Current Issues and Future Trends: DVB-RCS Satellite Systems

The telecommunications market is reaching new types of customers who require their applications anywhere and anyway. Therefore, the DVB-RCS standard, which had great success in the fixed version is changing in this direction. The main objective of this work is to give an overview of architectures and technologies that can be used for implementation of a first-generation of advanced mobile satellite systems for the provision of broadband multimedia services.     

Synthetic Aperture Radar

The general theory of side-looking synthetic aperture radar systems is developed. A simple circuit-theory model is developed; the geometry of the system determines the nature of the prefilter and the receiver (or processor) is the postfilter. The complex distributed reflectivity density appears as the input, and receiver noise is first considered as the interference which limits performance. Analysis and optimization are carried out for three performance criteria (resolution, signal-to-noise ratio, and least squares estimation of the target field). The optimum synthetic aperture length is derived in terms of the noise level and average transmitted power. Range-Doppler ambiguity limitations and optical processing are discussed briefly. The synthetic aperture concept for rotating target fields is described. It is observed that, for a physical aperture, a side-looking radar, and a rotating target field, the azimuth resolution is ?/? where ? is the change in aspect angle over which the target field is viewed, The effects of phase errors on azimuth resolution are derived in terms of the power density spectrum of the derivative of the phase errors and the performance in the absence of phase errors.     

Signal-to-Noise Ratio of Arbitrary Power-Series Nonlinear Amplifier

A general analysis of the effect of an arbitrary power-series nonlinear amplifier followed by a coherent mixing device on signal-to-noise ratio (SNR) is performed. An expression is derived for the improvement factor which is defined as the logarithm of the ratio of the output SNR to the input SNR. This expression is applicable to the coherent amplitude detector and phase locked loop as well as noncoherent amplifier by appropriate selections of the detection angle. Moreover, the improvement factor can be obtained for noise with an arbitrary amplitude distribution. To demonstrate the applicability of this analysis, the improvement factors of the nonlinear amplifiers such as a power-law amplifier and a power-series amplifier with positive and negative discriminations are numerically calculated for the cases where the input noise amplitude distributions are Rician and triangular.     

Alfvén Waves in the Solar Atmosphere

Alfvén waves are considered to be viable transporters of the non-thermal energy required to heat the Sun’s quiescent atmosphere. An abundance of recent observations, from state-of-the-art facilities, have reported the existence of Alfvén waves in a range of chromospheric and coronal structures. Here, we review the progress made in disentangling the characteristics of transverse kink and torsional linear magnetohydrodynamic (MHD) waves. We outline the simple, yet powerful theory describing their basic properties in (non-)uniform magnetic structures, which closely resemble the building blocks of the real solar atmosphere.     

Evaluation of early Archean volcaniclastic and volcanic flow rocks as possible sites for carbonaceous fossil microbes

Sedimentary rocks have traditionally been the focus of the search for Archean microfossils; the Earth's oldest fossil bacteria are associated with carbonaceous matter in sedimentary cherts in greenstone belts in the eastern Pilbara block of Western Australia and Barberton greenstone belt of South Africa. Reports of possible fossils in a martian meteorite composed of igneous rock and the discovery of modern bacteria associated with basalts have stimulated a new look at Archean volcanic rocks as possible sites for fossil microbes. This study examines silicified volcaniclastic rocks, near-surface altered volcanic flow rocks, and associated stromatolite- like structures from the Archean Barberton greenstone belt to evaluate their potential for the preservation of carbonaceous fossils. Detrital carbonaceous particles are widely admixed with current-deposited debris. Carbonaceous matter is also present in altered volcanic flow rocks as sparse particles in silica veins that appear to be fed by overlying carbonaceous chert layers. Neither microfossils nor mat-like material was identified in the altered volcanic rocks or adjacent stromatolite-like structures. Ancient volcanic flow and volcaniclastic rocks are not promising sites for carbonaceous fossil preservation.     

Microperturbations on the <Emphasis Type=Italic>International Space Station</Emphasis> during physical exercises of the crew

The results of a preliminary analysis of microperturbations on the International Space Station during physical exercises of the crew are presented. The goal of this paper is to identify the parameters of perturbations when physical exercises are performed. The results of measurements by sensors of microaccelerations of both Russian and American segments during physical exercises in the service module of the Russian segment are analyzed.     

Can identification of a fourth domain of life be made from sequence data alone, and could it be done on Mars?

A central question in astrobiology is whether life exists elsewhere in the universe. If so, is it related to Earth life? Technologies exist that enable identification of DNA- or RNA-based microbial life directly from environmental samples here on Earth. Such technologies could, in principle, be applied to the search for life elsewhere; indeed, efforts are underway to initiate such a search. However, surveying for nucleic acid-based life on other planets, if attempted, must be carried out with caution, owing to the risk of contamination by Earth-based life. Here we argue that the null hypothesis must be that any DNA discovered and sequenced from samples taken elsewhere in the universe are Earth-based contaminants. Experience from studies of low-biomass ancient DNA demonstrates that some results, by their very nature, will not enable complete rejection of the null hypothesis. In terms of eliminating contamination as an explanation of the data, there may be value in identification of sequences that lie outside the known diversity of the three domains of life. We therefore have examined whether a fourth domain could be readily identified from environmental DNA sequence data alone. We concluded that, even on Earth, this would be far from trivial, and we illustrate this point by way of examples drawn from the literature. Overall, our conclusions do not bode well for planned PCR-based surveys for life on Mars, and we argue that other independent biosignatures will be essential in corroborating any claims for the presence of life based on nucleic acid sequences.