运动声呐空时自适应处理中的多普勒影响分析

为了将空时自适应处理(STAP)理论更好地应用于水下环境,提高运动声呐对混响的抑制性能,研究了多普勒对运动声呐STAP的影响。具体分析了由多普勒造成的回波脉宽伸缩变换以及目标空时导向向量失配带来的影响。分析结果表明,回波包络脉宽变化对于匹配滤波输出信噪比影响较小,而对空时导向向量修正可以有效提高目标方位估计精度以及混响抑制能力。     

The Radio Observatory on the Lunar Surface for Solar studies

The Radio Observatory on the Lunar Surface for Solar studies (ROLSS) is a concept for a near-side low radio frequency imaging interferometric array designed to study particle acceleration at the Sun and in the inner heliosphere. The prime science mission is to image the radio emission generated by Type II and III solar radio burst processes with the aim of determining the sites at and mechanisms by which the radiating particles are accelerated. Specific questions to be addressed include the following: (1) Isolating the sites of electron acceleration responsible for Type II and III solar radio bursts during coronal mass ejections (CMEs); and (2) Determining if and the mechanism(s) by which multiple, successive CMEs produce unusually efficient particle acceleration and intense radio emission. Secondary science goals include constraining the density of the lunar ionosphere by searching for a low radio frequency cutoff to solar radio emission and constraining the low energy electron population in astrophysical sources. Key design requirements on ROLSS include the operational frequency and angular resolution. The electron densities in the solar corona and inner heliosphere are such that the relevant emission occurs at frequencies below 10 MHz. Second, resolving the potential sites of particle acceleration requires an instrument with an angular resolution of at least 2°, equivalent to a linear array size of approximately 1000 m. Operations would consist of data acquisition during the lunar day, with regular data downlinks. No operations would occur during lunar night.     

The Induced Magnetospheres of Mars, Venus, and Titan

This article summarizes and aims at comparing the main features of the induced magnetospheres of Mars, Venus and Titan. All three objects form a well-defined induced magnetosphere (IM) and magnetotail as a consequence of the interaction of an external wind of plasma with the ionosphere and the exosphere of these objects. In all three, photoionization seems to be the most important ionization process. In all three, the IM displays a clear outer boundary characterized by an enhancement of magnetic field draping and massloading, along with a change in the plasma composition, a decrease in the plasma temperature, a deflection of the external flow, and, at least for Mars and Titan, an increase of the total density. Also, their magnetotail geometries follow the orientation of the upstream magnetic field and flow velocity under quasi-steady conditions. Exceptions to this are fossil fields observed at Titan and the near Mars regions where crustal fields dominate the magnetic topology. Magnetotails also concentrate the escaping plasma flux from these three objects and similar acceleration mechanisms are thought to be at work. In the case of Mars and Titan, global reconfiguration of the magnetic field topology (reconnection with the crustal sources and exits into Saturn??s magnetosheath, respectively) may lead to important losses of plasma. Finally, an ionospheric boundary related to local photoelectron signals may be, in the absence of other sources of pressure (crustal fields) a signature of the ultimate boundary to the external flow.     

Pickup protons in the heliosphere

We calculate the conditions of pickup protons inside the termination shock. Outside 50 AU the partial pressure of pickup protons is greater than the magnetic pressure by a factor of > 10, and greater than the partial pressure of solar wind protons by a factor of > 100. Thus, pickup protons have a significant dynamical influence on the structures of the solar wind in the outer heliosphere.     

Bandwidth maximization for satellite laser communication

Free space optical communication between satellites networked together can make possible high speed communication between different places on Earth. The basic free space optical communication network includes at least two satellites. In order to communicate between them, the transmitter satellite must track the beacon of the receiver satellite and point the information optical beam in its direction. The pointing systems for laser satellite communication suffer during tracking from vibration due to electronic noise, background radiation from interstellar objects such as Sun, Moon, Earth, and Stars in the tracking field of view, and mechanical impact from satellite internal and external sources. Due to vibrations the receiver receives less power. This effect limits the system bandwidth for given bit error rate (BER). In this research we derive an algorithm to maximize the communication system bandwidth using the transmitter telescope gain as a free variable based on the vibration statistics model and the system parameters. Our model makes it possible to adapt the bandwidth and transmitter gain to change of vibration amplitude. We also present an example of a practical satellite network which includes a direct detection receiver with an optical amplifier. A bandwidth improvement of three orders of magnitude is achieved in this example for certain conditions, as compared with an unoptimized system     

Mathematical modeling of response of ecosystems with different structure to external impact.

A mathematical model was used to study the response of ecosystems of different structures to external impact. The response was measured as a sensitivity coefficient: the magnitude of the system's response vs. the change of the factor in the inflow. The formula has been obtained to calculate the sensitivity coefficient for ecosystems containing different numbers of trophic links. The derived sensitivity coefficients demonstrate that the degree of compensation for the external impact can differ depending on the type of system regulation and the length of the trophic chain. E. g. the sensitivity coefficient decreases with complexity of trophic links in an ecosystem for top-down controlled systems and impact of degree of openness on sensitivity e.g. closed ecosystems show higher sensitivity then fully open ecosystem to impacts also bottom-up control system show less sensitivity then top-down. Grant numbers: N99-04-96017, N25.     

RBE: mechanisms inferred from cytogenetics.

Cyclotron-accelerated heavy ion beams provide a fine degree of control over the physical parameters of radiation. Cytogenetics affords a view into the irradiated cell at the resolution of chromosomes. Combined they form a powerful means to probe the mechanisms of RBE. Cytogenetic studies with high energy heavy ion beams reveal three LET-dependent trends for 1) level of initial damage, 2) distribution of damage among cells, and 3) lesion severity. The number of initial breaks per unit dose increases from a low-LET plateau to a peak at approximately 180 keV/micrometer and declines thereafter. Overdispersion of breaks is significant above approximately 100 keV/micrometer. Lesion severity, indicated by the level of chromosomal fragments that have not restituted even after long repair times, increases with LET. Similar studies with very low energy 238Pu alpha particles (120 keV/micrometer) reveal higher levels of initial breakage per unit dose, fewer residual fragments and a higher level of misrepair when compared to high energy heavy ions at the same LET. These observations would suggest that track structure is an important factor in genetic damage in addition to LET.     

Atmospheric dynamics and bioregenerative technologies in a soil-based ecological life support system: initial results from Biosphere 2.

Biosphere 2 is the first man-made, soil-based, bioregenerative life support system to be developed and tested. The utilization and amendment of local space resources, e.g. martian soil or lunar regolith, for agricultural and other purposes will be necessary if we are to minimize the requirement for Earth materials in the creation of long-term off-planet bases and habitations. Several of the roles soil plays in Biosphere 2 are 1) for air purification 2) as a key component in created wetland systems to recycle human and animal wastes and 3) as nutrient base for a sustainable agricultural cropping program. Initial results from the Biosphere 2 closure experiment are presented. These include the accelerated cycling rates due to small reservoir sizes, strong diurnal and seasonal fluxes in atmospheric CO2, an unexpected and continuing decline in atmospheric oxygen, overall maintenance of low levels of trace gases, recycling of waste waters through biological regeneration systems, and operation of an agriculture designed to provide diverse and nutritionally adequate diets for the crew members.