Improved rainfall estimation over the Indian region using satellite infrared technique

The GOES Precipitation Index (GPI) technique (Arkin, 1979) for rainfall estimation has been in operation for the last three decades. However, its applications are limited to the larger temporal and spatial scales. The present study focuses on the augmentation on GPI technique by incorporating a moisture factor for the environmental correction developed by Vicente et al. (1998). It consists of two steps; in the first step the GPI technique is applied to the Kalpana-IR data for rainfall estimation over the Indian land and oceanic region and in the second step an environmental moisture correction factor is applied to the GPI-based rainfall to estimate the final rainfall. Detailed validation with rain gauges and comparison with Tropical Rainfall Measuring Mission (TRMM) merged data product (3B42) are performed and it is found that the present technique is able to estimate the rainfall with better accuracy than the GPI technique over higher temporal and spatial domains for many operational applications in and around the Indian regions using Indian geostationary satellite data. Further comparison with the Doppler Weather Radar shows that the present technique is able to retrieve the rainfall with reasonably good accuracy.     

Adaptive detection schemes in compound-Gaussian clutter

Radar detection of coherent pulse trains embedded in compound-Gaussian disturbance with partially known statistics is discussed. We first give a thorough derivation of two recently proposed adaptive detection structures. Next, we derive a different detection scheme exploiting the assumption that the clutter is wide-sense stationary. Resorting to the theory of circulant matrices, in fact, we demonstrate that the estimation of the structure of the clutter covariance matrix can be reduced to the estimation of its eigenvalues, which in turn can be (efficiently) done via fast Fourier transform codes. After a thorough performance assessment, mostly carried on via computer simulations, the results show that the newly proposed detector achieves better performance than the two previously introduced adaptive detectors. Moreover, a sensitivity analysis shows that, even though this detector does not strictly guarantee the constant false alarm rate property with respect to the clutter covariance matrix, it is robust, in the sense that its performance is only slightly affected by variations in the clutter temporal correlation     

Surface Composition of Vesta: Issues and Integrated Approach

The instruments on the Dawn spacecraft are exceptionally well suited to characterize and map the surface composition of Vesta in an integrated manner. These include a framing camera with multispectral capabilities, a high spectral resolution near-infrared imaging spectrometer, and a gamma-ray and neutron spectrometer. Three examples of issues addressed at Vesta are: (1) What is the composition of Vesta??s interior and differentiation state as exposed by the Great South Crater? (2) How has space weathering affected Vesta, both globally and at a local scale? and (3) Are volatiles or hydrated material present on Vesta??s surface? We predict that Dawn finds many surprises, such as an olivine-bearing mantle exposed near the south-pole, a weakly or un-weathered surface that has been relatively recently resurfaced, and a very thin layer of surficial volatiles derived from interaction with the solar wind.     

Binary Signal Detection with Decision and Information Feedbacks

This paper analyzes a sequential feedback detection scheme derived by Kramer's nonsequential intermittent one; in addition to the noiseless information feedback channel, it makes use of a noiseless decision feedback channel. Expressions for the normalized average evergy per symbol, average decision time, probability of error, and average power are derived. The comparison with the nonsequential intermittent scheme shows that the proposed one allows an appreciable gain in transmission rate and requires a smaller peak-to-average power ratio.     

Reconfigurable multiport amplifiers for in-orbit use

A highly reliable and economical system design is presented for a multiport amplifier (MPA) system, which has attracted considerable attention for its potential use in multibeam mobile satellite communications. An MPA is composed of multiple input/output ports and an array of multiple high-power amplifier (HPAs). Since the HPAs are shared among multiple beams, this design solves the problems of traffic imbalance among beams, traffic changes due to terminal movement, and changes in propagation conditions, and can efficiently utilize the total transmitting power with maximum flexibility and minimum hardware. The problem is that HPA failures degrade the MPA beam isolation, causing multipath interference in the surrounding area due to leakage through adjacent beams. To address this problem, we investigated the MPA performance and survival probability when HPAs fail, by taking the specific properties of the MPA configuration into account. Based on our analysis, we found that there is an optimum operational HPA on/off mode in the event of HPA failure, and we were able to obtain optimal performance and reliability by reconfiguring the HPA on/off states in orbit upon the occurrence of HPA failures. Our proposed self-redundant, reconfigurable MPA can achieve high reliability without any need for additional redundant units or switches.     

Rocket Exhaust Scattering Effects on Incident Microwave Propagation Characteristics

This paper considers the effect of the rocket exhaust turbulence and scattering within the surrounding medium upon the propagation characteristics of incident electromagnetic waves. The exhaust is represented by a cylindrical plasma beam, diffusing through the surrounding medium. The equations of propagation of EM waves are derived for both TE and TM modes. By using a quasi-linear perturbation technique the exhaust is further separated into an inner homogeneous cylindrical plasma beam, and an outer conical inhomogeneous turbulent region. The isotropic change in the temperature of the outer region and its effects on the fluctuations in the density of electrons, collision frequency, and plasma index of refraction are analyzed in detail. It is found that the exhaust turbulence and scattering effects produce linear fluctuations in the E and H fields computed from the exhaust inner region effect. The equations of this paper can be used in the prediction of the radar cross sections and the attenuation of microwaves by rocket exhaust plumes.     

The effects of microgravity on the skeletal system--a review

Exposure of astronauts to microgravity leads to the loss of calcium from weightbearing bones. Prolonged exposure, e.g., during a journey to Mars, may present problems on return to Earth, with increased risk of fractures and premature osteoporosis in later life. The precise mechanisms of calcium loss have yet to be determined although a key feature is the absence of mechanical loading. Countermeasures aimed at reducing calcium loss to acceptable levels include the use of exercise, drugs, dietary modifications and inertia suits such as the Soviet "Penguin" suit. Missions of a number of years may, however, require the development of artificial gravity on a spacecraft. The country that first solves the physiological problems of man in space and, in particular, skeletal calcium loss, will almost certainly be the first to be able to put a man on Mars.     

The Plasma Environment of Comet 67P/Churyumov-Gerasimenko Throughout the Rosetta Main Mission

The plasma environment of comet 67P/Churyumov-Gerasimenko, the Rosetta mission target comet, is explored over a range of heliocentric distances throughout the mission: 3.25 AU (Rosetta instruments on), 2.7 AU (Lander down), 2.0 AU, and 1.3 AU (perihelion). Because of the large range of gas production rates, we have used both a fluid-based magnetohydrodynamic (MHD) model as well as a semi-kinetic hybrid particle model to study the plasma distribution. We describe the variation in plasma environs over the mission as well as the differences between the two modeling approaches under different conditions. In addition, we present results from a field aligned, two-stream transport electron model of the suprathermal electron flux when the comet is near perihelion.     

航空发动机用金属橡胶隔振器动静态性能的研究

以某型号航空发动机阻尼减振需求为背景,对开发研制的金属橡胶隔振器与现用的橡胶隔振器进行了动、静态实验研究。研究结果表明:金属橡胶隔振器的能量耗散性能、静态承载能力、过临界的能力及提供振动防护的区域都远远大于橡胶隔振器,而且通过改变金属橡胶隔振器的结构参数和预压缩量可以优化其隔振效果。研究结果为利用金属橡胶隔振器来改善航空发动机的振动状况提供了依据。     

Ionization processes in the heliosphere — Rates and methods of their determination

The rates of the most important ionization processes acting in interplanetary space on interstellar H, He, C, O, Ne and Ar atoms are critically reviewed in the paper. Their long-term modulations in the period 1974 – 1994 are reexamined using updated information on relevant cross-sections as well as direct or indirect data on variations of the solar wind/solar EUV fluxes based on IMP 8 measurements and monitoring of the solar 10.7 cm radio emission. It is shown that solar cycle related variations are pronounced (factor of 3 between maximum and minimum) especially for species such as He, Ne, C for which photoionization is the dominant loss process. Species sensitive primarily to the charge-exchange (as H) show only moderate fluctuations 20% around average. It is also demonstrated that new techniques that make use of simultaneous observations of neutral He atoms on direct and indirect orbits, or simultaneous measurements of He⁺ and He⁺⁺ pickup ions and solar wind particles can be useful tools for narrowing the uncertainties of the He photoionization rate caused by insufficient knowledge of the solar EUV flux and its variations.