Random Characteristics of the Type II Phase-Locked Loop

The results of a study of the characteristics of the second-order Type II phase-locked loop with a Gaussian noise input, and obtained by digital computer simulation, are presented. The digital simulation is described and the random state variables are defined such that their characteristics can be interpreted in terms of existing phase portraits of autonomous phase-locked loops. The statistics associated with the state variables, which are phase error and a measure of frequency error, and those associated with the number of cycles skipped and the mean time to unlock, are given.     

Creating a productive international partnership in the Vision for Space Exploration

When US President George W. Bush on 14 January 2004 announced a new US “Vision for Space Exploration”, he called for international participation in “a journey, not a race”, a call received with skepticism and concern elsewhere. But, after a slow start in implementing this directive, during 2006 NASA has increased the forward momentum of action on the program and of discussions on international cooperation in exploring “the Moon, Mars, and beyond”. There are nevertheless a number of significant top-level issues that must be addressed if a cooperative approach to human space exploration is to be pursued. These include the relationship between utilization of the ISS and the lunar exploration plans, integration of potential partners’ current and future capabilities into the exploration plans, and the evolving space-related intentions of other countries.     

Space Telescope observations of aurorae on the giant planets

For the distant giant planets, Uranus and Neptune, the observation of aurorae may be the best astronomical technique for the detection of planetary magnetic fields, with implications for the structure and composition of their interiors. Aurorae may be detected by emssion of H I Ly α (1216 Å) and by H₂ bands near 1600 Å. The latter are important for very faint aurorae because there is essentially no planetary, interplanetary or geocoronal scattering of sunlight to contaminate the signal. For Uranus, present IUE results suggest the presence of a strong aurora at Ly α, but the background and instrument noise levels are very high compared to the apparent signal. At 1600 Å, the IUE instrument noise renders the H₂ emission bands on Uranus marginal at best. No aurora has yet been observed on Neptune. For Jupiter, where the existence and general characteristics of the magnetic field are well established, there is disagreement between ground-based infrared and space-borne ultraviolet observations of the location of the aurorae. For all four giant planets, Space Telescope can improve upon the quality of current optical observations. For spectroscopy, the low resolution mode of the High Resolution Spectrograph (HRS) is particularly well suited to auroral observations because of its spectral range, adequate resolution and high sensitivity. For ultraviolet imaging through appropriate filters, the ST spatial resolution, expected to be of order 5 hundredths of an arc second, is also well suited to determine the spatial properties of the aurorae.     

The future of space plasma simulation

Space plasma simulation is a subject which is in its infancy, but which is already having an important impact on space science. Its growth is being spurred onward by the continuing increase in capacity (speed and memory) of computers and by advances in the sophistication of numerical models. These advances are making it possible to simulate more realistic situations using more complex models. Already significant three-dimensional MHD calculations of the magnetosphere and its interaction with the solar wind have been carried out. In addition multi-dimensional particle simulations are illuminating many of the microscopic physics processes which go on (instabilities, saturation levels and wave nonlinearity, shock structure, etc.). Notwithstanding these advances, the surface has only been scratched; many challenges and opportunities are provided by simulation both for the space physicist and the model builders (also for computer designers). In MHD models more physics need to be included (Hall effect, gyroviscosity, accurate models of boundaries, how do we put microscopic physics effects into macroscopic codes, etc.). For model builders correct treatments of systems containing a large range of important space and time scales, magnetic field strengths, Alfven wave velocity, etc. present real challenges. What are the best ways to diagnose these complex models and obtain meaningful information? What quantities should be looked at? How should they be displayed? A discussion of the promises, the prospects, and the challenges of the above topics will be given with examples taken from recent work.     

Optimal aeroassisted return from high earth orbit with plane change

This paper gives a complete analysis of the problem of aeroassisted return from a high Earth orbit to a low Earth orbit with plane change. A discussion of pure propulsive maneuver leads to the necessary change for improvement of the fuel consumption by inserting in the middle of the trajectory an atmospheric phase to obtain all or part of the required plane change. The variational problem is reduced to a parametric optimization problem by using the known results in optimal impulsive transfer and solving the atmospheric turning problem for storage and use in the optimization process. The coupling effect between space maneuver and atmospheric maneuver is discussed. Depending on the values of the plane change i, the ratios of the radii, $\text{n =}\text{r}{}_1\text{r}{}_2$ between the orbits and $\text{a =}\text{r}{}_2\text{R}$ between the low orbit and the atmosphere, and the maximum lift-to-drag ratio E1 of the vehicle, the optimal maneuver can be pure propulsive or aeroassisted. For aeroassisted maneuver, the optimal mode can be parabolic, which requires only drag capability of the vehicle, or elliptic. In the elliptic mode, it can be by one-impulse for deorbit and one or two-impulse in postatmospheric flight, or by two-impulse for deorbit with only one impulse for final circularization. It is shown that whenever an impulse is applied, a plane change is made. The necessary conditions for the optimal split of the plane changes are derived and mechanized in a program routine for obtaining the solution.     

The effects of composting on the nutritional composition of fibrous bio-regenerative life support systems (BLSS) plant waste residues and its impact on the growth of Nile tilapia (Oreochromis niloticus)

Utilization of bio-regenerative life support systems (BLSS) plant waste residues as a nutritional source by Nile tilapia (Oreochromis niloticus) has proven problematic as a result of high concentrations of fibrous compounds in the plant waste residues. Nutritional improvement of plant waste residues by composting with the oyster mushroom (Pleurotus ostreatus), and the effects on growth and nutrient utilization of Nile tilapia fed such residues were evaluated. Five Nile tilapia (mean weight = 70.9 ± 3.1 g) were stocked in triplicate aquaria and fed one of two experimental diets, cowpea (CP) and composted cowpea (CCP), twice daily for a period of 8 weeks. Composting of cowpea residue resulted in reduced concentrations of nitrogen-free extract, hemi-cellulose and trypsin inhibitor activity, though trypsin inhibitor activity remained high. Composting did not reduce crude fiber, lignin, or cellulose concentrations in the diet. No significant differences (P < 0.05) were observed in weight gain, specific growth rate, survival rate, daily consumption, and food conversion ratio between tilapia fed CP and CCP. These results suggest that P. ostreatus is not a suitable candidate for culture in conjunction with the culture of Nile tilapia. Additional work is needed to determine what, if any, benefit can be obtained from incorporating composted residue as feed for Nile tilapia.     

Stellar winds of massive stars in M31

We have obtained the first UV high resolution spectra of hot luminous stars in M31 with the FOS onHubble Space Telescope. The spectra, combined with optical spectroscopic and photometric observations, enable us to study their stellar winds and photospheric parameters. We derive mass-loss rates and velocity laws from the wind line profiles, with the SEI method, as well as information on abundances. The wind lines and photospheric spectra are compared with galactic stars of the same spectral type.The spectra analyzed so far indicate that the stars have mass-loss rates comparable or slightly lower than galactic stars of the same spectral type, but possibly different velocity laws in their winds. The spectra of two stars are discussed here.     

Deceleration and heating constrained footprint of shuttle vehicles

With increasing frequency in shuttle operation, it is of interest to have more than one or two landing fields within the boundary of the reachable area of the reentry vehicle. This boundary, called the footprint, depends on the aerodynamic characteristics of the vehicle and is severely restricted by the deceleration and heating constraints imposed upon the atmospheric reentry trajectory. This paper gives a general assessment of the footprint as a function of various deceleration and heating constraints. The difficulties in the computation of the three-dimensional reentry trajectories with optimal modulation in both the angle-of-attack and the bank angle are alleviated by the following devices: (a) nondimensionalizing of the equations of motion and use of the density as the altitude variable; (b) use of the classical integrals of the motion; (c) transformation of the adjoint variables into physical variables; and (d) spherical rotation of the coordinates.