Tests of Direct-Sampling Coherent Detection with a Laboratory Analog-to-Digital Converter

Phase accuracy and simplicity advantages of a direct-sampling coherent detection (DCD) system were demonstrated by computer simulation in earlier correspondence. This correspondence reviews the DCD approach and reports the results of experiments involving digitized samples of analog signals taken by a high-quality, off-the-shelf A/Dconverter. IF signals near 10, 20, and 30 MHz were sampled at 5 MHzresulting in rms DCD phase errors generally below 0.50 compared witherrors less than 0.20 computed in the earlier simulations.     

Foliage attenuation and backscatter analysis of SAR imagery

The ability of a synthetic aperture radar (SAR) to detect targets under foliage is in part determined by the attenuation suffered by radiation propagating through the foliage and the backscatter from the foliage. MIT Lincoln Laboratory made measurements of foliage attenuation and backscatter using the NASA/JPL-UHF, L-, C-band fully-polarimetric SAR in July 1990. In this experiment, a 48 km² forested area near Portage, Maine was imaged. Twenty-seven 8 ft trihedral corner reflectors were arrayed throughout the imaged area in order to measure foliage attenuation. Ground truth was recorded at the time of the experiment in order to correlate the attenuation and backscatter results with foliage biophysical properties. The probability densities for foliage attenuation and for backscatter are determined as functions of frequency, polarization, and depression angle     

Adaptive Estimation for a System with Unknown Measu rement Bias

An adaptive state estimator for passive underwater tracking of maneuvering targets is developed. The state estimator is designed specifically for a system containing unknown or randomly switching biased measurements. In modeling the stochastic system, it is assumed that the bias sequence dynamics can be modeled by a semi-Markov process. By incorporating the semi-Markovian concept into a Bayesian estimation technique, an estimator consisting of a bank of parallel, adaptively weighted, Kalman filters has been developed. Despite the large and randomly varying measurement biases, the proposed estimator, provides an accurate estimate of the system states.     

Angle and polarization estimation in a coherent signal environment

It is shown how a uniform linear array of crossed dipoles may be used with the ESPRIT algorithm and spatial smoothing techniques to estimate the arrival directions and polarizations of incoming coherent plane waves. Some examples showing typical performance are presented. One method of smoothing can be used where it is necessary to estimate both the arrival angles and polarizations of signals. Two other methods can be used when only the arrival angles are of interest     

A model of radiation conditions during spacecraft flights in the interplanetary space and in the Earth's magnetosphere.

Based on the available measurement data, simulations of radiation conditions during spacecraft flights in the interplanetary space and in the Earth's and Jupiter's radiation belts has been carried out. The > or = 10 MeV and > or = 30 MeV solar flare proton fluence forecast has been proposed for Cycle 22. Radiation conditions due to both magnetospheric electrons and protons and to solar flare protons, magnetic rigidity cutoff being taken into account, have been evaluated on spacecraft trajectories in the Earth's and Jupiter's magnetospheres.     

Possible radio JETS in GX 1+4

GX 1+4 is an X-ray pulsar with a nominal period of 2 or 4 min. and has the highest of 0.22 yr⁻¹ among the X-ray pulsars. The source is identified with an optical counterpart V2116 Oph, however no binary period has been reported so far. There are several features of the source like , variable pulse shape, period history and the spectral changes which do not fit well with our present understanding of the source.

The radio observation of the galactic center region were made using Molonglo observatory Synthesis Telescope operating at a frequency of 843 MHz. A 12 hr observation was made with the field center at the position coordinates of the optical counterpart. No significant radio emission was seen corresponding to the center position.

The synthesis map however, showed the presence of two regions of radio enhancements almost equidistant from the optical source in the diametrically opposite directions/1/. The data is interpreted as due to synchrotron emission produced by the expanding jets of GX 1+4, similar to the radio jets observed in galaxies. Radio jets have been inferred for three galactic X-ray binary sources namely, Sco X-1, Cyg X-3, and SS 433. However, for all these sources no X-ray pulsation has been detected so far. The present observations if confirmed will be the first case of radio jets in a stellar system containing an X-ray pulsar.     

1E 1740.7-2942 revisited: Confirmation of its identification with the core of a double sided radio jet structure

After more than two years of successful in-orbit operations, the γ-ray coded aperture SIGMA telescope has accumulated 800 hours of live-time observations of the Galactic Center region, including the remarkable hard source identified with the X-ray source 1E 1740.7–2942. The long-term behavior of the soft γ-ray emission of 1E 1740.7–2942, as determined from the SIGMA survey, supplemented with previously available soft γ-ray data, leads to its identification with a singular radio source, which consists of a double sided radio jet emanating from a compact variable core whose variability is correlated with that of the soft γ-ray source. The compact radio core, which lies well inside the improved soft γ-ray (40–150 keV) error circle (27″ radius) derived from the high-resolution SIGMA survey, is also inside the ROSAT and TTM error circle derived respectively in the soft and hard X-ray bands.     

Detailed magnetic structure and stochastic threshold

Non-linear evolution of reconnection is too slow to explain-by itself-fast phenomena such as internal disruptions in tokamaks or flares in the solar atmosphere. It has been proposed that the change in topology could lead to field line stochastization, and hence to highly increased transport. An important objection to this idea is that the increase of transport coefficients should smoothly follow the amplitude of the perturbation, while the observations show a catastrophic transition. We have shown 1) that the shape of the structures (and not only their size or amplitude) evolve during non-linear evolution 2) that the stochastic threshold can be strongly influenced by the detailed shape. Therefore, sharp transitions can appear during the evolution, due to the combination of these two effects. We will first consider an idealized situation, namely the two-waves problem in slab geometry, for which we will study the effect of the separatrix shape on the stochastic threshold. In the second part, we will present an application to the internal disruption in tokamaks, with anm=1 perturbation in a toroidal magnetic configuration. In the last part, we will discuss possible applications of the studies on shape effects to the behaviour of trapped particles in structures of the solar atmosphere.     

Balloon-borne far-infrared spectrophotometry of the galactic center region

Far-infrared observations of the Galactic Center have been carried through with the MPE Im balloon-borne telescope “Golden Dragon”. The measurements are composed of photometric scanning (33–95 μm) of the inner 4′×4′ and low resolution spectroscopy (δ^ν = 10 cm^?1) of the center and of a position approximately 1.5′ to the north. A Mars spectrum has been obtained for calibration. The spatial resolution of the photometry map is increased using the Maximum Entropy Method and the resulting map is compared to other observations in the same and other spectral regions. A clear asymmetry in the ring-like structure around the center indicates the presence of noncircular motions. The shape of the spectra is fairly smooth with at least no prominent dust features. A simple modelling shows a drastic increase of column density within 2 pc from the center and a modest drop over the next 3 pc to the north.