Phase gradient autofocus-a robust tool for high resolution SARphase correction

The phase gradient autofocus (PGA) technique for phase error correction of spotlight mode synthetic aperture radar (SAR) imagery is examined carefully in the context of four fundamental signal processing steps that constitute the algorithm. We demonstrate that excellent results over a wide variety of scene content, and phase error function structure are obtained if and only if all of these steps are included in the processing. Finally, we show that the computational demands of the fun PGA algorithm do not represent a large fraction of the total image formation problem, when mid to large size images are involved     

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.     

The role of scientific ballooning for exploration of the magnetosphere

The magnetosphere is explored in situ by satellites, but measurements near the low altitude magnetospheric boundary by rockets, balloons and groundbased instruments play a very significant role. The geomagnetic field provides a frame with anisotropic wave and particle propagation effects, enabling remote sensing of the distant magnetosphere by means of balloon-borne and groundbased instruments. Examples will be given of successful studies, with coordinated satellite and balloon observations, of substorm, pulsation and other phenomena propagating both along and across the geomagnetic field. Continued efforts with sophisticated balloon-borne instrumentations should contribute substantially to our understanding of magnetospheric physics.     

Multidimensional Algebraic Polynomial Approximation

A computational algorithm to approximate an n-dimensional function of the form F(x,y,z, ...) by an algebraic polynomial in the least square (LSQ) sense is described. The basis of this algorithm is that the n-dimensional function can be approximated by a set of polynomials in each one of the independent variables x,y,z, ... . This technique is illustrated by deriving the algebraic polynomial approximation in the LSQ sense for three-dimensional functions of the form F(x,y,z).     

Merging Landsat and spaceborne radar data over Tunisia

Synthetic Aperture Radar data were acquired over Tunisia by Seasat in Aug. 1978 and by SIR-A in Nov. 1981. The radar images are contrasted to the Landsat scenes overlapping the same area. The Landsat images were taken in Aug. 1978 and in Sept. 1981. In this study, subareas of the SIR-A and MSS images are registered to the corresponding Seasat data (1313 lines by 1970 samples). The test site is located East of Kairouan, Tunisia. It is a low relief area with subdesertic climatic conditions. This region was selected for it has been surveyed by both Seasat and SIR-A providing perpendicular radar illumination directions. The multispectral and multitemporal coregistered data set enables comparisons between the systems (radar versus MSS, and Seasat versus SIR-A), and change detection in the desertification processes and on the surface of the playas.     

Contactless positioning,manipulation and shaping of liquids by gas bearing for microgravity applications

Contactless positioning, manipulation and shaping of liquids using the gas bearing principle is demonstrated by ground experiments. Extensions to microgravity applications are presented also associated with solidification of molten materials.     

The chromosphere-corona transition zone above an active region

Intensities and profiles of ion emission lines between 1170 A and 1700 A from an active region on the Sun are measured from spectra obtained with the Naval Research Laboratory's High Resolution Telescope and Spectrograph - HRTS. The measurements provide simultaneous determination of line intensities, wavelength shifts and Doppler widths at 50 separate positions in the active region, with spatial resolution of 1 arc second and spectral resolution 0.07 A. Fine structure variation of intensities and gas flow velocities in the temperature range 20,000–200,000 K are determined. The density sensitive line pair I(1486 N IV)/I(1548 C IV) has been used to measure electron pressures. Derived emission measures imply filling factors of 0.05–0.1 to balance the divergence of conductive flux width radiative losses above 60,000 K.     

Low-energy proton interaction with interplanetary magnetohydrodynamic discontinuities

Low energy proton measurements associated with interplanetary MHD discontinuities were detected by the DFH instrument on ISEE-3 spacecraft during the solar maximum period, September 1978–March 1980. The observations were made by three detectors, within eight sectors 45°wide each around the spacecraft, in eight energy channels extended from 35–1600Kev. They confirm that local magnetohydrodynamic conditions, especially discontinuities, significantly affect the propagation of low energy particles in the interplanetary medium.     

Observed and model N(h) profiles for the Bulgarian region

In this paper, combined bottom- and topside ionospheric N(h)-profiles are presented for the Bulgarian region. The profiles were constructed using ground (ionospheric observatories Sofia and Michurin) and satellite (Interkosmos-19) observations /1/.The observatories make quarter-hourly observations; in order to connect bottom and upper parts of the N(h) profile, we selected satellite orbits passing rather near to the observatory (zenith distance lsss than 100 km). Thus the time difference between ground station and satellite measurement was never more than 7.5 min.     

Sporadic emission of ultra high energy gamma rays from crab pulsar

In a further study of sporadic emission from pulsars we find evidence for short lived intense emission from the Crab pulsar.