A chirp scaling approach for processing squint mode SAR data

Image formation from squint mode synthetic aperture radar (SAR) is limited by image degradations caused by neglecting the range-variant filtering required by secondary range compression (SRC). Introduced here is a nonlinear FM chirp scaling, an extension of the chirp scaling algorithm, as an efficient and accurate approach to range variant SRC. Two methods of implementing the approach are described. The nonlinear FM filtering method is more accurate but adds a filtering step to the chirp scaling algorithm, although the extra computation is less than that of a time domain residual compression filter. The nonlinear FM pulse method consists of changing the phase modulation of the transmitted pulse, thus avoiding an increase in computation. Simulations show both methods significantly improve resolution width and sidelobe level, compared with existing SAR processors for squint angles above 10 deg for L-band and 20 deg for C-band     

Cramer-Rao bounds and estimation of the parameters of the Gumbeldistribution

Maximum Likelihood (ML) algorithms and Cramer-Rao (CR) bounds for the location and scale parameters of the Gumbel distribution are discussed. First we consider the case in which the scale parameter is known, obtaining the estimator of the location parameter by solving the likelihood equation and then evaluating its performance. We next consider the case where both the location parameter and the scale parameter are unknown and need to be estimated simultaneously from the reference samples. For this case, performance is analyzed by means of Monte Carlo simulation and compared with the asymptotic CR bound     

Bistatic RCS of Complex Objects near Forward Scatter

A technique is presented for predicting the bistatic radar cross section (RCS) of an opaque, arbitrarily shaped object at angles near forward scatter. Babinet's principle is employed to approximate the object's scattering pattern as the two-dimensional Fourier transform of its silhouette. Calculations for spheres and right-circular cylinders are shown to agree with predictions based on exact Mie theory, method of moments, and physical optics. Restrictions on the applicability of the method are presented.     

Comet Halley: Nucleus and jets (results of the VEGA mission)

The VEGA-1 and VEGA-2 spacecraft made their closest approach to Comet Halley on 6 and 9 March, respectively. In this paper those results of the onboard imaging experiment which were obtained around closest approach are discussed. The nucleus of the comet was clearly identifiable as an irregularly shaped object, with overall dimensions of (16±1)×(8±1)×(8±1) km. The nucleus rotates in the prograde sense about an axis nearly perpendicular to the orbital plane with a period of 53±2 hours. Its albedo is only $\text{0.04±}\text{0.02}\text{0.01}$ Many of the jet features observed during the second fly-by have been spatially reconstructed. Their sources form a quasi-linear structure on the surface. The dust above the surface is shown to be generally optically thin with the exception of certain specific dust jets. Brightness features on the surface are clearly seen. Correlating our data with other measurements, we conclude that the dirty snow-ball model will probably need to be revised.     

Low-harmonic three-phase inverters with nonpulsating terminalcurrents

A basic class of three-phase inverters with nonpulsating terminal currents, namely, the four-topology four-inductor four-capacitor class is derived and studied. The expressions of duty-ratios required for low-harmonic dc-to-three-phase-ac conversion, ranges of output voltage variation, and component stresses for given specifications of inverters are derived. The characteristics of inverters include low-harmonic power conversion, low component stresses, variable-frequency variable-amplitude output voltages of values both below and above the dc input value, and the use of minimum number of inductors to achieve nonpulsating terminal currents     

New atmospheric temperature results from the pioneer Venus entry probes

New Venus temperature profiles, derived from engineering measurements of the Small Probe Net Flux Radiometer (SNFR) instruments generally confirm to high accuracy the vertical structure and horizontal temperature contrast results of Seiff et al. /1/.     

Sharp Boundaries of Solar Wind Plasma Structures and an Analysis of Their Pressure Balance

This work is devoted to studying the sharp boundaries of small-scale structures of the solar wind according to the data of measurements with high time resolution onboard the INTERBALL-1 satellite and simultaneous measurements of the WIND spacecraft. Such issues as the character of change of various plasma and magnetic field parameters on these boundaries, the duration of boundary passage and the balance of the total (thermal plus magnetic) pressure on the boundaries of the structures are considered. On the basis of the vast statistical material available, the typical conditions in the solar wind are investigated, in which such sharp boundaries are observed.__________Translated from Kosmicheskie Issledovaniya, Vol. 43, No. 3, 2005, pp. 163–170.Original Russian Text Copyright © 2005 by Riazantseva, Khabarova, Zastenker, Richardson.     

Simulation of launch and re-entry acceleration profiles for testing of shuttle and unmanned microgravity research payloads

Microgravity experiments designed for execution in Get-Away Special canisters, Hitchhiker modules, and Reusable Re-entry Satellites will be subjected to launch and re-entry accelerations. Crew-dependent provisions for preventing acceleration damage to equipment or products will not be available for these payloads during flight; therefore, the effects of launch and re-entry accelerations on all aspects of such payloads must be evaluated prior to flight. A procedure was developed for conveniently simulating the launch and re-entry acceleration profiles of the Space Shuttle (3.3 and 1.7 × g maximum, respectively) and of two versions of NASA's proposed materials research Re-usable Re-entry Satellite (8 × g maximum in one case and 4 × g in the other). By using the 7 m centrifuge of the Gravitational Plant Physiology Laboratory in Philadelphia it was found possible to simulate the time dependence of these 5 different acceleration episodes for payload masses up to 59 kg. A commercial low-cost payload device, the “Materials Dispersion Apparatus” of Instrumentation Technology Associates was tested for (1) integrity of mechanical function, (2) retention of fluid in its compartments, and (3) integrity of products under simulated re-entry g-loads. In particular, the sharp rise from 1 g to maximum g-loading that occurs during re-entry in various unmanned vehicles was successfully simulated, conditions were established for reliable functioning of the MDA, and crystals of 5 proteins suspended in compartments filled with mother liquor were subjected to this acceleration load.     

On the generation of the stimulated electromagnetic emission. The computer simulation results

Results of computer simulations of the broad continuum (BC) feature of stimulated electromagnetic emissions (SEE) of the ionosphere are presented. The simulations were performed using the model of BC generation based on the double transformation of electromagnetic waves to upper hybrid (UH) waves and back with artificial small scale irregularities (ASI) of plasma density. An induced scattering of the UH waves by thermal ions provided a wide spectrum of SEE. For the simulations an empirical model of the ASI spectrum, development and decay was used.     

Space radiation quality factor investigations with “NAUSICAA”-device on board the “MIR” space station

The estimation of radiation quality factor in space flights is a serious problem of space dosimetry. The solution of this problem is based on LET-spectra measurements. The “NAUSICAA”-device installed on the “MIR” station has a detector with a large geometric factor, that gives the possibility of measuring LET-spectra with sufficient statistic in relatively short time interval and hence the corresponding Q. LET-spectra are accumulated through 24 hour periods and equivalent dose (H), absorbed dose (D), quality factor and flux (F) are measured every 10 minutes. The obtained data permit the estimate of the diurnal Q and Q in South Atlantic Anomaly. These values vary in the range of 1.4 – 2.2. The analysis of these values including their comparison with the calculated results is given in this paper.