The Low Metallicity Tail of the Halo Metallicity Distribution Function

Ongoing spectroscopy and photometry of stars selected in the HK objective-prism/interference-filter survey of Beers and colleagues has resulted in the identification of many hundreds of additional stars in the halo (and possibly the thick disk) of the Galaxy with abundances [Fe/H] -2.0. A new calibration of the technique for estimation of metal abundance based on a CaII K index as a function of broadband B - V color is applied to obtain metallicities for stars observed with the SSO 2.3m and INT 2.5m telescopes. This new data is combined with other samples of extremely metal-deficient stars (Ryan and Norris, 1991a; Beers et al., 1992; Carney et al., 1994) to form a large database of objects of low metallicity. The combined sample is examined and compared with expectations derived from a Simple Model of Galactic chemical evolution. There appears to be a statistically-significant deficit of stars more metal-weak than [Fe/H] = -3.0. An abundance of [Fe/H] -4.0 can be taken as the low-metallicity limit for presently-observable stars in the Galaxy.     

Output of a Balanced Frequency Discriminator Driven by a Sine Wave and Gaussian Noise

The output of a realizable balanced frequency discriminator is calculated for an input consisting of a sine wave plus Gaussian noise. Explicit autocorrelation and power spectra are found for one practical embodiment of the discriminator for various input carrier-to-noise ratios, with the carrier tuned to center frequency and also off-center. The formulas also permit the calculation of the output for the case of a spectrum of noise slowly swept through the discriminator. Although qualitatively similar to results previously obtained with an ideal discriminator, substantial differences are also found. Measurements are made that closely verified the theoretical results. No limiting is assumed.     

Soho Observations of Density Fluctuations in Coronal Holes

In recent UVCS/SOHO White Light Channel (WLC) observations we found quasi-periodic variations in the polarized brightness (pB) in the polar coronal holes at heliocentric distances of 1.9 to 2.45 solar radii. The motivation for the observation is the 2.5D MHD model of solar wind acceleration by nonlinear waves, that predicts compressive fluctuations in coronal holes. In February 1998 we performed new observations using the UVCS/WLC in the coronal hole and obtained additional data. The new data corroborate our earlier findings with higher statistical significance. The new longer observations show that the power spectrum peaks in the 10–12 minute range. These timescales agree with EIT observations of brightness fluctuations in polar plumes. We performed preliminary LASCO/C2 observations in an effort to further establish the coronal origin of the fluctuations. This revised version was published online in June 2006 with corrections to the Cover Date.     

Analysis of a Class of Nonlinear Sampled-Data Systems with a Gaussian Input Signal

The problem treated here is the analysis of a class of nonlinear sampled-data systems with a Gaussian input signal. The nonlinear element is a symmetrical limiter (memoryless device) that feeds the linear element. Preceding the limiter is a sampler and zero-order hold unit for data reconstruction. The analysis is predicated on finding an equivalent gain for the nonlinear element such that the mean-square error is a minimum. The nonlinearity is thus replaced by a linear component and the system is then analyzed by conventional techniques. It is also shown that for appropriate sampling rates the sampled-data system is equivalent to a continuous system due to the action of the sampler-hold combination. All calculations pertaining to the limiter input signal require the use of a transfer function that is interior to the feedback loop. However, in developing this transfer function, the overall system is considered so that the system output may be evaluated after the value of equivalent gain has been found.     

A Hybrid Navigation Concept Using a Spinning Satellite?Borne Interferometer and Self?Contained Equipment

This paper analyzes a hybrid navigation concept that uses signals from a radio interferometer mounted on a spinning geostationary satellite, preliminary position estimates from self-contained equipment, and stored a priori information on the past performance of this equipment. The craft-borne processor, optimum in the maximum a posteriori (MAP) sense, is designed to estimate position coordinates using only the incoming radio signals, although improved estimates result if the other two items are available. An error analysis starts with the derivation of an estimation error covariance matrix, whose elements depend on additive receiver noise and the physical parameters of the system. A minimum 1? estimation error in position is obtained by trading off these parameters. The effects of other major error sources, such as tropospheric phase fluctuations, multipath, and craft altitude uncertainty, are added to the estimation error to give a total 1? position error on the order of 3.7 to 5.6 km.     

Space Radiation Damage Measurements in the Earth Synchronous Orbit

For direct measurement of the integrated radiation dose experienced in Earth synchronous orbit, p-i-n diodes were flown as radiation dosimeters on LES-6. The diode, which has a lifetime of 10-4 seconds in the intrinsic region, was originally developed as a neutron dosimeter, but can detect 1-MeV electron fluences as low as 1013 e·cm-2. Observations over three years in orbit are presented.