A Method for Selection of an Optimum Communication Link

A method is presented for the selection of a communication system based upon optimization of both frequency-dependent and frequency independent parameters. The indicator of quality used is the effective radiated power per cycle per second of information bandwidth. The method involves consideration of the frequency-dependent qualities such as system temperature and propagation losses independently from the frequency-independent quantities such as modulation loss, signal-to-noise ratio, performance margin, etc. Except for temperature and propagation loss effects, the method is made frequency-independent by considering the spacecraft antenna in terms of area while the ground terminal antenna is treated in terms of gain, thus removing wavelength from the transmission equation. An example is presented of the application of the method to the selection of a system for transmission of wide-band video signals over a satellite to earth link. The example illustrates the exchanges that can be made between the frequency-dependent and -independent quantities in order to arrive at an optimum system.     

Error Probability for NCFSK with Linear FM Jamming

An expression is derived for the probability of error for a conventional binary, noncoherent, frequency-shift-key (NCFSK) communications system under the influence of bandpass Gaussian noise and a linear frequency-modulation jamming waveform. The resulting integral is expressed in terms of the well-known Q function, which depends upon average signal, noise, and jamming powers. The analytical procedures used can be applied to the analysis of the effects of other types of jamming.     

Measuring the matter distribution within z = 0.2 cluster lenses with XMM-Newton

We present an analysis of seven clusters observed by XMM-Newton as part of our survey of 17 most X-ray luminous clusters of galaxies at z  0.2 selected for a comprehensive and unbiased study of the mass distribution in massive clusters. Using the public software FTOOLS and XMMSAS we have set up an automated pipeline to reduce the EPIC MOS and pn spectro-imaging data, optimized for extended sources analysis. We also developped a code to perform intensive spectral and imaging analysis particularly focussing on proper background estimate and removal. XMM-Newton deep spectro-imaging of these clusters allowed us to fit a standard β-model to their gas emission profiles as well as a standard MEKAL emission model to their extracted spectra, and test their inferred characteristics against already calibrated relations.     

Computational fluid dynamics and material processing (buoyancy and thermocapillary convections in melts during directional crystallization)

Modelisation and solution of heat and mass transfer problems relevant for material processing are generally hard to be handled, as they often involve 3D unsteady flows, viscous mixtures, phase changes, moving liquid-solid fronts, deforming liquid-gas interfaces, etc.… For space applications, material processing benefits of reduced buoyancy convection but can be faced to a strongly increased complexity due to variable g, mainly in manned flight.

Computational techniques used to analyse fluid motions in material processing, accounting for free surface, crystallization front and bulk convection in melt, are reviewed with emphasis to directional crystallization. Hydrodynamics stability and bifurcation analysis are shown to be useful complementary tools for correlating data, and for a better understanding of the physical laws. This last point will be illustrated in the case of the onset of oscillations in metallic melts.     

Closed cycle coolers for space applications

Single stage Stirling coolers providing refrigeration at around 80 K have been developed for space use and are now being produced commercially. Development work is now concentrating on multistage coolers for temperatures below 30 K. This paper describes results from a two stage cooler built at the Rutherford Appleton Laboratory and preliminary tests on a closed cycle 4 K cooler.     

Multiposition alignment of strapdown inertial navigation system

The authors demonstrate that the stationary alignment of strapdown inertial navigation system (SDINS) can be improved by employing the multiposition/technique. Using an observability analysis, it is shown that an optimal two-position alignment not only satisfies complete observability conditions but also minimizes alignment errors. This is done by analytic rank testing of the stripped observability matrix and numerical calculation of the error covariance. It is also shown that an optimal three-position alignment accelerates the convergence of the alignment error compared with two-position alignment     

Lithium Depletion in the Sun: A Study of Mixing Based on Hydrodynamical Simulations

Based on radiation hydrodynamics modeling of stellar convection zones, a diffusion scheme has been devised describing the downward penetration of convective motions beyond the Schwarzschild boundary (overshoot) into the radiative interior. This scheme of exponential diffusive overshoot has already been successfully applied to AGB stars. Here we present an application to the Sun in order to determine the time scale and depth extent of this additional mixing, i.e. diffusive overshoot at the base of the convective envelope. We calculated the associated destruction of lithium during the evolution towards and on the main-sequence. We found that the slow-mixing processes induced by the diffusive overshoot may lead to a substantial depletion of lithium during the Sun's main-sequence evolution. This revised version was published online in June 2006 with corrections to the Cover Date.     

Galileo Probe Measurements of D/H and 3He/4He in Jupiter's Atmosphere

The Galileo Probe Mass Spectrometer measurements in the atmosphere of Jupiter give D/H = (2.6 ± 0.7) × 10-5 3He/4He = (1.66 ± 0.05) × 10-4These ratios supercede earlier results by Niemann et al. (1996) and are based on a reevaluation of the instrument response at high count rates and a more detailed study of the contributions of different species to the mass peak at 3 amu. The D/H ratio is consistent with Voyager and ground based data and recent spectroscopic and solar wind (SW) values obtained from the Infrared Spectroscopic Observatory (ISO) and Ulysses. The 3He/4He ratio is higher than that found in meteoritic gases (1.5 ± 0.3) × 10-4. The Galileo result for D/H when compared with that for hydrogen in the local interstellar medium (1.6 ± 0.12) × 10-5 implies a small decrease in D/H in this part of the universe during the past 4.55 billion years. Thus, it tends to support small values of primordial D/H - in the range of several times 10-5 rather than several times 10-4. These results are also quite consistent with no change in (D+3He)/H during the past 4.55 billion years in this part of our galaxy.     

Ship-mounted satellite tracking antenna with fuzzy logic control

A robust satellite tracking antenna is designed to cope with the sensor imprecision and the highly noisy sea environment. Fuzzy logic is utilized for the controller imprecision and the highly noisy sea environment. Fuzzy logic is utilized for the controller design as well as inaccurate data interpretation. The fuzzy-rule based controller eliminates the need to model the nonlinear and noisy ship-mounted antenna system. With Global Positioning System and the tracking controller the antenna can be brought to a neighborhood of the desired orientation. Spiral search with signal power feedback can then servo the antenna to the true orientation. Computer simulations and antenna experiments verify our design is indeed robust and effective