Some crucial X-ray observations — One or two SNR(s) in Crux?

A new EXOSAT (LE/CMA) observation of the region in Crux (R.A. 11h 45m, Dec. -62°) where Markert et al. (1981) reported the existence of two x-ray SNR's is presented. After cleaning the CMA field from the point source component, due to the UV emission of the numerous stars in the field, the smoothed x-ray contours are compared to the 408 MHz radio map of Caswell et al. (1983). The existence of two, well-separate x-ray emission regions is confirmed by EXOSAT, and the current x-ray/radio picture is not sufficent to distinguish clearly between the assumption of one or two (possibly interacting) SNR's in the region.     

An Upper Bound on Detection Probability for Fluctuating Signals

In the theory of signal detectability, the signal-to-noise ratio (SNR), defined as the quotient of the average received signal energy and the spectral density of the white Gaussian noise, is a fundamental parameter. For a signal which is exactly known, or known except for a random phase, this ratio uniquely defines the detection performance which can be achieved with a matched filter receiver. However, when the signal amplitude is a random parameter, the detection performance is changed and must be determined from the probability density function (pdf) of the amplitude. Relative to the case of a constant signal amplitude, such signal amplitude fluctuation usually degrades performance when a high probability of detection (Pd) is required, but improves performance at low values of Pd; the corresponding change in the required SNR is the so-called signal fluctuation loss Lf. Thus, since Lf in some cases represents an improvement in performance for low values of Pd, a question of at least theoretical interest is: how large might this improvement be, when the class of all signal amplitude pdf's is considered. The solution, presented here, results in a lower bound on the signal fluctuation loss Lf as a function of Pd, or equivalently an upper bound on Pd as a function of SNR. The corresponding most favorable pdf was determined using the Lagrange multiplier technique and results of a numerical maximization are included to provide insight into the general properties of the solution.     

Position and Velocity Estimation Via Bearing Observations

The problem of performing target motion analysis using noisy bearing measurements derived from multiple observation platforms or from a single moving observer is addressed. For the latter case, the properties of an estimator based on a Cartesian model of the process are detailed. Methods of providing estimates both before an observer maneuver, when the process is unobservable, and following an observer maneuver are developed. The results of an experimental study are presented.     

Wide-Aperture Digital VOR

The very high frequency (VHF) omnirange has undergone a number of evolutionary changes in the past 30 years. Early measurements of large errors led to the development of the Doppler VHF omnidirectional irectioal range (VOR) and its use. Further developments have been the precision multilobe VOR and the precision Doppler VOR. Interest in area navigation has led to the desire for a VOR greatly superior to any so far developed. Specifically, the objectives of the improved VOR would be a system that would significantly reduce magnitude of siting errors, provide greater accuracy and use digital techniques to simplify processing. ssing. The wide-aperture digital VOR described herein has been developed to meet these objectives. Its design is based on the crossed-pair interferometer principle where eight such pairs are energized successively by a set of pulses and phase shifts. The time multiplexed signal, detected by the aircraft receiver, is processed and simple digital computations are performedto determine the angular coordinates. The performance improvements have been achieved by the combination of using a 275-ft antenna aperture, and the use of digital techniques to generate the ground-radiated navigational signals and to process them in the airborne processor. Field tests of the feasibility model indicate it is a high performance ance system, capable of achieving an order of magnitude improvement in both site error reduction and in accuracy compared to present VOR systems. The measured accuracies were 0.11-degree standard deviation.     

Ultraviolet spectrometer experiment for the Voyager mission

The Voyager Ultraviolet Spectrometer (UVS) is an objective grating spectrometer covering the wavelength range of 500–1700 Å with 10 Å resolution. Its primary goal is the determination of the composition and structure of the atmospheres of Jupiter, Saturn, Uranus and several of their satellites. The capability for two very different observational modes have been combined in a single instrument. Observations in the airglow mode measure radiation from the atmosphere due to resonant scattering of the solar flux or energetic particle bombardment, and the occultation mode provides measurements of the atmospheric extinction of solar or stellar radiation as the spacecraft enters the shadow zone behind the target. In addition to the primary goal of the solar system atmospheric measurements, the UVS is expected to make valuable contributions to stellar astronomy at wavelengths below 1000 Å.     

Third-Order Loop Filter Design for Acceleration-Rate Tracking

A perfect third-order loop filter design that can be implemented as a digital filter is obtained which minimizes the noiseless steady-state acceleration rate (jerk) error for a fixed loop noise bandwidth. Simulations were performed to obtain transient responses of the third-order loop plus a sample fourth-order loop under a jerk input. The results enable one to obtain a loop design that minimizes the loop noise bandwidth required for a given steady-state jerk error and thus obtain better noise jitter performance.     

ATS-6 Energetic Particle Radiation Measurement at Synchronous Altitude

The Aerospace Corporation energetic electron-proton spectrometer operating on Applications Technology Satellite-6 (ATS-6) detects energetic electrons in four channels between 140 keV and greater than 32 MeV, and measures energetic protons in five energy channels between 2.3 and 80 MeV and energetic alpha particles in three channels between 9.4 and 94 MeV. After more than a year of operation in orbit, the experiment continues to return excellent data on the behavior of energetic magnetospheric electrons as well as information regarding the fluxes of solar protons and alpha particles.     

Calculation of Probability of Detection with Target Scintillation

This paper presents a computational method for the calculation of probability of detection using measured radar target cross-section data. The described method can also be used for probability of detection calculations when the radar target cross section follows a specified probability density function. Using the computational procedure of the paper, a number of curves are generated which can be used for probability of detection calculations with exponential and Gaussian radar target cross-section distributions. The results obtained using theoretical distributions are compared with the corresponding results using actual target cross-section measurements. The results of computer runs are compared to the corresponding values in the literature where available.     

A Concept of Position Fixing by a Passive Mode Navigation Satellite System

A position fix in a passive mode using satellites usually necessitates an expensive computer or lengthy hand calculation. This is the largest drawback of passive navigation and it would be more desirable if the user could find his position by a mere glance at a chart and table, as one uses Loran. The first step toward this goal is to use a synchronous satellite because it simplifies the problem. The next step is to find the position of the user by a Loran type of chart, which is universal, and correct this apparent position by looking at a special table which is made according to the amount of perturbation of both the satellite and the user's position. An example of the position fix along the route between Yokohama to Hawaii is shown. The concept can be extended to orbiting satellites due to the rules which govern the motion of satellites, if the fix accuracy is in the order of 2 to 5 miles. This method should be more accurate than the common sextant and more practical due to the fact that the satellite can be used at any time and in any weather. As a total system, it will be better than Omega because it could provide additional navigation information such as communication or traffic control by using satellites.     

The Solar Chemical Composition

We present our current knowledge of the solar chemical composition based on the recent significant downward revision of the solar photospheric abundances of the most abundant metals. These new solar abundances result from the use of a 3D hydrodynamic model of the solar atmosphere instead of the classical 1D hydrostatic models, accounting for departures from LTE, and improved atomic and molecular data. With these abundances, the new solar metallicity, Z, decreases to Z=0.012, almost a factor of two lower than earlier widely used values. We compare our values with data from other sources and analyse a number of impacts of these new photospheric abundances. While resolving a number of longstanding problems, the new 3D-based solar photospheric composition also poses serious challenges for the standard solar model as judged by helioseismology.