Stability and Control of a Trap Mode in Synchronous Orbit

The possibility of the existence of a trap mode in synchronous orbit is investigated. It is shown that when such a mode exists, certain parameters of the satellite orbit may soon attain undesired equilibrium values, thus complicating the normal stationkeeping operation of the spacecraft. A state-dependent control force which eliminates any such trap mode in the satellite orbit is then investigated.     

Limiting the Impulsive Noise in a PLL

This paper analyzes the operation of a phase-locked loop (PLL) preceded by a bandpass limiter (BPL) in the presence of impulsive noise. It is shown that the effect of the limiter consists essentially in a change of the statistics of the pulse strengths of the noise, so that the behavior of a BPL + PLL can be deduced from that of a simple PLL by suitable adjustment of the noise model. It is found that the limiter greatly enhances the PLL performance by reducing both the phase-error variance in the loop and the probability of cycle slippage. Finally, the design of the filter of the BPL is discussed, resulting in the conclusion that the best results are obtained by using a singletuned RLC circuit.     

Comparison of quiet-Sun radiances measured by CDS and SUMER on SOHO

Space Science Reviews - Since the beginning of the SOHO (Solar and Heliospheric Observatory) mission an intercalibration programme was carried out which included simultaneous observations of the...     

Probability Density of the Click Duration in an Ideal FM Discriminator

The expression of the probability density function PClick(T) of the duration r of the clicks is determined as a function of 1) the expected number of clicks per second, Felick 2) the expected number of times per second that the noise component x(t) in phase with the carrier has instantaneous amplitude larger than that of the carrier itself 3) the probability density function peross(T) of the duration of the time intervals in which x(t) < -A 4) the probability that the values of the noise component in quadrature with the carrier will be of opposite sign at the extremes of time intervals of duration v. Using for Pcross(T) an approximate formula that holds fairly well in the range of the values of the signal-to-noise power ratio ? usually encountered, PIick(T) has been calculated for several values of a and for two shapes of the noise spectrum, Gaussian and rectangular. Finally, an expression is given for the mean value of T.     

Space debris: Assessing risk and responsibility

We model the orbital debris environment by a set of differential equations with parameter values that capture many of the complexities of existing three-dimensional simulation models. We compute the probability that a spacecraft gets destroyed in a collision during its operational lifetime, and then define the sustainable risk level as the maximum of this probability over all future time. Focusing on the 900- to 1000-km altitude region, which is the most congested portion of low Earth orbit, we find that – despite the initial rise in the level of fragments – the sustainable risk remains below 10^-3${10}^{-3}$ if there is high (>98%) compliance to the existing 25-year postmission deorbiting guideline. We quantify the damage (via the number of future destroyed operational spacecraft) generated by past and future space activities. We estimate that the 2007 FengYun 1C antisatellite weapon test represents ≈1%$\approx 1\%$ of the legacy damage due to space objects having a characteristic size of ?10$?10$ cm, and causes the same damage as failing to deorbit 2.6 spacecraft after their operational life. Although the political and economic issues are daunting, these damage estimates can be used to help determine one-time legacy fees and fees on future activities (including deorbit noncompliance), which can deter future debris generation, compensate operational spacecraft that are destroyed in future collisions, and partially fund research and development into space debris mitigation technologies. Our results need to be confirmed with a high-fidelity three-dimensional model before they can provide the basis for any major decisions made by the space community.     

高能束流焊接技术在航空发动机耐热材料中的应用

我国高能束流焊接技术经过半个世纪,特别是近20年的迅速发展,逐渐形成了自己的研发体系.国内众多研究者进行的一系列基础理论和应用研究为高能束流焊接技术的发展奠定了基础.     

Comparison of self-healing ionomer to aluminium-alloy bumpers for protecting spacecraft equipment from space debris impacts

This paper discusses the impact behavior of a self-healing ionomeric polymer and compares its protection capability against space debris impacts to that of simple aluminium-alloy bumpers. To this end, 14 impact experiments on both ionomer and Al-7075-T6 thin plates with similar surface density were made with 1.5 mm aluminium spheres at velocity between 1 and 4 km/s.     

Cosmogenic Radionuclides as an Extension of the Neutron Monitor Era into the Past: Potential and Limitations

The cosmogenic radionuclides, ¹⁰Be, ¹⁴C and others, provide a record of the paleo-cosmic radiation that extends >10,000 years into the past. They are the only quantitative means at our disposal to study the heliosphere prior to the commencement of routine sunspot observations in the 17th century. The cosmogenic radionuclides are primarily produced by secondary neutrons generated by the galactic cosmic radiation, and can be regarded, in a sense, as providing an extrapolation of the neutron monitor era into the past. However, their characteristics are quite different from the man-made neutron monitor in several important respects: (1) they are sensitive to somewhat lower cosmic ray energies; (2) their temporal resolution is ～1 to 2 years, being determined by the rapidity with which they are sequestered in ice, biological, or other archives; (3) the statistical precision for annual data is very poor (～19%); however it is quite adequate (～5% for 22-year averages) to study the large variations (±40%) that have occurred in the paleo-cosmic ray record in the past between grand solar minima and maxima. The data contains “noise” caused by local meteorological effects, and longer-term climate effects, and the use of principal component analysis to separate these “system” effects from production effects is outlined. The concentrations of ¹⁰Be decreased by a factor of two at the commencement of Holocene, the present-day “interglacial”, due to a 100% increase in the ice accumulation rates in polar regions. The use of the ¹⁰Be flux to study heliospheric properties during the last glacial is discussed briefly.