Scanning Spaceborne Synthetic Aperture Radar with Integrated Radiometer

Spaceborne synthetic aperture radar systems are severely constrained to a narrow swath by ambiguity limitations. Here a vertically scanned-beam synthetic aperture system (SCANSAR) is proposed as a solution to this problem. The potential length of synthetic aperture must be shared between beam positions, so the along-track resolution is poorer; a direct tradeoff exists between resolution and swath width. The length of the real aperture is independently traded against the number of scanning positions. Design curves and equations are presented for spaceborne SCANSARs for altitudes between 400 and 1400 km and inner angles of incidence between 20° and 40°. When the real antenna is approximately square, it may also be used for a microwave radiometer. The combined radiometer and synthetic-aperture (RADISAR) should be useful for those applications where the poorer resolution of the radiometer is useful for some purposes, but the finer resolution of the radar is needed for others.     

Closed-loop, estimator-based model of human posture following reduced gravity exposure

A computational and experimental method is employed to provide an understanding of a critical human space flight problem, posture control following reduced gravity exposure. In the case of an emergency egress, astronauts' postural stability could be life saving. It is hypothesized that muscular gains are lowered during reduced gravity exposure, causing a feeling of heavy legs, or a perceived feeling of muscular weakness, upon return to Earth's 1 g environment. We developed an estimator-based model that is verified by replicating spatial and temporal characteristics of human posture and incorporates an inverted pendulum plant in series with a Hill-type muscle model, two feedback pathways, a central nervous system estimator, and variable gains. Results obtained by lowering the variable muscle gain in the model support the hypothesis. Experimentally, subjects were exposed to partial gravity (3/8 g) simulation on a suspension apparatus, then performed exercises postulated to expedite recovery and alleviate the heavy legs phenomenon. Results show that the rms position of the center of pressure increases significantly after reduced gravity exposure. Closed-loop system behavior is revealed, and posture is divided into a short-term period that exhibits higher stochastic activity and persistent trends and a long-term period that shows relatively low stochastic activity and antipersistent trends.     

Kelvin–Helmholtz Instability: Lessons Learned and Ways Forward

The Kelvin–Helmholtz instability (KHI) is a ubiquitous phenomenon across the Universe, observed from 500 m deep in the oceans on Earth to the Orion molecular cloud. Over the past two decades, several space missions have enabled a leap forward in our understanding of this phenomenon at the Earth’s magnetopause. Key results obtained by these missions are first presented, with a special emphasis on Cluster and THEMIS. In particular, as an ideal instability, the KHI was not expected to produce mass transport. Simulations, later confirmed by spacecraft observations, indicate that plasma transport in Kelvin–Helmholtz (KH) vortices can arise during non-linear stage of its development via secondary process. In addition to plasma transport, spacecraft observations have revealed that KHI can also lead to significant ion heating due to enhanced ion-scale wave activity driven by the KHI. Finally, we describe what are the upcoming observational opportunities in 2018–2020, thanks to a unique constellation of multi-spacecraft missions including: MMS, Cluster, THEMIS, Van Allen Probes and Swarm.     

Tracking of multiple maneuvering targets using multiscan JPDA and IMM filtering

The paper considers the problem of tracking multiple maneuvering targets in the presence of clutter using switching multiple target motion models. A novel suboptimal filtering algorithm is developed by applying the basic interacting multiple model (IMM) approach and the joint probabilistic data association (JPDA) technique. Unlike the standard single-scan JPDA approach, the authors exploit a multiscan joint probabilistic data association (mscan-JPDA) approach to solve the data association problem. The algorithm is illustrated via a simulation example involving tracking of four maneuvering targets and a multiscan data window of length two     

Costas array generation and search methodology

Costas arrays are permutation matrices that provide sequencing schemes for frequency hop in FSK waveforms. Such frequency-shift keying (FSK) waveforms can be designed to have nearly ideal ambiguity function properties in both the time and frequency directions: the Costas property permits at most one coincident tone in autocorrelations in both time and frequency. Costas arrays are found by number-theoretic generators and their extensions, and by exhaustive search methods. Two new extensions of number-theoretic methods are introduced here that find two new Costas arrays. All Costas arrays for orders 24, 25, and 26 are disclosed here, including previously unknown examples.     

Stability analysis of parallel DC-DC converters

We develop analytic methodologies for stability analyses (using nonlinear and linear methodologies) of parallel dc-dc converters (under unsaturated and saturated operating conditions) using their switching model, discrete model (based on nonlinear map), and averaged model. We describe the approach for investigating the behavior of the stable and unstable equilibrium solutions of a parallel dc-dc converter under parametric variations and illustrate the methodology using a load-sharing dc-dc buck converter. For unsaturated operating condition, using bifurcation analysis and Floquet theory, we predict the stability boundary of the nominal solution, determine its postinstability dynamics, and investigate the dependence of the converter dynamics on its initial conditions. Subsequently, we demonstrate the differences in the predictions of the instabilities and instability boundaries using (conventional) linearized averaged (small-signal) and discrete and switching models.     

Temperature effects in the ATIC BGO calorimeter

The Advanced Thin Ionization Calorimeter (ATIC) Balloon Experiment had a successful test flight and a science flight in 2000–01 and 2002–03 and an unsuccessful launch in 2005–06 from McMurdo, Antarctica, returning 16 and 19 days of flight data. ATIC is designed to measure the spectra of cosmic rays (protons to iron). The instrument is composed of a Silicon matrix detector followed by a carbon target interleaved with scintillator tracking layers and a segmented BGO calorimeter composed of 320 individual crystals totaling 18 radiation lengths to determine the particle energy. BGO (Bismuth Germanate) is an inorganic scintillation crystal and its light output depends not only on the energy deposited by particles but also on the temperature of the crystal. The temperature of balloon instruments during flight is not constant due to sun angle variations as well as differences in albedo from the ground. The change in output for a given energy deposit in the crystals in response to temperature variations was determined.     

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.     

Conjecture on superrotation in planetary atmospheres: A diffusion model with mixing length theory

Superrotation on Venus is discussed in the context of comparative planetary atmospheres. In our planetary system, the rigid shell component (global average) of superrotation is ubiquitous (Jupiter, Saturn, Earth, Venus, Mars, Titan). The largest equatorial values of the component are between 25 and 150 m/sec. We present a simplified, heuristic analysis, utilizing mixing length theory to describe the small scale non-linear advections of energy and angular momentum, thereby providing a closure of the dynamic system. This leads to the conjecture that the zonal velocity may be crudely estimated by

, approximating the observed planetary trends; with c the speed of sound, the parameter a being 1 or 2 for geostrophic or cyclostrophic conditions respectively, P_α an effective Prandtl number which becomes less than one when radiative cooling is important, S_o the average stability, Γ the adiabatic lapse rate and γ the ratio of specific heats.     

High-acceleration mass drivers

After testing an early, simplified mass-driver (35 gravity acceleration) a second, designated MD2, was designed and is now operating. MD2 is of axial geometry, with individually powered drive coils of 13.1 cm diameter. Timing is derived through the interruption of light beams by the moving armature (bucket). Electric power is provided by the resonant discharge of sector capacitor banks through silicon-controlled rectifiers in a two-phase, quadrature circuit. The bucket flies in vacuum, guided by passive dynamic eddy-current magnetic forces, those currents flowing in strip conductors lining the inside of a nonconducting vacuum pipe. The initial length of MD2 is 2.5 m, divided equally into acceleration and deceleration. Nominal acceleration is 5000 m/sec². For routine testing an ohmic bucket is used. Quantitative measurements are obtained with a solid bucket carrying two superconducting coils with a current density of 25 kA/cm². A cryogenic station for cooling the bucket to liquid helium temperature is connected to the vacuum pipe. The test program, now begun, will concentrate on guidance and acceleration forces, measurement of drive shielding losses, and possible couplings of drive forces into modes of oscillation.