microlevel both half-space parameters: apparent resistivity and apparent depth, followed by a recalculation of the secondary field components based on the half-space parameters levelled. Advantages and disadvantages of the diverse levelling techniques are discussed using a HEM data set obtained in a hilly region along the Saale River between the cities of Saalfeld and Jena in central Germany. It turns out from a comparison of apparent resistivity and apparent depth maps derived from levelled HEM data that manually advanced zero levelling of major level errors and automatic microlevelling of remaining minor level errors yield the best results.  相似文献   

Filamentary structures in planetary nebulae     

Hanna Dahlgren  Per Carlqvist  Gösta F. Gahm 《Astrophysics and Space Science》2007,310(1-2):65-72

We have studied small-scale, filamentary features in 14 planetary nebulae and found that some structures are recurrent and shaped like the letters V and Y, with the apex or stem pointing toward the central parts of the nebula. Two such filaments containing dust, one in NGC 3132 and one in NGC 7293, were investigated in more detail. The mass and density of the filaments were obtained from extinction measurements, and their physical properties were derived. We propose that the structures are confined by magnetic fields, and derive magnetic field strengths of about 10⁻⁸ T, in line with earlier estimates. We also estimate the magnitude of the electric currents that we expect are generated in these dynamic systems. We propose a theory where the magnetic fields control the sculpting and evolution of small-scale filaments. This theory demonstrates how the substructures may form magnetized flux ropes that are twisted around each other, in the shape of double helices. Similar structures, and with similar origin, are found in many other astrophysical environments.  相似文献   

Perturbative effects on a Mercurian orbiter due to the solar radiation pressure,solar wind and the coronal mass ejections     

《New Astronomy》2007,12(6):490-496

To explore the dynamics of a test particle in the near-Mercury’s environment, the orbital motion of an orbiter around Mercury is considered. Different perturbing forces, namely the Mercurian gravity field, the solar radiation pressure, the solar wind and the coronal mass ejections, are taken into account. The order of magnitude of each perturbing term is assessed. The equations of motion in canonical representation are obtained. The Hamiltonian in terms of Hansen coefficients is expressed. A procedure for solution is presented. The short and long periodic terms are removed from the Hamiltonian and the solution is obtained. Long periodic perturbations on the orbital dynamics of an orbiter around Mercury due to the solar events are found as revealed by Eq. (26) in the text. Resonance cases are discussed and the different resonant inclinations are obtained. A procedure for the computation of the position and velocity is presented.  相似文献   

An evaluation of the exposure in nadir observation of the JEM-EUSO mission     

《Astroparticle Physics》2013

We evaluate the exposure during nadir observations with JEM-EUSO, the Extreme Universe Space Observatory, on-board the Japanese Experiment Module of the International Space Station. Designed as a mission to explore the extreme energy Universe from space, JEM-EUSO will monitor the Earth’s nighttime atmosphere to record the ultraviolet light from tracks generated by extensive air showers initiated by ultra-high energy cosmic rays. In the present work, we discuss the particularities of space-based observation and we compute the annual exposure in nadir observation. The results are based on studies of the expected trigger aperture and observational duty cycle, as well as, on the investigations of the effects of clouds and different types of background light. We show that the annual exposure is about one order of magnitude higher than those of the presently operating ground-based observatories.  相似文献   

Dispersion measure variations and their effect on precision pulsar timing     

X. P. You  G. Hobbs  W. A. Coles  R. N. Manchester  R. Edwards  M. Bailes  J. Sarkissian  J. P. W. Verbiest  W. van Straten  A. Hotan  S. Ord  F. Jenet  N. D. R. Bhat  A. Teoh 《Monthly notices of the Royal Astronomical Society》2007,378(2):493-506

We present an analysis of the variations seen in the dispersion measures (DMs) of 20-ms pulsars observed as part of the Parkes Pulsar Timing Array project. We carry out a statistically rigorous structure function analysis for each pulsar and show that the variations seen for most pulsars are consistent with those expected for an interstellar medium characterized by a Kolmogorov turbulence spectrum. The structure functions for PSRs J1045−4509 and J1909−3744 provide the first clear evidence for a large inner scale, possibly due to ion–neutral damping. We also show the effect of the solar wind on the DMs and show that the simple models presently implemented into pulsar timing packages cannot reliably correct for this effect. For the first time we clearly show how DM variations affect pulsar timing residuals and how they can be corrected in order to obtain the highest possible timing precision. Even with our presently limited data span, the residuals (and all parameters derived from the timing) for six of our pulsars have been significantly improved by correcting for the DM variations.  相似文献   

Reliability based approach for offloading operation related to motion of two side-by-side moored LNG carriers     

《Applied Ocean Research》2015

A reliability based approach for analysis of offloading operations with two identical LNG carriers moored in a side-by-side configuration is suggested. The approach, although adopted for two tankers, is generally applicable to different types of LNG terminals. Different heading controls are conceivable for the discharging vessel in the case that the heading is not fixed: heading toward wind-sea, heading toward swell, heading toward wind and free weather vaning. The analyses show that heading toward the governing sea state is the most beneficial option for two identical LNG carriers moored in a side-by-side configuration. Strategies are discussed for estimation of limiting weather criteria that are consistent with prescribed target failure probabilities.  相似文献   

An Emulation Research on the Radio Occultation Exploration of Martian Ionosphere     

Hu Xiong  Wu Xiao-cheng  Gong Xiao-yan  Wang Xin  Xu Qin-chen 《Chinese Astronomy and Astrophysics》2010,34(1):100-112

The technical system of the Sino-Russian joint satellite-to-satellite Mars ionosphere occultation is analyzed and introduced. The analogue computation of the observed values of the radio waves of the ionosphere occultation event is carried out by adopting the three-dimensional ray tracking method and the electron density profile inversion is conducted by means of the simulated occultation observational data, with the result showing that the emulation algorithm is reliable. By taking advantage of the emulation method the case computation and analysis of the inversion errors caused by the observational error of the occultation radio wave phase and the satellite orbital error are respectively carried out, and it is obtained from the result that the effect of the phase measuring error of the 5% circle on the result of the daytime ionosphere occultation exploration may be neglected, while the absolute error of the night electron density measurement is less than 4 × 108 m^?3, and the main effect of the satellite orbital error on the occultation leads to the lifting or falling of the ionospheric height. The result shows that the technical system of the Sino-Russian joint Mars ionosphere occultation exploration is advanced. It can be expected that the high accuracy electron density profile is obtained and the technical system can be applied to the exploration of the lunar ionospheric environment.  相似文献   

Miniature ion mass analyzer     

《Planetary and Space Science》2007,55(9):1190-1196

The Swedish Institute of Space Physics is developing a miniature plasma analyzer for planetary missions (MIPA—Miniature ion precipitation analyzer). MIPA has been accepted to fly on-board both the ESA BepiColombo mission to Mercury (2014) and the Indian Chandrayaan—1 mission to the Moon (2007). The analyzer measures ions in the energy range 10 eV–15 keV and has a sufficient mass resolution to resolve the main groups of ions, namely $M/q=1,2,4,8,16,>30$. Field of view is $9^{\circ }\times {180}^{\circ }$. The instrument consists of the sensor head and a separate electronic board whose total mass is 300 g. The sensor head envelope is roughly $53\times 85\times 30{\mathrm{mm}}^3$ in volume and is designed for the extreme operation temperature range of $-100$ to $+{125}^{\circ }\mathrm{C}$. MIPA comprises an electrostatic scanner for angular resolution, a cylindrical electrostatic analyzer for energy discrimination and a time-of-flight (TOF) section for particle velocity measurement. Generic design allows using the instrument on various platforms including nano-satellite and multi-spacecraft missions. This document describes the design of the sensor part including ion optical as well as mechanical aspects.  相似文献   

A Constant Elevation Search Method for Narrow Beam Radar to Capture Space Targets     

《Chinese Astronomy and Astrophysics》2023,47(2):391-409

Position prediction is the basis of guiding radar to seek, capture, and track space targets, although there’s a risk of insufficient accuracy for narrow beam radar due to the influence of orbit determination and propagation error. Since the along-track error is the main factor that affects the accuracy of position prediction, this paper proposed a Constant Elevation Search (CES) method for narrow beam radar, based on the idea of along-track error compensation, to seek and capture transit targets. With along-track error estimation, all possible positions on a specified elevation can be observed, thus the success rate of capturing transit targets will be improved. Simulation shows that when the position prediction method fails, the CES method still works.  相似文献   

血管介入手术中的柔性虚拟力触觉系统研究     

冯安洋  陈笋  陈柏  耿令波  吴洪涛 《南京气象学院学报》2014,6(6):520-524

随着微创手术的发展,虚拟手术的应用前景越来越广阔.力反馈作为虚拟手术的核心技术,其实现的效果直接影响了虚拟手术的沉浸感.通过对经典包围盒碰撞检测算法进行研究比对,选择了球包围盒的碰撞检测算法,并建立了精确的反馈力计算模型,实现了介入血管中的柔性碰撞仿真.该仿真具有很高的实时性和精度.  相似文献